广州市96株血流感染金黄色葡萄球菌的基因分型及耐药性分析

目的 分析血流感染中分离金黄色葡萄球菌的分子分型特征和药物敏感试验结果,为耐药性金黄色葡萄球菌防治提供基础依据。方法 对中山大学附属第一医院2013—2017年血流感染分离的96株金黄色葡萄球菌应用MLST和spa技术进行分子分型,同时进行药物敏感试验分析血流感染中金黄色葡萄球菌对万古霉素、奎奴普丁/达福普汀、利奈唑胺等14种抗生素的药敏特征。结果 96株金黄色葡萄球菌中40株为甲氧西林耐药金黄色葡萄球菌（Methicillin resistant Staphylococcus aureus, MRSA）,其主要型别为ST59-t437型7株（占17.5%）,ST1-t114型4株（占10.0%）,ST239-t030型3株（占7.5%）,ST239-t037、ST45-t116、ST5-t2595和ST7-t091型均为2株（均占5.0%）;56株甲氧西林敏感金黄色葡萄球菌（Methicillin susceptible Staphylococcus aureus, MSSA）,其主要型别为ST188-t189型14株（占25.0%）,ST7-t803型3株（占5.4%）,ST188-t2883、ST6-t701、ST30-t338和ST59-t437型均2株（均占3.6%）。spa分型中发现4个新型,分别为t17040、t17041、t17044、t17046。40株MRSA菌株对红霉素和克林霉素耐药率分别为82.9%和51.2%,对万古霉素、利奈唑胺、奎奴普丁/达福普汀耐药率均为0;56株MSSA对所试验抗生素敏感率均较高,对奎奴普丁/达福普汀、万古霉素、利奈唑胺、呋喃妥因、替加环素的敏感率为100.0%,复方新诺明的敏感率为94.5%,对庆大霉素和利福平的敏感率均为96.4%。结论 血流感染中金黄色葡萄球菌分子分型型别较多,应重视金黄色葡萄球菌的感染控制。     

Organization and cellular arrangement of two neurogenic regions in the adult ferret (Mustela putorius furo) brain

In the adult mammalian brain, two neurogenic regions have been characterized, the subventricular zone (SVZ) of the lateral ventricle (LV) and the subgranular zone (SGZ) of the dentate gyrus (DG). Despite remarkable knowledge of rodents, the detailed arrangement of neurogenic regions in most mammals is poorly understood. In this study, we used immunohistochemistry and cell type‐specific antibodies to investigate the organization of two germinal regions in the adult ferret, which belongs to the order Carnivora and is widely used as a model animal with a gyrencephalic brain. From the SVZ to the olfactory bulb, doublecortin‐positive cells tended to organize in chain‐like clusters, which are surrounded by a meshwork of astrocytes. This structure is homologous to the rostral migratory stream (RMS) described in other species. Different from rodents, the horizontal limb of the RMS emerges directly from the LV, and the anterior region of the LV extends rostrally and reached the olfactory bulb. In the DG, glial fibrillary acidic protein‐positive cells with long radial processes as well as doublecortin‐positive cells are oriented in the SGZ. In both regions, doublecortin‐positive cells showed characteristic morphology and were positive for polysialylated‐neural cell adhesion molecule, beta‐III tubulin, and lamin B1 (intense staining). Proliferating cells were detected in both regions using antibodies against proliferating cell nuclear antigen and phospho‐histone H3. These observations demonstrate that the two neurogenic regions in ferrets have a similar cellular composition as those of other mammalian species despite anatomical differences in the brain. J. Comp. Neurol. 522:1818–1838, 2014. © 2013 Wiley Periodicals, Inc.     

