Built-in electric field effect on cyclotron mass of magnetopolarons in a wurtzite In_xGa_（1-x）N/GaN quantum well

The cyclotron mass of magnetopolarons in wurtzite In x Ga 1 x N/GaN quantum well is studied in the presence of an external magnetic field by using the Larsen perturbation method.The effects of the built-in electric field and different phonon modes including interface,confined and half-space phonon modes are considered in our calculation.The results for a zinc-blende quantum well are also given for comparison.It is found that the main contribution to the transition energy comes from half-space and interface phonon modes when the well width is very small while the confined modes play a more important role in a wider well due to the location of the electron wave function.As the well width increases,the cyclotron mass of magnetopolarons first increases to a maximum and then decreases either with or without the built-in electric field in the wurtzite structure and the built-in electric field slightly reduces the cyclotron mass.The variation of cyclotron mass in a zinc-blende structure is similar to that in a wurtzite structure.With the increase of external magnetic field,the cyclotron mass of polarons almost linearly increases.The cyclotron frequency of magnetopolarons is also discussed.     

Effects of Crossed Electric and Magnetic Fields on Shallow Donor Impurity Binding Energy in a Parabolic Quantum Well

We have calculated variationally the ground state binding energy of a hydrogenic donor impurity in a parabolic quantum well in the presence of crossed electric and magnetic fields. These homogeneous crossed fields are such that the magnetic field is parallel to the heterostructure layers and the electric field is applied perpendicular to the magnetic field. The dependence of the donor impurity binding energy to the well width and the strength of the electric and magnetic fields are discussed. We hope that the obtained results will provide important improvements in device applications, especially for a suitable choice of both fields in the narrow well widths.     

Optical response of an inverted InAs/GaSb quantum well in an in-plane magnetic field

The optical response of an inverted InAs/GaSb quantum well is studied theoretically. The influence of an in-plane magnetic field that is applied parallel to the quantum well is considered. This in-plane magnetic field will induce a dynamical polarization even when the electric field component of the external optical field is parallel to the quantum well.The electron–electron interaction in the quantum well system will lead to the de-polarization effect. This effect is found to be important and is taken into account in the calculation of the optical response. It is found that the main feature in the frequency dependence of the velocity–velocity correlation function remains when the velocity considered is parallel to the in-plane magnetic field. When the direction of the velocity is perpendicular to the in-plane magnetic field, the depolarization effect will suppress the oscillatory behavior in the corresponding velocity–velocity correlation function. The in-plane magnetic field can change the band structure of the quantum well drastically from a gapped semiconductor to a no-gapped semi-metal, but it is found that the distribution of the velocity matrix elements or the optical transition matrix elements in the wave vector space has the same two-tadpole topology.     

Multiple broadband magnetoelectric response in Terfenol-D/PZT structure

In this paper, a novel magnetoelectric(ME) composite structure is proposed, and the ME response in the structure is measured at the bias magnetic field up to 2000 Oe(1 Oe = 79.5775 A·m~(-1)) and the excitation frequency of alternating magnetic field ranging from 1 kHz to 200 kHz. The ME voltage of each PZT layer is detected. According to the measurement results, the phase differences are observed among three channels and the multi-peak phenomenon appears in each channel. Meanwhile, the results show that the ME structure can stay a relatively high ME response within a wide bandwidth.Besides, the hysteretic loops of three PZT layers are observed. When the frequency of alternating current(AC) magnetic field changes, the maximum value of ME coefficient appears in different layers due to the multiple vibration modes of the structure. Moreover, a finite element analysis is performed to evaluate the resonant frequency of the structure, and the theoretical calculating results accord well with the experimental results. The experiment results suggest that the proposed structure may be a good candidate for designing broadband magnetic field sensors.     

Large linear magnetoresistance in a new Dirac material BaMnBi_2

Dirac semimetal is a class of materials that host Dirac fermions as emergent quasi-particles.Dirac cone-type band structure can bring interesting properties such as quantum linear magnetoresistance and large mobility in the materials.In this paper,we report the synthesis of high quality single crystals of BaMnBi_2 and investigate the transport properties of the samples.BaMnBi_2 is a metal with an antiferromagnetic transition at T_N = 288 K.The temperature dependence of magnetization displays different behavior from CaMnBi_2 and SrMnBi_2,which suggests the possible different magnetic structure of BaMnBi_2.The Hall data reveals electron-type carriers and a mobility μ(5K)= 1500 cm~2/V·s.Angle-dependent magnetoresistance reveals the quasi-two-dimensional(2D) Fermi surface in BaMnBi_2- A crossover from semiclassical MR~H~2dependence in low field to MR~H dependence in high field,which is attributed to the quantum limit of Dirac fermions,has been observed in magnetoresistance.Our results indicate the existence of Dirac fermions in BaMnBi_2.     

