Temperature dependence of the properties of strong-coupling bipolaron in a quantum dot

Temperature dependence of the properties of strong-coupling bipolaron in a quantum dot (QD) is studied based on the Lee-Low-Pines-Huybrechts variational method and quantum statistical theory. Results of the numerical calculation show that the vibration frequency as well as the absolute value of the induced potential and the effective potential all increase with increasing coupling strength and temperature, respectively, and they also increase with decreasing relative distance of electrons. The bipolarons are closer and more stable when the temperature is higher and coupling strength is larger. The influence of radius of QD and dielectric constant ratio on the effective potential is little.     

量子点中弱耦合束缚极化子的内部激发态性质

采用线性组合算符和幺正变换方法研究库仑束缚势对量子点中弱耦合束缚极化子激发态性质的影响。计算了束缚极化子的振动频率、第一内部激发态能量、激发能量和共振频率随量子点的有效受限长度,电子-声子耦合强度和库仑束缚势的变化关系。结果表明:量子点中弱耦合束缚极化子的振动频率、第一内部激发态能量、激发能量和共振频率随量子点的有效受限长度的减少而迅速增大,随库仑束缚势的增加而增大。     

Effects of Thermal Lattice Vibration on the Effective Potential of Weak-Coupling Bipolaron in a Quantum Dot

Based on the Huybrechts' linear-combination operator,effects of thermal lattice vibration on the effective potential of weak-coupling bipolaron in semiconductor quantum dots are studied by using the LLP variational method and quantum statistical theory.The results show that the absolute value of the induced potential of the bipolaron increases with increasing the electron-phonon coupling strength,but decreases with increasing the temperature and the distance of electrons,respectively;the absolute value of the effective potential increases with increasing the radius of the quantum dot,electron-phonon coupling strength and the distance of electrons,respectively,but decreases with increasing the temperature;the temperature and electron-phonon interaction have the important influence on the formation and state properties of the bipolaron:the bipolarons in the bound state are closer and more stable when the electron-phonon coupling strength is larger or the temperature is lower;the confinement potential and coulomb repulsive potential between electrons are unfavorable to the formation of bipolarons in the bound state.     

磁场和库仑场对抛物量子点中弱耦合极化子激发态性质的影响

采用线性组合算符和幺正变换方法研究了磁场和库仑场对抛物量子点中极化子激发态性质的影响。导出了抛物量子点中弱耦合束缚磁极化子的振动频率、第一内部激发态能量、激发能量与量子点的有效受限长度、库仑束缚势和磁场的回旋频率之间的变化关系。通过数值计算,结果表明:抛物量子点中弱耦合束缚极化子的振动频率、第一内部激发态能量、激发能量均随量子点的有效受限长度减小而迅速增大。随库仑束缚势增大而增大。随磁场的回旋频率的增加而增大。     

库仑场对量子点中强耦合束缚极化子激发态性质的影响

采用线性组合算符和幺正变换方法研究量子点中强耦合束缚极化子的振动频率、第一内部激发态能量、激发能量和共振频率的性质。讨论了这些量随量子点的有效受限长度、电子-声子耦合强度和库仑束缚势的变化关系。通过数值计算结果表明:量子点中强耦合束缚极化子的振动频率、第一内部激发态能量、激发能量和共振频率随量子点的有效受限长度的减小而迅速增大,随库仑束缚势和电子-声子的耦合强度的增加而增大。     

Influence of inter-dot Coulomb repulsion and exchange interactions on conductance through double quantum dot

Conductance and other physical quantities are calculated in double quantum dots (DQD) connected in series in the limit of coherent tunnelling using a Green's function technique. The inter-dot Coulomb repulsion and the exchange interaction are studied by means of the Kotliar and Ruckenstein slave-boson mean-field approach. The crossover from the atomic to the molecular limit is analyzed in order to show how the conductance in the model depends on the competition between the level broadening (dot-lead coupling) and the dot-dot transmission. The double Kondo effect was found in the gate voltage characteristics of the conductance in the atomic limit. In the case, when each dot accommodates one electron, the Kondo resonant states are formed between dots and their adjacent leads and transport is dominated by hopping between these two resonances. In the molecular limit the conductance vanishes for sufficiently low gate voltages, which means the Kondo effect disappeared. For small dot-lead coupling the transport characteristics are very sensitive on the influence of the inter-dot Coulomb repulsion and the position of the local energy level. The resonance region is widened with increase of the inter-dot Coulomb interactions while the exchange interaction has opposite influence.     

