INAUGURAL ARTICLE by a Recently Elected Academy Member:Concepts relating magnetic interactions,intertwined electronic orders,and strongly correlated superconductivity

Unconventional superconductivity (SC) is said to occur when Cooper pair formation is dominated by repulsive electron–electron interactions, so that the symmetry of the pair wave function is other than an isotropic s-wave. The strong, on-site, repulsive electron–electron interactions that are the proximate cause of such SC are more typically drivers of commensurate magnetism. Indeed, it is the suppression of commensurate antiferromagnetism (AF) that usually allows this type of unconventional superconductivity to emerge. Importantly, however, intervening between these AF and SC phases, intertwined electronic ordered phases (IP) of an unexpected nature are frequently discovered. For this reason, it has been extremely difficult to distinguish the microscopic essence of the correlated superconductivity from the often spectacular phenomenology of the IPs. Here we introduce a model conceptual framework within which to understand the relationship between AF electron–electron interactions, IPs, and correlated SC. We demonstrate its effectiveness in simultaneously explaining the consequences of AF interactions for the copper-based, iron-based, and heavy-fermion superconductors, as well as for their quite distinct IPs.Emergence, the coming into being through evolution, is an important concept in modern condensed matter physics (1). Superconductivity is a classic example of emergence in the realm of quantum matter: as the energy scale decreases, the effective electron–electron interactions responsible for Cooper pairing and thus superconductivity evolves from the elementary microscopic Hamiltonian through unanticipated modifications (2). This evolution is why it is so difficult to derive superconductivity (SC) from first principles. Finding the microscopic mechanism of Cooper pairing means discovering the nature of the ultimate effective electron–electron interaction at the lowest energy scales.In the last three decades, unconventional (3–5) forms of SC have been discovered in many strongly correlated (repulsive electron–electron interaction) systems. These materials fascinate a lay person for their high superconducting transition temperatures and therefore the potential for revolutionary applications in power generation/transmission, transport, information technology, science, and medicine. They intrigue (and challenge) physicists to identify the mechanism of their high pairing-energy scale and because of the many intertwined (6, 7) electronic phases (IPs) that have been discovered in juxtaposition with the unconventional superconductivity. These IPs have been hypothesized to “arise together from one parent state” such that “the various order parameters are intertwined rather than simply competing with each other” (7). The best known and most widely studied examples of such materials include the copper-based (8–11) and iron-based (12–14) high-temperature superconductors, the heavy fermion superconductors (15–17), and the organic superconductors (18). One thing commonly noted in these systems is that SC normally borders antiferromagnetism (AF): in the phase diagram spanned by temperature and a certain control parameter (chemical-doping, pressure, etc.), an SC dome stands adjacent to the AF phase (Fig.1). However, the precise way the two phases are connected varies greatly from system to system.Open in a separate windowFig. 1.Schematic phase diagram of unconventional superconductors. Starting from a robust commensurate AF, a control parameter, such as carrier density or pressure, is varied so that the critical temperature T_AF of the AF phase diminishes. Eventually, an unconventional SC phase appears at higher values of the control parameter, and its critical temperature T_c is usually dome shaped. The intervening gray region is where the AF phase and the SC phase connect. It is here that the intertwined phases of electronic matter have typically been discovered. The characteristics of the IPs are highly distinctive to each system, as is the precise way (e.g., first order, coexistence, quantum critical) that the AF–SC connection occurs. By contrast, the appearance of unconventional SC phase on suppression of an AF state is virtually universal.Another very common observation is the appearance of other ordered phases of electronic matter that intertwine with the SC. These exotic intertwined phases (IPs) occur in the terra incognita between the SC and the AF (Fig. 1, gray). Examples include the charge/spin density wave (19–21) and intra-unit-cell symmetry breaking (21–23) orders in the copper-based superconductors and the nematic order (24, 25) in the iron-based superconductors. A key long-term objective for this field has therefore been to identify a simple framework within which to consider the relationship between the antiferromagnetic interactions, the intertwined electronic orders that appear at its suppression, and the correlated superconductivity.Because in all of the systems considered here SC emerges from the extinction of AF, it is widely believed that the effective electron–electron interaction triggering the Cooper pairing could be AF in form. In that case, of course, the same argument could apply to the other intertwined electronic phases. These ideas motivate the assertion that AF effective electron–electron interactions may drive both the correlated SC and the other IPs. Until recently, however, there has been little consensus on this issue. One reason is that the experimental evidence for many such intertwined states has only been firmly established in recent years. Another reason is that, although magnetism in proximity to unconventional SC appears universal, the nature of the IPs changes from system to system for reasons that appear mysterious.In this paper, we therefore explore the plausibility that an AF effective interaction could be the driving force for both the unconventional SC and the intertwined orders in the copper-based, iron-based, and heavy fermion superconductors. [We omit discussion of organic superconductors (26, 27) for the sake of brevity.] Here we will not try to rigorously solve for the ground state under different conditions. Our goal is to ask whether the known IPs are the locally stable mean-field phases when the sole effective electron–electron interaction is AF. We understand that the actual effective interactions may be more complex than this simplest AF form; we deliberately omit these details with the goal of identifying a simple framework within which all of the relevant phenomena can be considered. Two very recent preprints based on a related approach, but focusing only on the copper-oxide superconductors, have been published (28, 29).     

Ab initio study of hot electrons in GaAs

Hot carrier dynamics critically impacts the performance of electronic, optoelectronic, photovoltaic, and plasmonic devices. Hot carriers lose energy over nanometer lengths and picosecond timescales and thus are challenging to study experimentally, whereas calculations of hot carrier dynamics are cumbersome and dominated by empirical approaches. In this work, we present ab initio calculations of hot electrons in gallium arsenide (GaAs) using density functional theory and many-body perturbation theory. Our computed electron–phonon relaxation times at the onset of the Γ, L, and X valleys are in excellent agreement with ultrafast optical experiments and show that the ultrafast (tens of femtoseconds) hot electron decay times observed experimentally arise from electron–phonon scattering. This result is an important advance to resolve a controversy on hot electron cooling in GaAs. We further find that, contrary to common notions, all optical and acoustic modes contribute substantially to electron–phonon scattering, with a dominant contribution from transverse acoustic modes. This work provides definitive microscopic insight into hot electrons in GaAs and enables accurate ab initio computation of hot carriers in advanced materials.Hot carriers (HCs) generated by the absorption of light or injection at a contact are commonly found in many advanced technologies (1–9). In electronics, the operation of high-speed devices is controlled by HC dynamics, and HC injection is a key degradation mechanism in transistors (10, 11). In solar cells and plasmonics, recent work has focused on extracting the kinetic energy of HCs before cooling (7, 9), a process defined here as the energy loss of HCs, ultimately leading to thermal equilibrium with phonons. HC dynamics is also crucial to interpret time-resolved spectroscopy experiments used to study excited states in condensed matter (12). This situation has sparked a renewed interest in HCs in a broad range of materials of technological relevance.Experimental characterization of HCs is challenging because of the subpicosecond timescale associated with the electron–phonon (e-ph) and electron–electron (e-e) scattering processes regulating HC dynamics. For example, HCs can be studied using ultrafast spectroscopy, but microscopic interpretation of time-resolved spectra requires accurate theoretical models. However, modeling of HCs thus far has been dominated by empirical approaches, which do not provide atomistic details and use ad hoc parameters to fit experiments (13, 14). Notwithstanding the pioneering role of these early studies, the availability of accurate ab initio computational methods based on density functional theory (DFT) (15) and many-body perturbation theory (16) enables studies of HCs with superior accuracy, broad applicability, and no need for fitting parameters.Hot electrons in gallium arsenide (GaAs) are of particular interest because of the high electron mobility and multivalley character of the conduction band. Electrons excited at energies greater than ∼0.5 eV above the conduction band minimum (CBM) can transfer from the Γ to the L and X valleys, with energy minima at ∼0.25 and ∼0.45 eV above the CBM, respectively (17). Such intervalley scattering processes play a crucial role in hot electron cooling and transport at high electric fields.Ample experimental data exist on hot electron transport and cooling in GaAs (12, 18–21). The interpretation of these experiments relies on Monte Carlo simulations using multiple parameters fit to experimental results. For example, Fischetti and Laux (13) used two empirical deformation potentials to model electron scattering induced by optical and acoustic phonons. Additionally, Fischetti and Laux (13) used simplified band structure and phonon dispersions. We note that, because multiple parameter sets can fit experimental results, the HC scattering rates due to different physical processes obtained empirically are not uniquely determined (13, 14).Although heuristic approaches can provide some insight into HC dynamics of well-characterized materials (e.g., GaAs), there is a lack of generally applicable, predictive, and parameter-free approaches to study HCs.Here, we carry out ab initio calculations of hot electrons in GaAs with energies up to 5 eV above the CBM. Our ability to use extremely fine grids in the Brillouin zone (BZ) allows us to resolve hot electron scattering in the conduction band with unprecedented accuracy. We focus here on three main findings. First, our overall computed e-ph scattering rates are in excellent agreement with those in previous semiempirical calculations in ref. 13 that combine multiple empirical parameters. The advantage of our approach is the ability to compute the electronic band and momentum dependence of the e-ph scattering rates without fitting parameters. Second, we show that both optical and acoustic modes contribute substantially to e-ph scattering, with a dominant scattering from transverse acoustic (TA) modes. This result challenges the tenet that HCs lose energy mainly through longitudinal optical (LO) phonon emission. Third, our calculations provide valuable means for quantitative interpretation of experiments of hot electron cooling in GaAs. In particular, the ultrafast (∼50 fs) e-ph relaxation times that we compute at the onset of the X valley are in excellent agreement with the fastest decay time observed in ultrafast optical experiments (18, 19, 21). This signal was attributed by some (18) to e-e scattering and by others (21) to e-ph scattering. The excellent agreement with time decay signals in time-resolved experiments shows the dominant role of e-ph scattering for hot electron cooling at low carrier density.Our approach combines electronic band structures computed ab initio using the GW (where G is the Green function, W is the screened Coulomb potential, and GW is the diagram employed for the electron exchange-correlation interactions) method (16) with phonon dispersions from density functional perturbation theory (DFPT) (22), and it is entirely free of empirical parameters. We compute the e-ph matrix elements using a Wannier function formalism (23) on very fine BZ grids and are able to resolve e-ph scattering for the different conduction band valleys. The e-e rates for hot electrons—also known as impact ionization (II) rates—are computed using the GW method (16, 24), and thus include dynamical screening effects. Additional details of our calculations are discussed in Methods.     

