Symmetry of Lagrangians of nonholonomic controllable mechanical systems is studied. The definition and criterion of the symmetry of the system are presented. Under the condition that there exists a conserved quantity, the form of the conserved quantity is provided. An example is presented to illustrate the application of the results.

A field integration method for a weakly nonholonomic system is studied. The differential equations of motion of the system are established. The approximate solution of the holonomic system corresponding to the weakly nonholonomic system is obtained by using the field method. The restriction of nonholonomic constraint to initial conditions is added and the approximate solution of the weakly nonholonomic system is obtained. An example is given to demonstrate the application of the result.

An analytical expression of the Loschmidt echo in the Lipkin-Meshkov-Glick model is derived in the thermodynamical limit. It is used in the study of the decaying behaviour of the echo at the critical point of a quantum phase transition of the model. It is shown that the echo has a power law decay for relatively long times.

We present a quantum model of Bertrand duopoly and study the entanglement behavior on the profit functions of the firms. Using the concept of optimal response of each firm to the price of the opponent, we find only one Nash equilibirum point for the maximally entangled initial state. The presence of quantum entanglement in the initial state gives payoffs higher to the firms than the classical payoffs at the Nash equilibrium. As a result, the dilemma-like situation in the classical game is resolved.

We consider the problem often encountered in constructing large cluster states, that is, the δ_{x} measurements in the depolarizing noise, and present a regulation to calculate the error propagation and the accumulation. Rohde et al. first pointed out the question A quantitative analysis is given on the disadvantage influence of the probabilistic gate and noisy environment on the construction of two-dimensional (2D) scalable cluster states. Positivity of partial transpose criteria is used to predict whether the final state is distillable or not. A critical value on the error tolerance in constructing cluster states is determined, which could be used as a test-bed for successful construction of scalable 2D cluster states. Furthermore, our quantitative analysis would also be helpful to find optimal approach to suppress the noise accumulation.

We present two methods to solve the equation of motion of the cavityless optomechanical system and obtain an explicit formula of the covariance matrix of the evolved state when the initial state is Gaussian. We study the entanglement dynamics of this system initially in the vacuum state. It is shown that bipartite entanglement behaves in a periodic manner. In particular, we can easily generate a genuine three-mode continuous-variable entanglement.

The ground state energy curves of one-dimensional δ-function interacting bose and fermi gas are fitted with simple algebraic approximations according to their asymptotic behaviors.

Effect of delayed time in the logistic growth model subject to weak periodic signal and correlated multiplicative and additive white noise is investigated. Using small time delay approximation, we obtain the expression of the signal-to-noise ratio (SNR). It is found that the SNR is non-monotonic functions of the delayed times, the system parameters, the intensities of the multiplicative and additive noise, as well as the correlation strength of the two noises.

We demonstrate that Fokker-Planck equations with logarithmic factors in diffusion and drift terms can be straightforwardly derived from the class of "constant elasticity of variance" stochastic processes without appealing to any symmetry argument. Analytical closed-form solutions are available for some special cases of this class of Fokker-Planck equations. The dynamics of the underlying stochastic variables are examined. These Fokker-Planck equations have found a rather wide range of applications in various contexts. In particular, in the field of econophysics we have demonstrated their immediate relevance to modelling the exchange rate dynamics in a target zone, e.g. the linked exchange rate system of the Hong Kong dollar. Furthermore, the knowledge of exact solutions in some special cases can be useful as a benchmark to test approximate numerical or analytical procedures.

The free energy and the density profile of a hard disk liquid system under gravity are calculated by using the dimensional crossover of Rosenfeld hard sphere (3D) functional as well as the functional constructed from the scaled-particle theory which is considered to be very accurate. The two methods give the consistent results for a wide range of packing fractions.

The structural stability and elastic properties of wurtzite thallium nitride (TlN) under hydrostatic pressure are studied for the first time by first-principles calculations. The enthalpy calculations predict that TlN undergoes a phase transition from the wurtzite structure to the rocksalt structure at 19.2 GPa with a volume collapse of 13.0%. Our calculated results also show that this nitride is ductile in nature and exhibits high elastic anisotropy. Our ground-state results are in good agreement with the data of other theoretical calculations.

We study an N-dimensional system based on a sine square map and analyze the system behaviors of cases of dimension N≥3 with the tools of nonlinear dynamics. In the three-dimensional case, bifurcations in the parameter plane, invariant manifolds, critical manifolds and chaotic attractors are studied. Then we extend this study to the cases of higher dimension (N>3) to understand generalized properties of the system. The analysis and experimental results of the system demonstrate the existence of bounded chaotic orbits, which can be considered for secure transmissions.

We study the charged top-pion in the topcolor assisted technicolor model(TC2), and calculate the production of charged top-pion at e^{+}e^{-} and γγ colliders. At an e^{+}e^{-} collider, charged top-pion can be produced via the processes e^{+}e^{-} →tbπ_{t}^{-}(tbπt^{+}).At a γγ collider, it can be produced via the processes γγ → tbπ_{t}^{-}(tbπt^{+}). The cross section can reach to a few of fb depending on the mass of the charged top-pion.

The production of C, N, O elements in a standard big bang nucleosynthesis scenario is investigated. Using the up-to-date data of nuclear reactions in BBN, in particular the ^{8}Li (n,γ) ^{9}Li which has been measured in China Institute of Atomic Energy, a full nucleosynthesis network calculation of BBN is carried out. Our calculation results show that the abundance of ^{12}C is increased for an order of magnitude after addition of the reaction chain ^{8}Li(n,γ) ^{9}Li(α,n) ^{12}B(β) ^{12}C, which was neglected in previous studies. We find that this sequence provides the main channel to convert the light elements into C, N, O in standard BBN.