Binding Energies of Excitons in Square Quantum-Well Wires in the Presence of a Magnetic Field

The binding energies of the ground state of excitons in the GaAs/Ga1-xAlxAs square quantum-well wire in the presence of a magnetic field are investigated by using the variational method. It is assumed that the magnetic field is applied parallel to the axis of the wire. The calculations of the binding energy as a fimction of the wire size have been performed for infinite and finite confinement potentials. The contribution of the magnetic field makes the binding energy larger obviously, particularly for the wide wire, and the magnetic field is much more pronounced for the binding energy in a square quantum wire than that in a cylindrical quantum wire. The mismatch of effective masses between the well and the barrier is also considered in the calculation.     

Electron transport across a quantum wire embedding a saw-tooth superlattice

By developing the recursive Green function method, the transport properties through a quantum wire embedding a finite-length saw-tooth superlattice are studied in the presence of magnetic field. The effects of magnetic modulation and the geometric structures of the superlattice on transmission coefficient are discussed. It is shown that resonant peak splitting of this kind of structure is different from that of ‘magnetic' and ‘electric' superlattices in two-dimensional electron gas. The transmission spectrum can be tailored to match requirements through adjusting the size of saw-tooth quantum dot and field strength.     

Spatial Characteristics of Thomson Scattering Spectra in Laser and Magnetic Fields

Spatial characteristics of Thomson scattering spectra are studied for an electron moving in the circularly polarized laser field in the presence of a strong uniform magnetic field. The results show that the angular distributions of the spectra with respect to the azimuthal and polar angles exhibit different symmetries, respectively, which depend on the fields and electron parameters sensitively and significantly. Moreover, for relatively large parameters such as high laser intensity, high magnetic resonance parameter as well as large initial momentum of electron, the two lobes in spectra tend to the laser-propagating direction so that the radiation can be collimated in the forward direction. Furthermore, an important finding is that by choosing the appropriate fields and initial momentum of electron, the high frequency part of the Thomson scattering spectra can reach the frequency range of soft x-ray,in which a high radiation power per solid angle as ～10~(11) a.u. can be obtained.     

Double threshold behaviour of I－V characteristics of CoSi2/Si Schottky contacts

The forward current－voltage (I－V) characteristics of polycrystalline CoSi₂/n-Si(100) Schottky contacts have been measured in a wide temperature range. At low temperatures (≤200K), a plateau-like section is observed in the I－V characteristics around 10^-4A·cm^-2. The current in the small bias region significantly exceeds that expected by the model based on thermionic emission (TE) and a Gaussian distribution of Schottky barrier height (SBH). Such a double threshold behaviour can be explained by the barrier height inhomogeneity, i.e. at low temperatures the current through some patches with low SBH dominates at small bias region. With increasing bias voltage, the Ohmic effect becomes important and the current through the whole junction area exceeds the patch current, thus resulting in a plateau-like section in the I－V curves at moderate bias. For the polycrystalline CoSi₂/Si contacts studied in this paper, the apparent ideality factor of the patch current is much larger than that calculated from the TE model taking the pinch-off effect into account. This suggests that the current flowing through these patches is of the tunnelling type, rather than the thermionic emission type. The experimental I－V characteristics can be fitted reasonably well in the whole temperature region using the model based on tunnelling and pinch-off.     

Simple theoretical analysis of the Einstein’s photoemission from quantum confined superlattices

In this paper, we study the Einstein’s photoemission from III–V, II–VI, IV–VI and HgTe/CdTe quantum well superlattices (QWSLs) with graded interfaces and quantum well effective mass superlattices in the presence of a quantizing magnetic field on the basis of newly formulated dispersion relations in the respective cases. Besides, the same has been studied from the afore-mentioned quantum dot superlattices and it appears that the photoemission oscillates with increasing carrier degeneracy and quantizing magnetic field in different manners. In addition, the photoemission oscillates with film thickness and increasing photon energy in quantum steps together with the fact that the solution of the Boltzmann transport equation will introduce new physical ideas and new experimental findings under different external conditions. The influence of band structure is apparent from all the figures and we have suggested three applications of the analyses of this paper in the fields of superlattices and microstructures.     

Interband absorption and exciton binding energy in an inverse parabolic quantum well under the magnetic field

We have investigated the effects of the magnetic field which is applied perpendicular to the growth direction of the well on the interband absorption and on the binding energy of the excitons in an GaAs/Ga1−xAlxAs inverse parabolic quantum well (IPQW) with different widths as well as different Al concentrations at the well center. The calculations were performed within the effective mass approximation, using a variational method. We observe that IPQW structure turns into parabolic quantum well with the inversion effect of the magnetic field and the effective band gap of the system can be modified by changing Al concentration at the well center, the strength of the magnetic field and well dimensions. This case directly influences the nature of electronic and optical properties in this structure.     

Nonequilibrium transport through a Kondo dot in a magnetic field: perturbation theory and poor man's scaling

We consider electron transport through a quantum dot described by the Kondo model in the regime of large transport voltage V in the presence of a magnetic field B with max((V,B)>T(K). The electric current I and the local magnetization M are found to be universal functions of V/T(K) and B/T(K), where T(K) is the equilibrium Kondo temperature. We present a generalization of the perturbative renormali-zation group to frequency dependent coupling functions, as necessitated by the structure of bare perturbation theory. We calculate I and M within a poor man's scaling approach and find excellent agreement with experiment.     