Observation time scale,free-energy landscapes,and molecular symmetry

When structures that interconvert on a given time scale are lumped together, the corresponding free-energy surface becomes a function of the observation time. This view is equivalent to grouping structures that are connected by free-energy barriers below a certain threshold. We illustrate this time dependence for some benchmark systems, namely atomic clusters and alanine dipeptide, highlighting the connections to broken ergodicity, local equilibrium, and “feasible” symmetry operations of the molecular Hamiltonian.Free-energy surfaces or landscapes play a key role in the analysis of structure, dynamics, and thermodynamics in molecular science. In the present contribution we explain how comparison of theory and simulation with experiment should account for the time scale of observation, using atomic clusters and alanine dipeptide for illustration. First we explain the connection between the time scale and the free-energy barriers that separate different states. This connection is related to lumping schemes that group together local minima if they are connected by barriers below a certain threshold (1–4). To make this connection we use the Eyring–Polanyi formulation of the rate constant aswhere is the Boltzmann constant, h is Planck’s constant, T is the temperature, and is the activation free energy (5–7). Assuming first-order kinetics, the decay of an initial reactant concentration as a function of time t iswhere τ is the average lifetime. Hencerelates the free-energy barrier to the average lifetime, which we will associate with the observation time scale. If we lump together states connected by barriers less than a given threshold , as in the recursive regrouping scheme (1–4), then the number of free-energy minima and transition states will change when passes through values corresponding to barrier heights on the landscape in question. In the canonical ensemble the occupation probability and free energy of a group of minima J arewhere is the partition function for potential energy minimum j. The total rate constant for direct isomerization of minimum j to minimum l, , is obtained by summing over all of the transition states that directly connect the two minima, and the free energy of the transition states connecting groups J and L is thenwithHere a transition state is defined as a stationary point with a single negative Hessian eigenvalue (8).The importance of the experimental observation time scale has certainly been recognized before, in the context of local equilibration and hierarchical relaxation (9–12), broken ergodicity (13), and for single-molecule experiments (14–17). However, the connection between these applications for spin models, structural glass-formers, and proteins, to feasible operations of the Hamiltonian, introduced to analyze the spectra of nonrigid molecules (18), is perhaps less well appreciated. In the present contribution we connect these diverse topics using an explicit lumping scheme based on time scale or free-energy thresholds, with visualization using disconnectivity graphs (19, 20).The quantum-mechanical molecular Hamiltonian is invariant to operations of the complete nuclear permutation–inversion group, consisting of any combination of permutations of atoms of the same element, i.e., for element 1, for element 2, etc. (18, 21). The factor of 2 corresponds to inversion of all nuclear and electronic coordinates through the center of mass. The classification of molecular energy levels can be simplified by considering a subgroup whose elements are the feasible permutation–inversion operations (18). The feasible operations correspond to pathways in nuclear configuration space that are insurmountable on the given experimental time scale. These considerations are clearly the same principles that we must account for when analyzing broken ergodicity for glassy landscapes (22), where partition functions can be written explicitly as a function of the observation time scale (23), or for symmetry breaking by an applied field (24). The examples discussed below provide clear illustrations of how these ideas are connected.Disconnectivity graphs (19, 20) based on free energy (1, 25) provide a powerful tool for visualizing the effect of a changing observation time scale, and are related to the earlier “energy lid” representation (9). The “filling in” problems that arise for free-energy surfaces projected onto order parameters (26–29) are avoided by the regrouping scheme used in the present work and the mincut procedure of Krivov and Karplus (27–29). In the mincut procedure a single effective free-energy barrier between states is constructed that accounts for all direct and indirect paths. The present regrouping scheme employs free-energy barriers designed to reproduce the rates associated with the slowest transitions of interest for a transformed kinetic transition network. Indirect connections between states are explicitly included in this network if they correspond to time scales beyond the chosen regrouping threshold. All these paths are accounted for when the overall rates are calculated from the resulting master equation formulation. The observables, namely the phenomenological rate constants, are obtained from the appropriate effective free-energy barriers in each approach. All of the results presented below correspond to kinetic transition networks constructed using geometry optimization techniques, and expanded using discrete path sampling (30, 31) approaches, as detailed elsewhere (32, 33).     

Thermal conductivity of Fe-Si alloys and thermal stratification in Earth’s core

Light elements in Earth’s core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron–electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ∼100 to 110 W⋅m⁻¹⋅K⁻¹ for liquid Fe-9Si near the topmost outer core. If Earth’s core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core–mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core–mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core.

