Optical Properties of Nitrogen-Substituted Strontium Titanate Thin Films Prepared by Pulsed Laser Deposition

Perovskite-type N‑substituted SrTiO₃ thin films with a preferential (001) orientation were grown by pulsed laser deposition on (001)-oriented MgO and LaAlO₃ substrates. Application of N₂ or ammonia using a synchronized reactive gas pulse produces SrTiO_3-x:N_x films with a nitrogen content of up to 4.1 at.% if prepared with the NH₃ gas pulse at a substrate temperature of 720 °C. Incorporating nitrogen in SrTiO₃ results in an optical absorption at 370‑460 nm associated with localized N(2p) orbitals. The estimated energy of these levels is ≈2.7 eV below the conduction band. In addition, the optical absorption increases gradually with increasing nitrogen content.     

Symmetry and dynamics of molecular rotors in amphidynamic molecular crystals

Rotary biomolecular machines rely on highly symmetric supramolecular structures with rotating units that operate within a densely packed frame of reference, stator, embedded within relatively rigid membranes. The most notable examples are the enzyme FoF1 ATP synthase and the bacterial flagellum, which undergo rotation in steps determined by the symmetries of their rotators and rotating units. Speculating that a precise control of rotational dynamics in rigid environments will be essential for the development of artificial molecular machines, we analyzed the relation between rotational symmetry order and equilibrium rotational dynamics in a set of crystalline molecular gyroscopes with rotators having axial symmetry that ranges from two- to fivefold. The site exchange frequency for these molecules in their closely related crystals at ambient temperature varies by several orders of magnitude, up to ca. 4.46 × 10⁸ s^-1.     

Surface organization of aqueous MgCl2 and application to atmospheric marine aerosol chemistry

Inorganic salts in marine aerosols play an active role in atmospheric chemistry, particularly in coastal urban regions. The study of the interactions of these ions with water molecules at the aqueous surface helps to elucidate the role of inorganic cations and anions in atmospheric processes. We present surface vibrational sum frequency generation (SFG) spectroscopic and molecular dynamics (MD) studies of aqueous MgCl₂ surfaces as models of marine aerosol. Spectroscopy results reveal that the disturbance of the hydrogen bonding environment of the air/aqueous interface is dependent on the MgCl₂ concentration. At low concentrations (< 1 M) minor changes are observed. At concentrations above 1 M the hydrogen bonding environment is highly perturbed. The 2.1 M intermediate concentration solution shows the largest SFG response relative to the other solutions including concentrations as high as 4.7 M. The enhancement of SFG signal observed for the 2.1 M solution is attributed to a larger SFG-active interfacial region and more strongly oriented water molecules relative to other concentrations. MD simulations reveal concentration dependent compression of stratified layers of ions and water orientation differences at higher concentrations. SFG and MD studies of the dangling OH of the surface water reveal that the topmost water layer is affected structurally at high concentrations (> 3.1 M). Finally, the MgCl₂ concentration effect on a fatty acid coated aqueous surface was investigated and SFG spectra reveal that deprotonation of the carboxylic acid of atmospherically relevant palmitic acid (PA) is accompanied by binding of the Mg²⁺ to the PA headgroup.     

Structure of the floating water bridge and water in an electric field

The floating water bridge phenomenon is a freestanding rope-shaped connection of pure liquid water, formed under the influence of a high potential difference (approximately 15 kV). Several recent spectroscopic, optical, and neutron scattering studies have suggested that the origin of the bridge is associated with the formation of anisotropic chains of water molecules in the liquid. In this work, high energy X-ray diffraction experiments have been performed on a series of floating water bridges as a function of applied voltage, bridge length, and position within the bridge. The two-dimensional X-ray scattering data showed no direction-dependence, indicating that the bulk water molecules do not exhibit any significant preferred orientation along the electric field. The only structural changes observed were those due to heating, and these effects were found to be the same as for bulk water. These X-ray scattering measurements are supported by molecular dynamics (MD) simulations which were performed under electric fields of 10⁶ V/m and 10⁹ V/m. Directional structure factor calculations were made from these simulations parallel and perpendicular to the E-field. The 10⁶ V/m model showed no significant directional-dependence (anisotropy) in the structure factors. The 10⁹ V/m model however, contained molecules aligned by the E-field, and had significant structural anisotropy.     