Effects of differential habitat warming on complex communities

Food webs unfold across a mosaic of micro and macro habitats, with each habitat coupled by mobile consumers that behave in response to local environmental conditions. Despite this fundamental characteristic of nature, research on how climate change will affect whole ecosystems has overlooked (i) that climate warming will generally affect habitats differently and (ii) that mobile consumers may respond to this differential change in a manner that may fundamentally alter the energy pathways that sustain ecosystems. This reasoning suggests a powerful, but largely unexplored, avenue for studying the impacts of climate change on ecosystem functioning. Here, we use lake ecosystems to show that predictable behavioral adjustments to local temperature differentials govern a fundamental structural shift across 54 food webs. Data show that the trophic pathways from basal resources to a cold-adapted predator shift toward greater reliance on a cold-water refuge habitat, and food chain length increases, as air temperatures rise. Notably, cold-adapted predator behavior may substantially drive this decoupling effect across the climatic range in our study independent of warmer-adapted species responses (for example, changes in near-shore species abundance and predator absence). Such modifications reflect a flexible food web architecture that requires more attention from climate change research. The trophic pathway restructuring documented here is expected to alter biomass accumulation, through the regulation of energy fluxes to predators, and thus potentially threatens ecosystem sustainability in times of rapid environmental change.Natural systems are inherently complex entities, wherein organisms act as agents of material and biomass transport (1) weaving food webs through a mosaic thermal environment. Direct temperature effects on trophic interactions arise through thermal regulation of an organism’s physiology and behavior (2–5). For ecotherms (that is, organisms whose body temperature is aligned with ambient temperature), several biological rates show unimodal responses to temperature (2, 3, 6), and correspondingly, studies have shown that consumption rates initially rise with warming to a peak rate and then fall rapidly approaching a critical temperature (6). Understanding the ways that these organism responses alter food webs, and how these food web responses affect ecosystem function, are key requirements to predicting climate change impacts on ecosystems (7–11).A simple way to think about temperature’s effects on any single trophic interaction is through the general linear consumption function:Consumption(per?capita) = a?t_s?R, [1]where a is the attack rate, t_s is the time searching, and R is the resource biomass density. The direct effects of temperature on an organism’s ability to encounter and capture resources in a given habitat may largely depend on a, and t_s (with potential indirect effects relative to the consumer through temperature influences on R). The argument for the temperature dependence of attack rate (a) is relatively straightforward. Temperature mediates foraging velocity (3), and considering all else equal, velocity determines encounter rates and prey capture success. The influence of temperature on time searching is a little more complex, but the general expectation is that its influence will be shaped by the requirement that the organism allocate its feeding time in different patches or habitats to increase its fitness (5). Such thermal limitation of search time would lead to reductions of interaction strength in warming habitats—in effect, temperature would mediate prey availability (e.g., when temperature exceeds physiological limits). What remains to complete the consumption equation above is the effect of temperature on R, both the direct effects (for example, the impact of warming on R’s productivity) and indirect effects (for example, impact of warming on the number and consumption capabilities of consumers competing for R) (12, 13). Note that the numerical response (i.e., biomass accumulation) of the consumer may depend on additional vital rates (e.g., conversion efficiency). The conversion of prey biomass to predator biomass (often denoted e) may not change with temperature (2, 3), although recent research suggests that e may be temperature dependent if consumers switch among resources with different elemental composition to balance changing metabolic and somatic demands (14). Nevertheless, we focus on consumption (a, t_s, R) as a means to build an argument for temperature’s influence on trophic structure.Here, we extend the logic that underlies this simple representation of temperature-dependent consumption to develop hypotheses that link temperature differentials, through direct and indirect means, to spatial food web structure. Spatially simple laboratory studies of food webs suggest that larger-bodied, higher trophic level organisms are likely to have high extinction risk with ambient warming (15). In natural systems, these higher-order predators provide a spatially unifying component to food webs: their high mobility enables them to forage among different habitats, coupling food chains with unique basal resource groups (16–18). This coupling structure can be an important part of sustaining higher-order consumers with consequences for food chain length, trophic control, and ecosystem stability (16, 19–21). For example, theory argues that reduced access to a novel resource compartment may decrease a consumer’s biomass (19, 20), thereby increasing the chance of local extinction from a random event. When accessibility is limited, reduced coupling may alter food chain length if habitats contain prey that differ in trophic position (22) or if higher level prey increase, with reduced trophic control, and consequently predators become less omnivorous (19, 21). Given that temperature change can drive asymmetric responses in species that differ in thermal tolerance, the influence of spatially structuring elements on the response of a food web to warming will depend not only on the direct responses of consumers to temperature (2, 3, 5) but also those responses of other interacting community members (i.e., resources and competing predators) (12, 13, 23). We test notions of the structuring effects of differential temperature on spatially coupled food webs (thermal-accessibility hypothesis), using boreal lakes as a natural study system (Fig. 1). To make this test, we assembled one of the largest comparative food web datasets on record: 54 ecosystems, characterized using >3,000 isotope (N and C) samples.Open in a separate windowFig. 1.Simple schematic showing expected effects of differential warming on habitat coupling (horizontal axis) and habitat use (vertical axis) by lake trout in cold (Upper) and warm (Lower) lakes. A thermal accessibility argument predicts that lake trout couple into the thermally exposed near-shore resource channel less and should use (proportionally) that habitat less under warmer conditions (indicated by lake trout position). The arrow direction and thickness indicate coupling direction and strength. The letters in the upper diagram identify trophic groups used in both cold (Upper) and warm (Lower) lake depictions: lake trout (a), pelagic forage fish (b), pelagic invertebrates (c), pelagic phytoplankton (d), littoral fish (e), littoral invertebrates (f), and benthic algae (g). To the right in the diagram, we show thermal profile data contrasting temperature at depth from Victoria Lake [cold; summer air temperature, 15.5 °C; latitude (lat), 49.62306; longitude (long), −91.54889] and Charleston Lake (warm; summer air temperature, 19.7 °C; lat, 44.53611; long, −76.01194) taken at the time of sampling. Temperature is visually highlighted with darker blue (cold) and darker red (warm) hues. These lakes experience temperatures near the cold and warm endpoints for our dataset and are of the same order of magnitude in size, and both had thermal profiles recorded to 30 m.Freshwater lakes are particularly sensitive to climate change as lake habitats are structured by climate-driven water temperature and many biota are vulnerable to ambient temperature change (24). A key habitat feature of boreal lakes is thermal stratification, an effect of antagonistic physical forces of mixing by wind energy and resistance to mixing by solar heating that separates cold, more dense water (hypolimnion) from warmer, less dense surface water (epilimnion) (25). The stratification process creates a potential for temperature differentials between deeper offshore and shallower near-shore subhabitats within a lake, as temperatures remain relatively constant in deep habitats, whereas shallower near-shore temperatures are strongly influenced by air temperatures (26). Monitoring in the boreal region (27, 28) has shown that rising air temperature warms surface waters, accelerates the stratification process, and extends the duration of stratification; thus, air temperature is a primary determinant of lake thermal heterogeneity.Most aquatic organisms (e.g., invertebrates, amphibians, fish) are ectotherms; therefore, the demands of the thermal environment arguably form the most influential set of abiotic factors aquatic organisms must satisfy (29, 30) (including increased oxygen requirements in warmer water). Thermal differentiation in lakes typically corresponds with the species differences that characterize offshore and near-shore habitats. Conveniently, biomass flow from these habitats through a food web can be traced using stable carbon and nitrogen isotope ratios due to isotope differences at the base of the food web between phytoplankton (offshore) and benthic algae (near-shore) (18, 22, 31, 32).We focus our study on the trophic pathways that flow from basal resources to lake trout (Salvelinus namaycush), a vulnerable, cold-adapted (10–12 °C preference) apex predator (33) estimated to reside in 66,500 Canadian lakes (34) (Fig. 1). Previous studies show that lake trout play a keystone structural role in integrating resource pools in offshore and near-shore habitats (18, 21, 22, 31). In what follows, we test the direct and indirect effects of differential warming on this natural system (lake trout food web) across a summer mean temperature gradient ranging 15–20 °C. At the warmer end of this range, surface water temperatures will often exceed the physiological tolerance of lake trout and should restrict accessibility into the near-shore habitat (Fig. 1, Lower). Given this thermal mechanism, we predict that lake trout in warmer climates may change their habitat use to deeper waters and this spatial behavior may shift the degree that near-shore resource pools are coupled by this predator relative to cooler climates (Fig. 1). We further consider whether spatial responses are associated with a shift in the length of the apex predator’s food chain. This thermal-accessibility–mediated restructuring of fundamental food web structure is considered along with complementary notions of warm-tolerant competitor effects and relative prey abundance changes with climate.     

From the Cover: Correlating the motion of electrons and nuclei with two-dimensional electronic–vibrational spectroscopy