The geodynamo of Earth’s core is thought to be mainly driven by compositional (chemical) convection associated with the crystallization and light-element release of the inner core as well as thermal convection driven by a superadiabatic heat flow across the core–mantle boundary (CMB). The relative importance of these energy sources to the geodynamo, however, remains uncertain (1). The magnitudes of these energy sources can change throughout the evolution of the planet. The thermal gradient across the CMB can be constrained from both heat flow of the core and mantle, where a subadiabatic heat flow out of the core may hinder thermal convection and cause a thermally stratified layer at the top of the outer core (2). A global nonadiabatic structure at the top of the core has been inferred from seismic observations and geomagnetic fluctuations (3, 4), where the mechanisms for the origin rely on accurate determinations of the CMB heat flow and the core conductivity. Based on seismological observations and high-pressure and -temperature (P-T) mineral physics results, Earth’s outer and inner core are mainly composed of Fe (∼85 wt %) alloyed with Ni (∼5 wt %) and ∼8 to 10 wt % and 4–5 wt % of light elements, respectively, such as Si, O, S, C, and H (5–10). The effects of the candidate light elements on the electrical resistivity (ρ_e) and thermal conductivity (κ) of iron and their partitioning between the inner and outer core at relevant P-T conditions are thus of great importance for understanding the thermal state of the core as well as the generation and evolution of Earth’s magnetic field (2, 9, 11, 12). The thermal conductivity of the constituent core alloy controls the heat flow of the core, while the electrical resistivity of the constituent Fe alloy determines the ohmic dissipation rate of the magnetic field.Extensive studies on iron’s transport properties have been conducted via experiments and calculations (e.g., refs. 13–21), and recent studies report a thermal conductivity of ∼100 W⋅m⁻¹⋅K⁻¹ at conditions near the CMB (22, 23). Such a high thermal conductivity reduces the amount of heat that can be transported by convective flow (11) and raises a question as to what powered the convection prior to inner core growth over Earth’s history [the so-called new core paradox (24)]. Thus far, several hypotheses have been proposed to reconcile this paradox, including a possible large conductivity reduction due to nickel and light elements (25–28), a rapid core cooling rate (29), or exsolution of chemically saturated species from the core to the lowermost mantle, such as MgO, SiO₂, or FeO (e.g., refs. 30–32). The general consensus is that incorporation of light element(s) depresses high P-T thermal conductivity of iron by impurity scattering (12); this effect was assumed in our previous modeling of the high P-T transport properties of Fe-Ni alloyed with 1.8 wt % Si (25). The lowered thermal conductivity implies that thermal convection is easier to maintain. The rapid core cooling model would imply a young inner core and requires a hidden core heat source, such as radioactivity, which is not supported by geochemical evidence (29). The exsolution mechanism would offer an additional energy source to drive an early compositionally driven geodynamo (32), although some experiments find exsolution to be unlikely (33). The viability of each of these scenarios depends sensitively on the transport properties of iron alloyed with a significant amount of light element(s) (∼8 to 10 wt %) at core P-T conditions. Information on these electrical and thermal transport properties of iron alloys remain uncertain due to the sparsity of experimental and theoretical data.Here we focus on the geodynamic consequences of the transport properties of iron alloyed with 4 to 10 wt % silicon, which is considered to be one of the major light element candidates in the Earth’s core due to its geo- and cosmochemical abundance (5), high solubility in solid and liquid iron (34), and isotopic evidence (35). Fe-Si alloys have been the subject of previous studies focused on understanding the structural and physical properties of the core material, including its high P-T phase diagram (36–39), elasticity (40–44), melting behavior (36, 45, 46), and transport properties (25, 47–49). The observed density discontinuity of ∼4 to 5% across the inner-core boundary (ICB) indicates that excess light elements partition into the outer core during inner-core solidification (6, 50). We should note that the concentration of Si in the core remains uncertain. While some experiments have shown that Fe alloyed with ∼9 wt % Si can satisfy the density profile of the outer core and Fe alloyed with ∼4 wt % Si for the inner core, respectively (37, 40, 41, 51, 52), other studies indicate that a dominant Si light alloying component is unable to reproduce both the density and sound velocity distribution in the outer core (53, 54).High P-T diamond anvil cell (DAC) experiments had been previously conducted to constrain the electrical and thermal conductivity of Fe-Si alloys (28, 47, 55, 56), specifically their T-dependent resistivity and thermal conductivity at core pressures. The thermal conductivity of Fe-8 wt % Si (hereafter Fe-8Si) was measured using a high-P ultrafast optical pump probe and high P-T flash-heating methods (28). The results showed that 8 wt % silicon in solid hexagonal close-packed (hcp) Fe can strongly reduce the conductivity of pure iron by a factor of ∼2, i.e., giving ∼20 W⋅m⁻¹⋅K⁻¹ at ∼132 GPa and 3,000 K. However, the electrical resistivity of solid Fe-6.5Si at ∼99 GPa and 2,000 K was recently measured to be ∼73 µΩ⋅cm (56), which is higher than that of pure iron (22) by ∼60% at comparable conditions. The results imply a thermal conductivity of ∼66 W⋅m⁻¹⋅K⁻¹ using the Wiedemann–Franz law (TL = ρ_eκ) assuming an ideal Sommerfeld Lorentz number (L = L₀: 2.44 × 10⁻⁸ W⋅m⁻¹⋅K⁻²). Meanwhile, another recent study reported a moderate thermal conductivity of 50 to 70 W⋅m⁻¹⋅K⁻¹ for an Fe-5Ni-8Si alloy near CMB P-T conditions (∼140 GPa and 4,000 K) modeled from the measured resistivity of Fe-10Ni and Fe-1.8Si alloys using the four-probe van der Pauw method in laser-heated DACs (25). The results on Fe-10Ni and Fe-1.8Si alloys reveal a linear relationship between resistivity and temperature at a given high pressure, which is very similar to that of hcp Fe (22), over the range of measurements. In contrast, density functional theory (DFT)-based molecular dynamics simulations predict a small negative T dependence of the resistivity at high pressure when liquid Fe is alloyed with a significant amount of light elements (e.g., ∼13 wt % Si) (27). These experimental and computational results raise the possibility that the high P-T thermal transport behavior and its temperature dependence in Fe-Si alloys with a few wt % Si (e.g., 2 wt %) and a larger wt % Si (e.g., 8 to 10 wt %) can be quite different, making it difficult to evaluate the light element effects on the energetics of the core.In this study, we directly measured the electrical resistivities of polycrystalline hcp Fe-4.3 wt % Si (Fe-4.3Si, or Fe_0.92Si_0.08) and Fe-9 wt % Si (Fe-9Si, or Fe_0.84Si_0.16) alloys to ∼136 GPa and 3,000 K. We also computed the electrical resistivity and thermal conductivity of these Fe-Si alloys in solid and liquid phases using first-principles molecular dynamics (FPMD) and dynamical mean field theory (DMFT) calculations. The calculations include contributions from scattering off of Si as well as both electron–phonon (e-ph) and electron–electron (e-e) scattering. Our results are used to evaluate the Si impurity effects on the transport properties of Fe-Si alloy at P-T conditions of the topmost outer core. Assuming Si is the sole light element in the core, our results are used to constrain core thermal conductivity, which is in turn used to assess core heat flux, thermal state, and energy sources driving the geodynamo through geodynamical modeling.     

Atomic-scale probing of heterointerface phonon bridges in nitride semiconductor

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Rapid developments of various modern information technologies such as big data transmission, cloud computing, artificial intelligence technology, and the internet of things have put forward higher requirements on network transmission speed and capacity, demanding higher-power and higher-speed electronic devices (1, 2) such as nitride-based high-electron-mobility transistors (3). Thermal management in such devices becomes crucial as the high output power density results in a strong Joule self-heating effect, which increases the channel temperature and severely degrades device performance (4, 5). Solutions for thermal management include searching for high-thermal-conductivity materials (6, 7) and increasing interface thermal conductance (ITC) via interface engineering (8–10). The latter approach becomes increasingly important when the size of the device approaches nanoscale as the ITC dominates the device’s thermal resistance (11, 12). However, it is challenging to obtain precise knowledge of ITC due to the atomic size and the buried nature of heterointerfaces. The common methods to characterize thermal conductivity, including the time-domain thermoreflectance (13), the frequency-domain thermoreflectance (14), the 3-ω method (15), and coherent optical thermometry (16), suffer from a poor spatial resolution that is insufficient to measure thermal properties at the nanoscale.In fact, the thermal properties of semiconductor and insulator interfaces are largely governed by the interface phonons, and interface phonons also dominate the ITC of metal/semiconductor interfaces because electron–phonon coupling has little effect on the ITC of metal/semiconductor interfaces and can be ignored (17, 18). Previous calculations indicate that the interface can bridge the phonons with different energies and thus boost the inelastic phonon transport (19). Recent methods such as modal analysis were used to correlate the interface phonons with interfacial heat flow (20–23). Specifically, the interface phonon modes can be classified into four classes: extended modes (EMs), partially extended modes (PEMs), isolated modes (IMs), and localized modes (LMs), based on how the vibrational energy is distributed in space (22). The atomic vibrations are delocalized into both sides of the interface for EMs and are localized on one side for PEMs. IMs are not present at the interface, while LMs are highly localized at the atomically thin interface.Of the four modes, EMs and LMs can act as phonon bridges to increase the chances of phonons crossing the interface by elastic/inelastic scattering, while PEMs and IMs only have a small transmission probability for phonon transport. The delocalized EMs exhibit strong correlation between phonons of both sides, thus effectively serving as phonon bridges to support the phonons of one side to cross the interface to the other side via both elastic and inelastic scattering (22, 24). LMs, arising from the few atomic layers at the interface, exhibit extremely strong correlation with phonon modes of both sides, thus effectively serving as phonon bridges to facilitate frequency up and down conversion. As a result, the phonons in one side dump their energy to LMs at the interface region and then transfer it to the phonons in the other side (22, 25). This process can bridge the phonons with significantly different energies through inelastic scattering. Indeed, LMs usually have the highest contribution to the ITC on a per-mode basis (19, 22, 26, 27). Very recently, the localized modes for MoS₂/WSe₂ (28), Ge/Si (29), SrTiO₃/CaTiO₃ superlattices (30), and cubic-BN/diamond system (31) have been experimentally observed. At a Ge/Si interface, calculations suggested a small number of LMs make substantial contributions to ITC, acting as bridges to connect the bulk modes of two sides, while IMs hardly contribute to ITC (25). However, the dominant type of interface phonons for ITC is likely different in different material systems depending on the interface bonding. It is thus useful to experimentally study nanoscale phonon behaviors at various interfaces and correlate them to thermal conductance across the interface.In this work, we study interface phonons at the AlN/Si and AlN/Al heterointerfaces. Due to the wide bandgap, high-breakdown electric field, and high carrier mobility, the III-V nitride semiconductors such as GaN, AlN, and their ternary compounds are considered promising in the next-generation high-power and high-frequency electronic devices (32), which, however, requires excellent thermal conductivity, especially high ITC (33). For these nitride heterostructures, it remains largely unknown what types of interface phonons exist, not to mention how they impact the ITC and which one dominates. By using vibrational electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM), which has the ability to measure the phonon spectra (34–37) at atomic scale (28, 29, 38–42), we probe the interface phonons to gain insights into interface thermal properties.We observe multiple types of interface phonons at an AlN/Si interface and find that EMs that connect the bulk AlN phonon modes and bulk Si phonon modes, and LMs that promote a transverse acoustic (TA) mode of Si to penetrate into the AlN layer, act as bridges to mainly contribute to ITC. However, no such bridging effect is observed across the AlN/Al interface where PEMs dominate, while EMs and LMs only take up a small proportion in the total modes. The AlN/Si interface is therefore expected to have a much higher ITC than the AlN/Al interface since EMs and LMs contribute many times larger than PEMs to the ITC according to molecular dynamics (MD) simulations. Our results unveil the very different interface phonon modes at the heterointerfaces of AlN/Si and AlN/Al and find they have very different contributions to the ITC, providing insights into understanding and engineering the interface thermal properties.     

