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Chin. Phys. Lett.  
  Chin. Phys. Lett.--2016, 33 (05)   Published: 29 April 2016
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GENERAL

Effects of Pure Dzyaloshinskii–Moriya Interaction with Magnetic Field on Entanglement in Intrinsic Decoherence

Da-Chuang Li, Xian-Ping Wang, Hu Li, Xiao-Man Li, Ming Yang, Zhuo-Liang Cao
Chin. Phys. Lett. 2016, 33 (05): 050301 doi: 10.1088/0256-307X/33/5/050301
Full Text: [PDF 4613 KB] (Downloads:90)
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We investigate the effects of pure Dzyaloshinskii–Moriya (DM) interaction with magnetic field on entanglement in intrinsic decoherence, assuming that the system is initially in four Bell states $|\phi_{\pm}\rangle=(|00\rangle\pm|11\rangle)/\sqrt{2}$ and $|\psi_{\pm}\rangle=(|01\rangle\pm|10\rangle)/\sqrt{2}$, respectively. It is found that if the system is initially in the state $\rho_{1}(0)=|\phi_{+}\rangle\langle\phi_{+}|$, the entanglement can obtain its maximum when the DM interaction vector ${\boldsymbol D}$ is in the plane of $XOZ$ and magnetic field ${\boldsymbol B}={\boldsymbol B_{y}}$ with the infinite time $t$, moreover the entanglement is independent of $B_{y}$ and $t$ when ${\boldsymbol B_{y}}$ is perpendicular to ${\boldsymbol D}$. In addition, we obtain similar results when the system is initially in the states $\rho_{2}(0)=|\phi_{-}\rangle\langle\phi_{-}|$ or $\rho_{3}(0)=|\psi_{+}\rangle\langle\psi_{+}|$. However, we find that if the system is initially in the state $\rho_{4}(0)=|\psi_{-}\rangle\langle\psi_{-}|$, the entanglement can obtain its maximum for infinite $t$, when the DM vector is in the plane of $YOZ$, $XOZ$, or $XOY$, with the magnetic field parallel to $X$, $Y$, or $Z$ axis, respectively. Moreover, when the axial ${\boldsymbol B}$ is perpendicular to ${\boldsymbol D}$ for the initial state $\rho_{4}(0)$, the negativity oscillates with time $t$ and reaches a stable value, the larger the value of ${\boldsymbol B}$ is, the greater the stable value is, and the shorter the oscillation time of the negativity is. Thus we can adjust the direction and value of the external magnetic field to obtain the maximal entanglement, and avoid the adverse effects of external environment in some initial state. This is feasible within the current experimental technology.

Entanglement Dynamics of Two Qubits Coupled Independently to Cavities in the Ultrastrong Coupling Regime: Analytical Results

Wei-Ting Zhu, Qing-Bao Ren, Li-Wei Duan, Qing-Hu Chen
Chin. Phys. Lett. 2016, 33 (05): 050302 doi: 10.1088/0256-307X/33/5/050302
Full Text: [PDF 419 KB] (Downloads:37)
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Dynamics of quantum entanglement of two qubits in two identical quantum Rabi models is studied analytically in the framework of corrections to the rotating-wave approximations. A closed-form expression for the entanglement dynamics initiated from the well-known Bell states is derived, which is very close to the numerical exact results up to the ultrastrong coupling regime. It is found that the vanishing entanglement can be purely induced by the counter-rotating terms, and can be enhanced with the atom–cavity coupling.

A New Solution in Understanding Massive White Dwarfs

Zhen-Zhen Jing, De-Hua Wen
Chin. Phys. Lett. 2016, 33 (05): 050401 doi: 10.1088/0256-307X/33/5/050401
Full Text: [PDF 511 KB] (Downloads:35)
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The observed high over-luminous type-Ia supernovae imply the existence of super-Chandrasekhar limit white dwarfs, which raises a challenge to the classical white dwarf theories. By employing the Eddington-inspired Born–Infeld (EiBI) gravity, we reinvestigate the structures and properties of white dwarfs, and find out that the EiBI gravity provides a new way to understand the observations. It is shown that by choosing an appropriate positive Eddington parameter $\kappa$, a massive white dwarf with mass up to $2.8M_\odot$ can be supported by the equation of state of free electron gas. Unlike the classical white dwarf theory, the maximum mass of the white dwarf sequence in the EiBI gravity is not decided by the mass–radius relations, but is decided by the central density, $\rho_{\rm c}=4.3\times10^{14}$ kg/m$^3$, above which neutronization cannot be avoided and the white dwarf will transform into a neutron star. On the other hand, if the gravity in the massive white dwarf really behaves as the EiBI gravity predicts, then one can obtain a constraint on the Eddington parameter in the EiBI gravity, that is, $8\pi{\rho_0}\kappa G/c^2\geq 80$ (where $\rho_0=10^{18}$ kg/m$^3$) to support a massive white dwarf with mass up to $2.8M_\odot$. Moreover, we find out that the fast Keplarian frequency of the massive white dwarf raises a degeneration between the two kinds of compact stars, that is, one cannot distinguish whether the observed massive pulsar is a massive neutron star or a massive white dwarf only through the observed pulse frequency and mass.