The Effects of Grain Boundary Misorientation on the Mechanical Properties and Mechanism of Plastic Deformation of Ni/Ni3Al: A Molecular Dynamics Study

The effects of grain boundary misorientation angle (θ) on mechanical properties and the mechanism of plastic deformation of the Ni/Ni₃Al interface under tensile loading were investigated using molecular dynamics simulations. The results show that the space lattice arrangement at the interface is dependent on grain boundary misorientations, while the interfacial energy is dependent on the arrangement. The interfacial energy varies in a W pattern as the grain boundary misorientation increases from 0° to 90°. Specifically, the interfacial energy first decreases and then increases in both segments of 0–60° and 60–90°. The yield strength, elastic modulus, and mean flow stress decrease as the interfacial energy increases. The mechanism of plastic deformation varies as the grain boundary misorientation angle (θ) increases from 0° to 90°. When θ = 0°, the microscopic plastic deformation mechanisms of the Ni and Ni₃Al layers are both dominated by stacking faults induced by Shockley dislocations. When θ = 30°, 60°, and 80°, the mechanisms of plastic deformation of the Ni and Ni₃Al layers are the decomposition of stacking faults into twin grain boundaries caused by extended dislocations and the proliferation of stacking faults, respectively. When θ = 90°, the mechanisms of plastic deformation of both the Ni and Ni₃Al layers are dominated by twinning area growth resulting from extended dislocations.     

Electric field enhanced hydrogen storage on polarizable materials substrates

Using density functional theory, we show that an applied electric field can substantially improve the hydrogen storage properties of polarizable substrates. This new concept is demonstrated by adsorbing a layer of hydrogen molecules on a number of nanomaterials. When one layer of H₂ molecules is adsorbed on a BN sheet, the binding energy per H₂ molecule increases from 0.03 eV/H₂ in the field-free case to 0.14 eV/H₂ in the presence of an electric field of 0.045 a.u. The corresponding gravimetric density of 7.5 wt% is consistent with the 6 wt% system target set by Department of Energy for 2010. The strength of the electric field can be reduced if the substrate is more polarizable. For example, a hydrogen adsorption energy of 0.14 eV/H₂ can be achieved by applying an electric field of 0.03 a.u. on an AlN substrate, 0.006 a.u. on a silsesquioxane molecule, and 0.007 a.u. on a silsesquioxane sheet. Thus, application of an electric field to a polarizable substrate provides a novel way to store hydrogen; once the applied electric field is removed, the stored H₂ molecules can be easily released, thus making storage reversible with fast kinetics. In addition, we show that materials with rich low-coordinated nonmetal anions are highly polarizable and can serve as a guide in the design of new hydrogen storage materials.     

Distinct large-scale turbulent-laminar states in transitional pipe flow

When fluid flows through a channel, pipe, or duct, there are two basic forms of motion: smooth laminar motion and complex turbulent motion. The discontinuous transition between these states is a fundamental problem that has been studied for more than 100 yr. What has received far less attention is the large-scale nature of the turbulent flows near transition once they are established. We have carried out extensive numerical computations in pipes of variable lengths up to 125 diameters to investigate the nature of transitional turbulence in pipe flow. We show the existence of three fundamentally different turbulent states separated by two distinct Reynolds numbers. Below Re ₁ ≃ 2,300, turbulence takes the form of familiar equilibrium (or longtime transient) puffs that are spatially localized and keep their size independent of pipe length. At Re ₁ the flow makes a striking transition to a spatio-temporally intermittent flow that fills the pipe. Irregular alternation of turbulent and laminar regions is inherent and does not result from random disturbances. The fraction of turbulence increases with Re until Re ₂ ≃ 2,600 where there is a continuous transition to a state of uniform turbulence along the pipe. We relate these observations to directed percolation and argue that Re ₁ marks the onset of infinite-lifetime turbulence.     