Combining pressure and electrochemistry to synthesize superhydrides

Recently, superhydrides have been computationally identified and subsequently synthesized with a variety of metals at very high pressures. In this work, we evaluate the possibility of synthesizing superhydrides by uniquely combining electrochemistry and applied pressure. We perform computational searches using density functional theory and particle swarm optimization calculations over a broad range of pressures and electrode potentials. Using a thermodynamic analysis, we construct pressure–potential phase diagrams and provide an alternate synthesis concept, pressure–potential (P2), to access phases having high hydrogen content. Palladium–hydrogen is a widely studied material system with the highest hydride phase being Pd₃H₄. Most strikingly for this system, at potentials above hydrogen evolution and ∼ 300 MPa pressure, we find the possibility to make palladium superhydrides (e.g., PdH₁₀). We predict the generalizability of this approach for La-H, Y-H, and Mg-H with 10- to 100-fold reduction in required pressure for stabilizing phases. In addition, the P2 strategy allows stabilizing additional phases that cannot be done purely by either pressure or potential and is a general approach that is likely to work for synthesizing other hydrides at modest pressures.

Hydrides are a large class of materials containing hydrogen, the lightest and most abundant element in the universe. They have attracted much research interest due to their scientific significance and numerous applications. As important hydrogen storage media (1), they are able to store hydrogen at densities higher than that of liquid hydrogen (2). They also find applications in hydrogen compressors (3), refrigeration (4), heat storage (5), thermal engines (6), batteries (7), fuel cells (8), actuators (9), gas sensors (10), smart windows (11), H₂ purification (12), isotope separation (13), alloy processing (14), catalysis (15), semiconductors (16), neutron moderators (17), low-energy nuclear reactions (18), and recently possible high-temperature superconductors with a critical superconducting temperature T_c in the vicinity of room temperature in hydrogen-rich materials under pressure (19–38).In the late 1960s, Neil Ashcroft (19) and Vitaly Ginzburg (20) independently considered the possibility of high-temperature superconductivity in metallic solid hydrogen at high pressure. Later, the idea of chemical precompression was proposed in which chemical “pressure” is exerted to form hydrogen dominant metal hydrides stable at lower pressures (21). Following the successful prediction (22, 23) and confirmation (24) of very high T_c superconductivity in H₃S, near–room-temperature superconductivity was predicted (25, 26), synthesized (27), and discovered (28) in the superhydrides (defined as MH_n, for n > 6) in the La-H system. Later, comparable T_c values were observed experimentally for other La-H (29), Y-H (30–32), and La-Y-H (33) superhydrides, and room-temperature superconductivity was also reported in the C-S-H system (34). In addition, even higher T_c s have been theoretically predicted, such as Li₂MgH₁₆ with T_c as high as ∼ 470 K at 250 GPa (35).High pressures are needed to synthesize superhydrides (38). One major reason is that at lower pressures, the thermodynamic stability of superhydrides is weakened or no longer exists. To overcome such a challenge, it is obvious that more processing variables need to be introduced in addition to chemical composition and pressure. A processing variable that has been largely hidden is the electrical potential when utilizing electrochemistry for synthesis, which has been used in synthesizing palladium hydride at ambient pressure (39). In the present work we show that the synergetic use of pressure and electrical potential can dramatically extend the thermodynamic stability regime of superhydrides to modest pressures, an approach we term P2. This approach opens more opportunities for the creation of superhydrides and other materials by combining pressure and electrochemical loading techniques. We begin by outlining the general thermodynamic framework. We then apply the approach to the Pd-H system, where we also present density functional theory (DFT) predictions of palladium hydrides under pressure. This is followed by predictions for other metal hydride systems and then a discussion of the broad implications.     

Microscopic origins of the crystallographically preferred growth in evaporation-induced colloidal crystals

Unlike crystalline atomic and ionic solids, texture development due to crystallographically preferred growth in colloidal crystals is less studied. Here we investigate the underlying mechanisms of the texture evolution in an evaporation-induced colloidal assembly process through experiments, modeling, and theoretical analysis. In this widely used approach to obtain large-area colloidal crystals, the colloidal particles are driven to the meniscus via the evaporation of a solvent or matrix precursor solution where they close-pack to form a face-centered cubic colloidal assembly. Via two-dimensional large-area crystallographic mapping, we show that the initial crystal orientation is dominated by the interaction of particles with the meniscus, resulting in the expected coalignment of the close-packed direction with the local meniscus geometry. By combining with crystal structure analysis at a single-particle level, we further reveal that, at the later stage of self-assembly, however, the colloidal crystal undergoes a gradual rotation facilitated by geometrically necessary dislocations (GNDs) and achieves a large-area uniform crystallographic orientation with the close-packed direction perpendicular to the meniscus and parallel to the growth direction. Classical slip analysis, finite element-based mechanical simulation, computational colloidal assembly modeling, and continuum theory unequivocally show that these GNDs result from the tensile stress field along the meniscus direction due to the constrained shrinkage of the colloidal crystal during drying. The generation of GNDs with specific slip systems within individual grains leads to crystallographic rotation to accommodate the mechanical stress. The mechanistic understanding reported here can be utilized to control crystallographic features of colloidal assemblies, and may provide further insights into crystallographically preferred growth in synthetic, biological, and geological crystals.

As an analogy to atomic crystals, colloidal crystals are highly ordered structures formed by colloidal particles with sizes ranging from 100 nm to several micrometers (1–6). In addition to engineering applications such as photonics, sensing, and catalysis (4, 5, 7, 8), colloidal crystals have also been used as model systems to study some fundamental processes in statistical mechanics and mechanical behavior of crystalline solids (9–14). Depending on the nature of interparticle interactions, many equilibrium and nonequilibrium colloidal self-assembly processes have been explored and developed (1, 4). Among them, the evaporation-induced colloidal self-assembly presents a number of advantages, such as large-size fabrication, versatility, and cost and time efficiency (3–5, 15–18). In a typical synthesis where a substrate is immersed vertically or at an angle into a colloidal suspension, the colloidal particles are driven to the meniscus by the evaporation-induced fluid flow and subsequently self-assemble to form a colloidal crystal with the face-centered cubic (fcc) lattice structure and the close-packed {111} plane parallel to the substrate (2, 3, 19–23) (see Fig. 1A for a schematic diagram of the synthetic setup).Open in a separate windowFig. 1.Evaporation-induced coassembly of colloidal crystals. (A) Schematic diagram of the evaporation-induced colloidal coassembly process. “G”, “M”, and “N” refer to “growth,” “meniscus,” and “normal” directions, respectively. The reaction solution contains silica matrix precursor (tetraethyl orthosilicate, TEOS) in addition to colloids. (B) Schematic diagram of the crystallographic system and orientations used in this work. (C and D) Optical image (Top Left) and scanning electron micrograph (SEM) (Bottom Left) of a typical large-area colloidal crystal film before (C) and after (D) calcination. (Right) SEM images of select areas (yellow rectangles) at different magnifications. Corresponding fast-Fourier transform (see Inset in Middle in C) shows the single-crystalline nature of the assembled structure. (E) The 3D reconstruction of the colloidal crystal (left) based on FIB tomography data and (right) after particle detection. (F) Top-view SEM image of the colloidal crystal with crystallographic orientations indicated.While previous research has focused on utilizing the assembled colloidal structures for different applications (4, 5, 7, 8), considerably less effort is directed to understand the self-assembly mechanism itself in this process (17, 24). In particular, despite using the term “colloidal crystals” to highlight the microstructures’ long-range order, an analogy to atomic crystals, little is known regarding the crystallographic evolution of colloidal crystals in relation to the self-assembly process (3, 22, 25). The underlying mechanisms for the puzzling—yet commonly observed—phenomenon of the preferred growth along the close-packed <110> direction in evaporation-induced colloidal crystals are currently not understood (3, 25–29). The <110> growth direction has been observed in a number of processes with a variety of particle chemistries, evaporation rates, and matrix materials (3, 25–28, 30), hinting at a universal underlying mechanism. This behavior is particularly intriguing as the colloidal particles are expected to close-pack parallel to the meniscus, which should lead to the growth along the <112> direction and perpendicular to the <110> direction (16, 26, 31)^*.Preferred growth along specific crystallographic orientations, also known as texture development, is commonly observed in crystalline atomic solids in synthetic systems, biominerals, and geological crystals. While current knowledge recognizes mechanisms such as the oriented nucleation that defines the future crystallographic orientation of the growing crystals and competitive growth in atomic crystals (32–34), the underlying principles for texture development in colloidal crystals remain elusive. Previous hypotheses based on orientation-dependent growth speed and solvent flow resistance are inadequate to provide a universal explanation for different evaporation-induced colloidal self-assembly processes (3, 25–29). A better understanding of the crystallographically preferred growth in colloidal self-assembly processes may shed new light on the crystal growth in atomic, ionic, and molecular systems (35–37). Moreover, mechanistic understanding of the self-assembly processes will allow more precise control of the lattice types, crystallography, and defects to improve the performance and functionality of colloidal assembly structures (38–40).     

