Bound polaron in a quantum pseudodot under Rashba effect

In the present work, the influence of Rashba effect on bound polaron in a quantum pseudodot is studied. Using the Lee–Low–Pines unitary transformation method and the Pekar type variational procedure, we have derived an expression for the bound polaron ground state energy. The ground state energy as functions of the wave vector, the electron–phonon coupling strength, and quantum confinement size is obtained by considering different Coulomb bound potentials. It is found that (i) the ground state energy is decreased with raising the Coulomb bound potential, the electron–phonon coupling strength, and quantum confinement size. (ii) The ground state energy increases when the wave vector is increasing. (iii) The ground state energy splits into two branches (spin-up and spin-down) due to the Rashba effect.     

The energy levels of a two-electron two-dimensional parabolic quantum dot

This paper studies the two-electron total energy and the energy of the electron--electron interaction by using a variational method of Pekar type on the condition of electric--LO-phonon strong coupling in a parabolic quantum dot. It considers the following three cases: 1) two electrons are in the ground state; 2) one electron is in the ground state, the other is in the first-excited state; 3) two electrons are in the first-excited state. The relations of the two-electron total energy and the energy of the electron--electron interaction on the Coulomb binding parameter, the electron-LO-phonon coupling constant and the confinement length of the quantum dot are derived in the three cases.     

Squeezed magnetobipolarons in two-dimensional quantum dot

In this Letter, a different method was given for calculating the energies of the magnetobipolarons confined in a parabolic QD (quantum dot). We introduced single-mode squeezed states transformation, which are based on the Lee-Low-Pines and Huybrechts (LLP-H) canonical transformations. This method can provide results not only for the ground state energy but also for the excited states energies. Moreover, it can be applied to the entire range of the electron-phonon coupling strength. Comparing with the results of the LLP-H transformations, we have obtained more accurate results for the ground state energy, excited states energies and binding energy of the bipolarons. It shows that the magnetic field and the quantum dot can facilitate the formation of the bipolarons when η is smaller than some value.     

量子点中强耦合极化子的性质

采用Pekar类型的变分方法研究了抛物量子点中强耦合极化子的基态和激发态的性质。计算了基态和激发态极化子的结合能、光学声子平均数和极化子的共振频率。讨论了这些量对有效限制强度和电子 体纵光学声子耦合强度的依赖关系。结果表明:抛物量子点中极化子的共振频率、基态和激发态极化子的结合能以及光学声子平均数都随量子点的有效束缚强度的增大而减小。光学声子平均数随电子 体纵光学声子耦合强度的增加而增大。     

抛物量子点中强耦合磁极化子的性质

采用Pekar类型的变分方法研究了抛物量子点中强耦合磁极化子的基态和激发态的性质。计算了基态和激发态磁极化子的束缚能以及磁极化子的共振频率。讨论了这些量对回旋频率和有效限制强度的依赖关系,以及磁极化子光学声子平均数的性质,结果表明:由于Zeeman劈裂,抛物量子点中磁极化子的回旋共振频率劈裂为两支。基态和激发态磁极化子的束缚能以及磁极化子的共振频率都随回旋频率的增加而增大,随量子点的有效束缚强度的增大而减小。     

Rayleigh-Schrödinger perturbation theory for electron-phonon interaction in two dimensional quantum dots with asymmetric parabolic potential

Within the framework of the second-order Rayleigh-Schr?dinger perturbation theory, we investigate the effects of the interaction of the electron and longitudinal-optical phonons in two-dimensional semiconductive quantum dots with respect to a general potential. We propose a simple expression for the ground state energy, and compare it with those obtained by Landau-Pekar strong coupling theory. It is shown both analytically and numerically that the results obtained from the second-order Rayleigh-Schr?dinger perturbation theory could be better than those from Landau-Pekar strong coupling theory when the coupling constant is sufficiently small. Moreover, some interesting problems, such as polarons in quasi-one-dimensional quantum wires, and quasi-zero-dimensional asymmetric or symmetric quantum dots can be easily discussed only by taking different limits. After the numerical calculations, we find that there exists a simple dimensional scaling and symmetry relation for the ground state polaron energy. Furthermore, we apply our results to some weak-coupling polar semiconductors such as GaAs, CdS. It is shown that the polaronic effects are found to be quiet appreciable if the confinement lengths and smaller than a few nanometers. Received: 3 December 1997 / Revised: 6 July 1998 / Accepted: 17 September 1998     

