The effect of finiteness on the self-trapping of an electron in a one-dimensional lattice

The competition in the ground state between the “single site” symmetry due to the electron-lattice interaction and the “two-site” symmetry which exists in even-N lattices causes the electron to abruptly localize on a single site when the electron-phonon coupling exceeds a threshold. The relative sizes of the electronic wavefunction and of the lattice affect the dependence of the energy on N.     

Numerical simulation of the temperature dependence of the photoluminescence spectra of InAs/GaAs quantum dots

A mathematical model is proposed for describing the temperature dependence of the photoluminescence spectrum of self-ordered arrays of quantum dots with due regard for the electron-phonon interaction and different transfer processes in the “quantum dot-wetting layer-barrier” system. This model, as applied to analysis of the experimental spectra of InAs quantum dots grown on GaAs vicinal substrates, makes it possible to separate the manifestations of different mechanisms of excitation transfer in the photoluminescence spectra and to relate the observed temperature dependences of the spectra to the structural features of the quantum-dot array.     

Substrate influence in electron-phonon coupling measurements in thin Au films

Accurate understanding and measurement of the energy transfer mechanisms during thermal nonequilibrium between electrons and the surrounding material systems is critical for a wide array of applications. With device dimensions decreasing to sizes on the order of the thermal penetration depth, the equilibration of the electrons could be effected by boundary effects in addition to electron-phonon coupling. In this study, the rate of electron equilibration in 20 nm thick Au films is measured with the Transient ThermoReflectance (TTR) technique. At very large incident laser fluences which result in very high electron temperatures, the electron-phonon coupling factors determined from TTR measurements deduced using traditional models are almost an order of magnitude greater than predicted from theory. By taking excess electron energy loss via electron-substrate transport into account with a proposed three temperature model, TTR electron-phonon coupling factor measurements are more in line with theory, indicating that in highly nonequilibrium situations, the high temperature electron system looses substantial energy to the substrate in addition to that transferred to the film lattice through coupling.     

库的量子关联相干辅助系统能量提取的研究

基于单模微腔与二能级原子系综(库)构成的混合动力学模型,探索了非平衡库中量子关联相干(quantum correlated coherence, QCC)[Tan K C, et al. 2016 Phys. Rev. A 94, 022329])对系统动力学的影响.推导了量子关联相干库下系统演化的动力学方程.借助于含QCC的类GHZ库及其对应的参考库,清晰地揭示了非平衡库中QCC扮演着热力学资源的角色——能够有效辅助系统从库中提取更多能量.同时,结合解析与数值模拟方法研究了类GHZ库的有效温度和系统与库间的耦合参数对QCC能量效应的影响.研究发现, QCC对腔场的能量贡献不仅依赖于库的有效温度,而且也和系统与库间的耦合参数有关.这与二能级原子构成的传统的热库的情况(腔场从热库中提取的能量仅仅依赖于库的有效温度即二能级原子的热布局)完全不同.此外,研究发现QCC可视作一类优质的热力学资源,在特定条件下其对系统的能量贡献远大于原子热布局的贡献.因此, QCC将是高输出功率或高效率量子热机设计中的一类重要燃料.     

The polaron state of a crystal

The quantum mechanics of an electron-nuclear system with strong electron-phonon coupling is considered. First, a two-site model is treated in the adiabatic approximation. As the coupling constant increases, electron transfer undergoes qualitative changes; more specifically, a potential barrier forms in the adiabatic potential, the electron transfer becomes associated with the tunneling of nuclei through the barrier, and the level splitting in the system falls off exponentially. The properties of a similar crystal model are discussed. It is shown that electron transfer in a crystal in the case of strong coupling is likewise associated with the tunneling of nuclei through barriers in the deformation space. Strong coupling modifies the electron-electron interaction terms. The Hamiltonian (exchange) terms, which are not associated with electron transfer, are only weakly modified. At the same time, the terms involving transfer (the band terms) undergo exponential reduction and vanish in the limit as M → ∞ (M is the ion mass) and the carriers become small polarons. This reduction provides a basis for the natural mechanism of enhancement of the isotope effect.     

