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## COVER ARTICLE

2022, 71 (18): 186401. doi: 10.7498/aps.71.20221024
Abstract +
Two-dimensional topological insulator (2DTI) with a large bandgap is prerequisite for potentially observing quantum spin Hall and other quantum phenomena at room-temperature. At present, the synthesis of such materials possesses formidable challenge. In this work, we report our experimental results on synthesis of large-gap 2DTI stanene and bismuthene on B-faced InSb(111) substrate by using molecular beam epitaxy technology. We find that both the stanene and bismuthene can be synthesized by following the forming of a wetting layer on InSb(111) substrate, but with different prospects. On the one hand, it is found that the binding energy between Sn and the substrate is not strong enough to compete the binding force between Sn atoms during the post annealing, thus resulting in a wetting layer composed of many small domains. It significantly restricts the quality of the stanene epilayers. On the other hand, the Bi atoms on InSb(111) are found more stable than the Sn atoms on InSb(111), resulting in a uniform wetting layer which can be optimized by adjusting substrate temperature and post-annealing conditions. Large size and single crystal bismuthene domains have been observed under the STM measurement, which also indicates a bulk gap of ～0.15 eV and metallic edge states.

## EDITOR'S SUGGESTION

2022, 71 (18): 186701. doi: 10.7498/aps.71.20221322
Abstract +
As early as the 1950s, Prof. Yang and his collaborators realized that the most important interaction effects in a dilute quantum gas can be described by the s-wave scattering length between particles. This insight leads to universal descriptions of the interaction effects without the detailed knowledge of the interaction potential. They derived a formula expanding the energy density in terms of the gas parameter. This formula is later known as the Lee-Huang-Yang correction. However, it took forty years for the experimentalists to overcome several challenges and finally achieve degenerate quantum gases of atoms in 1995. The developments after 1995 have led to an exciting field known as “quantum gases” or “ultracold atomic gases”. The ultracold atom system has flexible tunability, allowing both the scattering length and the dimensionality to vary. The Lee-Huang-Yang corrections were observed from several experiments on ultracold atoms by increasing the scattering length. In addition, by reducing the dimensionality to one-dimension, several experiments on ultracold atoms have confirmed the Yang-Yang thermodynamics for one-dimensional bosons that Prof. Yang obtained in the 1960s and the large-N limit of one-dimensional fermions that Prof. Yang obtained around 2010. By increasing the dimensionality through using the idea of synthetic dimension, the experiment on ultracold atoms has also demonstrated the Yang monopole in the SU(2) non-abelian gauge field proposed by Prof. Yang in the 1970s. All of these experiments show the long-lasting impact of Prof. Yang’s theoretical work over several decades.

## EDITOR'S SUGGESTION

2022, 71 (18): 187802. doi: 10.7498/aps.71.20220801
Abstract +
In this work, we study the broadband manipulation of polarization states of terahertz (THz) waves with flexible metamaterial both theoretically and experimentally. Firstly, we construct a chiral THz metamaterial with asymmetric L-shaped metal-dielectric-metal structure, generating a series of electric dipoles via its interacting with terahertz waves. By changing the geometric parameters of the structure, the time responses of the electric dipoles in the two orthogonal directions are effectively modulated. Consequently, the chiral metamaterial efficiently converts linearly polarized terahertz wave into a circularly polarized one. The radiation of the metamaterial remains almost unaffected by the changing of the incident angle, which indicates that this chiral metamaterial can be used to realize a flexible terahertz circularly-polarized wave plate. Further, we present the working principle of this flexible terahertz circularly-polarized wave plate at the bending state based on the equivalent circuit model. Moreover, we fabricate a flexible metamaterial wave plate by using polymers as the dielectric layer. When the linearly polarized light is incident on the metamaterial, the circularly polarized output can be achieved in a wide frequency range of 0.46–0.62 THz. The polarization conversion remains quite stable even if the sample is bent. This flexible terahertz metamaterial wave plate is expected to be applied to 6G communication, molecular detection, etc.