New polaronic excitons in ferroelectric oxides: phenomenological theory

It is shown that three types of new polaronic excitons exist in ferroelectric oxides-charge transfer vibronic excitons (CTVEs). The electron-hole bipolaron pairs are common in all these cases, the difference having to do with the origin of charge transfer-lattice instabilities. These are the cases of the charge transfer and lattice distortions and their vibronic interactions induced in the harmonic (i) and anharmonic (ii) regimes both in the electronic excited state on the one hand, and the (iii) case of the CTVE excited vibronic state induced in the anharmonic regime but related with electronic ground state on the other. All these CTVEs are characterized by strong coupling with good self-localization conditions in separate orientation wells of their multi-well potentials. The relevant phenomenological theory is developed. While the (i) type CTVE is confirmed by luminescence experiments, the (iii) type CTVE with low excitation energy and with metastable intrinsic defect behaviour can be a candidate for the explanation of an order-disorder effect in the classical ferroelectric oxides discovered recently [Phys. Rev. Lett. 90 (2003) 037601].     

Electronic states in a Pöschl–Teller-like quantum well: Combined effects of electric field, hydrostatic pressure, and temperature

The work is devoted to present a theoretical study of the influences of external probes, such as applied electric field and hydrostatic pressure, on the electron and hole states in a Pöschl–Teller quantum well. The calculations have been done in the framework of the variational method. The dependence of the ground state energy of an electron and/or hole confined in the quantum well has been obtained as a function of the applied electric field and hydrostatic pressure. Different values of the asymmetry parameters of the Pöschl–Teller potential as well as temperature have been considered. It is shown that as a result of the increase in the electric field there is an augment of the ground state energy, and also that by increasing the quantum well width the effects of applied electric field are strengthened. It is obtained from the calculations that the ground state energy is a decreasing (increasing) function of the hydrostatic pressure (temperature). It is found that in the high pressure regime the energy grows with pressure, which is a previously unknown result. In the case of holes, the energy is always an increasing function both of the pressure and the temperature. Besides, the behavior of the photoluminescence peak energies associated to transitions between the ground states of electrons and heavy holes in the system is also reported.     

Vibration analog of a superradiant quantum transition

A vibrational analog of the superradiant quantum transition (SQT) in a classical system of weakly bound oscillators of van der Pole-Duffing (self-generators), in which the coupling element is a linear oscillator, is described. Such an analog is a strongly modulated oscillatory process of almost complete periodic energy exchange between the generators. This type of mode is alternative to nonlinear normal modes (NNM) and close in its character to limiting phase trajectories (LPTs), which have been introduced recently as applied to conservative systems, but in contrast to them, as the attractor. It is shown that the necessary condition of the transition to intense energy exchange in the classical system is the instability of one of the NNMs similarly to that when the condition of the superradiant transition is the instability of the ground state in a quantum model.     

半导体量子阱中弱耦合磁极化子的性质

采用线性组合算符和改进的LLP变分法,研究了在考虑电子自旋情况下无限深量子阱中弱耦合磁极化子的性质。导出了弱耦合磁极化子的声子平均数、基态能量和电子自旋能量与磁极化子基态能量之比的绝对值的表达式。并对两种不同阱材料的量子阱进行了数值计算,结果表明:磁极化子的声子平均数随电子-LO声子耦合常数和阱宽的增加而增大,并且最终随着阱宽的增加而趋于体情况下的极限值;由于电子自旋能的作用使磁极化子的基态能量由不考虑电子自旋下的一条分裂为两条,并且它随阱宽和电子-LO声子耦合常数的增加而减小,随回旋共振频率(磁场)的增加而增大。电子自旋作用能否忽略由回旋共振频率和阱材料本身的性质决定。     

抛物形量子点中弱耦合极化子的性质

采用线性组合算符和幺正变换方法研究了抛物形量子点中弱耦合极化子的基态能量和束缚能。计算结果表明,基态能量和束缚能随有效束缚强度增加而减小。随着有效束缚强度逐渐加大,最后逐渐趋于体结构极化子的基态能量。当有效束缚强度给定,基态能随电子-体纵光学声子耦合强度增加而减小。由于有效束缚强度与量子点受限强度平方根成反比,所以量子点受限越强,基态能和束缚能越大,电子-体纵光学声子耦合强度的变化对量子点的影响越小。当量子点受限变弱时,电子-体纵光学声子耦合强度变化对量子点的影响变大。所以在量子点弱受限区域,极化子对量子点的影响不容忽略。     