Phase transitions in phosphatidylcholine multibilayers

The ²H NMR spectrum of a multilamellar dispersion of 1-myristoyl-2-[14,14,14-²H₃]myristoyl-sn-glycero-3-phosphocholine with 1 mol% cholesterol in excess water has been recorded at temperatures between -15°C and 36°C. Motionally averaged quadrupole coupling constants ν_Q and motionally induced asymmetry parameters η are obtained by spectral analysis. Values of these quantities indicate that, at temperatures below -4°C, any rotational motion of the molecules about their molecular long axis is slow on the NMR time scale. At temperatures immediately above the pretransition these same parameters show that a fast-rotational motion is occurring about the molecular long axis. This rotational motion is hindered in that the molecules flip about a twofold symmetry axis. Between -4°C and the pretransition, spectra appear as the superposition of two powder patterns, one corresponding to the pattern observed below -4°C and the other to the pattern above the pretransition. The relative contribution of the latter increases with temperature until the pretransition is reached. These data have been interpreted in two ways: either the sample between -4°C and the pretransition contains two populations of rapidly and slowly rotating molecules, or there is only a single population of molecules undergoing a 180° flipping motion on the time scale of the NMR measurement. The latter interpretation is more consistent with other experimental findings. At the temperature of the main transition the hydrocarbon chains melt. In the absence of cholesterol, spectra are more complex in that the line shape is reproduced by the superposition of three spectral powder patterns between -4°C and the pretransition and by the superposition of two spectral patterns above the pretransition. It is postulated that these two patterns observed above the pretransition are in direct correspondence to the two ripple structures observed by freeze-fracture electron microscopy in the absence of cholesterol.     

Fast molecular tracking maps nanoscale dynamics of plasma membrane lipids

We describe an optical method capable of tracking a single fluorescent molecule with a flexible choice of high spatial accuracy (∼10–20 nm standard deviation or ∼20–40 nm full-width-at-half-maximum) and temporal resolution (< 1 ms). The fluorescence signal during individual passages of fluorescent molecules through a spot of excitation light allows the sequential localization and thus spatio-temporal tracking of the molecule if its fluorescence is collected on at least three separate point detectors arranged in close proximity. We show two-dimensional trajectories of individual, small organic dye labeled lipids diffusing in the plasma membrane of living cells and directly observe transient events of trapping on < 20 nm spatial scales. The trapping is cholesterol-assisted and much more pronounced for a sphingo- than for a phosphoglycero-lipid, with average trapping times of ∼15 ms and < 4 ms, respectively. The results support previous STED nanoscopy measurements and suggest that, at least for nontreated cells, the transient interaction of a single lipid is confined to macromolecular dimensions. Our experimental approach demonstrates that fast molecular movements can be tracked with minimal invasion, which can reveal new important details of cellular nano-organization.     

Design strategies to minimize the radiative efficiency of global warming molecules

A strategy is devised to screen molecules based on their radiative efficiency. The methodology should be useful as one additional constraint when determining the best molecule to use for an industrial application. The strategy is based on the results of a recent study where we examined molecular properties of global warming molecules using ab initio electronic structure methods to determine which fundamental molecular properties are important in assessing the radiative efficiency of a molecule. Six classes of perfluorinated compounds are investigated. For similar numbers of fluorine atoms, their absorption of radiation in the IR window decreases according to perfluoroethers > perfluorothioethers ≈ sulfur/carbon compounds > perfluorocarbons > perfluoroolefins > carbon/nitrogen compounds. Perfluoroethers and hydrofluorethers are shown to possess a large absorption in the IR window due to (i) the C─O bonds are very polar, (ii) the C-O stretches fall within the IR window and have large IR intensity due to their polarity, and (iii) the IR intensity for C-F stretches in which the fluorine atom is bonded to the carbon that is bonded to the oxygen atom is enhanced due to a larger C─F bond polarity. Lengthening the carbon chain leads to a larger overall absorption in the IR window, though the IR intensity per bond is smaller. Finally, for a class of partially fluorinated compounds with a set number of electronegative atoms, the overall absorption in the IR window can vary significantly, as much as a factor of 2, depending on how the fluorine atoms are distributed within the molecule.     

Mechanistic details of a protein–protein association pathway revealed by paramagnetic relaxation enhancement titration measurements