## EDITOR'S SUGGESTION

2022, 71 (18): 188702. doi: 10.7498/aps.71.20220752
Abstract +
Lipid rafts are small biomembrane functional units, resulting from the lateral phase separation of phospholipids. The phospholipid phase separation plays a crucial role in spatially organizing the biomolecules in life activities. Here, we study the kinetics of multi-component phospholipid phase separation quantitatively by using the single domain characterization methods including the movement tracking and radial fluctuation analyses, which provide valuable information about the physical and mechanical properties of the bulks and domains. The study is carried out in a low line tension condition similar to that in cells. The order of magnitude of line tension is ～0.1 pN as estimated from the radial fluctuation analysis. Fluorescence microscopy characterization shows that domains mainly coarsen through the coalescence pathways, while the evaporation-condensation is negligible. Through the tracking of domains, it is found that the bulk viscosity dominates the dynamics of domain coalescence. The coalescence of domains produces strong hydrodynamic flows in low viscosity bulk, which promotes the non-Brownian motion of surrounding domains, accelerating the lateral diffusion and coalescence of the domains. However, these hydrodynamic flows decrease significantly in high viscosity bulk. The domains rely mainly on Brownian motion to diffuse in this highly viscous medium, resulting in the slow lateral diffusion and low coalescence. Picking the domains following Brownian motion, the viscosities of liquid ordered bulk and liquid disordered bulk are determined to be, respectively, in a range of 10–8–10–7 Pa⋅s⋅m and 10–9 Pa⋅s⋅m from the Hughes-Pailthorpe-White empirical relation. Furthermore, we observe a bulk-viscosity-dependent scaling relation between the domain size and coarsening time experimentally. A theoretical model of domain diffusion and coalescence is established to understand the scaling relation. If the bulk viscosity is low, the hydrodynamic flow produces a high power exponent of 1.0. And if the bulk viscosity is high, the Brownian diffusion produces a low power exponent of 0.5. In addition, we demonstrate that the bulk viscosity can be regulated through the relative content of cholesterol. The 1,6-Diphenyl-1,3,5-hexatriene fluorescence anisotropy characterization exhibits that the increase of cholesterol in liquid ordered and liquid disordered bulks disorders and orders the phospholipid packing, thus reducing and increasing the bulk viscosity, respectively. It is expected that this viscosity regulation strategy can be used to control the multicomponent phospholipid phase separation. All in all, our study deepens the understanding of the physical mechanism behind the formation of lipid rafts. It also provides a reference for regulating the biomolecule distribution in cell membranes.

## EDITOR'S SUGGESTION

2022, 71 (18): 187304. doi: 10.7498/aps.71.20220529
Abstract +
Transparent conductive films (TCFs) play an indispensable role in optoelectronic devices because of their high conductivity and high optical transmittance. In order to obtain indium-free transparent conductive films with better performance, we need to improve the conductivity, while not damaging the transmittance. Metal mesh is highly conductive but prone to oxidation and abrasion, while transparent conductive oxide (TCO) is stable but less conductive. Thus, we composite the metal mesh with the stable TCO to achieve complementary advantages. In this work, we fabricate a hexagonal Cu mesh and then cover the Cu mesh with Al-doped ZnO (AZO) film by using lithography and magnetron sputtering. The line width and length of mesh are 15 µm and 150 µm, respectively, which are not visible to the naked eye. The effect of AZO growth temperature on the properties of such AZO/Cu mesh composite film is studied and the optimal temperature is 300 ℃. By designing the mesh and optimizing the process, the transmittance (400–800 nm), sheet resistance and FoM of AZO/ Cu mesh composite film reach 86.4%, 4.9 Ω/sq and 4.73 × 10–2 Ω–1, respectively, thus possessing both transparent and conductive property. Because of its low cost, competitive optoelectronic performance and stability, the potential applications of AZO/Cu mesh composite film in transparent electronics are fantastic. When used as a transparent conductor to connect LED to 3 V DC power, the luminance of LED in series with AZO/Cu mesh composite film is lighter than that of AZO film and Cu mesh. According to the Ohmic heating effect of electric current passing through a conductor, AZO/Cu mesh composite film can be designed as electric heating film. At low voltage safe for human body, AZO/Cu mesh composite film can implement fast, uniform and stabile heat. In the cyclic electric heating test, the AZO/Cu mesh composite film can be heated rapidly to 175 ℃ all the time, showing a fast temperature response and stable cyclic performance. More importantly, the AZO is itself transparent and conductive and prevents the metal from oxidizing effectively, thus ensuring the overall performance and maintaining the electric heating response. The experimental result and simulation application show that the AZO/Cu mesh composite film has a great potential application in transparent and heating film for defogging and defrosting glass.

