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Ground-State Properties of Superfluid Fermi Gas in Fourier-Synthesized Optical Lattices
CHEN Yan, ZHANG Ke-Zhi, WANG Xiao-Liang, CHEN Yong
Chin. Phys. Lett. 2014, 31 (03):
030301
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DOI: 10.1088/0256-307X/31/3/030301
By employing the balance condition between the lattice potential and the interatomic interaction, we study the ground state solutions of superfluid Fermi gases in Fourier-synthesized (FS) optical lattices. The average energy of the ground state, the atoms number, and the atom density distribution of the Fermi system are analytically derived along the Bose–Einstein condensation (BEC) side to the Bardeen–Cooper–Schrieffer (BCS) side. We analyze the properties of ground state solutions at both the BEC limit and unitarity in FS optical lattices. It is found that the relative phase α between the two lattice harmonics impacts greatly on the properties of the ground state of the superfluid Fermi gas. Especially in the BCS limit, when α=π/2, the average energy presents an exponential form with the increase of the potential depth of the lattice harmonics v2. Meanwhile, there exits a minimal value. Moreover, due to the Fermi pressure, the atom density distribution at unitarity is more outstretched than that in the BEC limit. The average energy at unitarity is apparently larger than that in the BEC limit. The properties of the ground state solution exhibit very different behaviors when the system transits from the BEC side to the BCS side.
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Floquet Topological Insulator in the BHZ Model with the Polarized Optical Field
ZHU Hua-Xin, WANG Tong-Tong, GAO Jin-Song, LI Shuai, SUN Ya-Jun, LIU Gui-Lin
Chin. Phys. Lett. 2014, 31 (03):
030503
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DOI: 10.1088/0256-307X/31/3/030503
Topological phase of newly found matter has aroused wide interests, especially related with the external periodical modulating. With the help of the Floquet theory, we investigate the possibility of externally manipulating the topological property in a HgTe/CdTe quantum well system with the polarized optical field. We give the phase diagram, showing that by modulating the parameters of the polarized optical field, especially the phase, the topological phase transition can be realized in the QW and lead to the so-called Floquet topological insulator. When the optical field is weak, the driven QSH state of QW is robust with the optical field. However, when the optical field is relatively larger, the group velocity of edge states and the gap between the bulk states exhibit certain oscillations. The implications of our results are discussed.
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Study of Thick Gaseous Electron Multipliers Gain Stability and Some Influencing Factors
ZHOU Xiao-Kang, LIU Qian, CHEN Shi, HUANG Wen-Qian, WANG Bin-Long, ZHANG Yong-Dong, GE Dong-Sheng, LIU Hong-Bang, ZHENG Yang-Heng, XIE Yi-Gang, DONG Yang, ZHANG Qiang, CHANG Jie, WANG Jing, LI Min, ZHOU Yan
Chin. Phys. Lett. 2014, 31 (03):
032901
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DOI: 10.1088/0256-307X/31/3/032901
The working stability of thinner-thick gaseous electron multipliers (THGEMs), which have been developed by the University of the Chinese Academy of Sciences and the Second Academy of China's Aerospace Science and Industry Corporation, is studied with an 8 keV x-ray on a Cu target. Gains of about 103–104 are achieved with a single board in Ar:iC4H10 (97:3). Environmental factors, such as pressure, temperature and humidity are considered. The thinner-THGEMs are shown to perform stably over two months of studies.
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Electron Impact Excitation of Xenon from the Ground State and the Metastable State to the 5p57p Levels
CHEN Zhan-Bin, DONG Chen-Zhong, XIE Lu-You, JIANG Jun
Chin. Phys. Lett. 2014, 31 (03):
033401
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DOI: 10.1088/0256-307X/31/3/033401
Electron impact excitation cross sections from the ground state and the lowest metastable state 5 p56 s J=2 to the excited states of the 5 p57 p configuration of xenon are calculated systematically using the fully relativistic distorted wave method. Special attention is paid to the configuration interaction effects in the wave-function expansion of target states. The results are in good agreement with the recent experimental data by Jung et al. [ Phys. Rev. A 80 (2009) 062708] over the measured energy range. These accurate theoretical results can be used in the modeling and diagnosis of plasmas containing xenon.
