Optical properties of an interacting large polaron gas

The normal state conductivity, , of a system of interacting large polarons is calculated within the Random Phase approximation and some numerical results are presented. The behaviour of the optical absorption as a function of the charge carrier density and of the temperature is analyzed for different values of the electron-phonon coupling constant. It is shown that exhibits features similar to those observed in the infrared spectra of the cuprates. Received 27 January 1999     

Normal state properties of an interacting large polaron gas

A simple approach to the many-polaron problem for both weak and intermediate electron-phonon coupling and valid for densities much smaller than those typical of metals is presented. Within the model the total energy, the collective modes and the single-particle properties are studied and compared with the available theories. It is shown the occurrence of a charge density wave instability in the intermediate coupling regime. Received 13 May 1998     

有限格点一维Holstein极化子研究

利用相干态正交化展开法,得到了一维Holstein模型基态能量的解析表达式.为了便于比较,将系统的基态试探波函数逐级展开到三级近似,计算了不同格点、不同耦合强度下的基态能量,在展开到3级近似时,所得结果与数值计算一致. 关键词： 相干态正交化展开 极化子 模拟退火方法     

Superconductivity mediated by the lattice fluctuations in the polaron system

The coupling between the charge transfer and the lattice fluctuations is proposed as a possible mechanism of the superconductivity in the polaron system near the cross-over from the quasi-localized to the delocalized regime. The corresponding self-energy matrix is deduced using the Holstein Hamiltonian and the equations for the off-diagonal order are formulated. The lower bound on the superconducting critical temperature is estimated and the possible relevance of the proposed mechanism for the superconducting copper oxides is discussed.     

The mass of a large polaron

We show that, beginning with the works of L. D. Landau and S. I. Pekar, the effective mass of a large polaron has been determined with a crucial error. Since all such research ignored the spatial dispersion of the lattice polarizability, the maximum group velocity of phonons is found to be zero, so that the phonon “cloud” of a polaron is unable to follow the polaron. We allow for the spatial dispersion of the lattice polarizability and derive an expression for the effective polaron mass valid over the entire velocity range in which a polaron can exist: from zero to the maximum group velocity of phonons. According to this expression, the polaron mass depends not only on the phonon frequency, reciprocal effective dielectric constant, and the carrier mass but also on the maximum group velocity of phonons interacting with the carrier and on the polaron velocity. Zh. éksp. Teor. Fiz. 112, 278–283 (July 1997)     

Effects of lattice dispersion and elastic anisotropy on the thermal properties of fcc metals

New parameters which separately take into account the effects of elastic anisotropy and phonon dispersion on the lattice specific heat in the case of fcc metals have been evaluated. A new graded mesh method which uses 162-direction approximation in (1/16) part of the Brillouin zone has been developed to evaluate the two new parameters.     

Effects of lattice dispersion and elastic anisotropy on the thermal properties of bcc metals

Two new parameters which take into account the effects of elastic anisotropy and phonon dispersion on lattice specific heat in the case of sodium and potassium have been evaluated. A new graded mesh method which uses a 162-direction approximation in (1/16) part of the Brillouin Zone (BZ) has been considered to evaluate the two parameters.     

The lattice polarizability of PbI2

We have measured the far-i.r. reflectivity spectrum of the layer crystal PbI₂ for the in-plane polarization. A strong reststrahlen band is observed, with TO and LO phonon frequencies located at 52 and 108 cm^?1. The large lattice contribution accounts for three-quarters of the low-frequency dielectric constant of 26. The deduced infrared effective charge and electronic polarizability of PbI₂ indicates that the intralayer bonding is closely related to the chemical bonding in the IV–VI rocksalt-structure crystals.     

An exactly solvable model for the large polaron

We present an exactly diagonalizable model Hamiltonian for the large polaron derived by analyzing the variational ansatz by Haga-Larsen (HL) for the Fröhlich Hamiltonian. The lowest energy eigenvalue of the model Hamiltonian for fixed wave numbers reproduces the energy of the variational ansatz by Haga-Larsen and is, therefore, an upper bound with respect to the corresponding energy eigenvalue of the Fröhlich Hamiltonian. This is valid for any momentum which is proven by extending the Haga-Larsen approach. Furthermore, since all integrations can be performed analytically, the model Hamiltonian is easily tractable. The energy eigenvalue spectrum of the model Hamiltonian is studied below and above the phonon-emission threshold. The quality of the model Hamiltonian is determined by the variational ansatz of Haga and Larsen. Incorporating an improved energy-momentum relation, a generalized model Hamiltonian is derived possessing a larger validity range with respect to the coupling strength. Furthermore, a second exactly diagonalizable model Hamiltonian based on improved Wigner-Brillouin perturbation theory due to Warmenbol, Peeters, and Devreese (WPD) is presented. It is briefly demonstrated that one is able to construct all mentioned model Hamiltonians also in the 2D polaron problem. In contrast to the 3D case, where the HL-type model Hamiltonian possesses the higher quality for any momentum, in the 2D case, it works well only for small momenta. For large momenta, only the WPD-type model Hamiltonian describes the energy-momentum relation correctly. We demonstrate the usefulness of the model Hamiltonian concept by exactly calculating the one-electron Green’s function for all mentioned model Hamiltonians and comment why significant advantages of the model Hamilton concept for the treating of low-dimensional systems (planar semiconducting quantum-well structures) can be expected.     