WANG Hai-Xia, ZHANG Yu-Hu, DING-Bing, ZHOU Xiao-Hong, ZHANG Ning-Tao, LIU Min-Liang, MA Ying-Jun, Y. Sasakiz, K. Yamada, H. Ohshima, S. Yokose, M. Ishizuka, T. Komatsubara, K. Furuno

Chin. Phys. Lett. 2010, 27 (8):
082701
.
DOI: 10.1088/0256-307X/27/8/082701

High-spin states in the odd-odd nucleus ^{128}I are investigated via the ^{124}Sn(^{7}Li,3n)^{128}I reaction at 28 and 32 MeV beam energies. A new level scheme of ^{128}I is established up to high-spin states at I^{π}=16, including 48 levels and 72γ transitions. The present level scheme is largely different from the one in a recent publication due to identification of several doublet and triplet γ transitions and their proper placements in the level scheme. The high-spin level structure exhibits no obvious collective properties and is possibly associated with two and multi-quasiparticle configurations.

We propose a scheme for generation of two-atom maximally entangled states and the quantum information transfer between two atoms via two identical atoms in resonation with ultrahigh-Q toroidal microcavities.

We propose a real-space Gutzwiller variational approach and apply it to a system of repulsively interacting ultracold fermions with spin-1/2 trapped in an optical lattice with a harmonic confinement. Using the real-space Gutzwiller variational approach, we find that in a system with balanced spin-mixtures on a square lattice, antiferromagnetism either appears in a checkerboard pattern or forms a ring, and the antiferromagnetic order is stable in the regions where the particle density is close to one, which is consistent with the recent results obtained by the real-space dynamical mean-field theory approach. We also investigate the imbalanced case and find that the antiferromagnetic order is suppressed there.

An atomic fountain with ^{85}Rb cold atoms is reported. A series of time-of-flight signals is obtained, and the measured temperature of the cold atomic cloud is about 2.4μK. It will have potential new applications in the precise measurement of fundamental constants and the proof of the Einstein's equivalence principle.

We investigate multiphoton ionization (MPI) of N_{2} exposed to femtosecond laser fields at 400 nm and 800 nm experimentally. Photoelectron energy spectra are measured with a high resolution photoelectron spectrometer at laser intensities up to 10^{14} W/cm^{2}. Prominent resonant peaks observed at 400 nm can be attributed to the resonance with molecular Rydberg states corresponding to different molecular orbitals, revealing the importance of the multiple orbitals contribution in strong field molecular dynamics.

High resolution lasers are necessary to derive the most information from molecular spectra. However, their use uncovers some photophysical processes that compromise the ability to resolve rotational structure. We study the influence of laser optical mode structure on the high resolution spectra of the S_{1} states of benzonitrile in a supersonic molecular beam using an Ar^{+} pumped cw ring dye laser which is amplified by a pulsed Nd:YAG laser. The latter could be operated either in one optical mode by injecting (seeding) its oscillator with a single mode diode laser, or with many optical modes by not using the seeder. Rotationally resolved lines are obtained when the oscillator of the YAG laser are operated in one single optical mode, but only a continuum is seen when the YAG laser has multiple modes. It is argued that the ac Stark effect is the most probable reason for broadening and blurring the rotational lines.

The vector correlations in the reaction F+H_{2} (v=0-3, j=0-3)→ HF(v',j')+H are investigated using the quasi-classical trajectory method on the Stark-Werner potential energy surface at a collision energy of 1.0 eV. The potential distribution P(θr)to angles between k and j', the distribution P(Ør) to dihedral angles, denoting k-k'-j' correlation and the polarization-dependent generalized differential cross sections, are calculated. The effect of reagent vibrational and rotational excitation on the F+H_{2} reaction is discussed in detail. The results suggest that the different vibrational and rotational quantum states of H_{2} have different influences on the product polarization.

FUNDAMENTAL AREAS OF PHENOMENOLOGY(INCLUDING APPLICATIONS)

We propose an ultracompact triplexer based on a shift of the cutoff frequency of the fundamental mode in a planar photonic crystal waveguide (PCW) with a triangular lattice of air holes. The shift is realized by modifying the radii of the border holes adjacent to the PCW core. Some defect holes are introduced to control the beam propagation. The numerical results obtained by the finite-difference time-domain method show that the presented triplexer can separate three specific wavelengths, i.e. 1310, 1490 and 1550 nm with the extinction ratios higher than -18 dB. The designed device with a size as compact as 12 μm × 6.5 μm is feasible for the practical application, and can be utilized in the system of fiber to the home.

The influence of atmospheric turbulence on the coherence of a dual-frequency laser beam is studied experimentally. An atmospheric turbulence simulator is inserted in one arm of a Mach-Zehnder interferometer. A single frequency laser beam and a dual-frequency laser beam with a frequency difference of 100 MHz travel through the interferometer, respectively. The visibilities of the interference fringes of the single and dual-frequency laser beams under different turbulent forces are compared. When the turbulence becomes stronger, the visibilities of the interference pattern of the single frequency interferometer decrease more rapidly. This shows that the atmospheric turbulence has less influence on the coherence of the dual-frequency laser beam. The linewidths broadened by the turbulence are calculated with the Wiener-Khintchine theory.