Electron correlation in two-dimensional systems: CHNC approach to finite-temperature and spin-polarization effects

Applying the classical-map hypernetted-chain method (CHNC) developed recently by Dharma-wardana and Perrot, we have studied the temperature and spin-polarization effects on electron correlation in the uniform quantum two-dimensional gas (2DEG) over a wide range of temperature T and spin-polarization ζ. The quantum fluid at the temperature T is mapped to a classical fluid at the temperature T_cf given by T_cf²=T²+T_q², where the quantum temperature T_q is determined by comparing the calculated correlation energy to that of Monte Carlo results for the fully spin-polarized quantum system at zero temperature. By the iterative solution of the modified HNC equation and the Ornstein-Zernike equation, we have obtained the pair distribution function (PDF) and correlation energy for the two-component classical 2DEG with a classical fluid temperature T_cf. The anti-parallel bridge function B₁₂(r) appearing in the modified HNC equation is determined by using the Monte Carlo correlation energy at T=0 or STLS (Singwi-Tosi-Land-Sjölander) result at T>0 and the numerical solution to the Percus-Yevick (PY) equation for the system of hard disks. By calculating the Pauli potential, the bridge function, PDFs, structure factors and correlation energy, we have shown that in some cases, the properties of the uniform quantum 2DEG depend remarkably on the temperature and spin-polarization.     

High energy limit in rearrangement collisions

The cross section for particle exchange from ground state to ground state in a three-particle collision is investigated. An asymptotic expansion for high incident energies can be given if the Fourier transforms of the interaction potentials allow an expansion in powers of 1/k for largek, which includes Coulomb interaction. It is shown that in general the first and second Born approximations provide the asymptotically leading terms. For special mass ratios, scattering into special (critical) angles becomes predominant. These angles, which are independent of energy, can also be determined employing classical arguments. Near the critical angles single terms of either the first or second Born approximation become dominant. If the exchanged mass is small, the interaction between the two heavy masses cancels near forward direction, implying the validity of the impact parameter theory. The results are discussed for Coulomb interaction as the simplest example (electron exchange). Here, the contributions by critical scattering are small unless the energies become exceedingly large.     

Stochastic electrodynamics. III. Statistics of the perturbed harmonic oscillator-zero-point field system

In this third paper in a series on stochastic electrodynamics (SED), the nonrelativistic dipole approximation harmonic oscillator-zero-point field system is subjected to an arbitrary classical electromagnetic radiation field. The ensemble-averaged phase-space distribution and the two independent ensemble-averaged Liouville or Fokker-Planck equations that it satisfies are derived in closed form without furtner approximation. One of these Liouville equations is shown to be exactly equivalent to the usual Schrödinger equation supplemented by small radiative corrections and an explicit radiation reaction (RR) vector potential that is similar to the Crisp-Jaynes semiclassical theory (SCT) RR potential. The wave function in this SED Schrödinger equation is shown to have thea priori significance of position probability amplitude. The other Liouville equation has no counterpart in ordinary quantum mechanics, and is shown to restrict initial conditions such that (i) The Wigner-type phase-space distribution is always positive, (ii) in the absence of an applied field, the only allowed solution of both equations is the quantum ground state, and (iii) if a previously applied field is suddenly turned off, then spontaneous transitions occur, with no need for a triggering perturbation as in SCT, until the system is in the ground state. It is also shown that the oscillator energy is a fluctuating quantity that must take on a continuum of values, with average value equal to the quantum ground-state energy plus a contribution due to the applied classical field.     

Ab initio theory of complex electronic ground state of superconductors: II.: Antiadiabatic state—Ground state of superconductors