Multidimensional nonlinear spectroscopy, in the electronic and vibrational regimes, has reached maturity. To date, no experimental technique has combined the advantages of 2D electronic spectroscopy and 2D infrared spectroscopy, monitoring the evolution of the electronic and nuclear degrees of freedom simultaneously. The interplay and coupling between the electronic state and vibrational manifold is fundamental to understanding ensuing nonradiative pathways, especially those that involve conical intersections. We have developed a new experimental technique that is capable of correlating the electronic and vibrational degrees of freedom: 2D electronic–vibrational spectroscopy (2D-EV). We apply this new technique to the study of the 4-(di-cyanomethylene)-2-methyl-6-p-(dimethylamino)styryl-4H-pyran (DCM) laser dye in deuterated dimethyl sulfoxide and its excited state relaxation pathways. From 2D-EV spectra, we elucidate a ballistic mechanism on the excited state potential energy surface whereby molecules are almost instantaneously projected uphill in energy toward a transition state between locally excited and charge-transfer states, as evidenced by a rapid blue shift on the electronic axis of our 2D-EV spectra. The change in minimum energy structure in this excited state nonradiative crossing is evident as the central frequency of a specific vibrational mode changes on a many-picoseconds timescale. The underlying electronic dynamics, which occur on the hundreds of femtoseconds timescale, drive the far slower ensuing nuclear motions on the excited state potential surface, and serve as a excellent illustration for the unprecedented detail that 2D-EV will afford to photochemical reaction dynamics.Two-dimensional electronic spectroscopy (2D-ES) has become an incisive tool to investigate the electronic relaxation and energy transfer dynamics of molecules, molecular aggregates, and nanomaterials (1–5). These studies have been able to separate the homogenous and inhomogeneous line widths, and identify cross-peaks associated with energy transfer between excitons of biological systems or different electronic states of systems that undergo fast nonradiative transitions. Two-dimensional IR spectroscopy (2D-IR) has proved an indispensible tool for studying vibrational couplings and ground-state structures of chemical and complex biological systems (6–8). Thus far, only 1D electronic–vibrational pump-probe spectroscopy, femtosecond stimulated Raman spectroscopy, and transient 2D-IR (t-2D-IR) are able to follow the evolution of nuclei on the ground or excited states subsequent to narrowband electronic excitation (9–12). t-2D-IR has the unique capability of being able to study the evolution of couplings between vibrations on excited potential energy surfaces (PESs).All of the aforementioned experimental techniques, however, are insensitive to the correlation between the initial absorption to an electronically excited state and the ensuing evolution of the nuclear modes on the excited PES(s). The vibrational manifolds on the ground and excited states are intrinsically linked to the electronic potentials: The coupling between these degrees of freedom is what determines the vertical Franck–Condon factors and therefore the electronic structure of excited molecules, complexes, and materials. The ability to correlate the initial excitation of the electronic–vibrational manifold with the subsequent evolution of high-frequency vibrational modes opens many potential avenues of fruitful study, especially in systems where electronic–vibrational coupling is important to the functionality of a system. This principle is paramount to understand the rapid nonradiative transfer between two (or more) electronic states via conical intersections where the Born–Oppenheimer approximation is not necessarily valid (13). For example, the primary steps in vision that involve the cis–trans isomerization of rhodopsin (14), or the photoprotective mechanisms that rapidly deliver excited DNA bases back to the ground state (15). The ability to directly measure these correlations has hitherto remained unexplored. Here, we demonstrate a new experimental technique, 2D electronic–vibrational spectroscopy (2D-EV), that combines the advantages of 2D-ES and 2D-IR, providing the ability to correlate the initial electronic absorption and subsequent evolution of nuclear motions.Degenerate multidimensional spectroscopy experiments have traditionally been performed in a background-free, four-wave mixing phase-matched geometry (1). With the advent and development of pulse-shaping technology, it is now possible to take advantage of techniques routinely used in NMR, such as phase cycling, and apply them to nonlinear optical spectroscopy (16). Here, we perform 2D optical measurements in a partially collinear geometry, the so-called pump-probe geometry, as pioneered in the mid-IR and electronic regimes by the groups of Zanni and Ogilvie, respectively (17, 18) and as originally envisaged by Jonas and coworker (19). Our pulse sequence and interpulse time delays are illustrated in Fig. 1A. The first visible excitation pulse, k₁, creates a coherent superposition of the ground and excited electronic states. After a coherence time, t₁, a second pulse, k₂, converts the system into a population state, either on the ground (Fig. 1B) or excited electronic state (Fig. 1C). Following a given value of the waiting time, t₂, the mid-IR probe pulse interrogates the vibrational quantum state of the system, which subsequently emits a signal, k_sig, after the echo time, t₃. In the pump-probe geometry, k₃ and k_sig are collinear, meaning that the k₃ probe pulse self-heterodynes the emitted field. This has the significant advantage that the signal has a well defined phase relationship with respect to the heterodyne, k₃, obviating the need to phase each 2D-EV spectrum using the projection-slice theorem (20). One disadvantage of this implementation is that the signal is not background free. The k₁ and k₂ pump pulse pair are created using an acousto-optic programmable dispersive filter pulse shaper (AOPDF) (21), which affords attosecond precision over interpulse time delay, t₁, and accurate control over the relative carrier-envelope phase (ϕ₁₂). In a pump-probe geometry, we can no longer take advantage of phase-matching conditions to separate all of the relevant Liouville pathways for the third-order response of the system, namely, the pump-probe signal, rephasing and nonrephasing signals (22, 23). The sum of the latter two signals comprise the total 2D spectrum of a system: a frequency–frequency correlation map of the initial absorption with the final emission. The pump-probe signal is insensitive to the relative phase of the visible pump pulses, whereas the rephasing (photon echo) and nonrephasing (free-induction decay) signals are sensitive to this phase, and thus a 2D spectrum can be obtained by phase cycling the pair of pump pulses. Each 2D-EV spectrum is created by collecting data for a series of t₁ time delays and ϕ₁₂ relative phases for a fixed t₂ waiting time. The mid-IR emission is frequency dispersed onto an array detector, and thereby t₃ is Fourier-transformed on the detector into its conjugate, ω_IR. The data are subsequently phase-cycled (17, 18), to create a time–frequency (t₁–ω_IR) map, which is then apodized, zero-padded (24), and Fourier-transformed along the t₁ time delay to create the frequency–frequency, ω_VIS–ω_IR, 2D-EV surface. A schematic of the full experimental setup is displayed in Fig. S1.Open in a separate windowFig. 1.(A) Pulse ordering of electronic–vibrational experiments. The green k₁ and k₂ pulses represent the electronic (visible) excitation pulses, and the gray k₃ and k_sig pulses the vibrational (mid-IR) probe and vibrational echo pulses, respectively. Feynman and energy level diagrams for evolution on the (B) ground and (C) excited PESs. (D) Calculated DFT minimum energy ground-state structure of DCM dye. Schematic PESs for (E) the case where the first optically excited state undergoes a surface crossing to a CT state in near proximity to the vertical Franck–Condon region; and (F) for a two-level system where the Stokes shift arises from a large anharmonic shift on the excited state. The green arrows represent the initial absorption in E and F, the blue arrows represent vibrational relaxation or motion along a reaction coordinate, Q, and the red arrows represent the fluorescence.Two-dimensional electronic spectroscopy is a penetrating tool to observe many different pathways including population transfer, electronic coupling, and coherent superpositions of various states of a system. This has the downfall that there is a degree of ambiguity in both the manifestation of pathways in a 2D spectrum and their interpretation. Fortunately 2D-EV, like other two-color 2D experiments (18, 25), has a reduced number of pathways that can contribute to a spectrum. For excitation bandwidths that are insufficient to excite one quantum of the mid-IR vibration probed (such as the experiments detailed here), we are unable to drive any wave packets in the vibration probed. Therefore, we are only sensitive to the two pathways displayed in the Feynman diagrams and associated energy level structure in Fig. 1 B and C: vibrational evolution on the ground or excited state. Note that we have only displayed the rephasing pathway for each signal. The respective nonrephasing signals of the pathways depicted in Fig. 1 B and C, which have the conjugate evolution in the t₁ coherence, are not displayed. In 2D-EV experiments, the rephasing and nonrephasing pathways contain identical information because the period of the high-frequency vibration is far longer than that of the electronic coherence.To demonstrate this new experimental technique, we apply it to the laser dye 4-(di-cyanomethylene)-2-methyl-6-p-(dimethylamino)styryl-4H-pyran (DCM), a model push-pull emitter (26). The calculated ground-state structure of DCM is displayed in Fig. 1D. The excited-state dynamics of DCM have been extensively studied, but the role of a charge-transfer (CT) state, especially in polar solvents, has remained inconclusive. DCM exhibits a substantial solvatochromatic Stokes shift. In dimethyl sulfoxide (DMSO), the static Stokes shift (λ) is 5,200 cm⁻¹ (see Fig. S2 for the absorption and fluorescence spectra in DMSO-d₆) but is only 3,200 cm⁻¹ in n-hexane (27). This difference in Stokes shift is also accompanied by a commensurate difference in the fluorescence quantum yield, which varies by two orders of magnitude between nonpolar and polar solvents (27, 28). There are currently two models for excited-state relaxation of DCM: (i) The first electronically excited state, S₁, is a valence or locally excited (LE) state that undergoes fast nonradiative decay into a lower-lying CT state (CT state between the dimethylaniline and pyran rings) and emission from CT state dominates the fluorescence quantum yield (29–34). This case is illustrated schematically in Fig. 1E. Some studies argue that the LE → CT surface crossing is accompanied by an excited-state isomerization or twisted intermediate (29, 30, 33, 34). The other proposed model (ii), illustrated in Fig. 1F, is that the S₁ state is CT in character and therefore has an anharmonically displaced potential compared with the S₀, which gives rise to the large vibrational Stokes shift (26, 35). The one prevailing conclusion from all of these studies is that the emissive state has some CT character.Here, we demonstrate the first implementation (to our knowledge) of the new 2D-EV experimental method, tracking the evolution of the electronic excitation and simultaneously the associated changes in nuclear geometry with femtosecond time resolution. We are able to differentiate between the two proposed mechanisms leading to the observed large Stokes shift of DCM in DMSO-d₆ and propose a mechanism based on observed shifts along the electronic and visible axes of 2D-EV spectra and their respective timescales.     