Effects of Reduced Frequency on Network Configuration and Synchronization Transition

Liu-Hua Zhu
Chin. Phys. Lett. 2016, 33 (05): 050501 doi: 10.1088/0256-307X/33/5/050501
Full Text: [PDF 3882 KB] (Downloads:19)
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Synchronization of networked phase oscillators depends essentially on the correlation between the topological structure of the graph and the dynamical property of the elements. We propose the concept of 'reduced frequency', a measure which can quantify natural frequencies of each pair of oscillators. Then we introduce an evolving network whose linking rules are controlled by its own dynamical property. The simulation results indicate that when the linking probability positively correlates with the reduced frequency, the network undergoes a first-order phase transition. Meanwhile, we discuss the circumstance under which an explosive synchronization can be ignited. The numerical results show that the peculiar butterfly shape correlation between frequencies and degrees of the nodes contributes to an explosive synchronization transition.

Travelling Wave in the Generalized Kuramoto Model with Inertia

Di Yuan, Dong-Qiu Zhao, Yi Xiao, Ying-Xin Zhang
Chin. Phys. Lett. 2016, 33 (05): 050502 doi: 10.1088/0256-307X/33/5/050502
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We study the dynamics of the generalized Kuramoto model with inertia, in which oscillators with positive coupling strength are conformists and oscillators with negative coupling strength are contrarians. By numerically simulating the model, we find that the model supports a modulated travelling wave state except for already displayed travelling wave states and $\pi$ state in previous literature. The modulated travelling wave state may be characterized by the phase distributions of oscillators. Finally, the modulated travelling wave state and the travelling wave state of the model in the parameter space are presented.

Phase-Sensitive Optical Time-Domain Reflectometer Based on a 120$^{\circ}$-Phase-Difference Michelson Interferometer

Yu-Long Cao, Fei Yang, Dan Xu, Qing Ye, Hai-Wen Cai, Zu-Jie Fang
Chin. Phys. Lett. 2016, 33 (05): 050701 doi: 10.1088/0256-307X/33/5/050701
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A phase-sensitive optical time domain reflectometer ($\phi$-OTDR) based on a 120$^{\circ}$-phase-difference Michelson interferometer is proposed. The Michelson interferometer with arm difference of 4 m is used to test the phase difference between the Rayleigh scattering from two sections of the fiber. A new demodulation method called the inverse transmission matrix demodulation scheme is utilized to demodulate the distributed phase from the backward scattering along the long fiber. The experimental results show that the 120$^{\circ}$-phase-difference interferometer $\phi$-OTDR can detect the phase along the 3 km fiber, and the acoustic signal within the whole human hearing range of 20 Hz–20 kHz is reproduced accurately and quickly.
NUCLEAR PHYSICS

Simulation of Suppression of Gamma Sensitivity of 'Fission Electron-Collection' Neutron Detector

Dong Wang, Chuan-Fei Zhang, Bo-Jun Li, Yi-Ping Cai, Xue-Bin Zhu, Fen-Ni Si, Zhi-Guo Xi
Chin. Phys. Lett. 2016, 33 (05): 052901 doi: 10.1088/0256-307X/33/5/052901
Full Text: [PDF 397 KB] (Downloads:13)
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The fission electron-collection neutron detector (FECND) is a current-type neutron detector. Based on the analysis of the generation process of the gamma signals of the FECND, a mechanism utilizing symmetrical structure is proposed and discussed to suppress the gamma signals. According to this mechanism, the electrons generated from the gamma rays can be well compensated for by the adjustment of the electrodes' thickness and distance. In this study, based on the Monte–Carlo simulation of the gamma signals of the FECND, the varying patterns are obtained between the gamma signals and the detector parameter settings. As indicated by the simulation results, the gamma electrons can be compensated for completely by simply adjusting the coated electrode substrate thickness and distance. Moreover, with a proposed optimal parameter setting, the gamma sensitivity can be as low as 3.39$\times$10$^{-23}$ C$\cdot$cm$^{2}$, while the signal-to-noise ratio can be higher than 200:1. The compensation results of the $\gamma$-rays in the FECND will be slightly affected by the manufacturing error or the assembly error.
ATOMIC AND MOLECULAR PHYSICS

Precision Frequency Measurement of $^{87}$Rb 5$S_{1/2}$ ($F=2$)$\to$5$D_{5/2}$ ($F''=4$) Two-Photon Transition through a Fiber-Based Optical Frequency Comb

Wei Xia, Shao-Yang Dai, Yin Zhang, Kun-Qian Li, Qi Yu, Xu-Zong Chen
Chin. Phys. Lett. 2016, 33 (05): 053201 doi: 10.1088/0256-307X/33/5/053201
Full Text: [PDF 608 KB] (Downloads:20)
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The absolute frequency of $^{87}$Rb 5$S_{1/2}$ ($F=2$)$\to$5$D_{5/2}$ ($F''=4$) two-photon transition at 778 nm is measured in an accuracy of 44 kHz. A home-made erbium-doped fiber laser frequency comb with frequency stability of $5.0\times10^{-13}$@1 s is employed for the light source. By using a periodically poled lithium niobate, the femtosecond pulse operating in 1556 nm is frequency-doubled to 778 nm to obtain the direct two-photon transition spectroscopy of thermal rubidium vapor. Through sweeping the carrier envelope offset frequency ($f_{\rm ceo}$), the 5$S_{1/2}$ ($F=2$)$\to $5$D_{5/2}$ ($F''=4$) two-photon transition line is clearly resolved and its absolute frequency is determined via the peak-finding of the fitting curve. After the frequency correction, the measured result agrees well with the previous experiment on this transition. The entire system configuration is compact and robust, providing a potential candidate of optical frequency standard for telecommunication applications.
FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

Differentiation of Positional Isomers of Propyl Alcohols Using Filament-Induced Fluorescence