Spin Splitting in Different Semiconductor Quantum Wells

We theoretically investigate the spin splitting in four undoped asymmetric quantum wells in the absence of external electric field and magnetic field. The quantum well geometry dependence of spin splitting is studied with the Rashba and the Dresselhaus spin-orbit coupling included. The results show that the structure of quantum well plays an important role in spin splitting. The Rashba and the Dresselhaus spin splitting in four asymmetric quantum wells are quite different. The origin of the distinction is discussed in this work.     

Thermal conductivity in 3D NJL model under external magnetic field

The thermal conductivity of the (2+1)-dimensional NJL model in the presence of a constant magnetic field is calculated in the mean-field approximation and its different asymptotic regimes are analyzed. Taking into account the dynamical generation of a fermion mass due to the magnetic catalysis phenomenon, it is shown that for certain relations among the theory's parameters (particle width, temperature and magnetic field), the profile of the thermal conductivity versus the applied field exhibits kink- and plateau-like behaviors. We point out possible applications to planar condensed matter.     

Spin-flip mesoscopic transport through a quantum dot coupled to carbon nanotube terminals

We investigate mesoscopic transport through a system that consists of a central quantum dot (QD) and two single-wall carbon nanotube (SWCN) leads in the presence of a rotating magnetic field. The spin-flip effect is induced by the rotating magnetic field, and the tunnelling current is sensitively related to the spin-flip effect. We present the calculations of charge and spin current components to show the intimate relations to the SWCN leads. Zeeman effect is important when the applied magnetic field is strong enough. The current characteristics are quite different when the source-drain bias is zero (eV=0) and nonzero (eV≠0). The asymmetric peak and valley of spin current versus gate voltage exhibit Fano resonance. Multi-resonant peaks of spin current versus photon energy ħω reflect the structure of CN quantum wires, as well as the resonant photon absorption and emission effect. The matching-mismatching of channels in the CN leads and QD results in novel spin current structure by tuning the frequency.     

Depending on the electric and magnetic field of the linear optical absorption and rectification coefficient in triple quantum well

In this study, the alteration of the potential profile, the energy levels, the dipole matrix element and the resonant peaks of the linear optical absorption (OA) and optical rectification (OR) coefficients in GaAs/GaAlAs triple quantum well (TQW) are calculated as dependent on the applied electric field and the magnetic field. The results show that the shape of confined potential profile, the energy levels and the dipole moment matrix elements are changed as dependent on the external fields. Also, the resonant peaks of the OA and OR coefficients depend on the applied external field effects. Therefore, I hope that these results will provide important improvement in semiconductor device applications, for suitable choice of electric and magnetic field values. It may particularly be useful in technological applications that the structure of TQW changes with the strength and direction of the external electric field.     

与光场依赖强度耦合下运动纠缠双原子的周期量子回声

本文将Tavis-Cummings (T-C)模型推广到同时考虑原子运动及与光场依赖强度耦合的情况.运用全量子理论,研究了与光场依赖强度耦合下运动纠缠双原子量子态保真度演化.采用数值计算的方法,探讨了双原子初始纠缠因子θ、光场平均光子数n以及场模结构参量p对双原子量子态保真度演化的影响,解析分析了双原子周期量子回声的形成规律,揭示了其物理实质.结果表明,在场模结构参量p=1的情况下,无论光强和双原子初始纠缠因子如何取值,双原子均产生周期2π的量子回声;在强相干场n=30条件下,改变P从1到小于800时,Bell态双原子产生2π/P周期量子回声.当p≥800时,无论光场是真空场、弱相干场或强相干场,双原子量子态保真度恒为1,即第一类Bell态原子持续处于保真态;而当双原子初始纠缠因子为3π/4时,无论n、p取何值,双原子量子态保真度保持为1,双原子持续处于第二类Bell态的保真态.其结论表明,该推广模型具有很好的双原子周期量子回声性质,为纠缠双原子信息高保真输出及噪音环境下量子信息处理的实验实现提供了理论参量和物理载体.     

Static magneto-polarizability of cylindrical nanostructures

The static polarizability of cylindrical systems is shown to have a strong dependence on a uniform magnetic field applied parallel to the tube axis. This dependence is demonstrated by performing exact numerical diagonalizations of simple cylinders (rolled square lattices), armchair and zig-zag carbon nanotubes (rolled honeycomb lattices) for different electron-fillings. At low temperature, the polarizability as function of the magnetic field has a discontinuous character where plateau-like region are separated by sudden jumps or peaks. A one to one correspondence is pointed out between each discontinuity of the polarizability and the magnetic-field induced cross-over between the ground state and the first excited state. Our results suggest the possibility to use measurements of the static polarizability under magnetic field to get important informations about excited states of cylindrical systems such as carbon nanotubes. Received 29 March 2001 and Received in final form 8 August 2001