We design and realize a 90°waveguide bend in two-dimensional triangular lattice silicon photonic crystal slabs by connecting linear waveguides along the orthogonal Γ - K and Γ- M directions. A pass band of 70 nm is realized by optimizing the geometry of the Γ - M waveguide. The connection region of the waveguide bend is optimized to improve the transmission efficiency of infrared light through the two different kinds of waveguides. The transmission efficiency of an optimized single bend is about 75% in simulation and 45% in measurement.

We study the formation of spatial solitons in an SBN:75 photorefractive crystal by a 532 nm continuous-wave laser beam. The output beam from the crystal cannot be compressed proportionally to the voltage of the applied electric field. Quasi-steady-state spatial solitons are formed instantaneously at a voltage of 900 V. Interestingly, the quasi-steady-state solitons exhibit a periodic behavior consisting of formation/broken/reformation cycles. If we increase the input intensity of the soliton beam but keep the same signal-to-background intensity ratio, the solitons stay for a longer time in the quasi-steady state and a longer period of soliton formation/broken/reformation cycle is also observed.

Horizontal correlation is one of the most important characteristic parameters in ocean acoustics. In a shallow water acoustic experiment, we observed that the transverse correlation coefficient is an oscillation function of frequency. A model based on adiabatic normal mode theory in a random inhomogeneous waveguide which is statistically isotropic in a horizontal plane is developed. The theory indicates that the oscillation of the transverse correlation coefficient is mainly due to the normal mode interference.

Magnetoacoustic tomography with magnetic induction has shown potential applications in imaging the electrical impedance for biological tissues. We present a novel methodology for the inverse problem solution of the 2-D Lorentz force distribution reconstruction based on the acoustic straight line propagation theory. The magnetic induction and acoustic generation as well as acoustic detection are theoretically provided as explicit formulae and also validated by the numerical simulations for a multilayered cylindrical phantom model. The reconstructed 2-D Lorentz force distribution reveals not only the conductivity configuration in terms of shape and size but also the amplitude value of the Lorentz force in the examined layer. This study provides a basis for further study of conductivity distribution reconstruction of MAT-MI in medical imaging.

Ocean acoustic tomography is an appealing technique for remote monitoring of the ocean environment. In shallow water, matched field processing (MFP) with a vertical line array is one of the widely used methods for inverting the sound speed profile (SSP) of water column. The approach adopted is to invert the SSP with a bottom mounted horizontal line array (HLA) based on MFP. Empirical orthonormal functions are used to express the SSP, and perturbation theory is used in the forward sound field calculation. This inversion method is applied to the data measured in a shallow water acoustic experiment performed in 2003. Successful results show that the bottom mounted HLA is able to estimate the SSP. One of the most important advantages of the inversion method with bottom mounted HLA is that the bottom mounted HLA can keep a stable array shape and is safe in a relatively long period.

Numerical investigation is made on the effect of streaky structures in transition by inviscid linear disturbance equation with temporal mode. Several disturbances with different streamwise wave numbers were induced, and the evolutions with time step were received. It suggests that the exponential growth and periodic variation of the waves are in existence. As the streamwise wave number increases, the disturbance growth rate begins by increasing, reaches a maximum at around α =0.4 with a disturbance frequency of 0.2186+0.001457i, and then decreases. Furthermore, the eigenfunctions of pressure disturbance are plotted.

Molecule dynamics simulation is now widely used in the study of nano pores, proteins and nano-scale devices. The limited friction in such a system requires the method of center-of-mass motion removal to be applied. We test the effect of different time period T of this method under osmotic pressure difference, and find that the impact on the net flux is very small together with the effective reduction of the accumulated numerical error when the period T is above 0.1 ps. The simulation results also show that the change of this time period of method has very little effect on the potential of mean force of the water inside the carbon nanotubes.

A generalized continuity equation extending the ordinary continuity equation is found using quanternions to show it is compatible with Dirac, Schrödinger, Klein-Gordon and diffusion equations. This generalized equation is Lorentz invariant. The transport properties of electrons are found to be governed by the Schrödinger-like equation and not by the diffusion equation.

PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES

The space-time conservation element and solution element (CESE) scheme is a new second order numerical scheme based on the concept of space-time conservation integration. In order to further overcome excessive numerical damping due to small Courant-Friedrichs-Lewy (CFL) number and to obtain a high quality solution, a Courant number insensitive (CNIS) scheme and a high-order scheme have been proposed by Chang et al. for fluid mechanics problems recently. In this study, to explore the potential capability of applications of the CNIS CESE scheme and the high-order CESE scheme to magnetohydrodynamics (MHD) equations, several benchmark MHD problems are calculated in one and two dimensions: (i) Brio and Wu's shock tube, (ii) Dai and Woodward's case, (iii) the Orszag-Tang vortex problem, (iv) the Riemann problem. The numerical results just prove that the CNIS scheme is more accurate and can keep the divergence free condition of the magnetic field, even if the CFL number is «1. Meanwhile, the tests show that the high order CESE scheme possesses the ability to solve MHD problems but is sensitive to the Courant number.

CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES

We investigate the morphology and wettability of [Bmim][PF_{6}] ionic liquid (IL) on a highly oriented pyrolitic graphite (HOPG) substrate using atomic force microscopy. Thin films, nanometer-sized droplets, and "drop-on-layer'' structures of the IL are found on the substrate. Films with a thickness of up to 2 nm (about 4 IL layers) show the solid-like behavior. In contrast, a dewetting phenomenon is observed for thicker IL films, indicating that the IL films retain liquid properties. The contact angle of a buck IL droplet on the HOPG is measured to be about 35^{}º. The wettability of the bulk IL droplet on the HOPG is found to be quite different from that of IL films. These results indicate that the IL molecules can be organized into various micro-morphologies when they are confined to a solid substrate and show characteristic behavior at nanometer scales.