Xiang-Ye Wei, Zhi-Wei Tu, Chang Liu, He-Long Li, Huai-Liang Xu
Chin. Phys. Lett. 2016, 33 (05): 054201 doi: 10.1088/0256-307X/33/5/054201
Full Text: [PDF 415 KB] (Downloads:9)
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We experimentally demonstrate the recognition of positional isomers of propyl alcohol vapor through nonlinear fluorescence induced by high-intensity femtosecond laser filaments in air. By measuring characteristic fluorescence of n-propyl and isopropyl alcohol vapors produced by femtosecond filament excitation, it is found that they show identical spectra, that is, those from molecular bands of CH, C$_{2}$, NH, OH and CN, while the relative intensities are different. By comparing the ratios of the CH and C$_{2}$ signals, the two propyl alcohol isomers are differentiated. The different signal intensities are ascribed to different ionization potentials of the two isomer molecules, leading to different production efficiencies of fluorescing fragments.

Dual-Wavelength Holmium-Doped Fiber Laser Pumped by Thulium–Ytterbium Co-Doped Fiber Laser

A. A. Latiff, A. Dhar, S. W. Harun, I. M. Babar, S. Das, M. C. Paul, H. Ahmad
Chin. Phys. Lett. 2016, 33 (05): 054202 doi: 10.1088/0256-307X/33/5/054202
Full Text: [PDF 517 KB] (Downloads:30)
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We present an all-fiber dual-wavelength holmium-doped fiber laser operating in 2 µm region using a newly developed holmium-doped fiber (HDF) as a gain medium. The proposed fiber laser is constructed by using a hybrid gain medium, i.e., a thulium–ytterbium co-doped fiber (TYDF) and an HDF in conjunction with a simple half-opened linear cavity, which is formed by a broadband mirror and an output coupler reflector. Without the HDF, the TYDF laser operates at wavelengths of 1991 and 1999 nm with a signal-to-noise ratio of more than 34 dB and the slope efficiency of 26.16 %. With the HDF, dual-wavelength output lines are obtained at 2075 and 2083 nm with signal-to-noise ratios of more than 17 dB, 3 dB bandwidth of less than 0.2 nm and the power difference between the two peaks of less than 1 dB at the TYDF laser pump power of 320 mW.

Wideband All-Polarization-Maintaining Yb-Doped Mode-Locked Fiber Laser Using a Nonlinear Optical Loop Mirror

Yi-Nan Lin, Wen-Tan Fang, Chun Gu, Li-Xin Xu
Chin. Phys. Lett. 2016, 33 (05): 054203 doi: 10.1088/0256-307X/33/5/054203
Full Text: [PDF 363 KB] (Downloads:6)
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We report on a wide-band and stable mode-locked all-polarization-maintaining fiber laser configuration using a nonlinear optical loop mirror. The central wavelength of the laser is 1080.14 nm and the 3 dB bandwidth is 20.29 nm. The repetition rate of the pulse is 3.28 MHz and the pulse width is 848 ps. By tuning the pump power, which is centered at 980 nm, from 300 mW to 380 mW, we obtain a linearly changed output power from 6 mW to 7.12 mW. The all-polarization-maintaining fiber configuration is fundamental to the stability of the output power.

Structured Illumination Chip Based on Integrated Optics

Yong Liu, Chen Wang, Anastasia Nemkova, Shi-Ming Hu, Zhi-Yong Li, Yu-De Yu
Chin. Phys. Lett. 2016, 33 (05): 054204 doi: 10.1088/0256-307X/33/5/054204
Full Text: [PDF 848 KB] (Downloads:14)
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A compact structured illumination chip based on integrated optics is proposed and fabricated on a silicon-on-insulator platform. Based on the simulation of Gaussian beam interference, we adopt a chirped diffraction grating to achieve a specific interference pattern. The experimental results match well with the simulations. The portability and flexibility of the structured illumination chip can be increased greatly through horizontal encapsulation. High levels of integration, compared with the conventional structured illumination approach, make this chip very compact, with a footprint of only around 1 mm$^{2}$. The chip has no optical lenses and can be easily combined with a microfluidic system. These properties would make the chip very suitable for portable 3D scanner and compact super-resolution microscopy applications.

Effects of Strong Turbulence on the Spiral Plane Mode of Whittaker–Gaussian Beam through Terrene-Atmosphere

Ye Li, Yi-Xin Zhang
Chin. Phys. Lett. 2016, 33 (05): 054205 doi: 10.1088/0256-307X/33/5/054205
Full Text: [PDF 447 KB] (Downloads:4)
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The analytic formulae of probability distribution of spiral plane modes for the Whittaker–Gaussian (WG) beams with orbital angular momentum (OAM) in strong turbulence regime are modeled based on the modified Rytov approximation. Numerical results show that the crosstalk range of OAM modes in the vicinity of signal mode increases with the increasing refractive-index construction parameter. However, effects of change of the width of the Gaussian envelope and the parameter ${W_0}$ of WG beams on normalization energy weight of signal mode can be ignored. We find theoretically that signal spiral plane mode of WG beams at each OAM level approximatively has the same normalization energy weight, implying that the channels with WG (pseudo non-diffraction) beam have higher channel capacity than the channels with the Laguerre–Gaussian beam.

