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Modulational Instability of Trapped Two-Component Bose–Einstein Condensates
Jian-Wen Zhou, Xiao-Xun Li, Rui Gao, Wen-Shan Qin, Hao-Hao Jiang, Tao-Tao Li, Ju-Kui Xue
Chin. Phys. Lett. 2019, 36 (9):
090302
.
DOI: 10.1088/0256-307X/36/9/090302
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.
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Experimental Realization of Degenerate Fermi Gases of $^{87}$Sr Atoms with 10 or Two Spin Components
Wei Qi, Ming-Cheng Liang, Han Zhang, Yu-Dong Wei, Wen-Wei Wang, Xu-Jie Wang, Xibo Zhang
Chin. Phys. Lett. 2019, 36 (9):
093701
.
DOI: 10.1088/0256-307X/36/9/093701
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.
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A Photon-Counting Full-Waveform Lidar
Bing-Cheng Du, Zhao-Hui Li, Guang-Yue Shen, Tian-Xiang Zheng, Hai-Yan Zhang, Lei Yang, Guang Wu
Chin. Phys. Lett. 2019, 36 (9):
094201
.
DOI: 10.1088/0256-307X/36/9/094201
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.
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Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce$_{2}$IrIn$_{8}$
Haijiang Liu, Yuanji Xu, Yigui Zhong, Jianyu Guan, Lingyuan Kong, Junzhang Ma, Yaobo Huang, Qiuyun Chen, Genfu Chen, Ming Shi, Yi-feng Yang, Hong Ding
Chin. Phys. Lett. 2019, 36 (9):
097101
.
DOI: 10.1088/0256-307X/36/9/097101
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.
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The 2D InSe/WS$_2$ Heterostructure with Enhanced Optoelectronic Performance in the Visible Region
Lu-Lu Yang, Jun-Jie Shi, Min Zhang, Zhong-Ming Wei, Yi-Min Ding, Meng Wu, Yong He, Yu-Lang Cen, Wen-Hui Guo, Shu-Hang Pan, Yao-Hui Zhu
Chin. Phys. Lett. 2019, 36 (9):
097301
.
DOI: 10.1088/0256-307X/36/9/097301
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.
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Superconducting Single-Layer T-Graphene and Novel Synthesis Routes
Qinyan Gu, Dingyu Xing, Jian Sun
Chin. Phys. Lett. 2019, 36 (9):
097401
.
DOI: 10.1088/0256-307X/36/9/097401
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.
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Magnetic and Magnetocaloric Properties of Polycrystalline and Oriented Mn$_{2-\delta}$Sn
Kun Li, Fanggui Wang, Youfang Lai, Mingzhu Xue, Xin Li, Jinbo Yang, Changsheng Wang, Jingzhi Han, Shunquan Liu, Wenyun Yang, Yingchang Yang, Honglin Du
Chin. Phys. Lett. 2019, 36 (9):
097502
.
DOI: 10.1088/0256-307X/36/9/097502
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.
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An Improved Room-Temperature Silicon Terahertz Photodetector on Sapphire Substrates
Xue-Hui Lu, Cheng-Bin Jing, Lian-Wei Wang, Jun-Hao Chu
Chin. Phys. Lett. 2019, 36 (9):
098501
.
DOI: 10.1088/0256-307X/36/9/098501
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.
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17 articles
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