Two-dimensional electronic–vibrational sum frequency spectroscopy for interactions of electronic and nuclear motions at interfaces

Interactions of electronic and vibrational degrees of freedom are essential for understanding excited-states relaxation pathways of molecular systems at interfaces and surfaces. Here, we present the development of interface-specific two-dimensional electronic–vibrational sum frequency generation (2D-EVSFG) spectroscopy for electronic–vibrational couplings for excited states at interfaces and surfaces. We demonstrate this 2D-EVSFG technique by investigating photoexcited interface-active (E)-4-((4-(dihexylamino) phenyl)diazinyl)-1-methylpyridin-1- lum (AP3) molecules at the air–water interface as an example. Our 2D-EVSFG experiments show strong vibronic couplings of interfacial AP3 molecules upon photoexcitation and subsequent relaxation of a locally excited (LE) state. Time-dependent 2D-EVSFG experiments indicate that the relaxation of the LE state, S₂, is strongly coupled with two high-frequency modes of 1,529.1 and 1,568.1 cm⁻¹. Quantum chemistry calculations further verify that the strong vibronic couplings of the two vibrations promote the transition from the S₂ state to the lower excited state S₁. We believe that this development of 2D-EVSFG opens up an avenue of understanding excited-state dynamics related to interfaces and surfaces.

Electronic and vibrational degrees of freedom are the most important physical quantities in molecular systems at interfaces and surfaces. Knowledge of interactions between electronic and vibrational motions, namely electronic–vibrational couplings, is essential to understanding excited-states relaxation pathways of molecular systems at interfaces and surfaces. Many excited-states relaxation processes occur at interfaces and surfaces, including charge transfer, energy transfer, proton transfer, proton-coupled electron transfer, configurational dynamics, and so on (1–11). These relaxation processes are intimately related to the electronic–vibrational couplings at interfaces and surfaces. Strong electronic–vibrational couplings could promote nonadiabatic evolution of excited potential energy and thus, facilitate chemical reactions or intramolecular structural changes of interfacial molecules (10, 12, 13). Furthermore, these interactions of electronic and vibrational degrees of freedom are subject to solvent environments (e.g., interfaces/surfaces with a restricted environment of unique physical and chemical properties) (9, 14, 15). Despite the importance of interactions of electronic and vibrational motions, little is known about excited-state electronic–vibrational couplings at interfaces and surfaces.Interface-specific electronic and vibrational spectroscopies enable us to characterize the electronic and vibrational structures separately. As interface-specific tools, second-order electronic sum frequency generation (ESFG) and vibrational sum frequency generation (VSFG) spectroscopies have been utilized for investigating molecular structure, orientational configurations, chemical reactions, chirality, static potential, environmental issues, and biological systems at interfaces and surfaces (16–52). Recently, structural dynamics at interfaces and surfaces have been explored using time-resolved ESFG and time-resolved VSFG with a visible pump or an infrared (IR) pump thanks to the development of ultrafast lasers (6–9, 13–15, 49, 53–61). Doubly resonant sum frequency generation (SFG) has been demonstrated to probe both electronic and vibration transitions of interfacial molecular monolayer (15, 62–71). This frequency-domain two-dimensional (2D) interface/surface spectroscopy could provide information regarding electronic–vibrational coupling of interfacial molecules. However, contributions from excited states are too weak to be probed due to large damping rates of vibrational states in excited states (62, 63). As such, the frequency-domain doubly resonant SFG is used only for electronic–vibrational coupling of electronic ground states. Ultrafast interface-specific electronic–vibrational spectroscopy could allow us to gain insights into how specific nuclear motions drive the relaxation of electronic excited states. Therefore, development of interface-specific electronic–vibrational spectroscopy for excited states is needed.In this work, we integrate the specificity of interfaces and surfaces into the capabilities of ultrafast 2D spectroscopy for dynamical electronic–vibrational couplings in excited states of molecules; 2D interface-specific spectroscopies are analogous to those 2D spectra in bulk that spread the information contained in a pump−probe spectrum over two frequency axes. Thus, one can better interpret congested one-dimensional signals. Two-dimensional vibrational sum frequency generation (2D-VSFG) spectroscopy was demonstrated a few year ago (72–74). Furthermore, heterodyne 2D-VSFG spectroscopy using middle infrared (mid-IR) pulse shaping and noncollinear geometry 2D-VSFG experiments have also been developed to study vibrational structures and dynamics at interfaces (31, 75–78). Recently, two-dimensional electronic sum frequency generation (2D-ESFG) spectroscopy has also been demonstrated for surfaces and interfaces (79). On the other hand, bulk two-dimensional electronic–vibrational (2D-EV) spectroscopy has been extensively used to investigate the electronic relaxation and energy transfer dynamics of molecules, biological systems, and nanomaterials (80–90). The 2D-EV technique not only provides electronic and vibrational interactions between excitons or different excited electronic states of systems but also, identifies fast nonradiative transitions through nuclear motions in molecules, aggregations, and nanomaterials. However, an interface-specific technique for two-dimensional electronic–vibrational sum frequency generation (2D-EVSFG) spectroscopy has yet to be developed.Here, we present the development of 2D-EVSFG spectroscopy for the couplings of electronic and nucleic motions at interfaces and surfaces. The purpose of developing 2D-EVSFG spectroscopy is to bridge the gap between the visible and IR regions to reveal how structural dynamics for photoexcited electronic states are coupled with vibrations at interfaces and surfaces. As an example, we applied this 2D-EVSFG experimental method to time evolution of electronic–vibrational couplings at excited states of interface-active molecules at the air–water interface.     

Electronic evidence of an insulator–superconductor crossover in single-layer FeSe/SrTiO3 films

In high-temperature cuprate superconductors, it is now generally agreed that superconductivity is realized by doping an antiferromagnetic Mott (charge transfer) insulator. The doping-induced insulator-to-superconductor transition has been widely observed in cuprates, which provides important information for understanding the superconductivity mechanism. In the iron-based superconductors, however, the parent compound is mostly antiferromagnetic bad metal, raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture. No evidence of doping-induced insulator–superconductor transition (or crossover) has been reported in the iron-based compounds so far. Here, we report an electronic evidence of an insulator–superconductor crossover observed in the single-layer FeSe film grown on a SrTiO₃ substrate. By taking angle-resolved photoemission measurements on the electronic structure and energy gap, we have identified a clear evolution of an insulator to a superconductor with increasing carrier concentration. In particular, the insulator–superconductor crossover in FeSe/SrTiO₃ film exhibits similar behaviors to that observed in the cuprate superconductors. Our results suggest that the observed insulator–superconductor crossover may be associated with the two-dimensionality that enhances electron localization or correlation. The reduced dimensionality and the interfacial effect provide a new pathway in searching for new phenomena and novel superconductors with a high transition temperature.The iron-based superconductors (1–4) represent the second class of high-temperature superconductors after the discovery of the first class of high-temperature cuprate superconductors. It is now generally agreed that the superconductivity in cuprates is realized by doping a Mott (charge transfer) insulator (5). In the iron-based superconductors, however, the parent compounds mostly exhibit a poor metallic behavior with an antiferromagnetic order, thus raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture (6–18), particularly whether the picture of doping a Mott insulator is relevant to the iron-based superconductors (1, 3, 11, 16, 17). Some theoretical calculations indicate that the iron-based superconductors may be in proximity to a Mott insulator (11, 16, 17), and attempts have also been made to unify cuprates and iron-based superconductors in theory (18). However, so far no clear experimental evidence of doping (or carrier concentration)-induced insulator–superconductor transition (or crossover) has been reported in the iron-based superconductors.The latest discovery of possible high-temperature superconductivity in the single-layer FeSe films grown on a SrTiO₃ substrate has attracted much attention both experimentally (19–27) and theoretically (28–32). The reduced dimensionality with enhanced interfacial effect makes this system distinct from its bulk counterpart. First, it has a simple crystal structure that consists of a single-layer Se-Fe-Se unit, which is an essential building block of the iron-based superconductors (19). Second, the superconducting single-layer FeSe/SrTiO₃ film possesses a distinct electronic structure that exhibits only electron pockets near the Brillouin zone corner without any Fermi crossing near the zone center (20–22). In particular, it was found that annealing in vacuum can tune the carrier concentration of the FeSe/SrTiO₃ films (21, 33), thus providing a good opportunity to investigate its carrier-dependent behaviors.In this paper, to our knowledge, we report the first observation of an insulator–superconductor crossover in the iron-based superconductors by performing systematic angle-resolved photoemission (ARPES) measurements on the single-layer FeSe/SrTiO₃ films at various carrier concentrations. At a very low carrier concentration, the spectral weight near the Fermi level is suppressed, accompanied with the opening of an insulating energy gap. When the carrier concentration increases, the spectral weight begins to fill in the insulating gap, resulting in a decrease in gap size with the formation and sharpening of the peak at the Fermi level. Eventually, when the carrier concentration increases to a critical value, the insulating gap closes and superconductivity starts to emerge. The overall evolution in the single-layer FeSe/SrTiO₃ film is quite similar to the insulator–superconductor transition observed in the cuprate superconductors (34–38). Our observations have established a clear case that an insulator–superconductor crossover takes place with increasing carrier concentration in a 2D iron-based superconductor. The similarities between the current observations and those in cuprates provide new insights in understanding the superconductivity mechanism in these systems. The observed insulator–superconductor crossover in the single-layer FeSe/SrTiO₃ film points to the significant role of the reduced dimensionality in dictating the physical properties and superconductivity.     