Anomalously robust valley polarization and valley coherence in bilayer WS2

We report the observation of anomalously robust valley polarization and valley coherence in bilayer WS₂. The polarization of the photoluminescence from bilayer WS₂ follows that of the excitation source with both circular and linear polarization, and remains even at room temperature. The near-unity circular polarization of the luminescence reveals the coupling of spin, layer, and valley degree of freedom in bilayer system, and the linearly polarized photoluminescence manifests quantum coherence between the two inequivalent band extrema in momentum space, namely, the valley quantum coherence in atomically thin bilayer WS₂. This observation provides insight into quantum manipulation in atomically thin semiconductors.Tungsten sulfide WS₂, part of the family of group VI transition metal dichalcogenides (TMDCs), is a layered compound with buckled hexagonal lattice. As WS₂ thins to atomically thin layers, WS₂ films undergo a transition from indirect gap in bulk form to direct gap at monolayer level with the band edge located at energy-degenerate valleys (K, K′) at the corners of the Brillouin zone (1–3). Like the case of its sister compound, monolayer MoS₂, the valley degree of freedom of monolayer WS₂ could be presumably addressed through nonzero but contrasting Berry curvatures and orbital magnetic moments that arise from the lack of spatial inversion symmetry at monolayers (3, 4). The valley polarization could be realized by the control of the polarization of optical field through valley-selective interband optical selection rules at K and K′ valleys as illustrated in Fig. 1A (4–6). In monolayer WS₂, both the top of the valence bands and the bottom of the conduction bands are constructed primarily by the d orbits of tungsten atoms, which are remarkably shaped by spin–orbit coupling (SOC). The giant spin–orbit coupling splits the valence bands around the K (K′) valley by 0.4 eV, and the conduction band is nearly spin degenerated (7). As a result of time-reversal symmetry, the spin splitting has opposite signs at the K and K′ valleys. Namely, the Kramer’s doublet |K ↑ ? and |K′ ↓ ? is separated from the other doublet |K′ ↑ ? and |K ↓ ? by the SOC splitting of 0.4 eV. The spin and valley are strongly coupled at K (K′) valleys, and this coupling significantly suppresses spin and valley relaxations as both spin and valley indices have to be changed simultaneously.Open in a separate windowFig. 1.(A) Schematic of valley-dependent optical selection rules and the Zeeman-like spin splitting in the valence bands of monolayer WS₂. (B) Diagram of spin–layer–valley coupling in 2H stacked bilayer WS₂. Interlayer hopping is suppressed in bilayer WS₂ owing to the coupling of spin, valley, and layer degrees of freedom.In addition to the spin and valley degrees of freedom, in bilayer WS₂ there exists an extra index: layer polarization that indicates the carriers’ location, either up-layer or down-layer. Bilayer WS₂ follows the Bernal packing order and the spatial inversion symmetry is recovered: each layer is 180° in plane rotation of the other with the tungsten atoms of a given layer sitting exactly on top of the S atoms of the other layer. The layer rotation symmetry switches K and K′ valleys, but leaves the spin unchanged, which results in a sign change for the spin–valley coupling from layer to layer (Fig. 1B). From the simple spatial symmetry point of view, one might expect that the valley-dependent physics fades at bilayers owing to inversion symmetry, as the precedent of bilayer MoS₂ (8). Nevertheless, the inversion symmetry becomes subtle if the coupling of spin, valley, and layer indices is taken into account. Note that the spin–valley coupling strength in WS₂ is around 0.4 eV (the counterpart in MoS₂ ∼ 0.16 eV), which is significantly higher than the interlayer hopping energy (∼0.1 eV); the interlayer coupling at K and K′ valleys in WS₂ is greatly suppressed as indicated in Fig. 1B (7, 9). Consequently, bilayer WS₂ can be regarded as decoupled layers and it may inherit the valley physics demonstrated in monolayer TMDCs. In addition, the interplay of spin, valley, and layer degrees of freedom opens an unprecedented channel toward manipulations of quantum states.Here we report a systemic study of the polarization-resolved photoluminescence (PL) experiments on bilayer WS₂. The polarization of PL inherits that of excitations up to room temperature, no matter whether it is circularly or linearly polarized. The experiments demonstrate the valley polarization and valley coherence in bilayer WS₂ as a result of the coupling of spin, valley, and layer degrees of freedom. Surprisingly, the valley polarization and valley coherence in bilayer WS₂ are anomalously robust compared with monolayer WS₂.For comparison, we first perform polarization-resolved photoluminescence measurements on monolayer WS₂. Fig. 2A shows the photoluminescence spectrum from monolayer WS₂ at 10 K. The PL is dominated by the emission from band-edge excitons, so-called “A” exciton at K and K′ valleys. The excitons carry a clear circular dichroism under near-resonant excitation (2.088 eV) with circular polarization as a result of valley-selective optical selection rules, where the left-handed (right handed) polarization corresponds to the interband optical transition at K (K′) valley. The PL follows the helicity of the circularly polarized excitation optical field. To characterize the polarization of the luminescence spectra, we define a degree of circular polarization as P=I(σ+)−I(σ−)I(σ+)+I(σ−), where I(σ±) is the intensity of the right- (left-) handed circular-polarization component. The luminescence spectra display a contrasting polarization for excitation with opposite helicities: P = 0.4 under σ+ excitation and P = −0.4 under σ− excitation on the most representative monolayer. For simplicity, only the PL under σ+ excitation is shown. The degree of circular polarization P is insensitive to PL energy throughout the whole luminescence as shown in Fig. 2A, Inset. These behaviors are fully expected in the mechanism of valley-selective optical selection rules (3, 4). The degree of circular polarization decays with increasing temperature and drops to 10% at room temperature (Fig. 2B). It decreases as the excitation energy shifts from the near-resonance energy of 2.088 to 2.331 eV as illustrated in Fig. 2C. The peak position of A exciton emission at band edges shifts from 2.04 eV at 10 K to 1.98 eV at room temperature. The energy difference between the PL peak and the near-resonance excitation (2.088 eV) is around 100 meV at room temperature, which is much smaller than the value 290 meV for the low temperature off-resonance excitation at 2.331 eV. However, the observed polarization for off-resonance excitation at 10 K (P = 16%) is much higher than the near-resonance condition at room temperature (P = 10%). It clearly shows that the depolarization cannot be attributed to single process, namely the off-resonance excitation or band-edge phonon scattering only (10).Open in a separate windowFig. 