磁场对非对称量子点中束缚磁极化子性质的影响

采用线性组合算符和幺正变换方法研究磁场对非对称量子点中弱耦合束缚磁极化子性质的影响。导出量子点中弱耦合束缚磁极化子振动频率和基态能量随量子点的横向和纵向有效受限长度、库仑束缚势、磁场的回旋共振频率和电子-声子耦合强度的变化关系。数值计算结果表明:非对称量子点中弱耦合束缚磁极化子的振动频率和基态能量随量子点的横向和纵向有效受限长度的减小而迅速增大。振动频率随库仑束缚势和磁场的回旋共振频率的增加而增大。基态能量随库仑束缚势和电子-声子耦合强度的增加而减小。     

Quantum dot multiexcitons in a magnetic field

Multiexcitons confined in InGaAs/GaAs quantum dots (QDs) with a lateral size slightly exceeding the exciton Bohr radius are investigated by magnetophotoluminescence spectroscopy at 2 K. The Coulomb correlations in the two-exciton complex result in an additional confinement, which increases with decreasing dot size, while a magnetic field reduces this effect. A three-exciton complex is confined only by the geometric confinement potential of the QD. The exciton-exciton repulsion increases with decreasing dot size, while a magnetic field decreases the repulsion strongly when the magnetic length becomes smaller than the lateral size of the QD. A shell model for the QD multiexciton states is proposed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 263–268 (25 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Effect of Parabolic Potential on Bipolaron

We apply a Feynman path-integral variational approach combining with the average for the relative motion to study the stability of bipolaron in a quantum dot. The binding energy is calculated in different parameters. We find that an optimum quantum potential favors the formation of bipolaron. Compared with other methods in literature, the present approach is better than Laudau Pekar one in all coupling regime and full path-integral one in the strong coupling regime.     

Effect of Parabolic Potential on Bipolaron

We apply a Feynman path-integral variational approach combining with the average for the relative motion to study the stability of bipolaron in a quantum dot. The binding energy is calculated in different parameters. We find that an optimum quantum potential favors the formation of bipolaron. Compared with other methods in literature, the present pproach is better than Laudau-Pekar one in all coupling regime and full path-integral one in the strong coupling regime.     

抛物量子点中弱耦合束缚极化子的相互作用能

研究了抛物量子点中弱耦合束缚极化子的性质,采用改进的线性组合算符和幺正变换方法导出了束缚极化子的振动频率、有效质量和相互作用能。讨论了量子点的有效受限长度、电子LO声子耦合强度和库仑场对抛物量子点中弱耦合极化子的振动频率、有效质量和相互作用能的影响。数值计算结果表明:弱耦合束缚极化子的振动频率和相互作用能随有效受限长度的减少而急剧增大,振动频率随库仑势以及电子LO声子耦合强度的增加而增加,而相互作用能随库仑势以及电子LO声子耦合强度的增加而减小。有效质量仅与电子LO声子耦合强度有关。     

Superlight small bipolarons in the presence of a strong Coulomb repulsion

We study a lattice bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte Carlo algorithm to solve the two-particle Coulomb-Fr?hlich model. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The bipolaron effective mass and radius are computed. Bipolarons on lattices constructed from triangular plaquettes have a novel crablike motion, and are small but very light over a wide range of parameters. We discuss the conditions under which such particles may form a Bose-Einstein condensate with high transition temperature, proposing a route to room temperature superconductivity.     

量子点中极化子的内部激发态性质

采用Huybrechts线性组合算符和幺正变换方法研究了抛物量子点中的强、弱耦合极化子的激发态性质。分别导出强、弱耦合情况下,抛物量子点中的极化子的第一内部激发态能量、激发能量、共振频率与量子点的有效受限长度和电子-声子耦合强度的关系。数值计算结果表明,量子点中弱耦合和强耦合极化子的内部激发态能量、激发能量和共振频率都随量子点的有效受限长度的减小而迅速增大。弱耦合极化子的第一内部激发态能量随电子-声子耦合强度的增加而减少;而强耦合极化子的振动频率随量子点的有效受限长度的减小而迅速增加。弱耦合极化子的第一内部激发态能量、激发能量和共振频率随电子-声子耦合强度的增加而减小。     

Bound polaron in a two- and three-dimensional quantum dot for arbitrary electron–phonon coupling strength

A variational approach is presented for calculating the ground-state (GS) binding energies of an electron bound to a Coulomb impurity in a polar semiconductor quantum dot (QD) with parabolic confinement in both two and three dimensions. We perform calculations for the entire range of the electron-phonon coupling constant and the Coulomb binding parameter and for arbitrary confinement length. It is found that the polaronic effect is stronger in a two dimensions (2D) dot than in a three dimensions (3D) dot and this trend is more pronounced with the increase of the coupling constant. Furthermore, the GS binding energy increases with increasing the Coulomb binding parameter in both 2D and 3D QDs for the same electron–phonon coupling constant. The results also indicate that this effect becomes much more pronounced with decreasing dimensionality.     