## EDITOR'S SUGGESTION

2022, 71 (18): 181101. doi: 10.7498/aps.71.20220475
Abstract +
By constructing the spatial distribution of external potential and incoherent pumping, a ${\cal{PT}}$ symmetrical model satisfied by the one-dimensional incoherent pumped exciton-polariton condensate system is designed. In the weakly nonlinear case, the ${\cal{PT}}$ symmetrical phase transition point is found, and the linear spectrum is shown. In the normal nonlinear case, found are the bright soliton with the zero background, the multi-poles dark solitons with zero background, the symmetry breaking dark solitons and symmetrical dark soliton with the homogeneous background, and the dip- and hump-type dark solitons with the homogeneous background, and discussed are the effects of inhomogeneous pumping and the imaginary part of external potential on the profiles and the stability of solitons. Through these results, the competition between ${\cal{PT}}$ symmetrical potential and the inhomogeneous pumping is understood, the scheme that how the bright and dark solitons are excited is presented, and the existence and stability regions of these solitons are determined. Finally, the symmetry breaking dark solitons are controlled by modulating the imaginary part of the ${\cal{PT}}$ symmetrical potential, which reveals the potential applications of the polariton condensate system in optical information processing, such as the all-optical switches.

## EDITOR'S SUGGESTION

2022, 71 (18): 184202. doi: 10.7498/aps.71.20220506
Abstract +
Owing to the potential applications in all-optical quantum information processing and quantum optical networks, magnet-free optical non-reciprocity transmission has attracted great interest and has been studied in many fields, such as parity-time-symmetry enhanced nonlinearity, optomechanical systems, photonic crystal, cold atomic Bragg lattices, chiral quantum optics, and hot atoms. In particular, the random thermal motion of hot atoms can be a useful resource to realize optical non-reciprocity. Here in this work, based on the susceptibility-momentum-locking of atomic thermal motion and the strong coupling characteristics of cavities, a magnetic-free optical reciprocity-nonreciprocity transmission conversion scheme is designed and realized through the atom-cavity compound system. Theoretical and experimental analysis show that the coupling field conditions determine the nonreciprocity of the system. Under the action of single traveling-wave field, the nonreciprocity in hot atoms depends on the propagation direction of the coupling field due to the Doppler effect. Therefore, by changing the opening and closing of the opposite coupling field, the two-way single channel optical nonreciprocal transmission based on intracavity electromagnetically induced transparency can be controlled. When the two coupling fields propagate simultaneously in the opposite directions, however, the cavity transmission changes from single-dark-state to double-dark-state peaks, in which the reciprocity outputs depend on the frequency difference between the two coupling fields. By tuning the frequency difference, the two-way multi-channel reciprocal-nonreciprocal transmission regulation based on double dark polar peaks can be realized. The study can be applied to all-optical quantum devices and quantum information processing, such as optical transistors, optical switching and routing, and quantum gate manipulation.

## EDITOR'S SUGGESTION

2022, 71 (18): 184203. doi: 10.7498/aps.71.20220575
Abstract +
Second harmonic generation (SHG) is an effective way to generate short wavelength laser with high power. The SHG is accompanied with the absorptions of fundamental waves and harmonic waves, which converts a fraction of the two waves deposit energy into heat, causing a temperature gradient along the radial direction of the periodically poled potassium titanyl phosphate (PPKTP) crystal. The inhomogeneous temperature distribution causes thermal lensing in the crystal. The thermal lensing effect will deform the spatial mode of the SHG cavity and result in the mode-mismatching of the fundamental wave to the SHG cavity, and therefore the conversion efficiency of SHG process is reduced. Moreover, with the increase of injected fundamental wave power, the influence caused by thermal lens becomes more and more serious. In order to obtain a high-efficiency frequency conversion, it is necessary to take the measure to minimize the effect caused by thermal lensing. In this paper, we report on a high efficiency generation of green laser at 532 nm by external cavity SHG process with a semi-monolithic standing cavity. The influences of thermal lens effect on the optimal conversion efficiency in different semi-monolithic cavities are theoretically analyzed. The variations of conversion efficiency with the pump power in “plane-concave” semi-monolithic cavity based on parallel crystal and also in “concave-concave” semi-monolithic cavity based on concave crystal are quantitatively analyzed. In experiments, two types of cavity structures are built to measure the variation of frequency doubling conversion efficiency with pump power. For the “plane-concave” semi-monolithic cavity, the maximum green laser power of 747 mW is obtained and the corresponding conversion efficiency reaches 93.4%±3%, with 800 mW infrared laser injected. For the “concave-concave” semi-monolithic cavity, the maximum green laser power of 529 mW is obtained and the corresponding conversion efficiency is 88.2% ± 3%, with 600 mW infrared laser injected. The results show that the thermal lens affects the optimal conversion efficiency more seriously in “concave-concave” semi-monolithic cavity than in “plane-concave” semi-monolithic cavity. Furthermore, the influence of thermal lens effect turns higher and higher with the increase of the loss in the cavity. It is obvious that the “plane-concave” semi-monolithic cavity is more suitable for the SHG process and has many potential applications in quantum optics and cold atom physics and provides a guidance for future research on high-efficiency SHG process.