2.Photoluminescence of monolayer WS₂ under circularly polarized excitation. (A) Polarization resolved luminescence spectra with σ+ detection (red) and σ− detection (black) under near-resonant σ+ excitation (2.088 eV) at 10 K. Peak A is the excitonic transition at band edges of K (K′) valleys. Opposite helicity of PL is observed under σ− excitation. Inset presents the degree of the circular polarization at the prominent PL peak. (B) The degree of the circular polarization as a function of temperature. The curve (red) is a fit following a Boltzmann distribution where the intervalley scattering by phonons is assumed. (C) Photoluminescence spectrum under off-resonant σ+ excitation (2.33 eV) at 10 K. The red (black) curve denotes the PL circular components of σ+ (σ−).Next we study the PL from bilayer WS₂. Fig. 3 shows the PL spectrum from bilayer WS₂. The peak labeled as “I” denotes the interband optical transition from the indirect band gap, and the peak A corresponds to the exciton emission from direct band transition at K and K′ valleys. Although bilayer WS₂ has an indirect gap, the direct interband optical transition at K and K′ valleys dominates the integrated PL intensity as the prerequisite of phonon/defect scattering is waived for direct band emission and the direct gap is just slightly larger than the indirect band gap in bilayers. Fig. 3A displays surprisingly robust PL circular dichroism of A exciton emission under circularly polarized excitations of 2.088 eV (resonance) and 2.331 eV (off resonance). The degree of circular polarization of A exciton emission under near-resonant σ± excitation is near unity (around 95%) at 10 K and preserves around 60% at room temperature. In contrast, the emission originating from indirect band gap is unpolarized in all experimental conditions.Open in a separate windowFig. 3.Photoluminescence of bilayer WS₂ under circularly polarized excitations. (A) Polarization-resolved luminescence spectra with components of σ+ (red) and σ− (black) under near-resonant σ+ excitation (2.088 eV) at 10 K. Peak A is recognized as the excitonic transition at band edge of direct gap. Peak I originates from the indirect band-gap emission, showing no polarization. Inset presents the circular polarization of the A excitonic transition around the PL peak. Opposite helicity of PL is observed under σ− excitation. (B) The degree of circular polarization as a function of temperature (black). The curve (red) is a fit following a Boltzman distribution where the intervalley scattering by phonons is assumed. (C) Photoluminescence spectrum of components of σ+ (red) and σ− (black) under off-resonance σ+ excitation (2.33 eV) at 10 K. A nonzero circular polarization P is only observed at emissions from A excitons.To exclude the potential cause of charge trapping or substrate charging effect, we study the polarization-resolved PL of bilayer WS₂ with an out-plane electric field. Fig. 4A shows the evolution of PL spectra in a field-effect-transistor-like device under circularly polarized excitations of 2.088 eV and an electric gate at 10 K. The PL spectra dominated by A exciton show negligible change under the gate bias in the range of −40 to 20 V. The electric-conductance measurements show that the bilayer WS₂ stays at the electrically intrinsic state under the above bias range. The PL spectra can be safely recognized as emissions from free excitons. As the gate bias switches further to the positive side (>20 V), the PL intensity decreases, and the emission from electron-bounded exciton “X⁻,” the so-called trion emerges and gradually raises its weight in the PL spectrum (11, 12). The electron–exciton binding energy is found to be 45 meV. Given only one trion peak in PL spectra, the interlayer trion (formed by exciton and electron/hole in different layers) and intralayer trion (exciton and electron/hole in the same layer) could not be distinguished due to the broad spectral width (13). Both the free exciton and trion show slight red shifts with negative bias, presumably as a result of quantum-confined stark effect (14). At all of the bias conditions, the degree of circular polarization of the free exciton and trion stays unchanged within the experiment sensitivity as shown in Fig. 4C.Open in a separate windowFig. 4.Electric-doping-dependent photoluminescence spectrum of bilayer WS₂ field-effect transistor. (A) Luminescence spectra of bilayer WS₂ at different gate voltage under near-resonant σ+ excitation (2.088 eV) at 10 K. X and X⁻ denote neutral exciton and trion, respectively. Green curve is a fitting consisting of two Lorentzian peak fits (peak I and X⁻) and one Gaussian peak fit (peak X). (B) Intensity of exciton and trion emissions versus gate. (Upper) The gate-dependent integral PL intensity consisting of exciton (X) and trion (X⁻). (Lower) The ratio of the integral PL intensity of exciton versus that of trion, as a function of the gate voltage. (C) Degree of circular polarization of exciton (X, red) and trion (X⁻, blue) versus the gate.It is also unlikely that the high polarization in bilayers results from the isolation of the top layer from the environments, as similar behaviors are observed in monolayer and bilayer WS₂ embedded in polymethyl methaccrylate (PMMA) matrix or capped with a 20-nm-thick SiO₂ deposition. The insensitivity of the circular-polarization degree on bias and environments rules out the possibility that the effects of Coulomb screening, charge traps, or charge transfers with substrates are the major causes for the robust circular dichroism in bilayers against monolayers.One potential cause may result from the shorter lifetime of excitons at K (K′) valley for bilayer system. The band gap shifts from K and K′ points of the Brillouin zone in monolayers to the indirect gap between the top of the valence band at Γ points and the bottom of the conduction band in the middle of K and Γ points in bilayers. Combining our time-resolved pump-probe reflectance experiments (Supporting Information) and the observed relative PL strength between monolayer and bilayer (10:1), we infer the exciton lifetime at K (K′) valleys around 10 ps, a fraction of that at monolayers. If we assume (i) the PL circular polarization P=P01+2ττk, where P₀ is the theoretical limit of PL polarization, and τ_k and τ denote the valley lifetime and exciton lifetime respectively; and (ii) the valley lifetime is the same for both monolayers and bilayers, the shorter exciton lifetime will lead to significantly higher PL polarization. However, the difference in exciton lifetime between bilayers and monolayers is not overwhelming enough to be the major cause of robust polarization observed in the time-integrated PL in bilayers.In monolayer WS₂ under circularly polarized resonant excitations, the depolarization mainly comes from the K ? K′ intervalley scattering. In bilayers, the depolarization could be either via K ? K′ intervalley scattering within the layer in a similar way as in monolayers, or via interlayer hopping, which also requires spin flip. As we discussed above, the interlayer hopping at K valley is suppressed in WS₂ as a result of strong SOC in WS₂ and spin–layer–valley coupling, which were experimentally proved by the circular dichroism in PL from bilayers. The robust polarization in bilayers implies that the intervalley scattering within a layer is diminished compared with that in monolayers. There are two prerequisites for intervalley scattering within layers: conservation of crystal momentum and spin flip of holes. The crystal momentum conservation could be satisfied with the involvement of phonons at K points in the Brillouin zone or atomic size defects, presumably sharing the similar strength in monolayers and bilayers. Spin-flip process could be realized by three different spin scattering mechanisms, namely D’yakonov–Perel (DP) mechanism (15), Elliot–Yaffet (EY) mechanism (16), and Bir–Aronov–Pikus (BAP) mechanism (17, 18). The DP mechanism acts through a Lamor precession driven by electron wavevector k dependent spin–orbit coupling. It is thought to be negligible for spin flip along out-plane direction as the mirror symmetry with respect to the plane of W atoms secures a zero out-plane crystal electric field. Another possible driving force behind the DP mechanism could be the asymmetry owing to the interface with the substrate. This can be excluded by the similar behaviors, where the monolayers and bilayers WS₂ are embedded in PMMA matrix or capped with a thin layer of SiO₂. The negligible effect of electric gating on polarization also implies that the DP mechanism is weak in monolayer and bilayer WS₂; the EY mechanism originates from scattering with phonons and defects. Its strength in bilayers and monolayers is likely to be at similar scale, and bilayers even have more low-frequency collective vibrational modes (19). Therefore, EY