Temperature dependence of biaxial strain and its influence on phonon and band gap of GaN thin film

Hexagonal GaN epilayer grown on sapphire substrate by metal organic chemical vapour deposition (MOCVD) is studied using Raman scattering and photoluminescence in a temperature range from 100\,K to 873\,K. The model of strain (stress) induced by the different lattice parameters and thermal coefficients of epilayer and substrate as a function of temperature is set up. The frequency and the linewidth of $E_2^{\rm high}$ mode in a GaN layer are modelled by a theory with considering the thermal expansion of the lattice, a symmetric decay of the optical phonons, and the strain (stress) in the layer. The temperature-dependent energy shift of free exciton A is determined by using Varshni empirical relation, and the effect of strain (stress) is also investigated. We find that the strain in the film leads to a decreasing shift of the phonon frequency and an about 10meV-increasing shift of the energy in a temperature range from 100\,K to 823\,K.     

铯原子束频标中的Majorana跃迁频移

本文对铯原子频标中的Majorana跃迁频移进行理论上的探讨。通过经受低频和微波多重共振的多能级系统的铯原子量子态变化的分析,认为产生这种频移的物理机制在于C_s¹³³原子穿过空间变化磁场时感应了塞曼子能级间的跃迁,扰动了频标赖以工作的两个能级相应态的相位,这个过程又相干于Ramsey共振过程中,最后导致Ramsey共振频移。基于此机制,解释一些有关实验现象并提出消除和减少Majorana跃迁的判断方法。     

Photoemission study of YBa(2)Cu(3)O(y) thin films under light illumination

Heterojunctions of Nb-doped SrTiO3 substrate and YBa(2)Cu(3)O(y) thin films show photoconductivity and photovoltaic effects due to photocarrier injection. Photocarrier injection is expected to be a new carrier doping method in strongly correlated systems instead of chemical substitution. We have studied the nature of photocarrier injection in YBa(2)Cu(3)O(y)/SrTiO(3):Nb using x-ray photoemission spectroscopy with pulsed laser excitation. The core-level spectra shift to higher binding energy by 0.78 eV under pulsed laser illumination at 30 Hz. The energy shift corresponds to the photovoltage, which arises at the interface. In addition, we have observed that the energy shift strongly depends on the frequency of the laser. The lifetime of the injected photoholes has been estimated to be 40 ms by analyzing the frequency dependence.     

Soliton self-frequency shift in a short tapered air-silica microstructure fiber

We report a soliton self-frequency shift of more than 20% of the optical frequency in a tapered air-silica microstructure fiber that exhibits a widely flattened large anomalous dispersion in the near infrared. Remarkably, the large frequency shift was realized in a fiber of length as short as 15 cm, 2 orders of magnitude shorter than those reported previously with similar input pulse duration and pulse energies, owing to the small mode size and the large and uniform dispersion in the tapered fiber. By varying the power of the input pulses, we generated compressed sub-100-fs soliton pulses of ~1-nJ pulse energy tunable from 1.3 to 1.65 mum with greater than 60% conversion efficiency.     

Tunable spectral shift of high-order harmonic generation in atoms using a sinusoidally phase-modulated pulse

High-order harmonic generation (HHG) from an atom illuminated by a sinusoidally phase-modulated pulse is investigated by solving the time-dependent Schrödinger equation. The spectral shift that occurs in atomic HHG can be achieved easily using our laser pulse. It is shown that the photon energy of the generated harmonics is controllable within the range of 1 eV. The shift of the frequency peak position is rooted in the asymmetry of the rising and falling parts of the laser pulse. We also show that by varying the phase parameters in the frequency domain of the laser one can adjust and control the shift in atomic harmonic spectra.     

Influence of a Single Frequency Electromagnetic Wave on Energy Spectrum of Nonpolariton System in a Kerr Nonlinear Blackbody

In a Kerr nonlinear blackbody, bare photons with opposite wave vectors and helieities are bound into pairs and unpaired photons are transformed into a different kind of quasiparticle, the nonpolariton. The present paper investigates the influence of a single frequency electromagnetic wave on the energy spectrum of the nonpolariton system. We find that the wave can lead to an energy shift of nonpolaritons. Moreover, we calculate the first-order energy shift on certain conditions.     

