We present a method for derivation of the density matrix of an arbitrary multi-mode continuous variable Gaussian entangled state from its phase space representation. An explicit computer algorithm is given to reconstruct the density matrix from Gaussian covariance matrix and quadrature average values. As an example, we apply our method to the derivation of three-mode symmetric continuous variable entangled state. Our method can be used to analyze the entanglement and correlation in continuous variable quantum network with multi-mode quantum entanglement states.

The modulational instability of two-component Bose–Einstein condensates (BECs) under an external parabolic potential is discussed. Based on the trapped two-component Gross–Pitaevskill equations, a time-dependent dispersion relation is obtained analytically by means of the modified lens-type transformation and linear stability analysis. It is shown that a modulational unstable time scale exists for trapped two-component BECs. The modulational properties—which are determined by the wave number, external trapping parameter, intra- and inter-species atomic interactions—are modified significantly. The analytical results are confirmed by direct numerical simulation. Our results provide a criterion for judging the occurrence of instability of the trapped two-component BECs in experiment.

Recently, a report from Elite Readers suggested that a strange phenomenon of 'square-shaped waves' had occurred at the beaches of the Isle of Rhe in the Bay of Biscay. Based on the hydrological and geological data of the Bay of Biscay, we find that the special phenomenon is closely related to a solitary wave that can be described by the shallow water wave equation. We discuss the formation mechanisms of the square-shaped waves by the Kadomtsev–Petviashvili equation. The combination of exact solutions and actual condition provides the simulated initial state. We then reproduce a square-shaped structure by a numerical method and obtain the result consistent with the observed picture from media. Our work enriches public understanding of strange water waves and has great significance for tourism development and shipping transportation.

We report the experimental realization of quantum degenerate Fermi gases of $^{87}$Sr atoms under controlled 10- and dual-nuclear-spin configurations. Based on laser cooling and evaporative cooling, we achieve an ultracold Fermi gas of 10$^{5}$ atoms equally distributed over 10 spin states, with a temperature of $T/T_{\rm F}=0.21$. We further prepare a dual-spin gas by optically pumping atoms to the $m_{\rm F}=9/2$ and $m_{\rm F}=7/2$ states and observe a slightly lower $T/T_{\rm F}$ than that for a 10-spin gas under the same trapping condition, showing efficient evaporative cooling under a decreasing number ${\cal N}$ of spin states (${\cal N}\geqslant 2$) despite the increasing importance of Pauli exclusion. Given that rethermalization becomes less efficient with ${\cal N}$ approaching unity, we evaporatively cool an almost polarized gas to 130 nK. The simple and efficient preparation of ultracold Fermi gases of $^{87}$Sr with tunable spin configurations provides a first step towards engineering topological quantum systems.

FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

We present the results of using a photon-counting full-waveform lidar to obtain detailed target information with high accuracy. The parameters of the waveforms (i.e., vertical structure, peak position, peak amplitude, peak width and backscatter cross section) are derived with a high resolution limit of 31 mm to establish the vertical structure and scattering properties of targets, which contribute to the recognition and classification of various scatterers. The photon-counting full-waveform lidar has higher resolution than linear-mode full-waveform lidar, and it can obtain more specific target information compared to photon-counting discrete-point lidar, which can provide a potential alternative technique for tomographic surveying and mapping.

We present a design of an acoustic levitator consisting of three pairs of opposite transducer arrays. Three orthogonal standing waves create a large number of acoustic traps at which the particles are levitated in mid-air. By changing the phase difference of transducer arrays, three-dimensional manipulation of particles is successfully realized. Moreover, the relationship between the translation of particles and the phase difference is experimentally investigated, and the result is in agreement with the theoretical calculation. This design can expand the application of acoustic levitation in many fields, such as biomedicine, ultrasonic motor and new materials processing.