Dynamic polarizability,dispersion coefficient C6 and dispersion energy in the effective fragment potential method

The development of a fragment–fragment dispersion energy expression, for the general effective fragment potential (EFP2) method is presented. C₆ dispersion coefficients, expressed in terms of the dynamic polarizabilties over the imaginary frequency range (α(iν)), were calculated for a set of homo and hetero dimers. Using these coefficients the dispersion energy has been calculated. The dispersion energy is expressed using a simple London series expansion terminated after the n=6 term and implemented using distributed localized molecular orbitals (LMOs). The EFP2 dispersion energy is compared to symmetry adapted perturbation theory (SAPT) values. From this comparison, it is apparent that one needs to include higher order terms in the dispersion energy. Adding an estimated C₈ term to the C₆ energy greatly improves the agreement with the benchmark SAPT energies.     

The thermoelectric properties of the small polaron

The high temperature behaviour of the thermoelectric transport coefficients for the Holstein small polaron model is investigated. An expression for the energy flux operator is derived, which includes the flow of the electron-lattice interaction energy. This inclusion is necessary for the small polaron model, because the electron-phonon coupling is assumed to be strong, while the kinetic energy of the electrons is understood as a small perturbation. Kubo formulae are used and approximated in lowest order perturbation theory. It follows that there are no dynamical corrections to the thermoelectric power. But the strong electron-lattice interaction leads to a contribution to the thermal conductivity.     

晶格热振动对准二维强耦合极化子有效质量的影响

采用Tokuda改变的线性组合算符法和改进的LLP变分法，研究了晶格热振动对无限势垒量子阱中电子与界面光学声子强耦合、与体纵光学声子弱耦合系统的影响，推导出作为阱宽和温度函数的极化子有效质量的表达式. 尤其得到了量子阱中极化子的振动频率及其随阱宽和温度变化的规律. 对KI/AgCl/KI量子阱进行了数值计算，结果表明，极化子的振动频率和有效质量随阱宽的增加而减小、随温度的升高而减小，但不同支声子与电子相互作用对极化子的振动频率和有效质量的贡献以及它们随阱宽和温度的变化情况大不相同. 关键词： 量子阱 强耦合极化子 振动频率 有效质量 温度依赖性     

High-accuracy measurement of atomic polarizability in an optical lattice clock

Presently, the Stark effect contributes the largest source of uncertainty in a ytterbium optical atomic clock through blackbody radiation. By employing an ultracold, trapped atomic ensemble and high stability optical clock, we characterize the quadratic Stark effect with unprecedented precision. We report the ytterbium optical clock's sensitivity to electric fields (such as blackbody radiation) as the differential static polarizability of the ground and excited clock levels α(clock) = 36.2612(7) kHz?(kV/cm)(-2). The clock's uncertainty due to room temperature blackbody radiation is reduced by an order of magnitude to 3×10(-17).     

Unified derivation of soliton, polaron and soliton lattice solutions in polyacetylene

The trans-(CH)_x polyacetylene system is studied in the framework of mean field approximation. In this paper it is shown that the phonon field satisfies a differential equation, which admits as solutions, besides the well-known solutions. found by other authors, many others.     

Spectral properties of the 2D Holstein polaron

The two-dimensional Holstein model is studied by means of direct Lanczos diagonalization preserving the full dynamics and quantum nature of phonons. We present numerical exact results for the single-particle spectral function, the polaronic quasiparticle weight, and the optical conductivity. The polaron band dispersion is derived both from exact diagonalization of small lattices and analytic calculation of the polaron self-energy.     

The width of the lattice distortion and the localization energy for a strong-coupled piezoelectric polaron

It is shown that the width of the lattice distortion for a strong-coupled piezoelectric polaron is proportional to $\text{a}{}^{\mathrm{<mtext>?3</mtext><mtext>2</mtext>}}{}_{\mathrm{<mtext>piezo</mtext>}}$. The well-known functional form of the localization energy follows from this result.