库仑场对非对称量子点中强耦合极化子声子平均数的影响

采用线性组合算符和幺正变换方法研究库仑场对非对称量子点中强耦合极化子振动频率和声子平均数的影响。导出量子点中强耦合束缚极化子振动频率和声子平均数随量子点的横向和纵向有效受限长度,库仑束缚势和电子-声子耦合强度的变化关系。数值计算结果表明:非对称量子点中强耦合束缚极化子的振动频率和声子平均数随量子点的横向和纵向有效受限长度的减小而迅速增大。随库仑束缚势和电子-声子耦合强度的增加而增大。     

EFFECT OF DIELECTRIC CONSTANT ON THE EXCITON GROUND STATE ENERGY OF CdSe QUANTUM DOTS

The B-spline technique is used in the calculation of the exciton ground state energy based on the effective mass approximation (EMA) model. The exciton is confined in CdSe microspherical crystallites with a finite-height potential wall (dots). In this approach, (a) the wave function is allowed to penetrate to the outside of the dots; (b) the dielectric constants of the quantum dot and the surrounding material are considered to be different; and (c) the dielectric constant of the dots are size-dependent. The exciton energies as functions of radii of the dots in the range 0.5-3.5 nm are calculated and compared with experimental and previous theoretical data. The results show that: (1) The exciton energy is convergent as the radius of the dot becomes very small. (2) A good agreement with the experimental data better than other theoretical results is achieved. (3) The penetration (or leaking) of the wave function and the difference of the dielectric constants in different regions are necessary for correcting the Coulomb interaction energy and reproducing experimental data. (4) The EMA model with B-spline technique can describe the status of excition confined in quantum dot very well.     

Inter-dot coupling effects on transport through correlated parallel coupled quantum dots

Transport through symmetric parallel coupled quantum dot system has been studied, using non-equilibrium Green function formalism. The inter-dot tunnelling with on-dot and inter-dot Coulomb repulsion is included. The transmission coefficient and Landaur-Buttiker like current formula are shown in terms of internal states of quantum dots. The effect of inter-dot tunnelling on transport properties has been explored. Results, in intermediate inter-dot coupling regime show signatures of merger of two dots to form a single composite dot and in strong coupling regime the behaviour of the system resembles the two decoupled dots.      

声子色散和库仑势对量子点中极化子基态能量的影响

本文在声子色散和库仑束缚势的影响下利用压缩态变分法计算了抛物量子点中弱耦合极化子的基态能量。采用的变分方法是基于逐次正则并且利用单模压缩态变换处理通常被我们所忽略的在第一次幺正变换中产生的声子产生湮灭算符的双线性项。计算得出了在考虑声子色散和库仑束缚势的情况下抛物量子点中弱耦合极化子的基态能量的数学表达式。讨论了在弱耦合情况下,受限长度,电子-声子耦合常数,色散系数,库仑结合参数与基态能量之间的依赖关系。     

Current bistability in resonant tunneling through a semiconductor quantum dot

We discuss resonant tunneling through quantum dot energy levels considering the charging energy of the dot. The hamiltonian of the system is reduced to a form of the Anderson hamiltonian of resonant tunneling. The mean-field approximation is applied and current–voltage characteristics are evaluated. The self-consistent solution is investigated for the low tunneling rate case in the low-temperature condition. The current bistability and the related current hysteresis are pointed out. The Coulomb staircase is shown in the current–voltage characteristics. These features are all due to Coulomb repulsion within the dot.     

库仑场对抛物量子点中强耦合极化子性质的影响

采用线性组合算符和幺正变换方法研究了在库仑场束缚下抛物量子点中强耦合束缚极化子的振动频率和基态能量。并对其进行了数值计算,结果表明:强耦合束缚极化子的振动频率和基态能量随量子点的有效受限长度的增加而减小,随电子-LO声子耦合强度的增加而增加,束缚极化子的基态能量随库仑势的增加而减小。