General theory of microscopic dynamical response in surface probe microscopy: from imaging to dissipation

We present a general theory of atomistic dynamical response in surface probe microscopy when two solid surfaces move with respect to each other in close proximity, when atomic instabilities are likely to occur. These instabilities result in a bistable potential energy surface, leading to temperature dependent atomic scale topography and damping (dissipation) images. The theory is illustrated on noncontact atomic force microscopy and enables us to calculate, on the same footing, both the frequency shift and the excitation signal amplitude for tip oscillations. We show, using atomistic simulations, how dissipation occurs through reversible jumps of a surface atom between the minima when a tip is close to the surface, resulting in dissipated energies of 1.6 eV. We also demonstrate that atomic instabilities lead to jumps in the frequency shift that are smoothed out with increasing temperature.     

EXPERIMENTAL STUDY OF AC ZEEMAN EFFECT IN 87Rb ATOMIC FREQUENCY STANDARD

We have proved experimentally that the frequency shift formula of AC Zeeman effect presented by us is correct. In a ⁸⁷Rb atomic frequency standard we add a new microwave field which causes the frequency shift of O-O transition frequency of ⁸⁷Rb atomic ground state. This frequency shift as a function of frequency and power of the added microwave field is measured. The experimental results agree with theory.     

第一性原理研究一维SiO2纳米材料的结构和性质

运用密度泛函理论，在6-31G(d)基组水平上，对二元环组成的SiO₂单链、双链纳米线和三元环组成的纳米管进行几何优化，并对其结合能、电子结构和振动频率进行计算.结果表明，平均结合能和能隙均随着SiO₂单元的增加单调变化.红外光谱的分析发现，径向和切向的振动频率与强度随着长度的变化有明显的移动.径向的振动频率向高频端移动，而切向的振动频率则向低频端移动，即SiO₂纳米材料中存在量子尺寸效应和各向异性现象. 关键词： 2纳米材料')" href="#">SiO₂纳米材料 能隙 振动光谱 尺寸效应     

Spectrum of Andreev bound states in a molecule embedded inside a microwave-excited superconducting junction

Nondissipative Josephson current through nanoscale superconducting constrictions is carried by spectroscopically sharp energy states, the so-called Andreev states. Although theoretically predicted almost 40 years ago, no direct spectroscopic evidence of these Andreev bound states exists to date. We propose a novel type of spectroscopy based on embedding a superconducting constriction, formed by a single-level molecule junction, in a microwave QED cavity environment. In the electron-dressed cavity spectrum we find a polariton excitation at twice the Andreev bound state energy, and a superconducting-phase-dependent ac Stark shift of the cavity frequency. Dispersive measurement of this frequency shift can be used for Andreev bound state spectroscopy.     

EXPERIMENTAL STUDY OF AC ZEEMAN EFFECT IN 87Rb ATOMIC FREQUENCY STANDARD

We have proved experimentally that the frequency shift formula of AC Zeeman effect presented by us is correct. In a ⁸⁷Rb atomic frequency standard we add a new microwave field which causes the frequency shift of O-O transition frequency of ⁸⁷Rb atomic ground state. This frequency shift as a function of frequency and power of the added microwave field is measured. The experimental results agree with theory.     

Dark soliton in one-dimensional Bose–Einstein condensate under a periodic perturbation of trap

The perturbation of a confining trap leads to the collective oscillation of a Bose--Einstein condensate, thereby the propagation of a dark soliton in the condensate is affected. In this study, periodic perturbation is employed to match the soliton oscillation. We find that the soliton dynamics depends sensitively on the coupling between the moving direction of the trap and that of the soliton. The soliton energy/depth evolves periodically, and a relevant shift in the soliton trajectory occurs as compared with the unperturbed case. Overall, the soliton oscillation frequency changes little even if the perturbation amplitude and frequency vary.     

Relative Energy Shift of a Two-Level Atom in a Cylindrical Spacetime

We investigate the evolution dynamics of a two-level atom system interacting with the massless scalar field in a Cylindrical spacetime. We find that both the energy shifts of ground state and excited state can be separated into two parts due to the vacuum fluctuations. One is the corresponding energy shift for a rest atom in four-dimensional Minkowski space without spatial compactification, the other is just the modification of the spatial compactified periodic length. It will reveal that the influence of the presence of one spatial compactified dimension can not be neglected in Lamb shift as the relative energy level shift of an atom.     