mechanism is unlikely to be the cause here; the BAP mechanism originates from the electron–hole exchange interaction. In monolayer and bilayer TMDCs, the optical features are dominated by the Wannier type, yet giant excitonic effect, and the exciton-binding energy in such intrinsic 2D semiconductors is estimated to be 0.6 ∼ 1 eV (20, 21). This giant exciton-binding energy indicates a mixture of electron and hole wavefunctions and, consequently, strong exchange interaction, which may contribute to the spin flip and intervalley scattering (5, 22). As the conduction band has a band mixing at K points, the spin flip of the electron would be a quick process. An analogous scenario is that the spin of holes relaxes in hundreds of femtoseconds or fewer in GaAs as a result of band mixing and spin–orbit coupling. The electron spin flip could lead to hole spin flip via strong exchange interaction accompanying intervalley scattering, which is realized by the virtual annihilation of a bright exciton in the K valley and then generation in the K′ valley or vice versa (22). This non-single-particle spin relaxation leads to valley depolarization instead of the decrease of luminescence intensity that results from coupling with dark excitons. Generally, the exciton-binding energy decreases with the relaxation of spatial confinement. However, first principle calculation shows that monolayer and bilayer WS₂ share the similar band dispersion and effective masses around K valley in their Brillouin zone as a result of spin–valley coupling (7). It implies that the binding energy of excitons around K valley in bilayer WS₂ is similar to or slightly less than that in monolayer WS₂. As the exchange interaction is roughly proportional to the square of exciton binding energy, the spin-flip rate and consequently intervalley scattering via exciton exchange interactions is presumably comparable or smaller to some extent in bilayer WS₂ (Supporting Information). Nevertheless, this is unlikely the major cause of the anomalously robust valley polarization in bilayer WS₂.Another possibility includes extra spin-conserving channels via intermediate intervalley-interlayer scatterings in bilayer WS₂, which are absent in monolayers (23). The extra spin-conserving channel may compete with the spin-flip process and reduce the relative weight of spin-flip intervalley scattering to some extent. However, the mechanism and the strength are unclear so far. Overall, the robust circular polarization in bilayers likely results from combined effects of the shorter exciton lifetime, smaller exciton-binding energy, extra spin-conserving channels, and the coupling of spin, layer, and valley degrees of freedom, indicating the relatively weak intervalley scattering in bilayer system. Further quantitative study is necessary to elaborate the mechanism.We also investigated the PL from bilayer WS₂ under a linearly polarized excitation. A linearly polarized light could be treated as a coherent superposition of two opposite-helicity circularly polarized lights with a certain phase difference. The phase difference determines the polarization direction. In semiconductors, a photon excites an electron–hole pair with the transfer of energy, momentum, and phase information. The hot carriers energetically relax to the band edge in a quick process around 10⁻¹ ∼ 10¹ ps through runs of inelastic and elastic scatterings, e.g., by acoustic phonons. During the quick relaxation process, generally the phase information randomizes and herein coherence fades. In monolayer TMDCs, the main channel for carrier relaxation is through intravalley scatterings including Coulomb interactions with electron (hole) and inelastic interactions with phonons, which are valley independent and preserve the relative phase between K and K′ valleys (24). In bilayer WS₂, the suppression of intervalley scattering consequently leads to the suppression of inhomogeneous broadening in carrier’s phase term. Subsequently, the valley coherence demonstrated in monolayer WSe₂ (24) is expected to be enhanced in bilayers (13). The valley coherence in monolayer and bilayer WS₂ could be monitored by the polarization of PL under linearly polarized excitations.Fig. 5A shows the linearly polarized components of PL under a linearly polarized excitation of 2.088 eV at 10 K. The emission from indirect band gap is unpolarized and A exciton displays a pronounced linear polarization following the excitation. The degree of linear polarization P=I(‖)−I(⊥)I(‖)+I(⊥) is around 80%, where I(∥)(I(⊥)) is the intensity of PL with parallel (perpendicular) polarization with respect to the excitation polarization. In contrast, the linear polarization is much weaker in monolayer samples (4% under the same experimental conditions, as shown in Fig. 5B). As presented in Fig. 5C, the polarization of A exciton is independent of crystal orientation and exactly follows the polarization of excitations. The degree of the linear polarization in bilayer WS₂ slightly decreases with the increased temperature and drops from 80% at 10 K to 50% at room temperature (Fig. 5D). This is the paradigm of the robust valley coherency in bilayer WS₂.Open in a separate windowFig. 5.Linearly polarized excitations on monolayer and bilayer WS₂. (A) Linear-polarization-resolved luminescence spectra of bilayer WS₂ under near-resonant linearly polarized excitation (2.088 eV) at 10 K. Red (black) presents the spectrum with parallel (cross) polarization with respect to the linear polarization of excitation source. A linear polarization of 80% is observed for exciton A, and the indirect gap transition (I) is unpolarized. (B) Linear-polarization-resolved luminescence spectra of monolayer WS₂ under near-resonant linearly polarized excitation (2.088 eV) at 10 K. Red (black) denotes the spectrum with the parallel (cross) polarization with respect to the linear polarization of excitation source. The linear polarization for exciton A in monolayer WS₂ is much weaker, with a maximum value of 4%. (C) Polar plot for intensity of the exciton A in bilayer WS₂ (black) as a function of the detection angle at 10 K. Red curve is a fit-following cos²(θ). (D) The degree of linear polarization of exciton A in bilayer WS₂ (black) as a function of temperature. The curve (red) is a fit following a Boltzmann distribution where the intervalley scattering by phonons is assumed. (E) Electric doping dependence of the linear polarization of exciton A in bilayer WS₂ at 10 K.The linear polarization of both exciton and trion in bilayer, contrasting to the circular polarization, which is insensitive to the electric field in the range, shows a weak electric gating dependence as shown in Fig. 5E. The PL linear polarization, presenting valley coherence, decreases as the Fermi level shifts to the conduction band. It does not directly affect intervalley scattering within individual layers and makes negligible change in circular dichroism. Nevertheless, the electric field between the layers induces a layer polarization and slightly shifts the band alignments between the layers by different amounts in conduction and valence bands (13, 25), although the shift is indistinguishable in the present PL spectra due to the broad spectral width. The layer polarization and the shift of band alignments may induce a relative phase difference between two layers and therefore affect the PL linear polarization via interference. Further study is needed to fully understand the mechanism.In summary, we demonstrated anomalously robust valley polarization and valley polarization coherence in bilayer WS₂. The valley polarization and valley coherence in bilayer WS₂ are the direct consequences of giant spin–orbit coupling and spin valley coupling in WS₂. The depolarization and decoherence processes are greatly suppressed in bilayer, although the mechanism is ambiguous. The robust valley polarization and valley coherence make bilayer WS₂ an intriguing platform for spin and valley physics.     