Protein–protein association generally proceeds via the intermediary of a transient, lowly populated, encounter complex ensemble. The mechanism whereby the interacting molecules in this ensemble locate their final stereospecific structure is poorly understood. Further, a fundamental question is whether the encounter complex ensemble is an effectively homogeneous population of nonspecific complexes or whether it comprises a set of distinct structural and thermodynamic states. Here we use intermolecular paramagnetic relaxation enhancement (PRE), a technique that is exquisitely sensitive to lowly populated states in the fast exchange regime, to characterize the mechanistic details of the transient encounter complex interactions between the N-terminal domain of Enzyme I (EIN) and the histidine-containing phosphocarrier protein (HPr), two major bacterial signaling proteins. Experiments were conducted at an ionic strength of 150 mM NaCl to eliminate any spurious nonspecific associations not relevant under physiological conditions. By monitoring the dependence of the intermolecular transverse PRE (Γ₂) rates measured on ¹⁵N-labeled EIN on the concentration of paramagnetically labeled HPr, two distinct types of encounter complex configurations along the association pathway are identified and dissected. The first class, which is in equilibrium with and sterically occluded by the specific complex, probably involves rigid body rotations and small translations near or at the active site. In contrast, the second class of encounter complex configurations can coexist with the specific complex to form a ternary complex ensemble, which may help EIN compete with other HPr binding partners in vivo by increasing the effective local concentration of HPr even when the active site of EIN is occupied.     

Evidence of Coulomb blockade behavior in a quasi-zero-dimensional quantum well on TiO2 surface

Line defects on the surface of rutile TiO₂(110) form in pairs separated by 1.2 nm creating a quantum well. The well is effectively closed by the presence of two charged structures at both ends separated by a distance in the 10–20 nm range. As expected for quantum confinement a long period oscillatory feature of the local density of states is observed and attributed to the formation of discrete quantum states inside the system. It is at first glance surprising that the lowest energy quantum state of the well can be observed at room temperature. The properties of the quantum state cannot be explained in an independent-electron, band-like theory. Instead, electron-electron correlation must be included to give a satisfactory picture of the spatial distribution of the charge density. Theory predicts charging energies of 1.30 eV and 1.14 eV for quantum well lengths of 14 nm and 16 nm, respectively, in good agreement with a classical calculation and the size dependence of the capacitance. This observation opens up the possibility of experimentally imaging the transition from a Coulomb blockade localized in a zero-dimensional system to an independent-particle or band-like behavior in an extended one-dimensional system.     

Nickel Oxide Films Deposited by Sol-Gel Method: Effect of Annealing Temperature on Structural,Optical, and Electrical Properties

In our study, transparent and conductive films of NiO_x were successfully deposited by sol-gel technology. NiO_x films were obtained by spin coating on glass and Si substrates. The vibrational, optical, and electrical properties were studied as a function of the annealing temperatures from 200 to 500 °C. X-ray Photoelectron (XPS) spectroscopy revealed that NiO was formed at the annealing temperature of 400 °C and showed the presence of Ni⁺ states. The optical transparency of the films reached 90% in the visible range for 200 °C treated samples, and it was reduced to 76–78% after high-temperature annealing at 500 °C. The optical band gap of NiOx films was decreased with thermal treatments and the values were in the range of 3.92–3.68 eV. NiO_x thin films have good p-type electrical conductivity with a specific resistivity of about 4.8 × 10⁻³ Ω·cm. This makes these layers suitable for use as wideband semiconductors and as a hole transport layer (HTL) in transparent solar cells.     

Alternating electron and proton transfer steps in photosynthetic water oxidation

Water oxidation by cyanobacteria, algae, and plants is pivotal in oxygenic photosynthesis, the process that powers life on Earth, and is the paradigm for engineering solar fuel–production systems. Each complete reaction cycle of photosynthetic water oxidation requires the removal of four electrons and four protons from the catalytic site, a manganese–calcium complex and its protein environment in photosystem II. In time-resolved photothermal beam deflection experiments, we monitored apparent volume changes of the photosystem II protein associated with charge creation by light-induced electron transfer (contraction) and charge-compensating proton relocation (expansion). Two previously invisible proton removal steps were detected, thereby filling two gaps in the basic reaction-cycle model of photosynthetic water oxidation. In the S₂ → S₃ transition of the classical S-state cycle, an intermediate is formed by deprotonation clearly before electron transfer to the oxidant (). The rate-determining elementary step (τ, approximately 30 µs at 20 °C) in the long-distance proton relocation toward the protein–water interface is characterized by a high activation energy (E_a = 0.46 ± 0.05 eV) and strong H/D kinetic isotope effect (approximately 6). The characteristics of a proton transfer step during the S₀ → S₁ transition are similar (τ, approximately 100 µs; E_a = 0.34 ± 0.08 eV; kinetic isotope effect, approximately 3); however, the proton removal from the Mn complex proceeds after electron transfer to . By discovery of the transient formation of two further intermediate states in the reaction cycle of photosynthetic water oxidation, a temporal sequence of strictly alternating removal of electrons and protons from the catalytic site is established.     