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Analysis and Measurement of the Displacement Sensor Based on an Up-tapered Mach–Zehnder Interferometer
WEN Xiao-Dong, NING Ti-Gang, YOU Hai-Dong, KANG Ze-Xin, LI Jing, LI Chao, FENG Ting, YU Shao-Wei, JIAN Wei
Chin. Phys. Lett. 2014, 31 (03):
034203
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DOI: 10.1088/0256-307X/31/3/034203
A displacement sensor based on an up-tapered Mach–Zehnder interferometer (MZI) is proposed and demonstrated experimentally. For this purpose, a fiber MZI is fabricated by using a commercial fusion splicer. Then the transmission spectra of the sensors with different middle fiber lengths are measured by bending the MZIs with different movements of the moving stage. The maximum sensitivity of 2.457 nm/mm is achieved while the shifting of the moving stage changes from 3 mm to 3.5 mm. Note that this kind of up-taper configuration is strong in strength, easy to fabricate and low in cost.
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Evaluation of Slow Light Periodic Signals Considering the Distortion in EDF
WANG Fu, WU Chong-Qing, WANG Zhi, LIU Guo-Dong, LIU Lan-Lan, SUN Zhen-Chao
Chin. Phys. Lett. 2014, 31 (03):
034207
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DOI: 10.1088/0256-307X/31/3/034207
Based on the fundamental harmonic phase delay, a new definition of fundamental harmonic fractional delay (FHFD) is proposed to evaluate slow light with the consideration of signal distortion, to eliminate the dependence on the choice of the reference point. By solving the rate equation of erbium-doped fiber (EDF), it is shown that the slow light always accompanies the signal distortion when the periodic signal propagates in EDF, and FHFD depends on the signal distortion, as well as the average input power, the modulation depth and the length of EDF. The results of simulations and experiments indicate that the definition of FHFD is reasonable and effective to evaluate the slow light of periodic signals.
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Real-Time Monitoring and Quantitative Evaluation of Cavitation Bubbles Induced by High Intensity Focused Ultrasound Using B-Mode Imaging
YU Jie, CHEN Chu-Yi, CHEN Gong, GUO Xia-Sheng, MA Yong, TU Juan, ZHANG Dong
Chin. Phys. Lett. 2014, 31 (03):
034302
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DOI: 10.1088/0256-307X/31/3/034302
A software-based method is proposed to eliminate the flooding interference strips in B-mode images, and to evaluate the cavitation bubbles generated during high intensity focused ultrasound (HIFU) exposures. In vitro tissue phantoms are exposed to 1.12 MHz HIFU pulses with a fixed 100 Hz pulse repetition frequency. HIFU-induced cavitation bubbles are detected as hyperechoic regions in B-mode images. The temporal evolution of cavitation bubbles, generated by HIFU pulses with varying driving amplitude and pulse length, is analyzed by measuring the time-varying area of the hyperechoic region. The results show that: first, it is feasible to monitor HIFU-induced cavitation bubble activity in real-time using B-mode imaging; second, more cavitation bubbles can be generated with higher acoustic energy delivered; third, the hyperechoic region is observed to shrink gradually after ceasing the HIFU pulses, which indicates the dissolution of cavitation bubbles. This work will be helpful for developing an effective tool to realize real-time monitoring and quantitative evaluation of HIFU-induced cavitation bubble activity using a current commercialized B-mode machine.
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Numerical Simulation on the Production Mechanism of Surface-Wave Plasmas Sustained along a Metal Rod
ZHU Long-Ji, CHEN Zhao-Quan, YIN Zhi-Xiang, WANG Guo-Dong, XIA Guang-Qing, HU Ye-Lin, ZHENG Xiao-Liang, ZHOU Meng-Ran, CHEN Ming, LIU Ming-Hai
Chin. Phys. Lett. 2014, 31 (03):
035203
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DOI: 10.1088/0256-307X/31/3/035203
For interpreting the production mechanism of surface-wave plasmas sustained along a metal rod, electromagnetic simulation on the electromagnetic field distributions and particle-in-cell/Monte Carlo collision (PIC/MCC) simulation of the ionization process are present. The results show that the enhanced electric field of surface plasmon polaritons (SPPs) can be excited in the ion sheath layer between the negative-voltage metal rod and the surface-wave plasmas, which is responsible for maintaining the plasma discharge. Moreover, the spatio-temporal evolutions of plasma density and electric fields are simulated by the PIC/MCC model. It is further suggested that the expanded ion sheath layer can extend the length of plasma domain by increasing the plasma absorbed energy from SPPs.