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。     

Statistical theory of electron transport in open electron-phonon systems

Within the framework of general statistical mechanics of irreversible processes the electrical resistivity of an open electron-phonon system is calculated. By means of the projection operator technique an evolution equation for coupled subsystems in a heat bath is derived and specialized for electrons and longitudinal phonons, the latter being coupled to a bath of transverse phonons. The influence of heating of the electron-phonon system is investigated and the question of validity of the linear response theory with or without inclusion of a dissipative mechanism is discussed. In the balance equation for the total electron momentum, terms of only third (and higher) order in the electrical field strength and the current density appear; consequently, the transverse phonons act only as a “momentum bath”. A general resistivity formula is derived containing the Bloch-Grüneisen law as a special case and including corrections due to phonon drag up to infinite order without a partial summation.     

Inelastic light scattering in the Eu and Yb monochalcogenides

Multiphonon inelastic light scattering has been investigated in the magnetically ordering Eu and in the diamagnetic Yb monochalocogenides. All aspects of this scattering suggest that the multi-phonon lines results from recombination during time resolved relaxation ot the excited “hot” electron. The multiphonon scattering in these compounds is therefore interpreted by a two-step process of absorption followed by emission (hot luminescence). The observation of zone-center and zone-boundary multiphonon scattering is related to the electron-phonon coupling which is dependent on the kinetic energy of the excited photo-electron. Hence, with excitation into the bottom of the conduction band the coupling to the phonon system is dominated by the Fröhlich polaron concept, leading to zone-center LO scattering, whereas at higher excitation energies the electron-phonon coupling is no longer selective and is dependent primarily on the LO phonon density of states. The relaxation process and the electron-phonon coupling are strongly dependent on magnetic order. However, by comparison of the Eu monochalcogenides with the corresponding Yb compounds it is evident that the general phenomenon of multiphonon scattering in these compounds is independent of a spin system and is determined alone by the unique band structure.     

On the stability of the polaron in one dimensional disordered systems

A one-dimensional disordered system of electrons described by a tight binding model interacting with vibrational degrees of freedom (in harmonic approximation) is considered. A stable configuration is determined by a numerical minimization of the total energy which is based on the adiabatic approximation. The behaviour of the electron density (charge density wave) and the density of states is analysed. The localization properties are investigated as well. In contrast to the corresponding disordered system with vanishing electron-phonon coupling the present model has an energy gap. The formation of the gap and the polaron band is shown to be quite different for both onsite and intersite types of coupling terms. For large disorder, the lattice distortion and the gap disappear if only the vibrational contribution to the intersite coupling is important. They increase, however, if only the vibrational contribution to the site energies is taken into account. In both cases the localization length decreases upon increasing the electron-phonon coupling energy. The results are discussed with respect to low dimensional organic materials and amorphous semiconductors.     

Observation of electron gas cooling in free sodium clusters

Free size-selected Na+(n) (n = 16-250) clusters have been studied by femtosecond pump-probe photoelectron and photofragmentation spectroscopy. Thermal electron emission from the hot electron gas was used to monitor the energy transfer from the electronic system to lattice vibrations. The electron-phonon coupling constants determined for the different sizes can be described by the radius dependent function g(R) = (2.3 + 114 A2/R2) X 10(16) W/m3K. No strong quantum size effect was observed even for the smallest cluster size.     

声子色散对量子点中弱耦合磁极化子声子平均数的影响

利用线性组合算符和幺正变换相结合的方法,研究了声子色散对抛物量子点中弱耦合磁极化子电子周围光学声子平均数的影响．计及纵光学(LO)声子色散,在抛物近似下导出了基态能量与量子点有效受限长度、声子色散系数、回旋共振频率以及电子-声子耦合常数之间的关系,电子周围光学声子平均数与声子色散系数以及电子-声子耦合常数的关系．数值计算结果表明在弱耦合情况下抛物量子点中磁极化子的基态能量随声子色散系数的增大而减小;电子周围光学声子平均数随声子色散系数增大而增大,随电子-声子耦合常数的增大而增大．     

