Observing Topological Charges and Dynamical Bulk-Surface Correspondence with Ultracold Atoms.

Quantum dynamics induced in quenching a d-dimensional topological phase across a phase transition may exhibit a nontrivial dynamical topological pattern on the (d-1)D momentum subspace, called band inversion surfaces (BISs), which have a one-to-one correspondence to the bulk topology of the postquench phase. Here we report the experimental observation of such dynamical bulk-surface correspondence through measuring the topological charges in a 2D quantum anomalous Hall model realized in an optical Raman lattice. The system can be quenched with respect to every spin axis by suddenly varying the two-photon detuning or phases of the Raman couplings, in which the topological charges and BISs are measured dynamically by the time-averaged spin textures. We observe that the total charges in the region enclosed by BISs define a dynamical topological invariant, which equals the Chern number of the postquench band and also characterizes the topological pattern of a dynamical field emerging on the BISs, rendering the dynamical bulk-surface correspondence. This study opens a new avenue to explore topological phases dynamically.

Highly Controllable and Robust 2D Spin-Orbit Coupling for Quantum Gases.

We report the realization of a robust and highly controllable two-dimensional (2D) spin-orbit (SO) coupling with a topological nontrivial band structure. By applying a retro-reflected 2D optical lattice, phase tunable Raman couplings are formed into the antisymmetric Raman lattice structure, and generate the 2D SO coupling with precise inversion and C_{4} symmetries, leading to considerably enlarged topological regions. The lifetime of the 2D SO coupled Bose-Einstein condensate reaches several seconds, which enables exploring fine-tuning interaction effects. These essential advantages of the present new realization open the door to explore exotic quantum many-body effects and nonequilibrium dynamics with novel topology.

Uncover Topology by Quantum Quench Dynamics.

Topological quantum states are characterized by nonlocal invariants. We present a new dynamical approach for ultracold-atom systems to uncover their band topology, and we provide solid evidence to demonstrate its experimental advantages. After quenching a two-dimensional (2D) Chern band, realized in an ultracold ^{87}Rb gas from a trivial to a topological parameter regime, we observe an emerging ring structure in the spin dynamics during the unitary evolution, which uniquely corresponds to the Chern number for the postquench band. By extracting 2D bulk topology from the 1D ring pattern, our scheme displays simplicity and is insensitive to perturbations. This insensitivity enables a high-precision determination of the full phase diagram for the system's band topology.

Softening of roton and phonon modes in a Bose-Einstein condensate with spin-orbit coupling.

Roton-type excitations usually emerge from strong correlations or long-range interactions, as in superfluid helium or dipolar ultracold atoms. However, in a weakly short-range interacting quantum gas, the recently synthesized spin-orbit (SO) coupling can lead to various unconventional phases of superfluidity and give rise to an excitation spectrum of roton-maxon character. Using Bragg spectroscopy, we study a SO-coupled Bose-Einstein condensate of ^{87}Rb atoms and show that the excitation spectrum in a "magnetized" phase clearly possesses a two-branch and roton-maxon structure. As Raman coupling strength Ω is decreased, a roton-mode softening is observed, as a precursor of the phase transition to a stripe phase that spontaneously breaks spatially translational symmetry. The measured roton gaps agree well with theoretical calculations. Furthermore, we determine sound velocities both in the magnetized and in the nonmagnetized phases, and a phonon-mode softening is observed around the phase transition in between. The validity of the f-sum rule is examined.

[Variations of δ13C in water-soluble compounds during spring phenology of typical tree species in the warm temperate zone]. / æš–æ¸©å¸¦å ¸åž‹æ ‘ç§æ˜¥å­£ç‰©å€™æœŸæ°´æº¶æ€§åŒ–åˆç‰©Î´13C变化规律.

In this study, we examined the regularity of phenological rhythmical change of plant water-soluble compound δ13C (δ13Cwsc) in spring for two typical tree species in the warm temperate zone of China, Pinus tabuliformis and Robinia pseudoacacia. The δ13Cwsc in each organ of those two species in the spring phenological period were measured to explore the relationship between δ13Cwsc and related environmental factors. The results showed that there were significant differences in δ13Cwsc values of each organ between P. tabuliformis and R. pseudoacacia, with higher δ13Cwsc(-25.03‰±0.01‰) in the new shoot of P. tabuliformis. The δ13Cwsc value in the non-photosynthetic organs were 0.83‰-1.8‰ higher than that in the photosynthetic organs, while the δ13Cwsc value in the aboveground part was generally lower than that in the underground part. As spring progressing, different carbon storage strategies were found between two species. When the terminal bud of P. tabuliformis opened, the carbon was obtained from the proximal old leaves. At the beginning of leaf development, photosynthetic products accumulated by old leaves could not meet the growth requirements for new leaves and roots, with 90% of which depending on the carbon reserve in branches and stems. When full leaf having developed, the photosynthetic function of both new and old leaves recovered and the carbon consumed by branches and stems was gradually replenished. For R. pseudoacacia, at the beginning of leaf bud opening and leaf spreading, branches were the main carbon source for new leaves and roots. When leaves were fully unfolded, mature leaves with high capacity of carbon sequestration became the primary carbon source. Results of principal component analysis showed that temperature during observation period, ≥10 ℃ accumulated temperature, sunshine duration and solar radiation were the main factors influencing δ13Cwsc, which could explained 86.3% of the total variation. The δ13Cwsc values of both species was negatively correlated with temperature and relative humidity, but positively correlated with the difference of saturated water pressure, ≥10 ℃ accumulated temperature and sunshine duration. The main environmental factors affecting plant δ13Cwsc varied during the phenological process. Our results could provide a reference for more accurate estimation of spring organ carbon distribution pattern of regional typical tree species, and also a theoretical basis for formulating scientific and reasonable forest management strategy.