The electronic structure, lattice dynamics, and electron-phonon coupling of gallium-doped germanium are investigated using a initio supercell methods. Our results indicate the superconducting transition in gallium-doped germanium can be explained within a standard phonon mediated mechanism. The contribution of acoustic modes to the coupling constant λ almost entirely comes from Ga related modes, and ~25% to λ. The calculated coupling constant is 0.35 and the corresponding transition temperature is 0.44 K for the gallium-doped germanium with 6.25% gallium content using Coulomb pseudopotential μ*=0.1, in agreement well with the experimental data.

The stress-strain response of Cu single-crystal compression micropillar containing initial dislocation network is investigated by three-dimensional discrete dislocation dynamics simulations. The results demonstrate that the stress-strain curves can be divided into three distinct types with increasing the sizes of micropillars: the three-stage exhaustion hardening, the multi-stage mixed hardening and the two-stage conventional forest hardening. The characteristic sizes of the micropillars is determined for the second type of the curves to be 500-700 nm.

To understand the molecular and electronic structure of alkali metal ions, we carry out the MP2 calculation and demonstrate that the maximal coordinator numbers for the hydrated K^{+} and Rb^{+} are 8, while those for the hydrated Cs^{+} and Fr^{+} are 10. Furthermore, on the basis of the binding energy, the HOMO-LUMO gap and the electron affinity, the stability of the molecular and electronic structures of M^{+}(H_{2}O)_{8} (M = K, Rb, Cs, Fr) decreases with the increasing alkali metal atomic number and the stability of the molecular structures of M^{+}(H_{2}O)_{8-10} (M = Cs, Fr) decreases with the increasing cluster size.

We report the formation energies of wurtzite zinc oxide (w-ZnO) nanowires (NWs) and nanotubes (NTs) with faceted morphologies, and show that hexagonal NWs (h-NWs) are energetically advantageous over the NWs with rhombic (r-), squared (s-), and triangular (t-) cross sections. The formation energies of h-NWs are proportional to the inverse of wire radius, whereas those of single-crystalline NTs are proportional to the inverse of wall thickness, irrespectively to tube radius. A simple model is presented to interpret these features.

Nanocrystalline cubic hafnium nitride (HfN_{x}) powders are prepared by the mechanical milling of Hafnium and hexagonal boron nitride (h-BN) powder mixtures. The prepared nanocrystalline HfN_{x} is analyzed and characterized using x-ray diffraction and Raman spectroscopy. HfN_{x} formation mechanisms in both mechanical milling and annealing processes are also studied in detail. It is found that HfN_{x} is probably formed via the phase separation of Hf(N) solid solution alloy driven by reactive mechanical milling in which Hf(N) solid solution alloy is formed by Hf and N atoms through a diffusion reaction process. Meanwhile, a phase separation can also be induced in Hf (N) solid solution as the N content exceeding its solubility limit, leading to an additional way to produce HfN_{x}. No HfB_{2} has been found during both milling and annealing processes.

Quasi-static and dynamic fracture properties and damage mechanism of glass fiber polymer composites embedded with different mass percentages of ZnO whiskers are investigated by using an Instron Testing machine and a Split-Hopkinson pressure bar. According to the experimental results and linear fracture mechanics, the quasi-static fracture toughness K_{Ic} and the dynamic fracture toughness K_{Id} under various impact velocities of specimens are obtained. Fracture mechanism is investigated by fractography analysis with a scanning electron microscope. The experimental results show that the mass percentage of ZnO_{w} has little influence on the quasi-static fracture toughness, but a little influence on the dynamic fracture toughness and time of initial fracture point of specimens by the reason of various fracture mechanisms.

The phase relations and pressure volume dependences of galena (PbS) under high pressure and high temperature are investigated by means of in situ observation using resistance heating in a diamond anvil cell and synchrotron radiation. The phase transition from NaCl type to TlI type takes place at approximately 2.4 GPa. A fit to the high temperature third-order Birch-Murnaghan equation of state yields an isothermal bulk modulus K_{0} = 37(3) GPa, and its pressure derivative K'_{0} =3.6(3), the temperature derivative of the bulk modulus (∂K/∂T)_{P=-0.022(9)} GPaK^{-1}, and the thermal expansion coefficient α_{0} =2.2(5)× 10^{-5} K^{-1} for TlI-type galena. The linear compressibilities β along a, b and c directions of TlI type is elastically anisotropic (β_{a} =3.4× 10^{-3} GPa^{-1}, β_{b} =1.4×10^{-4} GPa^{-1} and β_{c} =1.6× 10^{-3} GPa^{-1}). We obtain the temperature derivative of the bulk modulus (∂K/∂T)_{P} and thermal expansion coefficient α_{0} for TlI-type galena for the first time.

We investigate the heterojunction effect between para-sexiphenyl (p-6P) and copper phthalocyanine (CuPc) using Kelvin probe force microscopy. CuPc films are grown on the inducing layer p-6P by a weak epitaxy growth technique. The surface potential images of Kelvin probe force microscopy indicate the band bending in CuPc, which reduces grain boundary barriers and lead to the accumulation of holes in the CuPc layer. The electrical potential distribution on the surface of heterojunction films shows negligible grain boundary barriers in the CuPc layers. The relation between band bending and grain boundary barrier in the weak epitaxy growth thin films is revealed.