The vibrational energy flow transition in organic molecules: Theory meets experiment

Most large dynamical systems are thought to have ergodic dynamics, whereas small systems may not have free interchange of energy between degrees of freedom. This assumption is made in many areas of chemistry and physics, ranging from nuclei to reacting molecules and on to quantum dots. We examine the transition to facile vibrational energy flow in a large set of organic molecules as molecular size is increased. Both analytical and computational results based on local random matrix models describe the transition to unrestricted vibrational energy flow in these molecules. In particular, the models connect the number of states participating in intramolecular energy flow to simple molecular properties such as the molecular size and the distribution of vibrational frequencies. The transition itself is governed by a local anharmonic coupling strength and a local state density. The theoretical results for the transition characteristics compare well with those implied by experimental measurements using IR fluorescence spectroscopy of dilution factors reported by Stewart and McDonald [Stewart, G. M. & McDonald, J. D. (1983) J. Chem. Phys. 78, 3907–3915].     

A polarization scheme that resolves cross-peaks with transient absorption and eliminates diagonal peaks in 2D spectroscopy

Two-dimensional (2D) optical spectroscopy contains cross-peaks that are helpful features for determining molecular structure and monitoring energy transfer, but they can be difficult to resolve from the much more intense diagonal peaks. Transient absorption (TA) spectra contain transitions similar to cross-peaks in 2D spectroscopy, but in most cases they are obscured by the bleach and stimulated emission peaks. We report a polarization scheme, <0°,0°,+θ₂(t₂),-θ₂(t₂)>, that can be easily implemented in the pump-probe beam geometry, used most frequently in 2D and TA spectroscopy. This scheme removes the diagonal peaks in 2D spectroscopies and the intense bleach/stimulated emission peaks in TA spectroscopies, thereby resolving the cross-peak features. At zero pump-probe delay, θ₂ = 60° destructively interferes two Feynman paths, eliminating all signals generated by field interactions with four parallel transition dipoles, and the intense diagonal and bleach/stimulated emission peaks. At later delay times, θ₂(t₂) is adjusted to compensate for anisotropy caused by rotational diffusion. When implemented with TA spectroscopy or microscopy, the pump-probe spectrum is dominated by the cross-peak features. The local oscillator is also attenuated, which enhances the signal two times. This overlooked polarization scheme reduces spectral congestion by eliminating diagonal peaks in 2D spectra and enables TA spectroscopy to measure similar information given by cross-peaks in 2D spectroscopy.

