Polaron stability in molecular crystals

A semi-empirical Peierls–Holstein model is applied to studies of the stability of polarons in two-dimensional molecular crystal systems. Calculations for a broad range of intra- and inter-molecular parameters within this model were performed in order to obtain detailed knowledge concerning the stability of the polaron solution with respect to a rigid lattice band solution. For realistic values of the parameters the polaron solution is stable with a polaron energy in the range 50–100 meV. A metastable polaron solution is also identified. The polarons that result from our model are highly localized and it is questionable if adiabatic polaron transport can occur in the system.     

Formation of two-dimensional polarons that are absent in three-dimensional crystals

We report the direct time domain observation of a many-body process in a two-dimensional system: small polaron formation from the localization of a conduction band electron in NaCl thin films of unit cell thickness. Contrary to theoretical prediction for bulk NaCl crystal where an electron polaron does not exist, time-resolved two-photon photoemission reveals small polaron formation from delocalized conduction band electrons in crystalline NaCl thin films. The increased deformability and the reduced electronic bandwidth of a crystalline lattice in the thin film format are both responsible for the formation of small polarons that are absent in bulk solids.     

The effect of quasinesting and a magnon mode on the antiferromagnetic vector for the optical conductivity of a doped two-dimensional antiferromagnet

The optical spectrum of the normal state of a doped two-dimensional antiferromagnet is analyzed in a Kondo lattice model with regard to the complex structure of a spin polaron. The optical properties are determined by sharply anisotropic scattering of spin-polaron excitations by antiferromagnetic fluctuations of a system of localized spins. It is shown that the relaxation of carriers in the infrared range is mainly attributed to the strong coupling between these carriers and the mode of low-frequency spin excitations with the quasimomentum close to the antiferromagnetic vector Q=(π,π). The latter coupling is associated with the fact that the regions of the Fermi surface of the lower polaron band are close to the boundary of the antiferromagnetic Brillouin zone. The calculated optical characteristics are in qualitative agreement with experimental data for the normal state of high-temperature superconductors (HTSCs).     

极化子有效质量与温度的关系

本文计及离子晶格的原子结构,用紧束缚法导出了极化子的自陷能和有效质量,给出了有效质量与温度的关系。 关键词：     

Polaron on a one-dimensional lattice: II. A moving polaron

Continuum-limit equations for moving polarons on a one-dimensional lattice with a harmonic interaction potential between adjacent particles and a simple nonlinear potential with a cubic nonlinearity are derived for the first time; for some particular cases, their solutions are obtained. For a harmonic lattice in the continuum limit, a system of integrable nonlinear partial differential equations is derived. A one-soliton solution to this system describes a polaron moving with a constant velocity. The speed of this polaron is uniquely related to its amplitude, with its values ranging from zero to the speed of sound. For a nonlinear lattice, the resulting system of differential equations is integrable at a certain ratio of the problem parameters. The one-soliton solution to this system, as in the harmonic case, describes a polaron moving with a constant velocity. At arbitrary values of the lattice parameters, the nonlinear lattice was studied by numerical methods. It turned out that, in the entire range of parameters, the nonlinear lattice gives rise to moving polarons, with the speed of the polaron being determined by the competition between the electron-photon interaction parameter α and the nonlinearity parameter β. At α ? β, the behavior of the polaron is very close to the dynamics on the harmonic lattice. In the opposite case, the dynamic nonlinearity begins to dominate, giving rise to dynamics inherent to solitons, so that speed of the polaron can exceed the speed of sound. In a certain range of α and β, numerical calculations revealed a family of polaron-type stable solutions, the envelope of which can have several peaks. The numerical and exact analytical solutions are in very good agreement for a sufficiently large radius of the polaron, when the system of equations obtained in the continuum approximation has a solution.     

Optical phonon interaction effects in layered semiconductor structures

Various aspects of the electron-LO-phonon interaction effects on the electronic properties of a single two-dimensional electron layer (as occurring, for example, in artificially structured single quantum wells or heterojunctions made of III–V or II–VI semiconducting materials) are discussed theoretically. In particular, perturbation theory is carried to the second order in the coupling constant to obtain the two-dimensional polaron energy in the weak-coupling limit. Intermediate coupling (the so-called Lee-Low-Pines theory) and strong coupling theories for the two-dimensional polaron problem are developed and interpolation (Padé approximations) formulae valid for arbitrary coupling are derived. Effects of the band non-parabolicity and of the free-carrier screening on the weak-coupling theory are discussed. The real and the imaginary parts of the two-dimensional polaron self-energy are obtained in a many-body perturbation calculation. Comparison with the known three-dimensional results is made wherever possible, showing that the electron-LO-phonon interaction effects are substantially enhanced in confined structures. Explicit formulas valid for two-dimensional systems are given for various polaron parameters like the binding energy, the effective mass, the scattering rate, the average phonon density in the polaron cloud, etc.     

Self-consistent treatment of the polaron damping in the Holstein model

Using the Holstein model, the damping of the localized polaron state and the small polaron conductivity are represented by the series of contributions from the multiphonon processes in which the polaron damping is taken into account. The self-consistent treatment of the damping shows the enhancement of the small-polaron relaxation rate by the quantum fluctuations of the lattice corresponding to the energy non-conserving phonon processes. The conditions for the polaron localization as well as the dependence of the d.c. conductivity on the electron transfer integral are discussed.     