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Spatially Resolved N2(A3Σ+u, ν=0) Decay Studies in the Pulsed Direct-Current Nitrogen Discharge using the Laser-Induced-Fluorescence Technique
FENG Chun-Lei, YANG Hong-Liang, GAO Liang, DING Hong-bin
Chin. Phys. Lett. 2014, 31 (03):
035204
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DOI: 10.1088/0256-307X/31/3/035204
We focus on the investigation of the spatial distribution and temporal evolution of N2(A3Σ+u, ν=0) in a very early afterglow of a pulsed dc nitrogen discharge. The results indicate that a fast quenching process of N2(A3Σ+u, ν=0) exists in the very early afterglow. We study the dependence of this fast quenching process on the discharge pressure 20–40 torr. It seems that this fast quenching behavior of N2(A3Σ+u, ν=0) found in our experiment can be ascribed to the combined action of pooling reaction and collisions with N atoms through N2(A3Σ+u)+N2(A3Σ+u)→ N2?+N2(N2?=N2(B3Πg, C3Πu, C'3Πu, C"5Πu)) and N2(A3Σ+u)+N(4S)→N(2P)+N2, respectively. Meanwhile, the decay studies of N2(A3Σ+u, ν=0) near the anode and cathode infer that the production of N(4S) atoms does not distribute uniformly along the axis of the discharge gap at relatively low pressure, and this effect becomes gradually inconspicuous with the increasing discharge pressure.
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Helium-Implantation-Induced Damage in NHS Steel Investigated by Slow-Positron Annihilation Spectroscopy
LI Yuan-Fei, SHEN Tie-Long, GAO Xing, GAO Ning, YAO Cun-Feng, SUN Jian-Rong, WEI Kong-Fang, LI Bing-Sheng, ZHANG Peng, CAO Xing-Zhong, ZHU Ya-Bin, PANG Li-Long, CUI Ming-Huan, CHANG Hai-Long, WANG Ji, ZHU Hui-Ping, WANG Dong, SONG Peng, SHENG Yan-Bin, ZHANG Hong-Peng, HU Bi-Tao, WANG Zhi-Guang
Chin. Phys. Lett. 2014, 31 (03):
036101
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DOI: 10.1088/0256-307X/31/3/036101
Evolutions of defects and helium contained defects produced by atomic displacement and helium deposition with helium implantation at different temperatures in novel high silicon (NHS) steel are investigated by a slow positron beam. Differences of the defect information among samples implanted by helium to a fluence of 1×1017 ions/cm2 at room temperature, 300°C, 450°C and 750°C are discussed. It is found that the mobility of vacancies and vacancy clusters, a recombination of vacancy-type defects and the formation of the He-V complex lead to the occurrence of these differences. At high temperature irradiations, a change of the diffusion mechanism of He atoms/He bubbles might be one of the reasons for the change of the S-parameter.
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Effect of Mg and Fe Doping on Optical Absorption of LiNbO3 Crystal through First Principles Calculations
HUANG Duo-Hui, YANG Jun-Sheng, CAO Qi-Long, WAN Ming-Jie, LI Qiang, SUN Liang, WANG Fan-Hou
Chin. Phys. Lett. 2014, 31 (03):
037103
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DOI: 10.1088/0256-307X/31/3/037103
Using first principles calculations, we investigate the structural, optical, and electronic properties of LiNbO3 (LN) and M doped LN (M=Mg, Fe). The density of states are calculated to analyze the effect of doping Mg and Fe ions on the absorption spectra and electronic properties of LN. The results show an ultraviolet shift in the optical absorption edge of Mg-doped LN compared with that of intrinsic LN. On the contrary, the absorption edge of Fe-doped LN crystal reveals a red shift. The optical absorption spectra show an improved optical response in the visible range for Mg-doped LN, which significantly differs from that obtained for Fe-doped LN. The electronic excitations from the valence band to the conduction band of LN leads to an improved optical absorption response in the visible region as observed experimentally. The obvious changes of the doped LN crystal are found in some cases, which provide a helpful guide for preparing doped LN crystal.
