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Chin. Phys. Lett.  
  Chin. Phys. Lett.--2015, 32 (4)   Published: 23 March 2015
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GENERAL

Bursting Behavior in the Piece-Wise Linear Planar Neuron Model with Periodic Stimulation

JI Ying, WANG Ya-Wei
Chin. Phys. Lett. 2015, 32 (4): 040201 doi: 10.1088/0256-307X/32/4/040201
Full Text: [PDF 1596 KB] (85)
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A piece-wise linear planar neuron model with periodic stimulation which can mimic the behavior of bursting is explored. The periodic bursting with three frequencies can be observed in numerical simulation. We present an analysis of bursting in this non-smooth non-autonomous system by considering the system as a generalized autonomous system and introduce an appropriate form of the associated generalized equilibrium solution. The bifurcation mechanism of bursting as well as the coexistence of three frequencies is investigated in detail.

Realization of the Infinite-Dimensional 3-Algebras in the Calogero–Moser Model

YANG Yan-Xin, YAO Shao-Kui, ZHANG Chun-Hong, ZHAO Wei-Zhong
Chin. Phys. Lett. 2015, 32 (4): 040202 doi: 10.1088/0256-307X/32/4/040202
Full Text: [PDF 447 KB] (35)
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We investigate realization of the infinite-dimensional 3-algebras in the classical Calogero–Moser model. In terms of the Lax matrix of the Calogero–Moser model and the Nambu 3-brackets in which the variables are the coordinates qi, and canonically conjugate momenta pi and the coupling parameter β are an extra auxiliary phase-space parameter, we present the realization of the Virasoro–Witt, w and SDiff(T2) 3-algebras, respectively.

The Noncommutative Landau Problem in Podolsky's Generalized Electrodynamics

DIAO Xin-Feng, LONG Chao-Yun, KONG Bo, LONG Zheng-Wen
Chin. Phys. Lett. 2015, 32 (4): 040301 doi: 10.1088/0256-307X/32/4/040301
Full Text: [PDF 455 KB] (15)
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The Landau problem in Podolsky's generalized electrodynamics is studied by the method of diagonalization in noncommutative phase space and we find that the different noncommutative effects for a certain system led by the nonuniqueness of generalized Bopp shift can be avoided. The exact energy eigenvalues are found, and the result shows that the energy spectra are generically non-degenerate. Furthermore, we obtain the special energy spectra of noncommutative space and commutative space.

Dynamics of Open Systems with Affine Maps

ZHANG Da-Jian, LIU Chong-Long, TONG Dian-Min
Chin. Phys. Lett. 2015, 32 (4): 040302 doi: 10.1088/0256-307X/32/4/040302
Full Text: [PDF 439 KB] (27)
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Many quantum systems of interest are initially correlated with their environments and the reduced dynamics of open systems are an interesting while challenging topic. Affine maps, as an extension of completely positive maps, are a useful tool to describe the reduced dynamics of open systems with initial correlations. However, it is unclear what kind of initial state shares an affine map. In this study, we give a sufficient condition of initial states, in which the reduced dynamics can always be described by an affine map. Our result shows that if the initial states of the combined system constitute a convex set, and if the correspondence between the initial states of the open system and those of the combined system, defined by taking the partial trace, is a bijection, then the reduced dynamics of the open system can be described by an affine map.

Entanglement-Enhanced Two-Photon Delocalization in a Coupled-Cavity Array

TANG Shi-Qing, YUAN Ji-Bing, WANG Xin-Wen, KUANG Le-Man
Chin. Phys. Lett. 2015, 32 (4): 040303 doi: 10.1088/0256-307X/32/4/040303
Full Text: [PDF 674 KB] (30)
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We study the transport properties of two entangled photons which are initially injected into two nearest-neighbor coupling cavities in an one-dimensional coupled-cavity array (CCA). It is found that photonic transport dynamics in the two-photon CCA exhibits the entanglement-enhanced two-photon delocalization phenomenon. It is shown that the CCA can realize the localization-to-delocalization transition for two entangled photons.

Propagation and Interaction of Edge Dislocation (Kink) in the Square Lattice

JIA Li-Ping, Jasmina Tekić, DUAN Wen-Shan
Chin. Phys. Lett. 2015, 32 (4): 040501 doi: 10.1088/0256-307X/32/4/040501
Full Text: [PDF 1861 KB] (67)
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The propagation of kink or edge dislocations in the underdamped generalized two-dimensional Frenkel–Kontorova model with harmonic interaction is studied with numerical simulations. The obtained results show that exactly one line of atoms can be inserted into the lattice, which remains at standstill. However, if more than one line of atoms are inserted into the lattice, then they will split into several lines with α=1, where α presents the atoms inserted. In other words, only the kink with α=1 is stable, while the other kinks are unstable, and will split into α=1 kinks, which remain at standstill.