An integrative skeletal and paleogenomic analysis of stature variation suggests relatively reduced health for early European farmers

Human culture, biology, and health were shaped dramatically by the onset of agriculture ∼12,000 y B.P. This shift is hypothesized to have resulted in increased individual fitness and population growth as evidenced by archaeological and population genomic data alongside a decline in physiological health as inferred from skeletal remains. Here, we consider osteological and ancient DNA data from the same prehistoric individuals to study human stature variation as a proxy for health across a transition to agriculture. Specifically, we compared “predicted” genetic contributions to height from paleogenomic data and “achieved” adult osteological height estimated from long bone measurements for 167 individuals across Europe spanning the Upper Paleolithic to Iron Age (∼38,000 to 2,400 B.P.). We found that individuals from the Neolithic were shorter than expected (given their individual polygenic height scores) by an average of −3.82 cm relative to individuals from the Upper Paleolithic and Mesolithic (P = 0.040) and −2.21 cm shorter relative to post-Neolithic individuals (P = 0.068), with osteological vs. expected stature steadily increasing across the Copper (+1.95 cm relative to the Neolithic), Bronze (+2.70 cm), and Iron (+3.27 cm) Ages. These results were attenuated when we additionally accounted for genome-wide genetic ancestry variation: for example, with Neolithic individuals −2.82 cm shorter than expected on average relative to pre-Neolithic individuals (P = 0.120). We also incorporated observations of paleopathological indicators of nonspecific stress that can persist from childhood to adulthood in skeletal remains into our model. Overall, our work highlights the potential of integrating disparate datasets to explore proxies of health in prehistory.

The agricultural revolution—beginning ∼12,000 B.P. in the Fertile Crescent zone (1, 2) and then spreading (3–5) or occurring independently (6, 7) across much of the inhabited planet—precipitated profound changes to human subsistence, social systems, and health. Seemingly paradoxically, the agricultural transition may have presented conflicting biological benefits and costs for early farming communities (8, 9). Specifically, demographic reconstructions from archaeological and population genetic records suggest that the agricultural transition led to increased individual fitness and population growth (6, 10–12), likely due in part to new food production and storage capabilities. Yet, bioarchaeological analyses of human skeletal remains from this cultural period suggest simultaneous declines in individual physiological well-being and health, putatively from 1) nutritional deficiency and/or 2) increased pathogen loads as a function of greater human population densities, sedentary lifestyles, and proximity to livestock (9, 13–18).To date, anthropologists have used two principal approaches to study health across the foraging-to-farming transition in diverse global regions (13, 19, 20). The first approach involves identifying paleopathological indicators of childhood stress that persist into adult skeletal remains. For example, porotic hyperostosis (porous lesions on the cranial vault) and cribra orbitalia (porosity on the orbital roof) reflect a history of bone marrow hypertrophy or hyperplasia resulting from one or more periods of infection, metabolic deficiencies, malnutrition, and/or chronic disease (21–26). Meanwhile, linear enamel hypoplasia (transverse areas of reduced enamel thickness on teeth) occurs in response to similar childhood physiological stressors (e.g., disease, metabolic deficiencies, malnutrition, weaning) that disrupt enamel formation in the developing permanent dentition (27–30). Broadly, these paleopathological indicators of childhood stress tend to be observed at higher rates among individuals from initial farming communities relative to earlier periods, potentially reflecting their overall “poorer” health (14, 31–36).A second approach uses skeleton-based estimates of achieved adult stature as a proxy for health during childhood growth and development (37–39). Since stature is responsive to the influences of nutrition and disease burden alongside other factors, relatively short “height-for-age” (or “stunting”) has been used as an indicator of poorer health in both living and bioarchaeological contexts (39–43). When studying the past, individual stature can be estimated from long bone measurements and regression equations (44–47). Using these methods, multiple prior studies have reported a general profile of relatively reduced stature for individuals from early agricultural societies in Europe (15, 48–50), North America (51–53), the Levant (16, 32), and Asia (54, 55). For example, estimated average adult mean statures for early farmers are ∼10 cm shorter relative to those for preceding hunter-gatherers in both western Europe (females, −8 cm; males, −14 cm) (49, 50) and the eastern Mediterranean (females, −11 cm; males, −8 cm) (56). This pattern is not universal, as a few studies do not report such changes (57, 58); the variation could be informative with respect to identifying potential underlying factors (59).However, in addition to environmental effects like childhood nutrition and disease, inherited genetic variation can have an outsized impact on terminal stature, with ∼80% of the considerable degree of height variation within many modern populations explainable by heritable genetic variation (60–63). Moreover, migration and gene flow likely accompanied many subsistence shifts in human prehistory. For example, there is now substantial paleogenomic evidence of extensive population turnover across prehistoric Europe (64–69). Therefore, from osteological studies alone, we are unable to quantify the extent to which temporal changes in height reflect variation in childhood health vs. changes/differences in the frequencies of alleles associated with height variation.In this study, we have performed a combined analysis of ancient human paleogenomic and osteological data where both are available from the same n = 167 prehistoric European individuals representing cultural periods from the Upper Paleolithic (∼38,000 B.P.) to the Iron Age (∼2,400 B.P.). This approach allows us to explore whether “health,” as inferred from the per-individual difference between predicted genetic contributions to height and osteological estimates of achieved adult height, changed over the Neolithic cultural shift to agriculture in Europe. When craniodental elements were preserved and available for analysis (n = 98 of the 167 individuals), we also collected porotic hyperostosis, cribra orbitalia, and linear enamel hypoplasia paleopathological data in order to examine whether patterns of variation between osteological height and genetic contributions to height are explained in part by the presence/absence of these indicators of childhood or childhood-inclusive stress.     

Oxygen hole content,charge-transfer gap,covalency, and cuprate superconductivity

Experiments have shown that the families of cuprate superconductors that have the largest transition temperature at optimal doping also have the largest oxygen hole content at that doping [D. Rybicki et al., Nat. Commun. 7, 1–6 (2016)]. They have also shown that a large charge-transfer gap [W. Ruan et al., Sci. Bull. (Beijing) 61, 1826–1832 (2016)], a quantity accessible in the normal state, is detrimental to superconductivity. We solve the three-band Hubbard model with cellular dynamical mean-field theory and show that both of these observations follow from the model. Cuprates play a special role among doped charge-transfer insulators of transition metal oxides because copper has the largest covalent bonding with oxygen. Experiments [L. Wang et al., arXiv [Preprint] (2020). https://arxiv.org/abs/2011.05029 (Accessed 10 November 2020)] also suggest that superexchange is at the origin of superconductivity in cuprates. Our results reveal the consistency of these experiments with the above two experimental findings. Indeed, we show that covalency and a charge-transfer gap lead to an effective short-range superexchange interaction between copper spins that ultimately explains pairing and superconductivity in the three-band Hubbard model of cuprates.

Although several classes of high-temperature superconductors have been discovered, including pnictides, sulfur hydrides, and rare earth hydrides, cuprate high-temperature superconductors are still particularly interesting from a fundamental point of view because of the strong quantum effects expected from their doped charge-transfer insulator nature and single-band spin-one-half Fermi surface (1, 2).Among the most enduring mysteries of cuprate superconductivity is the experimental discovery, early on, that the hole content on oxygen plays a crucial role (2–5). Oxygen hole content (2np) is particularly relevant since NMR (5, 6) suggests a correlation between optimal Tc and 2np on the CuO2 planes: A higher oxygen hole content at the optimal doping of a given family of cuprates leads to a higher critical temperature. This is summarized in figure 2 of ref. 6. The charge-transfer gap also seems to play a central role for the value of Tc, as suggested by scanning tunneling spectroscopy (7) and by theory (8). Many studies have shown that doped holes primarily occupy oxygen orbitals (3, 9–11). This long unexplained role of oxygen hole content and charge-transfer gap on the strength of superconductivity in cuprates is addressed in this paper.The vast theoretical literature on the one-band Hubbard model in the strong-correlation limit shows that many of the qualitative experimental features of cuprate superconductors (12, 13) can be understood (14), but obviously not the above experimental facts regarding oxygen hole content. Furthermore, variational calculations (15) and various Monte Carlo approaches (16, 17) suggest that d-wave superconductivity in the one-band Hubbard model may not be the ground state, at least in certain parameter ranges (18, 19).It is thus important to investigate more realistic models, such as the three-band Emery-VSA (Varma–Schmitt-Rink–Abrahams) model that accounts for copper–oxygen hybridization of the single band that crosses the Fermi surface (20, 21). A variety of theoretical methods (8, 22–27) revealed many similarities with the one-band Hubbard model, but also differences related to the role of oxygen (28, 29).Investigating the causes for the variation of the transition temperature Tc for various cuprates is a key scientific goal of the quantum materials roadmap (30).^* We find and explain the above correlations found in NMR and in scanning tunnelling spectroscopy; highlight the importance of the difference between electron affinity of oxygen and ionization energy of copper (21, 31); and, finally, document how oxygen hole content, charge-transfer gap, and covalency conspire to create an effective superexchange interaction between copper spins that is ultimately responsible for superconductivity.We do not address questions related to intraunit-cell order (32, 33).     