Based on Q, P-dependent modification of the Born-Oppenheimer approximation (BOA), the ab initio theory of complex electronic ground state of superconductors is presented. As an illustrative example, application of the theory to superconductors of a different character and to the corresponding nonsuperconducting analogues is presented. It is shown that due to electron-phonon (EP) interactions, which drive system from adiabatic into antiadiabatic state, adiabatic translation symmetry is broken and system is stabilized in antiadiabatic state at distorted geometry with respect to adiabatic equilibrium high-symmetry structure. Stabilization effect in the antiadiabatic state is due to strong dependence of the electronic motion on the instantaneous nuclear kinetic energy, i.e. on the effect that is neglected on the adiabatic level within the BOA. At distorted geometry, antiadiabatic ground state is geometrically degenerated with fluxional nuclear configurations in the phonon modes that drive system into this state. It has been shown that until the system remains in antiadiabatic state, nonadiabatic polaron-renormalized phonon interactions are zero in the well-defined k-region of reciprocal lattice. This, along with geometric degeneracy of the antiadiabatic state, enables formation of mobile bipolarons that can move over lattice as supercarriers without dissipation. Moreover, it has been shown that due to EP interactions at transition into antiadiabatic state, k-dependent gap in one-electron spectrum has been opened. Gap opening is associated with shift of the original adiabatic Hartree-Fock orbital energies and with the k-dependent change in density of states of particular band(s) at Fermi level. Corrected one-particle spectrum enables to derive thermodynamic properties in full agreement with corresponding thermodynamic properties of superconductors.Based on the complex ab initio theory, it has been shown that Fröhlich's effective attractive electron-electron interaction term represents correction to electron correlation energy at transition from adiabatic into antiadiabatic state due to EP interactions. It has been shown that increased electron correlation is a consequence of stabilization of the system in superconducting electronic ground state, but not the reason for its formation.     

声子之间相互作用对量子点中极化子性质的影响

研究了抛物量子点中弱耦合极化子的性质。采用线性组合算符和微扰法,导出了抛物量子点中极化子的基态能量。当计及电子在反冲效应中发射和吸收不同波矢的声子之间的相互作用时,讨论了对量子点中极化子的基态能量的影响。通过数值计算,结果表明,量子点中极化子基态能量随量子点的有效受限长度的减小而迅速增大,随电子-LO声子的耦合强度的增加而减少。当l₀>1.4时,声子之间的相互作用不能忽略。     

Magnetic field effect on state energies and transition frequency of a strong-coupling polaron in an anisotropic quantum dot

By employing a variational method of the Pekar-type, which has different variational parameters in the x–y plane and the z-direction, we study the ground and the first excited state energies and transition frequency between the ground and the first excited states of a strong-coupling polaron in an anisotropic quantum dot (AQD) under an applied magnetic field along the z-direction. The effects of the magnetic field and the electron–phonon coupling strength are taken into account. It is found that the ground and the first excited state energies and the transition frequency are increasing functions of the external applied magnetic field. The ground state and the first excited state energies are decreasing functions, whereas transition frequency is an increasing function of the electron–phonon coupling strength. We find two ways of tuning the state energies and the transition frequency: by adjusting (1) the magnetic field and (2) the electron–phonon coupling strength.     

I–V characterization of a quantum well infrared photodetector with stepped and graded barriers

I–V characterization of an n-type quantum well infrared photodetector which consists of stepped and graded barriers has been done under dark at temperatures between 20–300 K. Different current transport mechanisms and transition between them have been observed at temperature around 47 K. Activation energies of the electrons at various bias voltages have been obtained from the temperature dependent I–V measurements. Activation energy at zero bias has been calculated by extrapolating the bias dependence of the activation energies. Ground state energies and barrier heights of the four different quantum wells have been calculated by using an iterative technique, which depends on experimentally obtained activation energy. Ground state energies also have been calculated with transfer matrix technique and compared with iteration results. Incorporating the effect of high electron density induced electron exchange interaction on ground state energies; more consistent results with theoretical transfer matrix calculations have been obtained.     

Spontaneous distortion in the spin-1/2 Ising-Heisenberg model on decorated planar lattices with a magnetoelastic coupling

Magnetoelastic properties of the spin-1/2 Ising-Heisenberg model on doubly decorated planar lattices partially amenable to lattice vibrations are examined within the framework of the harmonic approximation and decoration-iteration transformation. It is shown that the magnetoelastic coupling may lead to a spontaneous distortion of the vibrating decorating atoms and the mutual interplay between quantum spin fluctuations and local lattice deformations enhances typical quantum features like the quantum reduction of the magnetization in the ground state of the quantum antiferromagnetic phase, while it does not affect the ground-state behaviour of the classical ferromagnetic phase. It also turns out that the spontaneous distortion is responsible for a much more pronounced reduction of the critical temperature in the quantum antiferromagnetic phase than in the classical ferromagnetic phase.     

Exact solution of a large class of interacting quantum systems exhibiting ground state singularities

We demonstrate how to construct a large class of interacting quantum systems for which an exact solution may be found for the ground state wave function and ground state energy for some range of interaction parameters. It is shown that the ground state exhibits singularities in these cases, and in some simple instances the exact ground state phase diagram and critical indices are also found.