Robust Synchronization in an E/I Network with Medium Synaptic Delay and High Level of Heterogeneity

HAN Fang, WANG Zhi-Jie, FAN Hong, GONG Tao
Chin. Phys. Lett. 2015, 32 (4): 040502 doi: 10.1088/0256-307X/32/4/040502
Full Text: [PDF 680 KB] (15)
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It is known that both excitatory and inhibitory neuronal networks can achieve robust synchronization only under certain conditions, such as long synaptic delay or low level of heterogeneity. In this work, robust synchronization can be found in an excitatory/inhibitory (E/I) neuronal network with medium synaptic delay and high level of heterogeneity, which often occurs in real neuronal networks. Two effects of post-synaptic potentials (PSP) to network synchronization are presented, and the synaptic contribution of excitatory and inhibitory neurons to robust synchronization in this E/I network is investigated. It is found that both excitatory and inhibitory neurons may contribute to robust synchronization in E/I networks, especially the excitatory PSP has a more positive effect on synchronization in E/I networks than that in excitatory networks. This may explain the strong robustness of synchronization in E/I neuronal networks.
NUCLEAR PHYSICS

Residual Nuclides Induced in Cu Target by a 250 MeV Proton Beam

ZHANG Hong-Bin, ZHANG Xue-Ying, MA Fei, JU Yong-Qin, GE Hong-Lin, CHEN Liang, ZHANG Yan-Bin, WEI Ji-Fang, LI Yan-Yan, LUO Peng, WANG Jian-Guo, WAN Bo, XU Xiao-Wei, ZHOU Bin
Chin. Phys. Lett. 2015, 32 (4): 042501 doi: 10.1088/0256-307X/32/4/042501
Full Text: [PDF 704 KB] (15)
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Residual nuclide production is studied experimentally by bombarding a Cu target with a 250 MeV proton beam. The data are measured by the off-line γ-spectroscopy method. Six nuclides are identified and their cross sections are determined. The corresponding calculated results by the MCNPX and GEANT4 codes are compared with the experimental data to check the validity of the codes. A comparison shows that the MCNPX simulation has a better agreement with the experiment. The energy dependence of residual nuclide production is studied with the aid of MCNPX simulation, and it is found that the mass yields for the nuclides in the light mass region increase significantly with the proton energy.
ATOMIC AND MOLECULAR PHYSICS

Laser-Induced Graphite Plasma Kinetic Spectroscopy under Different Ambient Pressures

K. Chaudhary, S. Rosalan, M. S. Aziz, M. Bohadoran, J Ali, P. P. Yupapin, N. Bidin, Saktioto
Chin. Phys. Lett. 2015, 32 (4): 043201 doi: 10.1088/0256-307X/32/4/043201
Full Text: [PDF 1050 KB] (8)
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The laser induced plasma dynamics of graphite material are investigated by optical emission spectroscopy. Ablation and excitation of the graphite material is performed by using an 1064 nm Nd:YAG laser in different ambient pressures. Characteristics of graphite spectra as line intensity variations and signal-to-noise ratio are presented with a main focus on the influence of the ambient pressure on the interaction of laser-induced graphite plasma with an ambient environment. Atomic emission lines are utilized to investigate the dynamical behavior of plasma, such as the excitation temperature and electron density, to describe emission differences under different ambient conditions. The excitation temperature and plasma electron density are the primary factors which contribute to the differences among the atomic carbon emission at different ambient pressures. Reactions between the plasma species and ambient gas, and the total molecular number are the main factors influencing molecular carbon emission. The influence of laser energy on the plasma interaction with environment is also investigated to demonstrate the dynamical behavior of carbon species so that it can be utilized to optimize plasma fluctuations.

Generation of Linear Isolated Sub-60 Attosecond Pulses by Combining a Circularly Polarized Pulse with an Elliptically Polarized Pulse

XIA Chang-Long, MIAO Xiang-Yang
Chin. Phys. Lett. 2015, 32 (4): 043202 doi: 10.1088/0256-307X/32/4/043202
Full Text: [PDF 682 KB] (32)
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The two-color circularly polarized pulses scheme was proposed to generate isolated attosecond pulses in our previous work [Phys. Rev. A 87 (2013) 043406], while the polarization of the attosecond pulse was not investigated. We show a supplementary explanation of this scheme and present another scheme to generate linear isolated attosecond pulses by combining a circularly polarized pulse with an elliptically polarized pulse. High-order harmonic generation and quantum path control are investigated to compare these two schemes. Both schemes can obtain supercontinuum spectra plateau from about 200 eV to 550 eV, which belong to the water window region. It is found that the latter scheme can clearly eliminate the short quantum path and extend the harmonic plateau. A linear isolated attosecond pulse with a duration of sub-60 as can be generated by superposing a bandwidth of 70 eV.