Q-Switched Ytterbium-Doped Fiber Laser Using Black Phosphorus as Saturable Absorber

A. H. H. Al-Masoodi , M. H. M. Ahmed , A. A. Latiff , H. Arof , S. W. Harun 
Chin. Phys. Lett. 2016, 33 (05): 054206 doi: 10.1088/0256-307X/33/5/054206
Full Text: [PDF 902 KB] (Downloads:30)
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We demonstrate a Q-switched ytterbium-doped fiber laser (YDFL) using a newly developed multi-layer black phosphorous (BP) saturable absorber (SA). The BP SA is prepared by mechanically exfoliating a BP crystal and sticking the acquired BP flakes onto a scotch tape. A small piece of the tape is then placed between two ferrules and incorporated in a YDFL cavity to achieve a stable Q-switched operation in a 1.0 µm region. The laser has a pump threshold of 55.1 mW, a pulse repetition rate that is tunable from 8.2 to 32.9 kHz, and the narrowest pulse width of 10.8 µs. The highest pulse energy of 328 nJ is achieved at the pump power of 97.6 mW. Our results show that multi-layer BP is a promising SA for Q-switching laser operation.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

Hot Vertical-Displacement-Event Process due to Internal Energy Perturbations for HL-2M Tokamak Plasma

Lei Xue, Xu-Ru Duan, Guo-Yao Zheng, Yue-Qiang Liu, Shi-Lei Yan, V. N. Dokuka, V. E. Lukash, R. R. Khayrutdinov
Chin. Phys. Lett. 2016, 33 (05): 055201 doi: 10.1088/0256-307X/33/5/055201
Full Text: [PDF 1023 KB] (Downloads:11)
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During the tokamak operation, variation of the stored energy can cause internal perturbations of the plasma. These perturbations may develop into large-scale vertical movement of the whole column for the vertically elongated tokamak, eventually generating the hot vertical displacement event (VDE). It will cause considerable damage to the machine. In this work, the hot VDE process due to stored energy perturbations is investigated by a mature non-linear time-evolution code DINA. The influence on the vertical instability, the displacement direction and the electromagnetic loads on in-vessel components during the hot VDE are analyzed. It is shown that a larger perturbation leads to faster development of the vertical instability. Meanwhile the variation of the Shafranov shift, due to the energy change, is related to the VDE direction. The vertical electromagnetic force on the vacuum vessel and the halo current flowing in the divertor baffle become larger in the case of VDE moving towards the $X$ point.
CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES

Static and Dynamic Precipitation Behavior of the Al–20wt.%Zn Alloy

Chong-Yu Liu, Hong-Jie Jiang, Chun-Xia Wang, Hai-Quan Qi, Yi-Bing Li, Ming-Zhen Ma, Ri-Ping Liu
Chin. Phys. Lett. 2016, 33 (05): 056101 doi: 10.1088/0256-307X/33/5/056101
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The static and dynamic precipitation behavior of solution-treated binary Al–20 wt.% Zn alloy is investigated via artificial aging, cold rolling and artificial aging combined with cold rolling. The solution-treated Al–Zn alloy exhibits high thermal stability during aging, and low densities of nano-sized Zn particles are precipitated along with Al grain boundaries after aging at 200$^{\circ}\!$C for 13 h. Compared with static precipitation, dynamic precipitation occurs more easily in the Al–Zn alloy. Zn clusters are obtained after cold rolling at an equivalent plastic strain of 0.6, and the size of the Zn phase reaches hundreds of nanometers when the strain is increased to 12.1. The results show that the speed of static precipitation can be significantly enhanced after the application of 2.9 rolling strain. Grain refinement and defects induced by cold rolling are considered to promote Zn precipitation. The hardness of Al–Zn alloy is also affected by static and dynamic precipitations.

Photoluminescence Analysis of Injection-Enhanced Annealing of Electron Irradiation-Induced Defects in GaAs Middle Cells for Triple-Junction Solar Cells

Yong Zheng, Tian-Cheng Yi, Peng-Fei Xiao, Juan Tang, Rong Wang
Chin. Phys. Lett. 2016, 33 (05): 056102 doi: 10.1088/0256-307X/33/5/056102
Full Text: [PDF 554 KB] (Downloads:13)
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Photoluminescence measurements are carried out to investigate the injection-enhanced annealing behavior of electron radiation-induced defects in a GaAs middle cell for GaInP/GaAs/Ge triple-junction solar cells which are irradiated by 1.8 MeV with a fluence of $1\times10^{15}$ cm$^{-2}$. Minority-carrier injection under forward bias is observed to enhance the defect annealing in the GaAs middle cell, and the removal rate of the defect is determined with photoluminescence radiative efficiency recovery. Furthermore, the injection-enhanced defect removal rates obey a simple Arrhenius law. Therefore, the annealing activation energy is acquired and is equal to 0.58 eV. Finally, in comparison of the annealing activation energies, the E5 defect is identified as a primary non-radiative recombination center.

Current-Induced Reversible Resistance Jumps in La$_{0.8}$Ca$_{0.2}$MnO$_{3}$ Microbridge

Zhang-Yin Zhai, Qi-Yun Xie, Gui-Bin Chen, Xiao-Shan Wu, Ju Gao
Chin. Phys. Lett. 2016, 33 (05): 056103 doi: 10.1088/0256-307X/33/5/056103
Full Text: [PDF 545 KB] (Downloads:8)
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Two spatially confined La$_{0.8}$Ca$_{0.2}$MnO$_{3}$ (LCMO) microbridges with different widths, starting from a single LCMO film (3 mm$\times$5 mm), are fabricated by optical lithography. A second new and robust metal-insulator transition (MIT) peak at about 75 K appears, in addition to the normal MIT at 180 K observed in the standard LCMO film. When the two bridges are processed by currents of high densities, interesting reversible resistance jumps are excited only around the new peak. A stronger dependence of resistance jump on current excitation is found for the bridge with a smaller width. The temperature driven transition between new excited multiple metastable states are involved to explain the interesting low-temperature ultra-sharp jumps.