## EDITOR'S SUGGESTION

2022, 71 (18): 184201. doi: 10.7498/aps.71.20220665
Abstract +
Wave absorbing materials are widely used to prevent military equipment from being detected by radar wave and also serve as civil electromagnetic shielding. The absorbing properties of wave absorbing materials are determined by a combination of the electromagnetic parameters and the thickness of the composite material. In the actual case, the theoretically designed reflection loss peak intensity and the bandwidth of wave absorbing materials deviate from the engineered values. There are few reports on the mechanism about the variation of the intensity of the reflection loss absorption peak with thickness and the bandwidth of the reflection loss absorption peak. In this work, based on an interfacial reflection model, the reflective properties of radar wave at the air interface of the absorbing coating are investigated. The dependence of the matching impedance on the matching thickness of the absorbing material is determined, and the matching impedance parameters are further used to design the absorbing composites, which exhibit excellent microwave absorption properties, i.e. an average value of reflection loss is below –10 dB at 4–18 GHz in different thickness wave absorbing materials, and an average value of reflection loss is below –20 dB at 6–18 GHz in different thickness wave absorbing materials. The bandwidth of the reflection loss peak at the matched thickness is discussed in depth in principle based on the interface reflection model, and the theoretical calculations accord with the experimental results.

## EDITOR'S SUGGESTION

2022, 71 (18): 184702. doi: 10.7498/aps.71.20220212
Abstract +
A detailed three-dimensional thermal model is developed to examine the thermal behaviour of a lithium-ion battery. This model is a cross-flow liquid cooling model, which can make the heat dissipation of lithium-ion battery pack achieve higher safety. Two kinds of fluids are used for cooling, and the polynomial fitting function is used as the heat source term of lithium battery pack. The battery temperature distribution under the conditions of different Reynolds numbers, different numbers of micro-channel and different micro-channel radii are studied for this model. The simulation results show that the maximum battery temperature is 295.84 K, the minimum battery temperature is 293.14 K, and the maximum temperature difference of the battery pack is 2.7 K. The maximum temperature and temperature difference of the battery under the model are in line with the reasonable operating temperature range of lithium-ion battery pack. The maximum temperature of battery pack decreases with the increase of Reynolds number, but the effect of Reynolds number on heat dissipation of lithium-ion battery pack has a critical value. As the number of micro-channels increases, the maximum temperature of the battery string decreases. However, when the number of micro-channels increases to a certain value, the maximum temperature of the battery pack decreases slowly. The maximum temperature of the battery pack does not decrease monotonically as the radius of the micro-channel increases. Orthogonal analysis results show that the Reynolds number has the greatest influence on the cooling effect of the model, followed by the size of the micro-channel radius, and the number of micro-channels has the least influence. The optimized liquid cooling model can effectively reduce the maximum temperature of lithium-ion battery in theory, and the maximum temperature of lithium-ion battery decreases by 26.24 K in comparison with that of single battery at 2C discharge rate. The reliability of the cross-flow channel model is proved by numerical analysis, and it is also proved that the cross-flow channel has an equilibrium point between the perturbation gain and the flow retarding effect. The heat dissipation effect of lithium- ion battery pack is correlated with the number and radius of micro-channels, but not a single positive correlation. Reasonably increasing the number and size of micro-channels can effectively enhance the heat dissipation effect of battery pack.
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