Optical Response of CeB6 Nanoparticles with Different Sizes and Shapes from Discrete-Dipole Approximation

CHAO Luo-Meng, BAO Li-Hong, O. Tegus
Chin. Phys. Lett. 2015, 32 (4): 043301 doi: 10.1088/0256-307X/32/4/043301
Full Text: [PDF 500 KB] (25)
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The discrete dipole approximation is used to investigate the optical response of CeB6 nanoparticles with different sizes and different shapes. The extinction valley in the visible light range becomes narrower and the extinction peak at the near infrared region (NIR) is red-shifted with the increasing particle size. In addition, the extinction peak value of the spherical particle decreases more rapidly than that of cubic-shaped particle with an increase in the particle size, and the cubic-shaped particles exhibit better performance on blocking NIR radiation than spherical-shaped particles. The calculation results coincide well with the reported experimental results.
FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

Doppler Spectrum Analysis of Time-Evolving Sea Surface Covered by Oil Spills

YANG Peng-Ju, GUO Li-Xin, JIA Chun-Gang
Chin. Phys. Lett. 2015, 32 (4): 044101 doi: 10.1088/0256-307X/32/4/044101
Full Text: [PDF 713 KB] (11)
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Based on the model of a contaminated sea surface that was proposed by Lombardini et al., the influence of the damping effect of oil films on the sea surface roughness spectrum and the geometrical structure of the sea surface is examined in detail by comparing with a clean sea surface. Furthermore, based on a quasi-stationary algorithm, a time series of backscattered echoes from a time-evolving sea surface covered by oil slicks is obtained by utilizing the frequency-domain numerical method of the parallel fast multiple method. Then, the Doppler spectrum is evaluated by performing a standard spectral estimation technique. Finally, the influence of the oil film damping effect on the Doppler spectrum of the backscattered echoes from time-evolving sea surface is investigated in detail by making a comparison of the Doppler spectrum of an oil-covered sea surface with the Doppler spectrum of a clean sea surface. The numerical simulations show that the damping effect of oil films has an influence on the Doppler spectrum signature for both horizontal-to-horizontal and vertical-to-vertical polarizations.

Microwave Absorption Properties of Polyester Composites Incorporated with Heterostructure Nanofillers with Carbon Nanotubes as Carriers

LIU Hai-Tao, LIU Yang, WANG Bin-Song, LI Chen-Sha
Chin. Phys. Lett. 2015, 32 (4): 044102 doi: 10.1088/0256-307X/32/4/044102
Full Text: [PDF 718 KB] (13)
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Carbonaceous nanomaterials such as carbon nanotubes (CNTs), magnetic metal nanomaterials and semiconductor nanomaterials are superior candidates for microwave absorbers. Taking full advantage of the features of CNTs, nanophase cobalt and nanophase zinc oxide, whose main microwave absorption mechanisms are based on resistance loss, magnetic loss and dielectric loss, we fabricate CNT/Co and CNT/ZnO heterostructure nanocomposites, respectively. By using the CNTs, CNT/Co nanocomposites and CNT/ZnO nanocomposites as nanofillers, composites with polyester as matrix are prepared by in situ polymerization, and their microwave absorption performance is studied. It is indicated that the synergetic effects of the physic properties of different components in nano-heterostructures result in greatly enhanced microwave absorption performance in a wide frequency range. The absorption peak is increased, the absorption bandwidth is broadened, and the maximum peak shifts to a lower frequency.

Wavelength-Tunable Single Frequency Ytterbium-Doped Fiber Laser with Loop Mirror Filter

LU Bao-Le, HUANG Sheng-Hong, YIN Mo-Juan, CHEN Hao-Wei, REN Zhao-Yu, BAI Jin-Tao
Chin. Phys. Lett. 2015, 32 (4): 044201 doi: 10.1088/0256-307X/32/4/044201
Full Text: [PDF 583 KB] (22)
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By using a loop mirror filter, a novel wavelength-tunable single-frequency ytterbium-doped fiber laser is developed to select single longitudinal modes in a linear cavity. The output wavelength could be tuned 2.4 nm intervals range from 1063.3 to 1065.7 nm with the temperature change of the fiber Bragg grating. The maximum output power could reach 32 mW while the pump power increases to 120 mW. The corresponding optical-to-optical conversion efficiency is 26.7% and the slope efficiency is 33.9%, respectively. The output power fluctuation is below 2%, and its highest signal-to-noise ratio is 60 dB.