Preferable Orientations of Interacting C$_{60}$ Molecules inside Single Wall Boron Nitride Nanotubes

Zhen Yao, Jia-Yin Lv, Chun-Jian Liu, Hang Lv, Bing-Bing Liu
Chin. Phys. Lett. 2016, 33 (05): 056701 doi: 10.1088/0256-307X/33/5/056701
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This work focuses on the preferable orientation analysis of the hybrid system where the C$_{60}$ molecules are encapsulated inside the boron nitride nanotubes by using the two-molecule model. The low-energy state can be acquired in the contour map, which provides the visual information of the systematical van der Waals interaction potential for the C$_{60}$ molecules adopting different orientations. Our results show that the C$_{60}$ molecules exhibit the preferred pentagon and hexagon orientations with the tube's diameter smaller and larger than 13.55 Å, respectively. The preferred two-bond orientation obtained in the single-molecule model is absent in this study, indicating that the intermolecular interaction of adjacent C$_{60}$ molecules plays an important role in the orientational behaviors of this peapod structure.

Brightness of Blue/Violet Luminescent Nano-Crystalline AZO and IZO Thin Films with Effect of Layer Number: For High Optical Performance

Panagiota KORALLİ, Songül FİAT VAROL, Michael KOMPITSAS, Mihaela GIRTAN
Chin. Phys. Lett. 2016, 33 (05): 056801 doi: 10.1088/0256-307X/33/5/056801
Full Text: [PDF 791 KB] (Downloads:16)
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From different reports, it is realized that there is a need to consider all sides of aluminum-doped zinc oxide (AZO) and indium-doped zinc oxide (IZO) thin films with their optical, luminescence and surface properties including usage areas. We establish an assessment to carry out further information to summarize AZO and IZO nanocrystalline films with impact of the layer number.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

Investigations for the Surface of the Oxide Semiconductor Changes by Reduction

Bonghoon Kang, Sung-Tae Hwang
Chin. Phys. Lett. 2016, 33 (05): 057201 doi: 10.1088/0256-307X/33/5/057201
Full Text: [PDF 382 KB] (Downloads:8)
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Oxide semiconductor SrTiO$_{3}$ single crystals are exposed to a reducing atmosphere H$_{2}$/N$_{2}$ to induce the reduction of Ti$^{4+}$ to Ti$^{3+}$ and the release of oxygen from the lattice compensating the reduction of the Ti ions. In a reducing atmosphere H$_{2}$/N$_{2}$ the optical edge brings about a red shift. The infrared reflection spectra suggest that the (11) STO single crystal surface can be terminated by the domain of the SrO or TiO$_{2}$ alternative layer during the reduction. The anisotropy and asymmetry of optical second-harmonic intensity explain a slight shrinkage. The dielectric constant reaches about 6000 and shows almost frequency dependence at all temperatures. With the increasing temperature, the dielectric constant increases rapidly. The high temperature region and low temperature region have activation energies of 0.89 and 1.04, respectively.

Transport and Conductance in Fibonacci Graphene Superlattices with Electric and Magnetic Potentials

Yi-Heng Yin, Yan-Xiong Niu, Ming Ding, Hai-Yue Liu, Zhen-Jiang Liang
Chin. Phys. Lett. 2016, 33 (05): 057202 doi: 10.1088/0256-307X/33/5/057202
Full Text: [PDF 741 KB] (Downloads:14)
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We investigate the electron transport and conductance properties in Fibonacci quasi-periodic graphene superlattices with electrostatic barriers and magnetic vector potentials. It is found that a new Dirac point appears in the band structure of graphene superlattice and the position of the Dirac point is exactly located at the energy corresponding to the zero-averaged wave number. The magnetic and electric potentials modify the energy band structure and transmission spectrum in entirely diverse ways. In addition, the angular-dependent transmission is blocked by the potential barriers at certain incident angles due to the appearance of the evanescent states. The effects of lattice constants and different potentials on angular-averaged conductance are also discussed.

Influence of Annealing Temperature on Berthelot-Type Hopping Conduction Mechanism in Carbon-Nickel Composite Films

V. Dalouji, S. M. Elahi, A. Ghaderi, S. Solaymani
Chin. Phys. Lett. 2016, 33 (05): 057203 doi: 10.1088/0256-307X/33/5/057203
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Electrical conductivity of carbon-nickel composite films annealed at temperatures 300, 500 and 800$^{\circ}\!$C is studied over a temperature range of 50–300 K. While the conductivity data above room temperature show extended state conduction, lowering the temperature from 150 to 50 K leads to the Berthelot-type conduction mechanism. It can be seen that the films annealed at 500$^{\circ}\!$C have the maximum conductivity. The extent of the carrier wave function at 500$^{\circ}\!$C has the minima $2.87\times10^{-7}$ cm and $2.45\times10^{-7}$ cm in octahedral-metal stretching vibrations and intrinsic stretching vibrations of the metal at the tetrahedral site, respectively. The average distances between two vibration octahedral and tetrahedral sites at 500$^{\circ}\!$C also have the minima $1.13\times10^{-7}$ cm and $0.97\times10^{-7}$ cm, respectively. The Berthelot temperature for films annealed at 800$^{\circ}\!$C has the minimum of 94.3 K.