Relative Energy Shift of a Two-Level Atom in a Cylindrical Spacetime

We investigate the evolution dynamics of a two-level atom system interacting with the massless scalar field in a Cylindrical spacetime. We find that both the energy shifts of ground state and excited state can be separated into two parts due to the vacuum fluctuations. One is the corresponding energy shift for a rest atom in four-dimensional Minkowski space without spatial compactification, the other is just the modification of the spatial compactified periodic length. It will reveal that the influence of the presence of one spatial compactified dimension can not be neglected in Lamb shift as the relative energy level shift of an atom.     

声子和磁场对量子环中极化子性质的影响

采用求解能量本征方程、幺正变换和变分相结合的方法,研究声子和磁场对量子环中极化子性质的影响. 对KBr量子环的数值计算表明,电子或极化子的基态能量随量子环频率(或平均半径)的增大而增大,极化子基态能移随量子环频率的增大(或平均半径的减小)而减小,极化子中的平均声子数随量子环频率的增大(或平均半径的减小)而增大. 当有垂直磁场时,极化子基态能量和基态能移与外磁场及电子转动状态有关. 随着磁场强度的增大,基态能量出现简并且呈现非周期性振荡;能移随磁场强度的增大(或转动量子数绝对值的减小)而减小.     

Differential light-shift cancellation in a magnetic-field-insensitive transition of 87rb

The precise measurement of transition frequencies of trapped atomic samples is susceptible to inaccuracy arising from the inhomogeneous differential shift of the relevant energy levels in the presence of the trapping fields. We demonstrate near-complete cancellation of the differential ac Stark shift ("light shift") of a two-photon magnetic-field-insensitive microwave hyperfine (clock) transition in ^{87}Rb atoms trapped in an optical lattice. Up to 95(2)% of the differential light shift is cancelled while maintaining magnetic-field insensitivity. This technique should have applications in quantum information and frequency metrology.     

Determination of depths and frequency factors of traps by UV amplitude modulated excitation

A study is made of the photoluminescence of ZnS-type phosphors submitted to a UV excitation modulated at various frequencies. The resulting emission shows a modulation rate and a phase shift which are functions of the occupancy of the various groups of traps, and therefore depend on temperature. Starting from the phase shift, the activation energy as well as the frequency factor of various trapping levels in ZnS/Cu and CdS, ZnS/Mn, Au samples have been computed. These results are compared with those deduced from ordinary or fractional glow curves. The advantage of the present modulated excitation method is to yield simultaneously both the activation energy and the frequency factor.     

Dynamics of Dirac quasi-particles in lattice vibration and anomalous phonon frequency shift of graphene

By exact solution of time-dependent Schrödinger equation of electron in graphene under interaction with E_2g phonons, we investigate the dynamical behavior of Dirac quasi-particle in the process of lattice vibration. Due to the global geometric phases acquired by electron during lattice vibration, an anomalous shift of the vibration frequency is obtained. We calculate the Fermi energy dependence of frequency shift which is in consistence with experiment in case of small doping density.     

Analytical calculation of the space charge potential and the temperature of stored ions in an rf quadrupole trap

The use of a stored ion cloud in an rf quadrupole trap has shown its possibilities as a high-performance frequency standard. The most important source of systematic error of such a standard is the second-order Doppler shift, an effect of special relativity, which produces a relative frequency shift approximated by the ratio of the mean kinetic energy to the rest mass energy of an ion. Assuming that the ion cloud can be characterized by a temperatureT, the determination ofT is of paramount importance in evaluating the second-order Doppler shift. In this paper, an analytical expression of the ion-cloud temperature is established taking space-charge effects into account and involving only physical parameters which can be easily measured. The validity of this approximate expression is discussed and it is compared to numerically established results published elsewhere.     

Amplification of solitary optical waves in fibers with positive group velocity dispersion

Solitary optical waves with characteristic red frequency shift have been numerically identified in fibers with positive group velocity dispersion in the presence of amplification by semiconductor amplifiers. Influence of the Henry factor, the net gain, gain saturation characteristics, and frequency modulation on the frequency shift of the observed solitary optical waves has been examined. Normalized pulse energy as a function of the gain saturation characteristics has been studied.