Polarization-Stable 980 nm Vertical-Cavity Surface-Emitting Lasers with Diamond-Shaped Oxide Aperture

WU Hua, LI Chong, HAN Min-Fu, WANG Wen-Juan, SHI Lei, LIU Qiao-Li, LIU Bai, DONG Jian, GUO Xia
Chin. Phys. Lett. 2015, 32 (4): 044202 doi: 10.1088/0256-307X/32/4/044202
Full Text: [PDF 569 KB] (24)
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Polarization-stable 980 nm oxide-confined vertical-cavity surface-emitting lasers with 3 m diamond-shaped aperture are fabricated by comprehensively utilizing the anisotropic properties of wet etching and wet nitrogen oxidation of III–V semiconductor materials. Polarization-stable operation along the major axis of the diamond-shaped oxide aperture with 11 dB orthogonal polarization suppression ratio is achieved in a temperature range of 15–55°C from the threshold to 4 mA.

Preliminary Study on Underwater Ambient Noise Generated by Typhoons

WANG Jing-Yan, LI Feng-Hua
Chin. Phys. Lett. 2015, 32 (4): 044301 doi: 10.1088/0256-307X/32/4/044301
Full Text: [PDF 721 KB] (7)
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We present a typhoon-generated noise model with which the noise intensity during typhoons can be estimated accurately. The model is verified through experimental study, and the simulation results agree reasonably with the experimental data. The measured noise intensity is approximately proportional to the cube of the local wind speed.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

On the Gradient of the Electron Pressure in Anti-Parallel Magnetic Reconnection

WANG Huan-Yu, HUANG Can, LU Quan-Ming, WANG Shui
Chin. Phys. Lett. 2015, 32 (4): 045201 doi: 10.1088/0256-307X/32/4/045201
Full Text: [PDF 1490 KB] (16)
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We first perform a two-dimensional particle-in-cell simulation of anti-parallel magnetic reconnection to verify that in the electron diffusion region the reconnection electric field is mainly balanced by the gradient of the electron pressure. Then, by following typical electron trajectories in the fixed electromagnetic field of anti-parallel reconnection, we calculate the gradient of the electron pressure. We find that the resulted gradient of the electron pressure is equal to the reconnection electric field. This indicates that in the electron diffusion region the reconnection electric field is balanced by the gradient of the electron pressure, which results from the electron nongyrotropic motions. Our result gives a microphysical explanation of the balance between the reconnection electric field and the gradient of the electron pressure.

Reduction of Reactive-Ion Etching-Induced Ge Surface Roughness by SF6/CF4 Cyclic Etching for Ge Fin Fabrication

MA Xue-Zhi, ZHANG Rui, SUN Jia-Bao, SHI Yi, ZHAO Yi
Chin. Phys. Lett. 2015, 32 (4): 045202 doi: 10.1088/0256-307X/32/4/045202
Full Text: [PDF 839 KB] (12)
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An SF6/CF4 cyclic reactive-ion etching (RIE) method is proposed to suppress the surface roughness and to optimize the morphology of Ge fin, aiming at the fabrication of superior Ge FinFETs for future CMOS technologies. The surface roughness of the Ge after RIE can be sufficiently reduced by introducing SF6-O2 etching steps into the CF4-O2 etching process, while maintaining a relatively large ratio of vertical etching over horizontal etching of the Ge. As a result, an optimized rms roughness of 0.9 nm is achieved for Ge surfaces after the SF6/CF4 cyclic etching with a ratio of greater than four for vertical etching over horizontal etching of the Ge, by using a proportion of 60% for SF6-O2 etching steps.
CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES

Acoustic Response and Micro-Damage Mechanism of Fiber Composite Materials under Mode-II Delamination

ZHOU Wei, LV Zhi-Hui, WANG Ya-Rui, LIU Ran, CHEN Wei-Ye, LI Xiao-Tong
Chin. Phys. Lett. 2015, 32 (4): 046201 doi: 10.1088/0256-307X/32/4/046201
Full Text: [PDF 879 KB] (7)
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Realizing the accurate characterization for the dynamic damage process is a great challenge. Here we carry out testing simultaneously for dynamic monitoring and acoustic emission (AE) statistical analysis towards fiber composites under mode-II delamination damage. The load curve, AE relative energy, amplitude distribution, and amplitude spectrum are obtained and the delamination damage mechanism of the composites is investigated by the microscopic observation of a fractured specimen. The results show that the micro-damage accumulation around the crack tip region has a great effect on the evolutionary process of delamination. AE characteristics and amplitude spectrum represent the damage and the physical mechanism originating from the hierarchical microstructure. Our finding provides a novel and feasible strategy to simultaneously evaluate the dynamic response and micro-damage mechanism for fiber composites.