Periodic nanostripe arrays are observed on In/Si(113) surface using scanning tunneling microscopy. The stripe superstructures are identified as N×1 reconstructions elongating in [21 1] or [121] direction and consisting of one vacancy line, one Si adatom row, and N-2 In rows, in which N=5 is predominant. The vacancy line formation relieves the strain induced by the Si adatom row and In rows, and plays an important role in stabilizing the stripe structures. The stability of nanostripe structures is demonstrated by analyzing the strain-mediated interaction of vacancy lines in the framework of the Frenkel-Kontorova model, which indicates that the predominant vacancy line period of N=5 corresponds to the minimum Frenkel-Kontorova energy.

CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES

The optimization of ion beam sputtering deposition process for Sb_{2}Te_{3} thin films deposited on BK7 glass substrates is reported. The influence of composition ratio on the thermoelectric properties is investigated. X-ray diffraction shows that the major diffraction peaks of the films match with those of Sb_{2}Te_{3}. Hall effect and Seebeck coefficient measurement reveal that all the samples are of p-type. The Sb_{2}Te_{3} thin films exhibit the Seebeck coefficient of 190 μVk^{-1} and the electrical conductivity of 1.1×10^{3} Scm^{-1} when the atomic ratio of Sb to Te is 0.65. Carrier concentration and motility of the films increase with the increasing atomic ratio of Sb to Te. The Sb_{2}Te_{3} film with a maximum power factor of 2.26×10^{-3} Wm^{-1}K^{-2} is achieved when annealed at 400^{}°C. Raman measurement shows that the main peaks are at about 120 cm^{-1}, 252 cm^{-1}and 450 cm^{-1}, in agreement with those of V-VI compound semiconductors.

The Kondo effect in two-dimensional manganese phthalocyanine (MnPc) self-assembled monolayer films on Pb(111) islands is studied by low-temperature scanning tunneling microscopy. Variation of the Kondo temperature from 50 K to 300 K at different molecule adsorption sites is revealed. It is shown that the variation is mainly due to the change in the width of d orbital, rather than the shift of its energy. The two-dimensional dI/dV mapping reveals the periodic modulation of the Kondo resonance in the self-assembled MnPc monolayer.

Terahertz signals emitted from three photoconductive antennae based on semi-insulating GaAs and with different gap sizes are tested. These signals represent that the distribution of electrical field between electrode gaps and electrical field enhancement on the anodes is testified. Two main causes of this phenomenon are the different movabilities of electrons and holes and the induced current which is brought by the electrons on arriving at the anodes. The electrical field distributes in a large region, which extends from tens to hundreds of micrometers and it is decided by the gap size.

Alternating-current small-signal admittances of armchair graphene nanoribbons are investigated using the method of non-equilibrium Green's function. The calculated ac admittances show an oscillatory response between inductive and capacitive behaviors, which is a result of the finite length of the graphene nanoribbon. The effects of hydrogen-passivated edges on ac response are demonstrated. At low frequency, the edge effects transform the inductive behavior in a metallic graphene nanoribbon into a capacitive one. Finally, the effects of variations in the width and bandgap of a graphene nanoribbon on its dynamic response are investigated.

Spin-dependent transport properties of the zigzag graphene nano-ribbon (zGNR) based structure Al-zGNR-Al are investigated by abinitio technique where density functional calculation is carried out within the Keldysh non-equilibrium Green's function formalism. The energy band structure of the infinite zigzag ribbon is sensitive to the dangling bonds of carbon atoms on both edge sides. For the three-circle-width zigzag ribbon with one edge monohydrogenated and the other edge dihydrogenated (zGNR(H-H2)), strongly spin-polarized energy bands are found. A spin-down branch is obtained just below the Fermi level while a spin up band appears above it. For the structure Al-zGNR(H-H2)-Al, where three-circle-width and seven-circle-length (3×7) zGNR(H-H2) is coupled by two (100) aluminium electrodes, an obvious spin filter property is found as the bias voltage changes. When the length of the sandwiched zGNR(H-H2) ribbon increases, the spin-up current is strongly restrained especially under higher bias voltage.

ZHANG Xian-Gao, CHEN Kun-Ji, FANG Zhong-Hui, QIAN Xin-Ye, LIU Guang-Yuan, JIANG Xiao-Fan, MA Zhong-Yuan, XU Jun, HUANG Xin-Fan, JI Jian-Xin, HE Fei, SONG Kuang-Bao, ZHANG Jun, WAN Hui, WANG Rong-Hua

Chin. Phys. Lett. 2010, 27 (8):
087301
.
DOI: 10.1088/0256-307X/27/8/087301

A nonvolatile memory device with nitrided Si nanocrystals embedded in a floating gate was fabricated. The uniform Si nanocrystals with high density (3×10^{11} cm^{-2}) were deposited on ultra-thin tunnel oxide layer (~ 3 nm) and followed by a nitridation treatment in ammonia to form a thin silicon nitride layer on the surface of nanocrystals. A memory window of 2.4V was obtained and it would be larger than 1.3V after ten years from the extrapolated retention data. The results can be explained by the nitrogen passivation of the surface traps of Si nanocrystals, which slows the charge loss rate.

A nonvolatile memory device with nitrided Si nanocrystals embedded in a floating gate was fabricated. The uniform Si nanocrystals with high density (3×10^{11} cm^{-2}) were deposited on ultra-thin tunnel oxide layer (~ 3 nm) and followed by a nitridation treatment in ammonia to form a thin silicon nitride layer on the surface of nanocrystals. A memory window of 2.4 V was obtained and it would be larger than 1.3 V after ten years from the extrapolated retention data. The results can be explained by the nitrogen passivation of the surface traps of Si nanocrystals, which slows the charge loss rate.