Transient absorption (TA) spectroscopy and microscopy are ubiquitously used for measuring kinetics in chemical, biological, and material sciences. TA spectroscopy initiates excited-state dynamics with a pump pulse and tracks their evolution with a probe pulse, yielding kinetic information. The polarization of the pump and probe fields strongly affects the utility and interpretation of TA data (1–5). The choice of pulse polarization can be employed to ease interpretation or extract particular information. For example, under three-dimensional (3D) isotropic conditions, kinetics measured at 54.7° relative angle (magic angle) between pump and probe polarizations are insensitive to molecular rotation (6–11). Alternatively, the anisotropy can be calculated after independently measuring parallel and perpendicularly polarized pulses, giving a signal that depends on rotational diffusion and not population relaxation (6–11). Magic angle and anisotropy measurements are textbook experiments.A technique closely related to TA spectroscopy is two-dimensional (2D) spectroscopy, such as 2D infrared (IR) and 2D electronic spectroscopy. TA and 2D spectroscopies are alike in that they both measure a signal created by three electric field interactions from the pulse sequence, which makes them third-order nonlinear techniques (3, 12, 13). Because TA and 2D spectroscopy are both third-order techniques, the polarization dependence of their spectra is identical. However, the way in which the experiments are implemented puts physical limitations on the polarizations that can be applied. For TA spectroscopy, the first two interactions (E₁ and E₂) are created by the pump pulse and third by the probe pulse (E₃), followed by the emitted field (E_emit) that ultimately becomes the signal. For time-domain 2D spectroscopy, there are also three interactions, one each from two separate pump pulses (E₁ and E₂) and the third from the probe pulse (E₃), followed by E_emit. The signals of both experiments depend on the orientational average of four electric fields with the sample, which is often written as the four-point orientational average <E₁,E₂,E₃,E_emit>.Since TA spectroscopy only uses two laser pulses, polarization control is traditionally limited to the relative angle between pump and probe polarizations. When 2D spectroscopy was first developed, it was implemented in a four-wave mixing geometry that allowed all three pulse polarizations to be individually set along with the polarization of the emitted field (14–16). This new capability led to the derivation of the full fourth-order orientational correlation function (4, 10, 17) and more advanced polarization schemes that determined the angle between coupled oscillators (17), suppressed peaks (18), and enhanced signal-to-noise (19, 20). One of the most unique polarization schemes was <E₁,E₂,E₃,E_emit> = <−45°,+45°,90°,0°>, with the angles defined in the laboratory-fixed frame. This polarization scheme eliminates the diagonal peaks from the 2D spectrum under 3D isotropic conditions, isolating the desired cross-peak features (18). This method works by destructively interfering the <0°,90°,0°,90°> and the <0°,90°,90°,0°> signals in situ. In the same publication, Hochstrasser and coworkers proposed <−60°,+60°,0°,0°>, which removes diagonal peaks but does not compensate for rotational diffusion (18, 21). Ginsberg et al. and Read et al. implemented <−60°,+60°,0°,0°> in the visible (22, 23). One can independently measure and subtract spectra collected for each of these polarizations (24), but subtraction afterward is usually less accurate and sometimes very difficult, such as when measuring the kinetics of protein aggregation. Besides polarization schemes, peaks can be removed by fitting or by isolating interstate coherences (25–27).Diagonal peak suppression was widely implemented in 2D spectroscopy (18, 22, 23, 28–38) until pump-probe beam geometries began replacing the four-wave mixing geometry. The most common pump-probe implementation of 2D spectroscopy is using a pulse shaper to create the two pump pulses (39, 40). Pulse-shaping 2D spectroscopy has many advantages over four-wave mixing 2D spectroscopy, such as phase stability, shot-to-shot readout, and absorptive line shapes (41, 42). One drawback has been, like TA spectroscopy, that the pump pulses are collinear and so their polarizations are difficult to control independently (43). As a result, the <−45°,+45°,90°,0°> polarization scheme that was so useful for visualizing cross-peaks is now less utilized. We note that <−45°,+45°,90°,0°> can be implemented in pump-probe 2D spectroscopies that use interferometers (44–46), birefringent wedges (37, 47), and polarization pulse shapers (43).In this paper we report a polarization scheme that can be implemented in the pump-probe geometry used by 2D and TA spectroscopies. Spectra collected in this polarization scheme only contain features from nonparallel transition dipoles. For 2D spectroscopy, this scheme eliminates the diagonal peaks so that only cross-peaks remain in the spectra. For TA spectroscopy, this scheme means that TA spectra provide the same information as the cross-peaks in 2D spectra, as we demonstrate. The polarization scheme we implement is <0°,0°,+60°,−60°>, or more generally, <0°,0°,+θ₂(t₂),−θ₂(t₂)> [where θ₂(t₂) depends on the pump-probe delay, t₂]. Permutational symmetry holds for the fourth-rank orientational response at t₂ = 0 and so <0°,0°,+60°,−60°> gives an equivalent 2D spectrum to <−60°,+60°,0°,0°>. What has been overlooked is that <0°,0°,+60°,−60°> can be experimentally implemented in the pump-probe geometry by adding two polarizers in the probe beam (SI Appendix, Fig. S1), whereas <−60°,+60°,0°,0°> cannot. As a result, the <0°,0°,+60°,−60°> polarization allows TA spectroscopy to obtain coupling information that, until now, could only be resolved by 2D spectroscopy. The method promises to revive 2D spectra of cross-peaks, enable TA spectroscopy to measure couplings, and permit new experiments like TA imaging of coupled modes. In what follows, we first qualitatively describe the method and experimentally demonstrate it then present the theoretical underpinnings and discussion of its strengths, weaknesses, and potential uses.     