[Responses of plant community diversity to human disturbance in temperate grassland with different soil parent materials]. / ä¸åŒæ¯è´¨æ¸©å¸¦è‰åœ°æ¤ç‰©ç¾¤è½å¤šæ ·æ€§å¯¹äººä¸ºå¹²æ‰°çš„å“åº”.

Based on community investigation data from grasslands on two different soil parent material types (loess and sand parent materials) and under three human utilization modes in the Saihan Ullah Reserve, we calculated human disturbance index (HDI) and biodiversity indices and analyzed the interactions between species diversity and degradation levels. The results showed that degradation status varied across different soil parent material types and human utilization modes, and that degradation levels of loess and sand parent materials both increased with the enhancement of human utilization intensification. HDI of loess parent material grasslands (mean value of 1.21) was lower than sand parent material grasslands (mean value of 1.48) in the same human utilization. Biodiversity indices declined with soil sandy degree and the utilization intensification. The mean values of Margarlef richness index, Shannon diversity index, Simpson dominance index and Pielou evenness index were between 1.57-4.27, 1.16-2.39, 0.76-0.87, and 0.71-0.80, respectively. The Margalef richness index, Shannon diversity index and Simpson dominance index decreased with increasing HDI, while Pielou evenness index increased. Overgrazing could lead to serious threat on both grasslands with soil parent material types, and the optimum utilization mode of loess and sand parent material grasslands were enclosure with mowing and seasonal grazing. In the future works of biodiversity conservation, it is important to consider the influence of both different soil patent material and human utilization modes of grassland. It is urgent to develop different utilization modes for grassland under different soil parent material types, which would enhance the matchness of grassland restoration and management with local conditions.

Acute phase serum cathepsin S level and cathepsin S/cystatin C ratio are the associated factors with cerebral infarction and their diagnostic value for cerebral infarction.

Cathepsin S plays an important role in the pathogenesis of several cardiovascular diseases; however, the relationship between serum cathepsin S and cerebral infarction (CI) is still unknown. This study aimed to investigate the relationship between acute phase serum cathepsin S level and cerebral infarction. A total of 202 stroke patients were enrolled into this study, and were divided into cerebral infarction (n = 140) group and non-cerebral infarction group (non-CI, n = 62). Fifty healthy individuals were recruited as the control group. Serum levels of cathepsin S and cystatin C were measured at days 1, 7, and 14 posthospitalization. Compared to the non-CI group, the CI group had significantly higher rates of hypertension, dyslipidemia, and smoking (all P < 0.05). The CI group had significantly higher cathepsin S levels and cathepsin S to cystatin C ratio (CatS/CysC) at both days 1 and 7 posthospitalization (both P < 0.05). Multivariate logistic regression analysis demonstrated that cathepsin S level (day 7) and CatS/CysC (days 1 and 7) were the associated factors with CI (all P < 0.05). Receiver operating characteristic (ROC) curve analysis revealed that the Area Under Curve (AUC) value of CatS-day7, CatS/CysC-day1, and CatS/CysC-day7 were 0.726 (95% CI: 0.652-0.800, P < 0.001), 0.641 (95% CI: 0.559-0.723, P = 0.001), and 0.721 (95% CI: 0.645-0.797, P = 0.039), respectively. Cathepsin S and CatS/CysC were associated with acute CI, and may have the potential to be the diagnostic biomarkers for CI. Our findings help to better understand the role of serum cathepsin S level in CI.

Ultra-low noise magnetic field for quantum gases.

A ultralow noise magnetic field is essential for many branches of scientific research. Examples include experiments conducted on ultracold atoms, quantum simulations, and precision measurements. In ultracold atom experiments specifically, a bias magnetic field will often serve as a quantization axis and be applied for Zeeman splitting. As atomic states are usually sensitive to magnetic fields, a magnetic field characterized by ultralow noise as well as high stability is typically required for experimentation. For this study, a bias magnetic field is successfully stabilized at 14.5 G, with the root mean square value of the noise reduced to 18.5 µG (1.28 ppm) by placing µ-metal magnetic shields together with a dynamical feedback circuit. Long-time instability is also regulated consistently below 7 µG. The level of noise exhibited in the bias magnetic field is further confirmed by evaluating the coherence time of a Bose-Einstein condensate characterized by Rabi oscillation. It is concluded that this approach can be applied to other physical systems as well.

Precision mapping the topological bands of 2D spin-orbit coupling with microwave spin-injection spectroscopy.

To investigate the band structure is one of the key approaches to study the fundamental properties of a novel material. We report here the precision band mapping of a 2-dimensional (2D) spin-orbit (SO) coupling in an optical lattice. By applying the microwave spin-injection spectroscopy, the band structure and spin-polarization distribution are achieved simultaneously. The band topology is also addressed with observing the band gap close and re-open at the Dirac points. Furthermore, the lattice depth and the Raman coupling strength are precisely calibrated with relative errors in the order of 10-3. Our approach could also be applied for exploring the exotic topological phases with even higher dimensional system.

Realization of two-dimensional spin-orbit coupling for Bose-Einstein condensates.

Cold atoms with laser-induced spin-orbit (SO) interactions provide a platform to explore quantum physics beyond natural conditions of solids. Here we propose and experimentally realize two-dimensional (2D) SO coupling and topological bands for a rubidium-87 degenerate gas through an optical Raman lattice, without phase-locking or fine-tuning of optical potentials. A controllable crossover between 2D and 1D SO couplings is studied, and the SO effects and nontrivial band topology are observed by measuring the atomic cloud distribution and spin texture in momentum space. Our realization of 2D SO coupling with advantages of small heating and topological stability opens a broad avenue in cold atoms to study exotic quantum phases, including topological superfluids.