Indium tin oxide (ITO) layers are prepared on polymethylmethacrylate and polyethylene terephthalate by rf-magnetron sputtering at room temperature. Their 3D-AFM images and visible transmittance spectra are contrastively characterized and studied. An interesting morphological effect of the polymer substrates on their top ITO layers is observed. Dominant direct and indirect transition types deduced from optical spectra are surprisingly found in ITO films on different polymers. Furthermore, qualitative band structures are figured, and some theoretical discussions about the correlation of optical band structure, interface/surface morphology and carrier density are also presented.

High quality Pb-doped Bi_{2}Sr_{2}CuO_{6+}_{δ} (Pb-Bi2201) single crystals are grown by the traveling solvent floating zone technique, with dimensions as large as ~50 mm×~5.0 mm×~2 mm}. The Pb-Bi2201 single crystals with different doping levels are obtained by the annealing process which covers a wide doping range of the overdoped region. We describe in detail the growth and annealing procedures and the characterization and physical property measurements of the Pb-Bi2201 crystals. The availability of these crystals provides a good opportunity to experimentally investigate high-Tc cuprate superconductors, particularly in the overdoped region.

We present the investigations of non-adiabatic effects by including vertex corrections in the standard Eliashberg theory and show that high phonon frequency is unfavorable to superconductivity in the regime of strong vertex correction. This means that it is hard to find high-transition-temperature superconductors in the compounds with light elements if the non-adiabatic effects are strong. The interplay interaction between non-adiabatic effect and Coulomb interaction makes the transition temperature of silane superconductor not so high as predicted by the standard Eliashberg theory.

Recently, Liu et al. proposed a so-called extended Anderson-Higgs mechanism by studying the (2+1)-dimensional Ginzburg-Landau model in the pseudogap region of high-T_{c} superconductor (Phys. Rev. B 65 (2002) 132513). We revisit this problem based on a general decomposition of the U(1) gauge potential. Using the bulk superconductor and superconduct ring as examples, we obtain a simpler expression for the extended Anderson-Higgs mechanism. In the former case we indicate that all the phase field can always be "eaten up'' by the pure gauge term A_{||}. In the latter case, we decompose the phase field as θ(x)=θ_{1}(x)+θ_{2}(x) and find that only the phase field θ_{1} connected with Anderson-Higgs mechanism can be canceled by the pure-gauge term A_{||}. On the other hand, the remaining phase field θ_{2} connected with A_{^}is multi-valued, which can induce new physical effects such as A-B effect and flux quantization. It is natural to conclude that there is no longitudinal phase fluctuation effect in high-temperature superconductors since longitudinal phase \theta_{1} is connected with pure-gauge term.

We present our lab cryocooler-based superconducting nanowire single photon detection (SNSPD) system. The dark count rate and system quantum efficiency are investigated at the bath temperature of 3.1 K with a 300-mK temperature fluctuation. The polarization sensitivity of the SNSPD is also measured, and the system counting rate and the timing jitter are discussed.

The film thickness should be known for extracting the intrinsic surface resistance from the effective surface resistance as measured by using the dielectric resonator method. Thicknesses of 70 nm to 360 nm are measured for YBa_{2}Cu_{3}O_{7-δ} films in a non-invasive way by using the two-resonant-mode dielectric resonator (TDR) method. A rutile resonator with the respective resonant frequencies of 15.25-15.61 GHz and 15.10-15.37 GHz for the TE_{021} and the TE_{012} modes is used for this purpose. Differences between the values as measured by using the TDR technique and those measured by using a step profilometer appear to be less than 3%, which is smaller than the previous value of 5% as measured by using a 8.6 GHz single-resonance mode rutile resonator [Lee et al. J. Korean Phys. Soc. 54(2009)1619]. Merits of using the TDR method are discussed.

Small and thin mesa structures of intrinsic Josephson junctions 1-2 μm on a side and 1.5-7.5 nm in thickness are fabricated from single crystals of Bi2-xPbxSr2CaCu2O8+δ with a nominal Pb content x from 0.2 to 0.5. The Josephson critical current density jc is found to be pronouncedly large, ranging from 10 to 50 kA/cm2. Switching current probability measurements for these mesa samples show an indication of a crossover to the macroscopic quantum tunneling region.

Ion implantation technique is used to study the magnetic properties of Cu-doped ZnO. The room temperature ferromagnetism in the Cu-implanted ZnO samples is observed. From the photoluminescence spectrum of implanted samples we observe a broad green emission around 510 nm, which is related to defects in the samples. X-ray photoelectron spectroscopy measurement shows that Cu ions are in the mixed oxidation state of +1 or +2 and substitute for the Zn^{2+} ions of the ZnO matrix. We argue that the ferromagnetism is related to these defects, and the substitution of Cu^{2+} into Zn^{2+} sites in crystal ZnO could contribute to the observed ferromagnetism.

Magnetic properties of spin-ladder compounds Sr14(Cu1-yFey)24O41 (0 ≤ y ≤ 0.05) are investigated in the temperature range from 10 to 300 K. The result reveals that all the samples exhibit magnetic crossover behavior in the paramagnetic range, and Fe3+ doping can efficiently increase the susceptibility due to the large moment of Fe3+. Both the observations are consistent with our previous investigation on transport behaviors, indicating the strong correlation between the magnetism and transport behaviors. The spin gap is evidenced in all the samples, and strengthens as Fe3+ doping level increases, which can be associated with the antiferromagnetic interaction between Fe3+ and Cu cations.