Superconductivity at ～100?K in dense SiH4(H2)2 predicted by first principles

Motivated by the potential high-temperature superconductivity in hydrogen-rich materials, the high-pressure structures of SiH₄(H₂)₂ in the pressure range 50–300 GPa were extensively explored by using a genetic algorithm. An intriguing layered orthorhombic (Ccca) structure was revealed to be energetically stable above 248 GPa with the inclusion of zero-point energy. The Ccca structure is metallic and composed of hydrogen shared SiH₈ dodecahedra layers intercalated by orientationally ordered molecular H₂. Application of the Allen-Dynes modified McMillan equation yields remarkably high superconducting temperatures of 98–107 K at 250 GPa, among the highest values reported so far for phonon-mediated superconductors. Analysis reveals a unique superconducting mechanism that the direct interactions between H₂ and SiH₄ molecules at high pressure play the major role in the high superconductivity, while the contribution from H₂ vibrons is minor.     

Enhanced detection of abnormalities in heart rate variability and dynamics by 7‐day continuous ECG monitoring

BackgroundThe analysis of heart rate variability (HRV) and heart rate (HR) dynamics by Holter ECG has been standardized to 24 hs, but longer‐term continuous ECG monitoring has become available in clinical practice. We investigated the effects of long‐term ECG on the assessment of HRV and HR dynamics.MethodsIntraweek variations in HRV and HR dynamics were analyzed in 107 outpatients with sinus rhythm. ECG was recorded continuously for 7 days with a flexible, codeless, waterproof sensor attached on the upper chest wall. Data were divided into seven 24‐h segments, and standard time‐ and frequency‐domain HRV and nonlinear HR dynamics indices were computed for each segment.ResultsThe intraweek coefficients of variance of HRV and HR dynamics indices ranged from 2.9% to 26.0% and were smaller for frequency‐domain than for time‐domain indices, and for indices reflecting slower HR fluctuations than faster fluctuations. The indices with large variance often showed transient abnormalities from day to day over 7 days, reducing the positive predictive accuracy of the 24‐h ECG for detecting persistent abnormalities over 7 days. Conversely, 7‐day ECG provided 2.3‐ to 6.5‐fold increase in sensitivity to detect persistent plus transient abnormalities compared with 24‐h ECG. It detected an average of 1.74 to 2.91 times as many abnormal indices as 24‐h ECG.ConclusionsLong‐term ECG monitoring increases the accuracy and sensitivity of detecting persistent and transient abnormalities in HRV and HR dynamics and allows discrimination between the two types of abnormalities. Whether this discrimination improves risk stratification deserves further studies.     

Depression of reactivity by the collision energy in the single barrier H?+?CD4?→?HD?+?CD3 reaction

Crossed molecular beam experiments and accurate quantum scattering calculations have been carried out for the polyatomic H + CD₄ → HD + CD₃ reaction. Unprecedented agreement has been achieved between theory and experiments on the energy dependence of the integral cross section in a wide collision energy region that first rises and then falls considerably as the collision energy increases far over the reaction barrier for this simple hydrogen abstraction reaction. Detailed theoretical analysis shows that at collision energies far above the barrier the incoming H-atom moves so quickly that the heavier D-atom on CD₄ cannot concertedly follow it to form the HD product, resulting in the decline of reactivity with the increase of collision energy. We propose that this is also the very mechanism, operating in many abstraction reactions, which causes the differential cross section in the backward direction to decrease substantially or even vanish at collision energies far above the barrier height.     

Mechanism of the order–disorder phase transition,and glassy behavior in the metal-organic framework [(CH3)2NH2]Zn(HCOO)3

Transitions associated with orientational order–disorder phenomena are found in a wide range of materials and may have a significant impact on their properties. In this work, specific heat and ¹H NMR measurements have been used to study the phase transition in the metal-organic framework (MOF) compound [(CH₃)₂NH₂]Zn(HCOO)₃. This compound, which possesses a perovskite-type architecture, undergoes a remarkable order–disorder phase transition at 156 K. The (DMA⁺) cationic moieties that are bound by hydrogen bonds to the oxygens of the formate groups (N─H⋯O ∼ 2.9 Å) are essentially trapped inside the basic perovskite cage architecture. Above 156 K, it is the orientations of these moieties that are responsible for the disorder, as each can take up three different orientations with equal probability. Below 156 K, the DMA⁺ is ordered within one of these sites, although the moiety still retains a considerable state of motion. Below 40 K, the rotational motions of the methyl groups start to freeze. As the temperature is increased from 4 K in the NMR measurements, different relaxation pathways can be observed in the temperature range approximately 65–150 K, as a result of a “memory effect.” This dynamic behavior is characteristic of a glass in which multiple states possess similar energies, found here for a MOF. This conclusion is strongly supported by the specific heat data.