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Effects of Thickness and Polarization Field on the Photovoltaic Properties of BiFeO3 Thin Films
ZHOU Yin-E, TAN Xin-Yu, YU Ben-Fang, LIU Li, YUAN Song-Liu, JIAO Wei-Hong
Chin. Phys. Lett. 2014, 31 (03):
037304
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DOI: 10.1088/0256-307X/31/3/037304
We study the effects of film thickness on lattice parameters, direct band gap and photovoltaic outputs in the sol-gel derived BiFeO3 thin films. With the change of the film thickness, the great transitions will take place in the preferred orientation and lattice parameters. Furthermore, the photovoltaic outputs are significantly dependent on the film thickness. The results show that the open circuit voltage gradually increases and the short circuit current reciprocally decreases with the increase of film thickness. In particular, we demonstrate for the first time that there are tunable photovoltaic outputs with external electric field polarization switching in the polycrystalline BiFeO3 film, which is critical for the future device applications based on the photovoltaic properties of BiFeO3 films.
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Simulation and Experimentation for Low Density Drain AlGaN/GaN HEMT
WANG Chong, HE Yun-Long, DING Ning, ZHENG Xue-Feng, ZHANG Peng, MA Xiao-Hua, ZHANG Jin-Cheng, HAO Yue
Chin. Phys. Lett. 2014, 31 (03):
038501
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DOI: 10.1088/0256-307X/31/3/038501
In order to improve the breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs), we report a feasible method of low density drain (LDD) HEMT. The fluoride-based plasma treatment using CF4 gas is performed on the drain-side of the gate edge. The channel two-dimensional electron gas (2DEG) concentrations are modulated by fluoride plasma treatment, and the peak electric field at the gate edge is effectively reduced, so the breakdown voltage is improved. The electric field distributions of the LDD-HEMTs are simulated using the Silvaco software, and the peak of the electric field on the gate edge is effectively reduced. Experimental results show that, compared with the conventional HEMT, LDD-HEMTs have a lower reverse leakage current of the gate, and the breakdown voltage is increased by 36%. The current collapse characteristics of the LDD-HEMTs are confirmed by dual-pulse measurement, and an obvious pulse current reduction is due to the surface states by implanting F ions between the gate and the drain.
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Calculation of the Physical and Microdosimetric Parameters of Electron and Alpha-Particle Radiation Using Monte Carlo Simulations
GENG Jin-Peng, CAO Tian-Guang, LI Duo-Fang, AN Hai-Long, HAN Ying-Rong, LI Jin, HU Jin-Shan, LI Nan-Nan, ZHAN Yong
Chin. Phys. Lett. 2014, 31 (03):
038701
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DOI: 10.1088/0256-307X/31/3/038701
Various ionizing radiations, such as electrons and alpha particles, transfer their energy to media by produced secondary electrons and induce double- or single-strand break of DNA, which result in variable effects. To understand how the ionizing radiations interact with DNA and break it, several models have been developed, most of them consider the water as a vapor state. Actually, the ionizing particles interact with DNA which is solved in liquid water. To compare the difference of vapor and liquid water models, we calculate the stopping power, continuous slowing down approximation (CSDA) range and S value of electrons and alpha particles at cellular scale in liquid and vapor by Monte Carlo simulations, respectively. Our data show that the stopping power and CSDA range are different in liquid and vapor water in a special energy range. For many S values, the liquid model is better than the vapor model when the energy of the electrons is higher than 100 keV and the vapor model is higher than the liquid model for the 1 MeV alpha particles.
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47 articles
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