We report the temperature-dependent resistivity and dielectric function (1 kHz~3 MHz) of a charge ordering system La_{1.5}Sr_{0.5}NiO_{4-δ} The primary data of ac impedance was measured by a four-terminal pair arrangement. Above 180 K, the resistivity is independent of frequency. At lower temperatures, the dielectric function could not be fitted by a Debye model with a single relaxation time, it should have a distribution. At T=130 K, there is a sudden increase in the ρ-T relation, meanwhile an anomaly dielectric response occurs on the temperature dependant dielectric spectrum in the whole measured frequency range, in which the dielectric constant has a high value even at high frequency region. Our analysis suggests that the current system would be more consistent with the Zener polaron model.

Nano-carbon films with large density of caterpillar-like clavae are synthesized by microwave plasma-assisted chemical vapor deposition using a mixture of methane and hydrogen gases on Mo film substrates. The films are characterized by Raman spectra, optical microscopy and field emission scanning electron microscopy. Field electron emission measurements of nano-carbon films are also carried out to show the turn-on field as low as 1.5 V/μm and the high current density of 2.2 mA/cm^{2} at electric field of 5.7 V/μm, the uniformly distributed emission site density from a broad well-proportioned emission area of about 4.0 cm^{2} is also obtained. The field-emission current density J versus macroscopic electric field E does not follow the original Fowler-Nordheim (F-N) relation since they are not well represented in the F-N plot by a straight line. A modified F-N relation is applied successfully to explain all the field-emission data observed for E<6.0 V/μm.

CROSS-DISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY

We report our laser-driven method used to make large quantities of straight thin silver nanowires, and experimentally demonstrate that femtosecond laser pulse polarization has a prominent effect on formation of non-spherical shapes of nanoscale particles. Further, our experiment directly reveals that the underlying mechanism is plasmon-plasmon interaction, which can be controlled by polarization and plays a decisive role in this non-synthetic method for metal nanowire formation.

Molecular dynamical simulation is carried out to investigate the effects of the incident energy on a-C:H film growth from C and H atomic flux. Our simulations show that the film growth at low incident energy (1 eV) is dominated by the adsorption of H and C atoms. At moderate incident energy (10 and 20 eV), the abstraction reaction of incident H atoms with H atoms adsorbed at the surface becomes important. At high incident energy (30 and 40 eV), the a-C:H film growth is a two-step process: one is the adsorption and the shallow implantation of C atoms, and the other is the deep implantation of H atoms.

Diamond films are deposited on Mo substrates by dc hot-cathode plasma chemical vapor deposition method using a CH_{4}-H_{2}-CO_{2} gas mixture. Adjusting the flow of CO_{2}, we study the relevant influence on surface morphology, grain orientation and crystalline quality of films with scanning electron microscopy, x-ray diffraction, Raman spectroscopy, respectively. The results show that grain orientation of the films has a transition with the increasing CO_{2} addition, from (100) orientation to (110) orientation and then (111) orientation. The crystalline quality is improved but the growth rate is decreased by raising the flow of CO_{2}. The experimental results are also discussed briefly.

A critical Biot number, which determines both the sensitivity of spherical ceramics to quenching and the durations of the temperature-wave propagation and the thermal stresses in the ceramics subjected to thermal shock, is theoretically obtained. The results prove that once the Biot number of a ceramic sphere is greater than the critical number, its thermal shock failure will be such a rapid process that the failure only occurs in the initial regime of heat conduction, whereas the thermal shock failure of the ceramic sphere is uncertain in the course of heat conduction. The presented results provide a guide to the selection of the ceramics applied in the thermostructural engineering with thermal shock.

We demonstrate an ultra-violet light-emitting diode (UV-LED) fabricated on a bulk GaN substrate with electroluminescence (EL) emission centered at about 340 nm. The UV-LED exhibits low reverse leakage current on the order of 10^{-9} A under -5 V at room temperature, which can be explained by the low defect density in the epi-structure. The evolution of EL spectra as a function of injection current levels reveals the improved heat dissipation of the LEDs with vertical geometry on the bulk GaN substrate. The unusual increase of EL intensity at elevated temperatures can be explained by thermally assisted p-dopant ionization.

We numerically investigate the effects of the exciton generation rate G, temperature T, the active layer thickness d and the position of LUMO level E_{L} related to the cathode work function W_{c} at a given energy gap on the open-circuit voltage V_{oc} of single layer organic solar cells with Schottky contact. It is demonstrated that open-circuit voltage increases concomitantly with the decreasing cathode work function W_{c} for given anode work functions and exciton generation rates. In the case of given cathode and anode work functions, the open-circuit voltage first increases with the exciton generation rate and then reaches a saturation value, which equals to the built-in voltage. Additionally, it is worth noting that a significant improvement to V_{oc} could be made by selecting an organic material which has a relative high LUMO level (low |E_{L}| value). However, V_{oc} decreases as the temperature increases, and the decreasing rate reduces with the enhancement of exciton generation rate. Our study also shows that it is of no benefit to improve the open-circuit voltage by increasing the device thickness because of an enhanced charge recombination in thicker devices.

Simulation of the heat consumption in phase change random access memories (PCRAMs) is investigated by a three-dimensional finite element model. It is revealed that the thermal conductivity and electrical conductivity of the buffer layer are crucial in controlling the heating efficiency in RESET process. The buffer layer materials W, TiN, WO_{3}, TiO_{2} and poly-germanium (poly-Ge) are applied in the simulation respectively, and compared with each other. The simulation results show that limitation of electrical conductivity is effective on heating efficiency and the limitation of thermal conductivity is important on the reliable RESET process.