人体消化道内微机电系统线圈耦合系数分析

在人体消化道微机电系统无线能量传输系统中,线圈间的耦合程度是影响传输效率的关键因素之一.本文对空间任意位置的两个线圈建立耦合模型,提出了一种计算线圈耦合系数的方法,分析了轴向偏移、径向偏移、角度偏移对耦合程度的影响,与实验结果吻合较好.最后利用这种方法,比较分析了能量传输系统中发射线圈的两种布置方式.     

Visual and visuomotor processing of hands and tools as a case study of cross talk between the dorsal and ventral streams

A major principle of organization of the visual system is between a dorsal stream that processes visuomotor information and a ventral stream that supports object recognition. Most research has focused on dissociating processing across these two streams. Here we focus on how the two streams interact. We tested neurologically-intact and impaired participants in an object categorization task over two classes of objects that depend on processing within both streams—hands and tools. We measured how unconscious processing of images from one of these categories (e.g., tools) affects the recognition of images from the other category (i.e., hands). Our findings with neurologically-intact participants demonstrated that processing an image of a hand hampers the subsequent processing of an image of a tool, and vice versa. These results were not present in apraxic patients (N?=?3). These findings suggest local and global inhibitory processes working in tandem to co-register information across the two streams.     

Warning signals for eruptive events in spreading fires

Spreading fires are noisy (and potentially chaotic) systems in which transitions in dynamics are notoriously difficult to predict. As flames move through spatially heterogeneous environments, sudden shifts in temperature, wind, or topography can generate combustion instabilities, or trigger self-stabilizing feedback loops, that dramatically amplify the intensities and rates with which fires propagate. Such transitions are rarely captured by predictive models of fire behavior and, thus, complicate efforts in fire suppression. This paper describes a simple, remarkably instructive physical model for examining the eruption of small flames into intense, rapidly moving flames stabilized by feedback between wind and fire (i.e., “wind–fire coupling”—a mechanism of feedback particularly relevant to forest fires), and it presents evidence that characteristic patterns in the dynamics of spreading flames indicate when such transitions are likely to occur. In this model system, flames propagate along strips of nitrocellulose with one of two possible modes of propagation: a slow, structured mode, and a fast, unstructured mode sustained by wind–fire coupling. Experimental examination of patterns in dynamics that emerge near bifurcation points suggests that symptoms of critical slowing down (i.e., the slowed recovery of the system from perturbations as it approaches tipping points) warn of impending transitions to the unstructured mode. Findings suggest that slowing responses of spreading flames to sudden changes in environment (e.g., wind, terrain, temperature) may anticipate the onset of intense, feedback-stabilized modes of propagation (e.g., “blowup fires” in forests).Multistable systems can, when sufficiently perturbed, undergo “critical transitions” in which they shift abruptly between dynamically distinct states. Such transitions represent important steps in the progression of many natural processes [e.g., the sudden demise of ecosystems or populations (1, 2), the onset of climatic shifts (3, 4), the crash of financial markets (5, 6), the collapse of power grids or of Internet communication networks (7, 8), transitions from life to death (9, 10)], and the identification of phenomena that trigger or presage their onset remains an intellectually challenging and practically important goal of research on the dynamics of complex systems.Recent evidence suggests that a set of generic statistical indicators may warn of impending transitions in a wide range of systems (11, 12). Briefly, as systems approach catastrophic bifurcations, they exhibit slower rates of recovery from perturbations (13), a phenomenon referred to as “critical slowing down;” as the duration of influence associated with those perturbations increases, the fluctuations to which they give rise can become larger (increased variance) (14), more correlated (increased autocorrelation) (15), and/or more asymmetric (increased skewness) (16). Many studies of critical transitions in natural systems have identified corresponding trends in individual variables of state [e.g., increased variance in electrical signals before an epileptic seizure (17)] (2–4, 18), but similar patterns have proven difficult to detect in systems for which variables of state are noisy, interdependent, or poorly defined (as in interconnected, cyclic, or chaotic systems) (11, 12). Warning signals—or, more generally, transitions between alternative stable states—in such systems have, as a result, eluded experimental examination.Spreading fires are noisy [and potentially chaotic (19)] systems for which warning signals of transitions in dynamics could aid in the development of improved practices for control and suppression. In large-scale natural fires (i.e., wildfires), for example, slowly moving flames can spontaneously erupt into blowup fires—large, rapidly moving fires stabilized by feedback between wind and spreading flames (i.e., wind–fire coupling) (20, 21). Such events, which are not captured by operative models of fire behavior, pose enormous risks to fire response teams, and complicate efforts in fire suppression (22–24).To examine patterns in dynamics associated with the onset of intense, feedback-stabilized modes of propagation, we built a simple physical model for blowup-like fires based on a bistable combustion system. In this system, flames propagate along strips of nitrocellulose either as slow, structured flames (characterized by well-defined heights and shapes) or as fast, unstructured flames (marked by aperiodic oscillations in size and shape) in which a form of wind–fire coupling sustains 5- to 10-fold faster rates of propagation. Transitions between these modes can be induced by topographical features of the strip: structured flames can, upon encountering folds in the strip become unstructured; similarly, unstructured flames can, upon encountering the same folds (hereafter referred to as “bumps”), become structured and slow. By using this model system to examine (i) conditions that influence the likelihood of perturbation-initiated transitions between modes of propagation and (ii) patterns in dynamics that emerge as these transitions become more likely, we addressed this question: “Do slowly spreading fires exhibit detectable symptoms of critical slowing down prior to transitioning to intense, feedback-stabilized fires?”     