Anisotropic Ballistic Transport through a Potential Barrier on Monolayer Phosphorene

Fang Cheng, Bing He
Chin. Phys. Lett. 2016, 33 (05): 057301 doi: 10.1088/0256-307X/33/5/057301
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We demonstrate theoretically the anisotropic quantum transport of electrons through a single barrier on monolayer phosphorene. Using an effective ${\boldsymbol k}\cdot{\boldsymbol p}$ Hamiltonian, we find that the transmission probability for transport through n–n–n (or n–p-n) junction is an oscillating function of the incident angle, the barrier height, as well as the incident energy of electrons. The conductance in such systems depends sensitively on the transport direction due to the anisotropic effective mass. By tuning the Fermi energy and gate voltage, the channels can be transited from opaque to transparent, which provides us with an efficient way to control the transport of monolayer phosphorene-based microstructures.

The 45K Onset Superconductivity and the Suppression of the Nematic Order in FeSe by Electrolyte Gating

Wei-Ke Wang, Yan Liu, Ji-Yong Yang, Hai-Feng Du, Wei Ning, Lang-Sheng Ling, Wei Tong, Zhe Qu, Zhao-Rong Yang, Ming-Liang Tian, Yu-Heng Zhang
Chin. Phys. Lett. 2016, 33 (05): 057401 doi: 10.1088/0256-307X/33/5/057401
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The electronic doping effect on both the superconductivity and the nematic order in the FeSe nanoflake are investigated by using the electric-double-layer transistor configuration. The superconductivity can be effectively controlled by electronic doping, and the onset superconducting transition temperature $T_{\rm c}$ reaches as high as 45 K at a gate voltage of $V_{\rm g}=4$ V. Meanwhile, the nematic phase is gradually suppressed with the increase of electronic doping (or $V_{\rm g}$). The results provide an effective method with variable charge doping for investigation of the rich physics in the FeSe superconductor.

Fano Resonances Can Provide Two Criteria to Distinguish Majorana Bound States from Other Candidates in Experiments

Ye Xiong
Chin. Phys. Lett. 2016, 33 (05): 057402 doi: 10.1088/0256-307X/33/5/057402
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There are still debates on whether the observed zero energy peak in the experiment by Stevan et al. [Science 346 (2014) 602] reveals the existence of the long pursued Majorana bound states (MBSs). We propose that, by mounting two scanning tunneling microscopic tips on top of the topological superconducting chain and measuring the transmission spectrum between these two metallic tips, there are two kinds of characteristics on the spectrum that are caused by MBSs uniquely. One is symmetric peaks with respect to zero energy and the other is $4\pi$ period caused by a nearby Josephson junction. The former refers to the fact that MBSs are composited by Majorana fermions which distributed in the particle and hole subspaces equally. The latter is based on the well known $4\pi$ period of Josephson effect in topological superconductor. We think that such two characteristics can be used as criteria to distinguish MBSs from other candidates, such as impurities, Kondo effect and traditional Andreev bound states.

Oxygen Scavenging Effect of LaLuO$_{3}$/TiN Gate Stack in High-Mobility Si/SiGe/SOI Quantum-Well Transistors

Jin-Feng Feng, Chang Liu, Wen-Jie Yu, Ying-Hong Peng
Chin. Phys. Lett. 2016, 33 (05): 057701 doi: 10.1088/0256-307X/33/5/057701
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Higher-$\kappa$ dielectric LaLuO$_{3}$, deposited by molecular beam deposition, with TiN as gate stack is integrated into high-mobility Si/SiGe/SOI quantum-well p-type metal-oxide-semiconductor field effect transistors. Threshold voltage shift and capacitance equivalent thickness shrink are observed, resulting from oxygen scavenging effect in LaLuO$_{3}$ with Ti-rich TiN after high temperature annealing. The mechanism of oxygen scavenging and its potential for resistive memory applications are analyzed and discussed.

Phonon Confinement Effect in Two-dimensional Nanocrystallites of Monolayer MoS$_2$ to Probe Phonon Dispersion Trends Away from Brillouin-Zone Center

Wei Shi, Xin Zhang, Xiao-Li Li, Xiao-Fen Qiao, Jiang-Bin Wu, Jun Zhang, Ping-Heng Tan
Chin. Phys. Lett. 2016, 33 (05): 057801 doi: 10.1088/0256-307X/33/5/057801
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The fundamental momentum conservation requirement $q\sim0$ for the Raman process is relaxed in the nanocrystallites (NCs), and phonons away from the Brillouin-zone center will be involved in the Raman scattering, which is well-known as the phonon confinement effect in NCs. This usually gives a downshift and asymmetric broadening of the Raman peak in various NCs. Recently, the $A'_1$ mode of 1L MoS$_2$ NCs is found to exhibit a blue shift and asymmetric broadening toward the high-frequency side [Chem. Soc. Rev. 44 (2015) 2757 and Phys. Rev. B 91 (2015) 195411]. In this work, we carefully check this issue by studying Raman spectra of 1L MoS$_2$ NCs prepared by the ion implantation technique in a wide range of ion-implanted dosage. The same confinement coefficient is used for both $E'$ and $A'_1$ modes in 1L MoS$_2$ NCs since the phonon uncertainty in an NC is mainly determined by its domain size. The asymmetrical broadening near the $A'_1$ and $E'$ modes is attributed to the appearance of defect-activated phonons at the zone edge and the intrinsic asymmetrical broadening of the two modes, where the anisotropy of phonon dispersion curves along ${\it \Gamma}$–$K$ and ${\it \Gamma}$–$M$ is also considered. The photoluminescence spectra confirm the formation of small domain size of 1L MoS$_2$ nanocrystallites in the ion-implanted 1L MoS$_2$. This study provides not only an approach to quickly probe phonon dispersion trends of 2D materials away from ${\it \Gamma}$ by the Raman scattering of the corresponding NCs, but also a reference to completely understand the confinement effect of different modes in various nanomaterials.