In recent years, superconducting quantum interference devices (SQUIDs) have been demonstrated to be useful in the low field nuclear magnetic resonance (NMR) measurements. The high temperature superconducting (HTS) SQUID used in our experiments has a frequency-independent sensitivity of 40-50 fT/Hz^{1/2}. When a liquid nitrogen cooled LC circuit is employed to form a tuned circuit with the SQUID, the sensitivity of the system can be further enhanced. The LC circuit consists of a capacitor and a coil made of copper wire or HTS tape, which is inductively coupled to the SQUID. However, the homogeneity of the measurement field deteriorates because of the HTS tape coil in the proximity of the sample. In contrast, the thin film SQUID with a washer area of 1 cm^{2} has no effect on the NMR signal. Therefore, the impairment of the measurement field homogeneity in the case of different superconducting elements nearby is discussed by examining the free induction decay signals at 9 kHz. It is found that a square superconducting film with an area of 1 cm^{2} may compensate for the inhomogeneity of the measurement field after the adjustment of its position.

We develop magnetic metallic contaminant detectors using high-temperature superconducting quantum interference devices (HTS-SQUIDs) for industrial products. Finding ultra-small metallic contaminants is an important issue for manufacturers producing commercial products such as lithium ion batteries. If such contaminants cause damages, the manufacturer of the product suffers a big financial loss due to having to recall the faulty products. Previously, we described a system for finding such ultra-small particles in food. In this study, we describe further developments of the system, for the reduction of the effect of the remnant field of the products, and we test the parallel magnetization of the products to generate the remnant field only at both ends of the products. In addition, we use an SQUID gradiometer in place of the magnetometer to reduce the edge effect by measuring the magnetic field gradient. We test the performances of the system and find that tiny iron particles as small as 50×50 μm^{2} on the electrode of a lithium ion battery could be clearly detected. This detection level is difficult to achieve when using other methods.

Hole transport characteristics in N,N'-bis(naphthalen-1-yl)-N,N'-bis(pheny) benzidine (NPB) and 4,4',4''-tri(N-carbazolyl)triphenylamine (TCTA) are comparatively investigated. The current density-voltage (J-V) characteristics of hole-only devices based on NPB and TCTA at different temperatures and thicknesses show that the hole current is dominated by the bulk conduction with an exponential trap distribution. Detailed analyses of the J-V characteristics give the trap densities N_{t} of (6.3±0.3)×10^{18} and (1.9±0.02)×10^{18} cm^{3}, characteristic trap depths of 135±6 and 117±5 meV, hole mobilities of (8.1±0.5)×10^{-5} and (1.9±0.1)×10^{-4} cm^{2}V^{-1}s^{-1} for NPB and TCTA, respectively. It is found that TCTA exhibits higher hole mobility. Obviously, this is directly related to the lower trap density and shallow trap depth in TCTA films, leading to good charge carrier transport.

A higher sensitivity is achieved by making use of a high electron mobility transistor (HEMT) as the piezoresistive device. The temperature dependence on the electromechanical coupling effect of accelerometers is reported. The current in the structure of our study decreases at the rate of 3.15 mA/^{}ºC with the temperature going up at every region, and piezoresistance coefficient decreases because of the shift of energy and expansion of lattice.

We perform the density functional theory and Brownian dynamics simulations based on the three-dimensional structure of the WT KcsA channel and its mutants. Our data suggest that the electrostatic interactions between the channels and cations, within the signature sequence of K^{+} channels, determine the selectivity of the channel.

We study the effects of the strength of coupling between neurons on the spiking regularity and coherence in a complex network with randomly connected Hodgkin-Huxley neurons driven by colored noise. It is found that for the given topology realization and colored noise correlation time, there exists an optimal strength of coupling, at which the spiking regularity of the network reaches the best level. Moreover, when the temporal regularity reaches the best level, the spatial coherence of the system has already increased to a relatively high level. In addition, for the given number of neurons and noise correlation time, the values of average regularity and spatial coherence at the optimal strength of coupling are nearly independent of the topology realization. Furthermore, there exists an optimal value of colored noise correlation time at which the spiking regularity can reach its best level. These results may be helpful for understanding of the real neuron world.

We study the multiband non-thermal emission from two pulsar wind nebulae (PWNe), the Crab nebula and the PWN in MSH 15-52. Both of them have been recently detected by the Fermi large area telescope (LAT) and powered by central gamma-ray pulsars. Motivated by the Fermi LAT results, we use a simplified time-dependent injection model to study the non-thermal emission from radio to very high energy gamma-ray radiation from these two sources. In this model, the relativistic electrons are accelerated in pulsar magnetosphere and at pulsar wind termination shocks and can be described by a broken power law. Those high energy particles evolve with time and produce non-thermal emission through synchrotron radiation and inverse Compton scattering of soft photons. For Crab nebula, using the GeV emission from 100 MeV to 10 GeV given by Fermi LAT, we can constrain the maximum energy of the electrons and other parameters. The non-thermal emission can be well explained by this model. We also use this model to explain the non-thermal emission from the PWN in MSH 15-52.

We investigate the Bianchi type-V magnetized string cosmological model with variable magnetic permeability for viscous fluid distribution. The magnetic field is due to an electric current produced along the x-axis. Thus the magnetic field is in yz-plane and F_{23} is the only non-vanishing component of electromagnetic field tensor F_{ij}. To obtain the deterministic model in terms of cosmic time t, we assume the condition ξθ= const where ξ is the coefficient of bulk viscosity and θ the expansion in the model.