Elasticity,friction, and pathway of γ-subunit rotation in FoF1-ATP synthase

We combine molecular simulations and mechanical modeling to explore the mechanism of energy conversion in the coupled rotary motors of F_oF₁-ATP synthase. A torsional viscoelastic model with frictional dissipation quantitatively reproduces the dynamics and energetics seen in atomistic molecular dynamics simulations of torque-driven γ-subunit rotation in the F₁-ATPase rotary motor. The torsional elastic coefficients determined from the simulations agree with results from independent single-molecule experiments probing different segments of the γ-subunit, which resolves a long-lasting controversy. At steady rotational speeds of ∼1 kHz corresponding to experimental turnover, the calculated frictional dissipation of less than k_BT per rotation is consistent with the high thermodynamic efficiency of the fully reversible motor. Without load, the maximum rotational speed during transitions between dwells is reached at ∼1 MHz. Energetic constraints dictate a unique pathway for the coupled rotations of the F_o and F₁ rotary motors in ATP synthase, and explain the need for the finer stepping of the F₁ motor in the mammalian system, as seen in recent experiments. Compensating for incommensurate eightfold and threefold rotational symmetries in F_o and F₁, respectively, a significant fraction of the external mechanical work is transiently stored as elastic energy in the γ-subunit. The general framework developed here should be applicable to other molecular machines.F_oF₁-ATP synthase is essential for life. From bacteria to human, this protein synthesizes ATP from ADP and inorganic phosphate P_i in its F₁ domain, powered by an electrochemical proton gradient that drives the rotation of its membrane-embedded F_o domain (1–5). Its two rotary motors, F₁ and F_o, are coupled through the γ-subunit forming their central shaft (2). ATP synthase is a fully reversible motor, in which the rotational direction switches according to different sources of energy (2, 6). In hydrolysis mode, the F₁ motor pumps protons against an electrochemical gradient across the membrane-embedded F_o part, converting ATP to ADP and P_i (7, 8).F₁ has a symmetric ring structure composed of three αβ-subunits with the asymmetric γ-subunit sitting inside the ring (9, 10). Each αβ-subunit has a catalytic site located at the αβ-domain interface. The F₁ ring has a pseudothreefold symmetry with the three αβ-subunits taking three different conformations, E (empty), TP (ATP-bound), and DP (ADP bound) (9–11). The F_o part is composed of a c ring and an a subunit (3, 12). Driven by protons passing through the interface of the c ring and the a subunit, the c ring rotates together with the γ-subunit (rotor) relative to the a subunit, which is connected to the F₁ ring through the peripheral stalk of the b subunit (stator) (12). Interestingly, in nature, one finds a large variation in the number of subunits in the c ring. In animal mitochondria, one finds c₈ rings, requiring a minimal number of eight proton translocations for the synthesis of three ATP, at least 20% fewer protons than in bacteria and plant chloroplasts with c₁₀–c₁₅ rings (13, 14). The resulting symmetry mismatches between F₁ and F_o (15–17) clearly distinguish the biomolecular motor from macroscopic machines.Key open questions concern the detailed rotational pathway of the two coupled rotary motors, the impact of the rotational symmetry mismatch between the F_o and F₁ motors on the motor mechanics, the resulting need for transient energy storage, the role of frictional dissipation, and the molecular elements associated with stepping of the F₁ motor (18–24). Here we explore these questions by building a dissipative mechanical model of the F₁ motor on the basis of atomistic molecular dynamics (MD) simulations. Friction and torsional elasticity of the γ-subunit are central to the efficient function of the coupled F_oF₁ nanomotors (15, 25, 26). For γ-subunits cross-linked with the α₃β₃-ring, estimates have been obtained by monitoring thermal angle fluctuations in single-molecule experiments (16, 27) and MD simulations (28). To probe the elastic and frictional properties under mechanical load over broad ranges of rotation angles and angular velocities, we induce torque-driven γ-subunit rotation in MD simulations (20, 29). From the resulting mechanical deformation and energy dissipation, we construct a fully quantitative viscoelastic model. We account for the torsional elasticity and friction by describing the rotational motion of the γ-subunit as overdamped Langevin dynamics on a 2D harmonic free energy surface. The model quantifies the magnitude of transient elastic energy storage compensating for the incommensurate rotational symmetries of the F_o and F₁ motors (30). The resulting energetic constraints allow us to map out a detailed pathway for their coupled rotary motions, and to rationalize the finer stepping of the mammalian F₁ motor seen in recent experiments (31), with only eight c subunits in the corresponding F_o motor. By quantifying the frictional dissipation, we identify a key contributor to the high thermodynamic efficiency of the F₁ motor. The general framework developed here for F₁ should be applicable also to other molecular machines.