Controllable Nucleation of Nanobubbles at a Modified Graphene Surface

MA Wang-Guo, ZHANG Meng, NIE Xue-Chuan, WANG Chun-Lei, FANG Hai-Ping, HE Meng-Dong, ZHANG Li-Juan
Chin. Phys. Lett. 2015, 32 (4): 046801 doi: 10.1088/0256-307X/32/4/046801
Full Text: [PDF 1137 KB] (52)
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The properties of nanoscale gas bubbles at the solid/water interface have been investigated for more than 20 years. However, the stability of nanobubbles remains far from being understood. How to control the formation of nanobubbles is the key issue for understanding their long lifetime. In this work, using molecular dynamics simulations we modify the substrate (graphene) with charge dipoles in which the local properties of the surface could be changed. Nanobubbles could be stabilized on the local hydrophobic area and modified area with the hydrophilic boundary where gas nuclei are deposited beforehand. Those results provide two methods to control the nucleation of gas nanobubbles and fix them on a target area.

Low-Temperature Deposition of nc-SiOx:H below 400°C Using Magnetron Sputtering

LI Yun, YIN Chen-Chen, JI Yun, SHI Zhen-Liang, JIN Cong-Hui, YU Wei, LI Xiao-Wei
Chin. Phys. Lett. 2015, 32 (4): 046802 doi: 10.1088/0256-307X/32/4/046802
Full Text: [PDF 1377 KB] (14)
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Silicon oxide films containing nanocrystalline silicon (nc-SiOx:H) are deposited by co-sputtering technology at low temperatures (<400°C) that are much lower than the typical growth temperature of nc-Si in SiO2. The microstructures and bonding properties are characterized by Raman and FTIR. It is proven that an optimum range of substrate temperatures for the deposition of nc-SiOx:H films is 200–400°C, in which the ratio of transition crystalline silicon decreases, the crystalline fraction is higher, and the hydrogen content is lower. The underlying mechanism is explained by a competitive process between nc-Si Wolmer–Weber growth and oxidation reaction, both of which achieve a balance in the range of 200–400°C. We further implement this technique in the fabrication of multilayered nc-SiOx:H/a-SiOx:H films, which exhibit controllable nc-Si sizes with high crystallization quality.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

Electron Correlation Effects in Polaron-Pair Recombination in Conjugated Polymers

ZHAO Hong-Xia, ZHAO Hui, CHEN Yu-Guang, YAN Yong-Hong
Chin. Phys. Lett. 2015, 32 (4): 047201 doi: 10.1088/0256-307X/32/4/047201
Full Text: [PDF 1038 KB] (12)
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Within a Su–Schriffer–Heeger model modified to include electron-electron interaction and an external electric field, we investigate the dynamics of oppositely charged polarons in a polymer chain in the presence of both electron-phonon and electron-electron interactions under the influence of an external electric field. We adopt a multi-configurational time-dependent Hartree–Fock method for the time-dependent Schrödinger equation and the Newtonian equation of motion for a lattice. Our results show that the on-site Coulomb interaction is of fundamental importance and favors the recombination between the pairs of polarons, and the yield of excitons depends crucially on the strength of the on-site Coulomb interaction U. Furthermore, the influence of the nearest neighbor interaction V is also discussed.

Potential Barrier Behavior of BaTiO3–(Bi0.5Na0.5)TiO3 Positive Temperature Coefficient of Resistivity Ceramic

LENG Sen-Lin, SHI Wei, LI Guo-Rong, ZHENG Liao-Ying
Chin. Phys. Lett. 2015, 32 (4): 047202 doi: 10.1088/0256-307X/32/4/047202
Full Text: [PDF 743 KB] (8)
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High-Curie-temperature (Tc) lead-free Y-doped 90 mol%BaTiO3–10 mol%(Bi0.5Na0.5)TiO3 ceramic with positive temperature coefficient of resistivity (PTCR) is prepared by the conventional solid state reaction in nitrogen atmosphere. The PTCR ceramic exhibits a room-temperature resistivity (ρ25) of ∼500 Ω⋅cm and a high PTCR effect (maximum resistivity (ρmax)/minimum resistivity (ρmin)) of ∼4.5 orders of magnitude. A capacitance-voltage approach is first employed to calculate the potential barrier (ϕ) of the grain boundary of PTCR ceramic above Tc. It is found that the potential barrier changes from 0.17 to 0.77 eV as the temperature increases from 180 to 220°C, which is very close to the predictions of the Heywang–Jonker model, suggesting that the capacitance-voltage method is valid to estimate the potential barrier of PTCR thermistor ceramics.