BiS$_{2}$-Layer Gives Giant Birefringence: First-Principles Calculations

Hai Wang
Chin. Phys. Lett. 2016, 33 (05): 057802 doi: 10.1088/0256-307X/33/5/057802
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Birefringence attracts increasing attention due to its extensive applications in the imaging spectrometer, laser devices and optical components. Stimulated by the discovery of the giant birefringence (GBF) in LaOBiS$_{2}$ and AFBiS$_{2}$, we explore its origin by checking three compounds with and without the BiS$_{2}$ layer: Bi$_{2}$OS$_{2}$ (i.e., BiOBiS$_{2})$, Bi$_{2}$O$_{2}$S and LaOAlS$_{2}$. It is demonstrated that GBF appears only in the compounds with the BiS$_{2}$ layer and is furthermore confirmed in $M$OBiS$_{2}$ ($M$=Al, Ga, In and Sc) compounds. The result is useful for discovering/developing new-type birefringent crystal or even multifunctional materials.
CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

P-Type Nitrogen-Doped ZnO Films Prepared by In-Situ Thermal Oxidation of Zn$_{3}$N$_{2}$ Films

Yu-Ping Jin, Bin Zhang, Jian-Zhong Wang, Li-Qun Shi
Chin. Phys. Lett. 2016, 33 (05): 058101 doi: 10.1088/0256-307X/33/5/058101
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P-type nitrogen-doped ZnO films are prepared successfully by in-situ thermal oxidation of Zn$_{3}$N$_{2}$ films. The prepared films are characterized by x-ray diffraction, non-Rutherford backscattering (non-RBS) spectroscopy, x-ray photoelectron spectroscopy, and photoluminescence spectrum. The results show that the Zn$_{3}$N$_{2}$ films start to transform to ZnO at 400$^{\circ}\!$C and the total nitrogen content decreases with the increasing annealing temperature. The p-type films are achieved at 500$^{\circ}\!$C with a low resistivity of 6.33 $\Omega$$\cdot $cm and a high hole concentration of +8.82 $\times$ 10$^{17}$ cm$^{-3}$, as well as a low level of carbon contamination, indicating that the substitutional nitrogen (N$_{\rm O}$) is an effective acceptor in the ZnO:N film. The photoluminescence spectra show clear UV emissions and also indicate the presence of oxygen vacancy (V$_{\rm O}$) defects in the ZnO:N films. The p-type doping mechanism is briefly discussed.

Thermal Analysis Simulation of Germanium Zone Refining Process Assuming a Constant Radio-Frequency Heating Source

Mahdi Ezheiyan, Hossein Sadeghi, Mohammad-Hossein Tavakoli
Chin. Phys. Lett. 2016, 33 (05): 058102 doi: 10.1088/0256-307X/33/5/058102
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Three-dimensional thermal analysis simulation of a horizontal zone refining system is conducted for germanium semiconductor materials. The considered geometry includes a graphite boat filled with germanium placed in a cylindrical quartz tube. A flow of Ar and H$_{2}$ gas mixture is purged through the tube. A narrow section of the boat is assumed to be exposed to a constant heat rate produced by an rf coil located outside the quartz tube. The results of this analysis provide essential information about various parameters such as the shape of the molten zone, required power and temperature gradient in the system.

A Method for Preparation of Ordered Porous Silicon Based on a 2D SiO$_{2}$ Template

Ying Wu, Xiao-Xia Zhai, Cong-Mian Zhen, Xiao-Wei Liu, Li Ma, Deng-Lu Hou
Chin. Phys. Lett. 2016, 33 (05): 058103 doi: 10.1088/0256-307X/33/5/058103
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A new method for fabricating ordered porous silicon is reported. A two-dimensional silica nanosphere array is used as a template with a hydrofluoric acid–hydrogen peroxide solution for etching the nanospheres. The initial diameter and distribution of the holes in the resulting porous silicon layer are determined by the size and distribution of the silica nanospheres. The corrosion time can be used to control the depths of the holes. It is found that the presence of a SiO$_{2}$ layer, formed by the oxidation of the rough internal surface of the hole, is the primary reason allowing the corrosion to proceed. Ultraviolet reflection and thermal conductivity measurements show that the diameter and distribution of the holes have a great influence on properties of the porous silicon.

Influence of Post-Annealing on Electrical Characteristics of Thin-Film Transistors with Atomic-Layer-Deposited ZnO-Channel/Al$_{2}$O$_{3}$-Dielectric