声子色散对量子点中弱耦合磁极化子声子平均数的影响

利用线性组合算符和幺正变换相结合的方法,研究了声子色散对抛物量子点中弱耦合磁极化子电子周围光学声子平均数的影响。计及纵光学（ LO）声子色散,在抛物近似下导出了基态能量与量子点有效受限长度、声子色散系数、回旋共振频率以及电子－声子耦合常数之间的关系,电子周围光学声子平均数与声子色散系数以及电子－声子耦合常数的关系。数值计算结果表明在弱耦合情况下抛物量子点中磁极化子的基态能量随声子色散系数的增大而减小;电子周围光学声子平均数随声子色散系数增大而增大,随电子－声子耦合常数的增大而增大。     

Theory of resonant tunneling through quantum dots in the presence of electron-phonon interaction

Structures where the electrons of a two-dimensional electron gas are confined to disconnected regions can be fabricated by the use of appropriate gate geometries. The transport between these electrostatically defined quantum dots takes place by tunneling. Using the tunneling Hamiltonian approach we present a theoretical model of the system including electron-phonon interaction. The relevant coupling constants are determined from realistic wave functions for the expected confinement potentials. The phonon part of the Hamiltonian is diagonalized using a canonical transformation. Starting from the determination of the transmission matrix for the interacting system we calculate the current-voltage characteristics for different temperatures and phonon coupling strengths.     

New polaron effect in magnetooptic phenomena in a quantum well

It is predicted that resonance coupling between two discrete electron energy levels corresponding to different size-quantization quantum numbers and different Landau quantum numbers can occur in a quantum well in a quantizing magnetic field. The resonance coupling is due to the interaction of an electron with LO phonons and results in the formation of polaron states of a new type. It is shown that for a certain value of the magnetic field, which depends on the splitting of the electron size-quantization levels, the absorption peak and the two-phonon resonance Raman scattering peak split into two components, the separation between which is determined by the electron-phonon coupling constant. The resonance coupling between size-quantization levels with the same Landau quantum numbers is also studied. The splitting of the peaks in this case is virtually independent of the magnetic field and can be observed in much weaker fields. The experimental observation of the effect will make it possible to determine the relative position of the electronic levels and the electron-phonon coupling constant. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 7, 511–515 (10 April 1997)     

非对称量子点中电子的激发能量和跃迁谱线频率

研究了非对称量子点中与声子强耦合的电子的性质.采用线性组合算符和幺正变换方法研究非对称量子点中与声子强耦合的电子的第一内部激发态能量、激发能量和第一内部激发态到基态的跃迁谱线频率随量子点的横向和纵向有效受限长度,电子-声子耦合强度的变化关系。数值计算结果表明：非对称量子点中与声子强耦合的电子的第一内部激发态能量、激发能量和第一内部激发态到基态的跃迁谱线频率随量子点的横向和纵向有效受限长度的减小而迅速增大,表现出奇特的量子尺寸效应。非对称量子点中与声子强耦合的电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小。非对称量子点中与声子强耦合的电子的激发能量和第一内部激发态到基态的跃迁谱线频率随电子-声子耦合强度的增加而增大。     

Electronic spectrum and localization of electronic states in aperiodic quantum dot chains

The electronic energy spectra of aperiodic Thue-Morse, Rudin-Shapiro, and double-periodic quantum dot chains are investigated in the tight-binding approximation. The dependence of the spectrum on all parameters of a “mixed” aperiodic chain model is studied: the electronic energy at quantum dots and the hopping integrals. The electronic degree of localization in the chains under consideration is determined by analyzing the inverse participation ratio. Its spectral distribution and the dependence of the band-averaged degree of localization on these model parameters have been calculated. It is shown that a transition of the system’s sites to a resonant state in which the degree of electron localization decreases, while an overlap between the subbands occurs in the spectrum is possible when the parameters are varied.     

Hydrogenic impurity bound polaron in a parabolic quantum dot with arbitrary electron-phonon coupling strength

A variational approach is employed to obtain the ground and the first excited state binding energies of an electron bound to a hydrogenic impurity in a polar semiconductor quantum dot (QD) with symmetric 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 binding energy of ground and first excited state is larger in a two-dimension (2D) dot than in a three-dimension (3D) dot and this trend is more pronounced with the increase of the electron-phonon coupling constant for the same value of the Coulomb binding parameter and confinement length. Furthermore, the ground and the first excited state binding energy increases with increasing the Coulomb binding parameter in both 2D and 3D QDs for the same electron-phonon coupling constant.