Fabrication and Characterization of a Single Electron Transistor Based on a Silicon-on-Insulator

SU Li-Na, LV Li, LI Xin-Xing, QIN Hua, GU Xiao-Feng
Chin. Phys. Lett. 2015, 32 (4): 047301 doi: 10.1088/0256-307X/32/4/047301
Full Text: [PDF 757 KB] (20)
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A single electron transistor based on a silicon-on-insulator is successfully fabricated with electron-beam nanolithography, inductively coupled plasma etching, thermal oxidation and other techniques. The unique design of the pattern inversion is used, and the pattern is transferred to be negative in the electron-beam lithography step. The oxidation process is used to form the silicon oxide tunneling barriers, and to further reduce the effective size of the quantum dot. Combinations of these methods offer advantages of good size controllability and accuracy, high reproducibility, low cost, large-area contacts, allowing batch fabrication of single electron transistors and good integration with a radio-frequency tank circuit. The fabricated single electron transistor with a quantum dot about 50 nm in diameter is demonstrated to operate at temperatures up to 70 K. The charging energy of the Coulomb island is about 12.5 meV.

Frequency Performance of Ring Oscillators Based on a-IGZO Thin-Film Transistors

YU Guang, WU Chen-Fei, LU Hai, REN Fang-Fang, ZHANG Rong, ZHENG You-Dou, HUANG Xiao-Ming
Chin. Phys. Lett. 2015, 32 (4): 047302 doi: 10.1088/0256-307X/32/4/047302
Full Text: [PDF 607 KB] (14)
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Ring oscillators based on indium gallium zinc oxide thin film transistors are fabricated on glass substrates. The oscillator circuit consists of seven delay stages and an output buffer inverter. The element inverter exhibits a voltage gain higher than −6 V/V and a wide output swing close to 85% of the full swing range. The dynamic performance of the ring oscillators is evaluated as a function of supply voltage and at different gate lengths. A maximum oscillation frequency of 0.88 MHz is obtained for a supply voltage of 50 V, corresponding to a propagation delay of less than 85 ns/stage.

Single- and Few-Electron States in Deformed Topological Insulator Quantum Dots

LI Jian, ZHANG Dong
Chin. Phys. Lett. 2015, 32 (4): 047303 doi: 10.1088/0256-307X/32/4/047303
Full Text: [PDF 1020 KB] (24)
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We theoretically investigate the single- and few-electron states in deformed HgTe quantum dots (QDs) with an inverted band structure using the full configuration interaction method. For the circular and deformed QD, it is found that the energy of edge states is robust against the shape from the circular QD in various elliptic ones. For the few electron states, electrons will firstly fill the edge states localized at the short axis, then the states localized at the long axis of the QD before filling the bulk states. The filling of the edge states can be controlled by tuning the dot size or the deformation of the geometry of the HgTe QD, respectively.

Thermoelectric Transport by Surface States in Bi2Se3-Based Topological Insulator Thin Films

LI Long-Long, XU Wen
Chin. Phys. Lett. 2015, 32 (4): 047304 doi: 10.1088/0256-307X/32/4/047304
Full Text: [PDF 530 KB] (37)
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We develop a tractable theoretical model to investigate the thermoelectric (TE) transport properties of surface states in topological insulator thin films (TITFs) of Bi2Se3 at room temperature. The hybridization between top and bottom surface states in the TITF plays a significant role. With the increasing hybridization-induced surface gap, the electrical conductivity and electron thermal conductivity decrease while the Seebeck coefficient increases. This is due to the metal-semiconductor transition induced by the surface-state hybridization. Based on these TE transport coefficients, the TE figure-of-merit ZT is evaluated. It is shown that ZT can be greatly improved by the surface-state hybridization. Our theoretical results are pertinent to the exploration of the TE transport properties of surface states in TITFs and to the potential application of Bi2Se3-based TITFs as high-performance TE materials and devices.

Influence of Temperature on the Conductivity of Multi-walled Carbon Nanotube Interconnects

LU Qi-Jun, ZHU Zhang-Ming, YANG Yin-Tang, DING Rui-Xue
Chin. Phys. Lett. 2015, 32 (4): 047305 doi: 10.1088/0256-307X/32/4/047305
Full Text: [PDF 574 KB] (30)
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We propose a new conductivity model of multi-walled carbon nanotube (MWCNT) interconnects considering the influence of temperature. For each shell of MWCNT interconnects, it may present the property of ballistic transport or may suffer from acoustic photo and optical phonon (OP) scattering depending on their mean free paths (MFPs) and the wire length. Furthermore, since the MFPs are proportional to the shell diameter, five regions exist in the wire length in which the factors influencing the conductivity are determined. Thus the conductivity is modeled in five cases according to their lengths, and the final obtained model is a 5-piecewise function. By using this model, the influence of temperature on the conductivity is examined and analyzed. It is shown that the conductivity demonstrates different, changing behaviors with the increase of temperature in the five cases. Additionally, the influence of OP scattering on the conductivity does not need to be taken into account at room temperatures, whereas this influence will produce a decline region in the conductivity at high temperatures.