Submersed micropatterned structures control active nematic flow,topology, and concentration

Coupling between flows and material properties imbues rheological matter with its wide-ranging applicability, hence the excitement for harnessing the rheology of active fluids for which internal structure and continuous energy injection lead to spontaneous flows and complex, out-of-equilibrium dynamics. We propose and demonstrate a convenient, highly tunable method for controlling flow, topology, and composition within active films. Our approach establishes rheological coupling via the indirect presence of fully submersed micropatterned structures within a thin, underlying oil layer. Simulations reveal that micropatterned structures produce effective virtual boundaries within the superjacent active nematic film due to differences in viscous dissipation as a function of depth. This accessible method of applying position-dependent, effective dissipation to the active films presents a nonintrusive pathway for engineering active microfluidic systems.

Active fluids are inherently out of equilibrium; they locally transform internal energy into material stresses that can result in spontaneous hydrodynamic motion. An increasing number of biophysical systems, including colonies of bacilliform microbes (1–4), cellular monolayers (5–9), and subcellular filaments (10–12), display such collective active motion, orientational order, and topological singularities. Controlling active dynamics is essential not only to fully understanding how such biological systems employ self-generated stresses but also, in order to develop active microfluidic devices.To this end, recent work considers how confining walls (13–15), arrangements of obstacles (16, 17), and the dynamics of topological defects (18) dictate active nematic flow. Control of active material concentration has been studied from the perspectives of coexistence of phases in self-propelled rods (19–21) and motility-induced phase separation (22–24). Controlled accumulation and depletion of active matter have been engineered in bacterial systems to concentrate cells (25, 26) and to drive bacterial-ratchet motors (27–29). Similarly, substrate gradients modify cellular motility, driving density variation (30) and directed migration (31, 32).In addition to varying concentration and flow, topology has been controlled by including externally driven flows (33–35) and curvature (36, 37). Recent work shows that locally altering activity modifies defect populations (38–41), and anisotropic smectic sublayers below active nematic sheets can constrain orientation (42). Such studies demonstrate how underlying sublayer properties have pronounced effects on active dynamics and suggest approaches for engineering control of active matter.We propose a micropattern-based method for controlling active nematic dynamics without contiguous contact with active films. By patterning oil-submersed solid substrates below two-dimensional (2D) active nematic films with geometrical structures of differing height, we achieve effective virtual boundaries within active films that control topological defect populations, collective flow, and concentration of active nematic material without penetrating the film. By implementing underlying submersed patterned microstructures, we tune the depth of the oil layer to adjust dissipation within the superjacent film and thereby, generate a highly tunable technique for controlling the active dynamics. Presently, we introduce four initial submersed structures: micropatterned trenches (Fig. 1 A–C), undulated substrates (SI Appendix, Fig. S1), stairways (Fig. 1 D–F), and pillars (Fig. 1 G–I).Open in a separate windowFig. 1.Submersed micropatterns control active nematic dynamics. (A–C) Trench setup. An active film resides at the oil–water interface above different substrate depths. The active flows drag the underlying oil layer, but viscous dissipation is depth dependent, affecting active nematic film dynamics. (B) Fluorescence microscope image of the active nematic bundled microtubule film above a submersed trench. (Scale bar: 250 μm.) (C) Simulation results for the vorticity field within the superjacent active nematic layer. The flow behaviors within the low-friction region (between the dashed lines) are distinct from the behavior in the high friction region (beyond the dashed lines). Plus-half (minus-half) defects denoted by dark green (magenta) symbols behave differently in the two regions. (D–F) Stairway setup. (E) Fluorescence microscope image of the micromilled stairway and the superjacent bundled microtubule film. Step location is indicated by dashed lines. The oil depth increases from left to right. The differences in oil depth alter the length scale of the active turbulence above each step. (Scale bar: 250 μm.) (F) Simulations results for discrete steps in the effective friction (dashed lines). The effective friction coefficient decreases from left to right. The color bar is shared with C. (G–I) Pillar setup. (H) Fluorescence microscope image of the bundled microtubule film above the SU-8 micropillar. (Scale bar: 100 μm.) (I) Simulation results show that the active nematic concentration ϕ is depleted within the high-friction region encircled by the pillar perimeter (dashed line).     

From the Cover: The evolution of parasitism from mutualism in wasps pollinating the fig,Ficus microcarpa,in Yunnan Province,China

Theory identifies factors that can undermine the evolutionary stability of mutualisms. However, theory’s relevance to mutualism stability in nature is controversial. Detailed comparative studies of parasitic species that are embedded within otherwise mutualistic taxa (e.g., fig pollinator wasps) can identify factors that potentially promote or undermine mutualism stability. We describe results from behavioral, morphological, phylogenetic, and experimental studies of two functionally distinct, but closely related, Eupristina wasp species associated with the monoecious host fig, Ficus microcarpa, in Yunnan Province, China. One (Eupristina verticillata) is a competent pollinator exhibiting morphologies and behaviors consistent with observed seed production. The other (Eupristina sp.) lacks these traits, and dramatically reduces both female and male reproductive success of its host. Furthermore, observations and experiments indicate that individuals of this parasitic species exhibit greater relative fitness than the pollinators, in both indirect competition (individual wasps in separate fig inflorescences) and direct competition (wasps of both species within the same fig). Moreover, phylogenetic analyses suggest that these two Eupristina species are sister taxa. By the strictest definition, the nonpollinating species represents a “cheater” that has descended from a beneficial pollinating mutualist. In sharp contrast to all 15 existing studies of actively pollinated figs and their wasps, the local F. microcarpa exhibit no evidence for host sanctions that effectively reduce the relative fitness of wasps that do not pollinate. We suggest that the lack of sanctions in the local hosts promotes the loss of specialized morphologies and behaviors crucial for pollination and, thereby, the evolution of cheating.