We investigate the granular flow states in a channel with bottleneck by molecular dynamics simulations. Our study is restricted only on a selected key area rather than on the whole system to focus on the flow properties of a single granular state. A random force field is introduced to control the granular temperature. It is also pointed out that the flow rate in the granular flow can be correlated with the pressure, which leads us to carry out a comprehensive study similar to the classical study for general liquid-gas phase transition. Our results show that the dilute flow state and the dense flow state of the granules are similar to the gas state and the liquid state of general substances, respectively, and the properties of phase transition and critical phenomenon are also similar to those occurring in general substances.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

We utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce$_{2}$IrIn$_{8}$. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature $T_{\rm coh}\sim 40$ K, which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization. However, the Fermi vector and the Fermi surface volume have little change around $T_{\rm coh}$, which challenges the widely believed evolution from a high-temperature small Fermi surface to a low-temperature large Fermi surface. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Two-dimensional (2D) InSe and WS$_2$ exhibit promising characteristics for optoelectronic applications. However, they both have poor absorption of visible light due to wide bandgaps: 2D InSe has high electron mobility but low hole mobility, while 2D WS$_2$ is on the contrary. We propose a 2D heterostructure composed of their monolayers as a solution to both problems. Our first-principles calculations show that the heterostructure has a type-II band alignment as expected. Consequently, the bandgap of the heterostructure is reduced to 2.19 eV, which is much smaller than those of the monolayers. The reduction in bandgap leads to a considerable enhancement of the visible-light absorption, such as about fivefold (threefold) increase in comparison to monolayer InSe (WS$_2$) at the wavelength of 490 nm. Meanwhile, the type-II band alignment also facilitates the spatial separation of photogenerated electron-hole pairs; i.e., electrons (holes) reside preferably in the InSe (WS$_2$) layer. As a result, the two layers complement each other in carrier mobilities of the heterostructure: the photogenerated electrons and holes inherit the large mobilities from the InSe and WS$_2$ monolayers, respectively.

Single-layer superconductors are ideal materials for fabricating superconducting nano devices. However, up to date, very few single-layer elemental superconductors have been predicted and especially no one has been successfully synthesized yet. Here, using crystal structure search techniques and ab initio calculations, we predict that a single-layer planar carbon sheet with 4- and 8-membered rings called T-graphene is a new intrinsic elemental superconductor with superconducting critical temperature ($T_{\rm c}$) up to around 20.8 K. More importantly, we propose a synthesis route to obtain such a single-layer T-graphene, that is, a T-graphene potassium intercalation compound (C$_4$K with $P4/mmm$ symmetry) is firstly synthesized at high pressure ($>$11.5 GPa) and then quenched to ambient condition; and finally, the single-layer T-graphene can be either exfoliated using the electrochemical method from the bulk C$_4$K, or peeled off from bulk T-graphite C$_4$, where C$_4$ can be obtained from C$_4$K by evaporating the K atoms. Interestingly, we find that the calculated $T_{\rm c}$ of C$_4$K is about 30.4 K at 0 GPa, which sets a new record for layered carbon-based superconductors. The present findings add a new class of carbon-based superconductors. In particular, once the single-layer T-graphene is synthesized, it can pave the way for fabricating superconducting devices together with other 2D materials using the layer-by-layer growth techniques.

Magnetization switching is one of the most fundamental topics in the field of magnetism. Machine learning (ML) models of random forest (RF), support vector machine (SVM), deep neural network (DNN) methods are built and trained to classify the magnetization reversal and non-reversal cases of single-domain particle, and the classification performances are evaluated by comparison with micromagnetic simulations. The results show that the ML models have achieved great accuracy and the DNN model reaches the best area under curve (AUC) of 0.997, even with a small training dataset, and RF and SVM models have lower AUCs of 0.964 and 0.836, respectively. This work validates the potential of ML applications in studies of magnetization switching and provides the benchmark for further ML studies in magnetization switching.