Universality of a Critical Magnetic Field in a Holographic Superconductor

D. Momeni, R. Myrzakulov
Chin. Phys. Lett. 2015, 32 (4): 047401 doi: 10.1088/0256-307X/32/4/047401
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We study aspects of holographic superconductors analytically in the presence of a constant external magnetic field. It is shown that the critical temperature and the critical magnetic field can be calculated at nonzero temperature. We detect the Meissner effect in such superconductors. A universal relation between black hole mass M and critical magnetic field Hc is proposed to be Hc/M2/3≤0.687365. We also discuss some aspects of phase transition in terms of black hole entropy and Bekenstein's entropy to the energy upper bound.

Dependence of Switching Current Distribution of a Current-Biased Josephson Junction on Microwave Frequency

ZHAI Ji-Quan, LI Yong-Chao, SHI Jian-Xin, ZHOU Yu, LI Xiao-Hu, XU Wei-Wei, SUN Guo-Zhu, WU Pei-Heng
Chin. Phys. Lett. 2015, 32 (4): 047402 doi: 10.1088/0256-307X/32/4/047402
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We investigate the distribution of the switching current of a current-biased Josephson junction (CBJJ) and its dependence on the microwave frequency using two theoretical methods, one of which is the quantum trajectory method and the other is the master equation method. Both the methods show that the distribution of the switching current of CBJJ will exhibit double peaks in a certain range of microwave frequency if proper microwave power is given, and the gap between the two peaks will increase with the microwave frequency. The obtained results can be used to identify the energy difference of the ground and first excited states in a Josephson junction for any bias current.

Degradation Mechanism of the Superconducting Transition Temperature in Nb Thin Films

SONG Xiao-Hui, JIN Yi-Rong, FAN Zhen-Jun, MI Zhen-Yu, ZHANG Dian-Lin
Chin. Phys. Lett. 2015, 32 (4): 047403 doi: 10.1088/0256-307X/32/4/047403
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Systemic measurements show that there is no 3D to 2D crossover in the reduction of the superconducting transition temperature Tc in Nb thin films. This result is consistent with all previous measurements while it is contrary to the prevailing understanding based on the interplay between proximity, localization, and lifetime broadening. Our study indicates that the decrease of Tc can be interpreted by the combined effects of electron-phonon coupling parameter λ and the defect scattering rate ρw, being uniquely determined by their ratio λ/ρw. Other factors such as film thickness and irradiation do not produce additional effects beyond these two parameters.

Doping Effect of Co at Ag Sites in Antiperovskite Mn3AgN Compounds

CHU Li-Hua, WANG Cong, SUN Ying, LI Mei-Cheng, WAN Zi-Pei, WANG Yu, DOU Shang-Yi, CHU Yue
Chin. Phys. Lett. 2015, 32 (4): 047501 doi: 10.1088/0256-307X/32/4/047501
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Antiperovskite compounds Mn3Ag1−xCoxN (x=0.2, 0.5 and 0.8) are synthesized and the doping effect of the magnetic element Co at the Ag site is investigated. The crystal structure is not changed by the introduction of Co. However, with the increase of the content of Co, the spin reorientation gradually disappears and the antiferromagnetic transition changes to the ferromagnetic transition at the elevated temperature when x=0.8. In addition, all of the magnetic phase transitions at the elevated temperature are always accompanied by the abnormal thermal expansion behaviors and an entropy change. Moreover, when x=0.8, the coefficient of linear expansion is −1.89×10−6 K−1 (290–310 K, ΔT=20 K), which is generally considered as the low thermal expansion.

The Magnetic Anisotropy and Complete Phase Diagram of CuFeO2 Measured in a Pulsed High Magnetic Field up to 75 T

ZUO Hua-Kun, SHI Li-Ran, XIA Zheng-Cai, HUANG Jun-Wei, CHEN Bo-Rong, JIN Zhao, WEI Meng, OUYANG Zhong-Wen, CHENG Gang
Chin. Phys. Lett. 2015, 32 (4): 047502 doi: 10.1088/0256-307X/32/4/047502
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The magnetization behavior of a CuFeO2 single crystal grown by the floating zone technique is investigated with a pulsed high magnetic field. We observe a series of field-induced multi-step-like transitions with hysteresis, of which the critical magnetic fields are temperature-dependent and show anisotropy. By using a pulsed high magnetic field up to 75 T, the magnetization behavior shows that the critical transition magnetic fields of spin-flip/flop shift to lower field regions with an increase in temperature. According to the magnetization curves, a complete magnetic phase diagram is depicted.