You-Hang Wang, Qian Ma, Li-Li Zheng, Wen-Jun Liu, Shi-Jin Ding, Hong-Liang Lu, Wei Zhang
Chin. Phys. Lett. 2016, 33 (05): 058501 doi: 10.1088/0256-307X/33/5/058501
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High-performance thin-film transistors (TFTs) with a low thermal budget are highly desired for flexible electronic applications. In this work, the TFTs with atomic layer deposited ZnO-channel/Al$_{2}$O$_{3}$-dielectric are fabricated under the maximum process temperature of 200$^{\circ}\!$C. First, we investigate the effect of post-annealing environment such as N$_{2}$, H$_{2}$-N$_{2}$ (4%) and O$_{2}$ on the device performance, revealing that O$_{2}$ annealing can greatly enhance the device performance. Further, we compare the influences of annealing temperature and time on the device performance. It is found that long annealing at 200$^{\circ}\!$C is equivalent to and even outperforms short annealing at 300$^{\circ}\!$C. Excellent electrical characteristics of the TFTs are demonstrated after O$_{2}$ annealing at 200$^{\circ}\!$C for 35 min, including a low off-current of $2.3\times10^{-13}$ A, a small sub-threshold swing of 245 mV/dec, a large on/off current ratio of 7.6$\times$10$^{8}$, and a high electron effective mobility of 22.1 cm$^{2}$/V$\cdot$s. Under negative gate bias stress at $-$10 V, the above devices show better electrical stabilities than those post-annealed at 300$^{\circ}\!$C. Thus the fabricated high-performance ZnO TFT with a low thermal budget is very promising for flexible electronic applications.

Mechanical Analysis and Measurements of a Multicomponent NbTi/Cu Superconducting Magnets Structure for the Fully Superconducting Electron Cyclotron Resonance Ion Source

Ming-Zhi Guan, Qiang Hu, Pei-Feng Gao, Xing-Zhe Wang, Tong-Jun Yang, Wei Wu, Can-Jie Xin, Bei-Min Wu, Li-Zhen Ma
Chin. Phys. Lett. 2016, 33 (05): 058502 doi: 10.1088/0256-307X/33/5/058502
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A fully superconducting electron cyclotron resonance (ECR) ion source (SECRAL II) is currently being built in the Institute of Modern Physics, Chinese Academy of Sciences. Its key components are three superconducting solenoids (Nb-Ti/Cu) and six superconducting sextupoles (Nb-Ti/Cu). Different from the conventional superconducting ECR magnetic structure, the SECRAL II includes three superconducting solenoid coils that are located inside the superconducting sextupoles. The SECRAL II can significantly reduce the interaction forces between the sextupole and the solenoids, and the magnets can also be more compact in size. For this multi-component SECRAL II generating its self field of $\sim$8 T and being often exposed to the high self field, the mechanical analysis has become the main issue to keep their stress at $ < $200 MPa on coils. The analytical and experimental results in mechanics are presented in the SECRAL II structure. To improve the accuracy and efficiency of analysis, according to the composite rule of micromechanics, the equivalent uniform windings are used to simulate the epoxy-impregnated Nb-Ti/Cu coils. In addition, using low temperature strain gauges and a wireless fast strain acquisition system, a fundamental experiment on the strains developments of a sextupole is reported. Finally, based on our analysis, the stresses and deformations for its assembly of each SECRAL II coil will be further optimized.

Excess Carrier Lifetime Improvement in c-Si Solar Cells by YAG:Ce$^{3+}$-Yb$^{3+}$

Talib Hussain, Hui-Qi Ye, Dong Xiao
Chin. Phys. Lett. 2016, 33 (05): 058801 doi: 10.1088/0256-307X/33/5/058801
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Ce$^{3+}$-Yb$^{3+}$ doped Y$_{3}$A$_{l5}$O$_{12}$ (YAG) is a luminescent down-conversion material which could convert visible photons to near infrared photons. In this work, YAG:Ce$^{3+}$-Yb$^{3+}$ is applied on the front surface of mass-produced mono crystalline Si solar cells. For the coated cells, the external quantum efficiency from the visible to the near infrared is improved, and the energy conversion efficiency enhances from 11.70% to 12.2% under AM1.5G. Furthermore, the phosphor down-conversion effect on the solar cell is characterized by the microwave detected photoconductivity technique on the n-type silicon wafer under the 977 nm excitation. The down-conversion materials improve the average excess carrier lifetime from 22.5 µs to 24.2 µs and the average surface recombination velocity reduces from 424.5 cm/s to 371.6 cm/s, which reveal the significant reduction in excess carrier recombination by the phosphors.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS

Dynamical Study of a Constant Viscous Dark Energy Model in Classical and Loop Quantum Cosmology

Sara Benchikh, Noureddine Mebarki, Dalel Aberkane
Chin. Phys. Lett. 2016, 33 (05): 059501 doi: 10.1088/0256-307X/33/5/059501
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Dynamical behaviors and stability properties of a flat space Friedmann–Robertson–Walker universe filled with pressureless dark matter and viscous dark energy are studied in the context of standard classical and loop quantum cosmology. Assuming that the dark energy has a constant bulk viscosity, it is found that the bulk viscosity effects influence only the quintessence model case leading to the existence of a viscous late time attractor solution of de-Sitter type, whereas the quantum geometry effects influence the phantom model case where the big rip singularity is removed. Moreover, our results of the Hubble parameter as a function of the redshift are in good agreement with the more recent data.

Test of the Cosmic Transparency with the Baryon Acoustic Oscillation and Type Ia Supernova Data

He-Kun Li, Pu-Xun Wu, Hong-Wei Yu
Chin. Phys. Lett. 2016, 33 (05): 059801 doi: 10.1088/0256-307X/33/5/059801
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We use the latest baryon acoustic oscillation and Union 2.1 type Ia supernova data to test the cosmic opacity between different redshift regions without assuming any cosmological models. It is found that the universe may be opaque between the redshift regions 0.35–0.44, 0.44–0.57 and 0.6–0.73 since the best fit values of cosmic opacity in these regions are positive, while a transparent universe is favored in the redshift region 0.57–0.63. However, in general, a transparent universe is still consistent with observations at the $1\sigma$ confidence level.
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