Mn-based Heusler alloys have attracted significant research attention as half-metallic materials because of their giant magnetocrystalline anisotropy and magnetocaloric properties. We investigate the crystal structure and magnetic properties of polycrystalline, [101]-oriented, and [100]-oriented Mn$_{2-\delta}$Sn prepared separately by arc melting, the Bridgeman method, and the flux method. All of these compounds crystallize in a Ni$_{2}$In-type structure. In the Mn$_{2-\delta}$Sn lattice, Mn atoms occupy all of the 2$a$ and a fraction of the 2$d$ sites. Site disorder exists between Mn and Sn atoms in the 2$c$ sites. In addition, these compounds undergo a re-entrant spin-glass-like transition at low temperatures, which is caused by frustration and randomness within the spin system. The magnetic properties of these systems depend on the crystal directions, which means that the magnetic interactions differ significantly along different directions. Furthermore, these materials exhibit a giant magnetocaloric effect near the Curie temperature. The largest value of maximum of magnetic entropy change ($-\Delta S_{\rm M})$ occurs perpendicular to the [100] direction. Specifically, at 252 K, maximum $-\Delta S_{\rm M}$ is 2.91 and 3.64 J$\cdot$kg$^{-1}$K$^{-1}$ for a magnetic field of 5 and 7 T, respectively. The working temperature span over 80 K and the relative cooling power reaches 302 J/kg for a magnetic field of 7 T, which makes the Mn$_{2-\delta}$Sn compound a promising candidate for a magnetic refrigerator.

The Debye relaxation of dielectric spectroscopy exists extensively in monohydroxy alcohols. We model the relaxation based on the infinite-pseudospin-chain Ising model and the Glauber dynamics, and the corresponding complex permittivity is obtained. The model results are in good agreement with the experimental data of 3,7-dimethyl-1-octanol, 2-ethyl-1-hexanol and 5-methyl-2-hexanol in a wide temperature range. Moreover, in the model parameters, the sum of the mean-field interaction energy and two times the orientation is nearly twice the hydrogen bond energy, which further states the rationality of this model.

CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Quasicrystals have long-range quasi-periodic translational ordering and non-crystallographic rotational symmetry. Al–Cu–Fe quasicrystals have great potential for lithium storage because of their high Al content and a large number of defects in the structure. In our previous study (J. Alloys Compd. 805 (2019) 942) we showed that Al–Cu–Fe quasicrystals have good initial capacity whereas its cycle stability is poor. In the present study, graphite/AlCuFe is prepared by the mechanical alloying method. The results show that graphite/AlCuFe quasicrystal composites are successfully synthesized by planetary ball milling at 550 rpm for 80 h. The quasicrystal particle size decreases and the amorphous graphite forms onion-like carbon (OLC) when the two phases mix evenly. OLC forms on the surface of the Al–Cu–Fe quasicrystalline powder. Charge and discharge tests show that graphite/AlCuFe quasicrystal composites have high-stability capacity of 480 mAh/g after 20 cycles, which is larger than the sum of capacities of graphite and Al–Cu–Fe quasicrystals.

The critical adsorption of semi-flexible polymer chains on attractive surfaces is studied using Monte Carlo simulations. The results reveal that the critical adsorption point of a free polymer chain is the same as that of an end-grafted one. For the end-grafted polymer, we find that the finite-size scaling relation and the maximum fluctuation of adsorbed monomers are equivalent in estimating the critical adsorption point. The effect of chain stiffness on the critical adsorption is also investigated. The surface attraction strength for the critical adsorption of semi-flexible polymer chain decreases exponentially with an increase in the chain stiffness; In other words, lower adsorption energy is needed to adsorb a stiffer polymer chain. The result is explained from the viewpoint of the free energy profile for the adsorption.

We design and fabricate a good performance silicon photoconductive terahertz detector on sapphire substrates at room temperature. The best voltage responsivity of the detector is 6679 V/W at frequency 300 GHz as well as low voltage noise of 3.8 nV/Hz$^{1/2}$ for noise equivalent power 0.57 pW/Hz$^{1/2}$. The measured response time of the device is about 9 μs, demonstrating that the detector has a speed of $>$110 kHz. The achieved good performance, together with large detector size (acceptance area is 3 μm$\times 160$ μm), simple structure, easy manufacturing method, compatibility with mature silicon technology, and suitability for large-scale fabrication of imaging arrays provide a promising approach to the development of sensitive terahertz room-temperature detectors.