Temperature Dependence of Raman Scattering in 4H-SiC Films under Different Growth Conditions

WANG Hong-Chao, HE Yi-Ting, SUN Hua-Yang, QIU Zhi-Ren, XIE Deng, MEI Ting, Tin C. C., FENG Zhe-Chuan
Chin. Phys. Lett. 2015, 32 (4): 047801 doi: 10.1088/0256-307X/32/4/047801
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The microRaman scattering of 4H-SiC films, fabricated by low pressure chemical vapor deposition under different growth conditions, is investigated at temperatures ranging from 80 K to 550 K. The effects of growth conditions on E2(TO), E1(TO) and A1(LO) phonon mode frequencies are negligible. The temperature dependences of phonon linewidth and lifetime of E2(TO) modes are analyzed in terms of an anharmonic damping effect induced by thermal and growth conditions. The results show that the lifetime of E2(TO) mode increases when the quality of the sample improves. Unlike other phone modes, Raman shift of A1 (longitudinal optical plasma coupling (LOPC)) mode does not decrease monotonously when the temperature increases, but tends to blueshift at low temperatures and to redshift at relatively high temperatures. Theoretical analyses are given for the abnormal phenomena of A1(LOPC) mode in 4H-SiC.
CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

Two-Photon Fluorescence Properties and Ultrafast Responses of a Hyperbranched Diketo-Pyrrolo-Pyrrole Polymer with Triphenylamine as the Core

WANG Yao-Chuan, JIANG Yi-Hua, WANG Gui-Qiu, LIU Da-Jun, LI Bo, HUA Jian-Li
Chin. Phys. Lett. 2015, 32 (4): 048201 doi: 10.1088/0256-307X/32/4/048201
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The two-photon fluorescence properties and ultrafast responses of a hyperbranched polyyne (hb-DPP-J2) with triphenylamine as the central core, Diketo-Pyrrolo-Pyrrole as the connecting unit and electron acceptor are studied. The polymer has a D-π-A-π-D conjugated structure along the extended polyyne π-bridge systems, and the effective conjugated unit repeats itself in the whole hyperbranched polymer chain. The polymer exhibits a large two-photon absorption cross section and high fluorescence quantum yields. The ultrafast dynamic results give a deep understanding of the excited energy transfer processes under excitation, and reveal a long relaxation lifetime of the intramolecular charge transfer (ICT) state.

Numerical Investigation on the Propagation Mechanism of Steady Cellular Detonations in Curved Channels

LI Jian, NING Jian-Guo, ZHAO Hui, HAO Li, WANG Cheng
Chin. Phys. Lett. 2015, 32 (4): 048202 doi: 10.1088/0256-307X/32/4/048202
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The propagation mechanism of steady cellular detonations in curved channels is investigated numerically with a detailed chemical reaction mechanism. The numerical results demonstrate that as the radius of the curvature decreases, detonation fails near the inner wall due to the strong expansion effect. As the radius of the curvature increases, the detonation front near the inner wall can sustain an underdriven detonation. In the case where detonation fails, a transverse detonation downstream forms and re-initiates the quenched detonation as it propagates toward the inner wall. Two kinds of propagation modes exist as the detonation is propagating in the curved channel. One is that the detonation fails first, and then a following transverse detonation initiates the quenched detonation and this process repeats itself. The other one is that without detonation failure and re-initiation, a steady detonation exists which consists of an underdriven detonation front near the inner wall subject to the diffraction and an overdriven detonation near the outer wall subject to the compression.

Analysis of Capacitance-Voltage-Temperature Characteristics of GaN High-Electron-Mobility Transistors

ZHAO Miao, LIU Xin-Yu
Chin. Phys. Lett. 2015, 32 (4): 048501 doi: 10.1088/0256-307X/32/4/048501
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The capacitance-voltage characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTS) are measured in the temperature range of 223–398 K. The dependence of capacitance on frequency at various temperatures is analyzed. At lower temperatures, the capacitance decreases only very slightly with frequency. At higher frequencies the curves for all temperatures tend to one capacitance value. Such behavior can be attributed to the interface states or the dislocations.

Spectroscopic Characterization of Staphylococcal Nuclease Mutants with Tryptophan at Internal Sites

GAO Guang-Yu, LI Yu, WANG Wei, ZHONG Dong-Ping, WANG Shu-Feng, GONG Qi-Huang
Chin. Phys. Lett. 2015, 32 (4): 048701 doi: 10.1088/0256-307X/32/4/048701
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Tryptophan (Trp) is an intrinsic fluorescent probe for detecting the site-specified dynamics inside/outside protein. It is found that the Trp can easily be inserted in desired sites of protein, which affects the integrity of the overall structure. To evaluate this effect, we design thirteen double point mutants of staphylococcal nuclease, each of which has a single Trp residue planted at an internal site. The studies on Trp fluorescence, ANS-binding fluorescence, far- and near-UV CD spectra, and enzymatic activity are carried out. It is found that the mutation at the hydrophobic core of protein generates molten globular state conformation, which is a loose structure compared to their original compactness in wild type (WT). Its enzyme activity and surface hydrophobicity are also affected. The studies show that by proper site designing and external binding, Trp mutagenesis is a suitable method for carrying out the study on site specified dynamics of proteins.
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