Mutualisms are defined by the net benefits that are usually provided to individuals of each interacting species. These interactions often have influences far beyond the partner species directly interacting, and commonly provide many fundamental ecosystem services (1, 2). For example, in most cases, mycorrhizal fungi provide nutrients to forest trees, pollinators help flowering plants set fruit, intestinal bacteria promote nutrient uptake across diverse animal taxa, bacteria in lucinid clams help detoxify benthic sediments, and photosynthetic algae help maintain the coral reefs that structure nearshore marine environments around the world (3–6).However, while both partners in a mutualism usually receive net benefits from the interaction, mutualisms also usually impose costs on one or both partners interacting mutualistically. In the absence of fitness-aligning mechanisms between the partners (e.g., vertical transmission of symbionts, or repeated interactions with immediate fitness benefits), theory suggests that other mechanisms are needed to maintain a mutualism’s stability. Specifically, it has been proposed that a mutualism’s long-term stability often depends on mechanisms that limit the invasion of “cheater” individuals into the populations of either partner species (2, 3, 7–14). Broadly, cheaters can be defined as individuals (or species) that do not provide a beneficial service to their partners. By not providing a potentially costly service to their partners, cheaters are thought to benefit themselves relative to “cooperating” individuals or species in the short term (12–14). Invasion by such cheaters potentially erodes the net benefits resulting from the interaction, and therefore can lead to a breakdown of the mutualism itself.Consistent with this viewpoint, data suggest that in many cases the hosts (the larger of the two partners in the mutualism) can effectively promote cooperation by selectively allocating more resources to those symbionts that provide them with greater benefits. For example, some legumes have been shown to selectively allocate more resources to nodules containing rhizobia that are better at providing fixed nitrogen (14–16). In other studies, some host plants allocate more carbon to strains of mycorrhizal fungi that provide their hosts with more phosphorus (17–19).However, other authors question the biological relevance of much of this experimental evidence to natural species interactions, the direction of cause and effect, and the actual costs for providing benefits. A central question is the degree to which evidence for cheaters, defined as receiving fitness benefits by not providing services (relative to a mutualist that does provide benefits), exist at all (12, 13, 20). Key empirical issues concern whether or not individuals with a cheating phenotype do, in fact, cheat (impose a reproductive cost on their partner, relative to a cooperating mutualist). In addition, are cheating individuals that fail to benefit their host at least as fit as cooperating (mutualistic) individuals that do? Does the host allocate relatively more resources to more beneficial partners (effectively expressing sanctions against cheaters relative to cooperators)? Ultimately, this becomes a set of specific empirical questions: What is the relative fitness of cooperators and cheaters that interact with the same partner (host)? And, does the host effectively sanction cheaters relative to cooperators, and if so, to what degree (21, 22)? At a fundamental level, the relative fitness of cheaters and cooperators is only measurable and relevant within the context of a given host’s responses to them (3, 21, 22).To resolve these questions, it is useful to study those mutualistic host–symbiont interactions in which it is straightforward to measure and experimentally manipulate both benefits and costs to each partner under natural conditions (22–32). Ideally, we should be able to comparatively assess experimental results across a diversity of host–symbiont mutualisms that differ in what theory suggests should be key metrics (e.g., strength of host sanctions, existence and relative abundance of cheaters, and so forth).The over 750 species of host figs (Ficus: Moraceae) and their obligately pollinating wasps (Agaonidae: Hymenoptera) provide such a range of both experimental and comparative options that can be exploited to address these questions (22–32) (SI Appendix, Supplementary Text and Fig. S1). Ovipositing female fig wasps deposit a drop of fluid from their poison sac into the ovules of flowers into which they lay their eggs. This fluid initiates the formation of gall tissue upon which the developing larvae feed (33) (SI Appendix). At any given site, each fig species is typically pollinated by only one or two fig wasp species (24, 26). Morphological and molecular studies broadly support coevolution between genera of pollinating wasps and their respective sections of figs, while functional studies demonstrate coadaptation between them (33–51).For example, different groups of figs are characterized by either active or passive pollination (43–45) (SI Appendix). Passive pollination does not require specialized wasp morphologies or behaviors. In contrast, active pollination requires specialized female wasp morphologies and behaviors (44). The wasps collect pollen in their natal fig using coxal combs on their forelegs and store it in pollen pockets on their thoraxes (Fig. 1). After emerging from their natal figs, female wasps use volatile chemical scent cues produced by receptive figs to identify them (35–37). Dispersal flights from the natal fig are aided by prevailing winds and routinely cover scores of kilometers (38–41). Upon finding and entering a receptive fig of an appropriate host species, the foundress wasps repeatedly remove a few grains of pollen from their pockets and place them on the stigmatic surfaces of the individual flowers on which they attempt to lay eggs. Active pollination provides clear benefits for the host fig. Pollination is more efficient in actively pollinated fig species relative to passively pollinated species. This is reflected in the dramatically lower (∼1/10) amounts of pollen that active species typically produce (43–45). Conversely, active pollination appears to be costly for the wasps in terms of specialized body structures, energy, and time (22, 42, 45).Open in a separate windowFig. 1.Receptive F. microcarpa fig and pollinating structures of E. verticillata compared with Eupristina sp. (A) A cheater wasp (Eupristina sp.) laying eggs in a receptive fig of her host F. microcarpa. Pollinator wasps (E. verticillata) (B and C) have specialized morphological structures such as pollen pockets (black arrow) on the underside of their thorax and coxal combs on their forelegs (white arrows) that facilitate pollination. Pollen is stored in the pockets and coxal combs facilitate pollen transfer (43, 44). Cheater wasps (Eupristina sp.) (D and E) retain pollen pockets (black arrow) but lack coxal combs (white arrow).The most basic mutualistic services (e.g., the wasp’s ability to pollinate) can be experimentally manipulated. By allowing or restricting the female pollinator wasps’ access to, and ability to actively collect pollen, pollinators that either do (P+) or do not (P−) carry pollen can be produced and then introduced into receptive figs (22). Furthermore, the effects on pollinator wasp fitness (i.e., lifetime reproductive success) of pollinating the host fig (or not) can be quantified by counting their relative number of offspring in naturally occurring figs (22–32). Moreover, the many existing experimental studies using the same methodologies provide context for the findings of any given experiment (22–32). In previous experiments on actively pollinated fig species, wasps that do not pollinate (P−) have lower fitness than wasps that pollinate (P+) due to increased rates of fig abortion (killing all wasp larvae) and increased larval mortality reducing the number of P− offspring that emerge. These “host sanctions” are likely caused by selective resource allocation by the tree to better-pollinated figs (28). Although pollination typically leads to a higher number of wasp offspring, pollination is not an absolute requirement for wasp offspring to develop (28). Finally, there are at least two known cases of cheating wasp species, in which species of wasps that lack both morphologies and behaviors that permit efficient, active pollination of their host co-occur with a congeneric pollinator possessing these traits. Importantly, the species that lack these traits have clearly evolved within lineages of wasps that otherwise possess these apparently costly traits that permit them to actively pollinate their host (52, 53) (SI Appendix).Here, we exploit the opportunity provided by a third case (54, 55), in which a mutualistic active pollinator and a congeneric cheater species co-occur on the same monoecious host fig. Specifically, we conducted a combination of behavioral, morphological, phylogenetic, and experimental studies to compare these wasps and the outcomes of their interactions with their shared host fig, Ficus microcarpa (subgenus Urostigma: section Urostigma: subsection Conosycea), in and near the Xishuangbanna Tropical Botanical Garden (XTBG), China. Eupristina verticillata is the described active pollinator of F. microcarpa at this location, while an undescribed coexisting wasp species (Eupristina sp.) lacks the necessary adaptation for active pollination and appears to be a cheater (54, 55).In this study, we address and answer the following questions: 1) Does the undescribed Eupristina sp. wasp associated with F. microcarpa impose a reproductive cost on its host? We find that it does, and that the cost for host reproductive success is large. 2) Does the cheater exhibit significantly higher levels of reproductive success than the pollinator in their host? Yes, in both direct and indirect competition. Combined with the reproductive loss it imposes on the host, this species meets the strictest definition of cheater. 3) Is this cheater closely related (possibly a sister species) to the mutualist pollinator of their shared host? We find that within the context of other sympatric Eupristina species associated with seven fig hosts in this area, it is. Furthermore, it represents an independent loss of pollination structures from another case previously reported in this genus. 4) Does the host (F. microcarpa) locally exhibit detectable host sanctions against wasps that do not pollinate it? In sharp contrast with all 15 other cases of actively pollinated Ficus species that have been reported (22, 29–32), we find that it does not. 5) Given that cheaters exhibit equal or greater fitness than the pollinator, how do they coexist? Although deserving further study, we suggest that regular seasonal fluctuations in the relative abundances of the two wasp species facilitate their coexistence at this site (54, 55). Seasonal changes in the prevalence of westerly winds cause regional spatial heterogeneity in source pools of pollinators and cheaters that immigrate to the local host, F. microcarpa.     

Specific contribution of Reelin expressed by Cajal–Retzius cells or GABAergic interneurons to cortical lamination

The extracellular protein Reelin, expressed by Cajal–Retzius (CR) cells at early stages of cortical development and at late stages by GABAergic interneurons, regulates radial migration and the “inside-out” pattern of positioning. Current models of Reelin functions in corticogenesis focus on early CR cell–derived Reelin in layer I. However, developmental disorders linked to Reelin deficits, such as schizophrenia and autism, are related to GABAergic interneuron–derived Reelin, although its role in migration has not been established. Here we selectively inactivated the Reln gene in CR cells or GABAergic interneurons. We show that CR cells have a major role in the inside-out order of migration, while CR and GABAergic cells sequentially cooperate to prevent invasion of cortical neurons into layer I. Furthermore, GABAergic cell–derived Reelin compensates some features of the reeler phenotype and is needed for the fine tuning of the layer-specific distribution of cortical neurons. In the hippocampus, the inactivation of Reelin in CR cells causes dramatic alterations in the dentate gyrus and mild defects in the hippocampus proper. These findings lead to a model in which both CR and GABAergic cell–derived Reelin cooperate to build the inside-out order of corticogenesis, which might provide a better understanding of the mechanisms involved in the pathogenesis of neuropsychiatric disorders linked to abnormal migration and Reelin deficits.

Correct lamination of the cerebral cortex is essential for normal brain function. In this regard, abnormal neuronal migration is common among many neurodevelopmental and neuropsychiatric disorders linked to cognitive impairment (1). Since the discovery that Reln is responsible for the reeler mutation (2), numerous studies support the idea that the extracellular protein Reelin is essential for neuronal migration and the layering of laminated structures, including the cerebral cortex and cerebellum (3–7). Inactivation of genes encoding for components of the Reelin pathway [e.g., the APOE2 and VLDL receptors (8), the adaptor Dab1 (9, 10), and the proteasome component Cullin-5 (11)] results in reeler-like phenotypes. These observations reinforce the relevance of this pathway for these developmental processes. This gene is also essential for the structural and functional organization of the blood–brain barrier (12).In the developing cerebral cortex, Reelin is largely expressed by Cajal–Retzius (CR) cells, an early pioneer neuronal population, which, because of its strategic location in the marginal zone/layer I, Reelin secretion, morphogenetic-like functions, and transient nature, are considered essential in corticogenesis (13–15). Current models of Reelin action on migrating neurons, including Cadherin-mediated adhesion, Integrin-regulated detachment of radial glia, long-range and contact attraction/stop signaling, and Ephrin signaling, have mainly taken into consideration Reelin expressed by CR cells at the cortical surface (16–25). However, several studies report that CR cell ablation causes contradictory effects on corticogenesis (26–28). Reelin is also expressed by cortical GABAergic interneurons in the cortical plate (CP), long before the completion of cortical migration, persisting into adulthood (29). Recent studies have implicated Reelin in adult plasticity (30–33), as well as in the pathogenesis of neurodevelopmental, neuropsychiatric, and neurodegenerative diseases, notably in Alzheimer’s disease, schizophrenia/bipolar disorder, autism spectrum disorder, and epilepsy (34–41), at stages when Reelin is mainly expressed by GABAergic neurons. Despite this, the contribution of Reelin expressed by GABAergic interneurons in cortical development is largely unknown and, consequently, the role of CR cell–derived Reelin might be overinterpreted. Only one recent study reports very mild migration deficits in the hippocampus of mice lacking Reelin in GABAergic interneurons (42).Here we selectively inactivated the Reln gene in CR cells or GABAergic interneurons. We found both overlapping and specific functions of Reelin expressed by these two cell populations in distinct features of the reeler phenotype.