The Electronic and Lattice Structures of the Groundand the Polaron States of Polymer Poly (Phenylene Vinylene) (PPV)

The electronic and lattice structures of poly (phenylene vinylene) (PPV) are studied theoretically. Both the electron-electron and electron-phonon interactions are taken into account in the Pariser-Parr-Pople model. The electronic band and the lattice structure of the ground state and the polaronic state are calculated by means of the unrestricted Hartree-Fock method. In the ground state, there exist eight bands in PPV including four valence bands and four conduction bands, and the benzenes can be considered to be rigid. The polaron induces the split of energy bands. There are four localized electronic states within the energy gap. The defect of the polaron appears to extend over about 5 units. The benzenes are strongly affected by the electron-phonon interaction. Our calculation for the energy band structure of the ground and polaron states are consistent with experimental absorption spectra. The results of our calculation show that the electron-phonon and inter-site electron-electron interactions play an important role in determining the electronic and lattice structures.     

Effect of Temperature on Polaron Stability in a One-Dimensional Organic Lattice

Effect of temperature on the polaron stability in a one-dimensional organic lattice is investigated within the Su-Schrieffer-Heeger model. The temperature effect is simulated by introducing random forces to the equation of the lattice motion. It is found that the localized polaron state becomes delocalized even at low temperatures. The time of polaron keeping localized depends on the magnitude of temperatures. By taking into account the thermal effect, we find that the dissociation field is weaker as compared with earlier works.     

The Electronic and Lattice Structures of the Ground and the Polaron States of Polymer Poly (Phenylene Vinylene) (PPV)

The electronic and lattice structures of poly (phenylene vinylene) (PPV) are studied theoretically. Both the electron-electron and electron-phonon interactions are taken into account in the Pariser-Parr-Pople model. The electronic band and the lattice structure of the ground state and the polaronic state are calculated by means of the unrestricted Hartree-Fock method. In the ground state, there exist eight bands in PPV including four valence bands and four conduction bands, and the benzenes can be considered to be rigid. The polaron induces the split of energy bands. There are four localized electronic states within the energy gap. The defect of the polaron appears to extend over about 5 units. The benzenes are strongly affected by the electron-phonon interaction. Our calculation for the energy band structure of the ground and polaron states are consistent with experimental absorption spectra. The results of our calculation show that the electron-phonon and inter-site electron-electron interactions play an important role in determining the electronic and lattice structures.     

Polarons in quantum chains with XY exchange and Dzyaloshinsky-Moriya interaction

Excitations of the polaron types are investigated in the spin-1/2 quantum chain with XY exchange and Dzyaloshinsky-Moriya interaction, both coupled to acoustic vibrations of the substrate lattice. The study is carried out via Jordan-Wigner transformation with the help of which the spin chain is mapped onto a chain of spinless fermions. From the resulting effective fermion-lattice Hamiltonian, the discrete equations of motion are derived. These equations are solved in the continuum limit for self-trapped states near the bottom of the fermion spectrum interacting with long-wavelength acoustic lattice modes. The associate polaron solution, which has a pulse shape, is shown to propagate bound to the induced lattice kink distortion by translation along the chain at a constant velocity v. The pair can also experience an additional acceleration ϑ₀ when the free fermion charge is excited above its groundstate. The polaron binding energy is strongly reduced, depending quadratically on the ratio D/J of the Dzyaloshinsky-Moriya interaction strength D to the isotropic XY exchange interaction J. It is also found that polaron parameters depend only on the XY spin-lattice coupling but not on the Dzyaloshinsky-Moriya contribution.     

二维声学极化子的基态能量和有效质量

自陷电子对了解光电材料的光学性质非常重要.近些年来,形变晶格中电子自陷的问题受到研究人员的广泛关注.电子既与声学模耦合,也与光学模相互作用,但电子由自由态向自陷态的转变缘于近程的电子-声学声子耦合.研究表明:声学极化子在大多数半导体以及Ⅲ-Ⅴ族化合物,甚至碱卤化物中都不可能自陷.另一方面,电子-声子耦合在束缚结构,如二维、一维系统中,会有所增强.换言之,电子在低维结构中更容易自陷.Farias等人指出:声学极化子在二维系统中自陷的临界电子-声子耦合常数为定值,不随声子截止波矢的变化而改变.这种结论在物理上不尽合理.通过计算二维系统中的声学极化子基态能量和有效质量,讨论了二维声学极化子自陷问题.研究发现,二维声学极化子自陷转变的临界耦合常数随声子截止波矢的增加朝电子-声子耦合较弱的方向变化.这一特征与前人关于体和表面极化子研究获得的结论定性一致.所得二维声学极化子基态能量的表达式与Farias等人一致,但自陷的结果与Farias等人的结果在定性和定量上均有不同,我们认为Farias等人关于二维声学极化子自陷转变点的确定方式有不妥之处.通过改进自陷转变点的确定方式,得到了在物理上更合理的结果.     

The inertial mass tensor of a polaron in an isotropic medium

Owing to a funadmentally erroneous approach to calculations of the effective polaron mass (calculations that use a model without spatial dispersion of the lattice polarizability), the polaron inertial mass has never before been distinguished from the mass as a measure of kinetic energy. In this paper we derive an expression for the tensor of the inertial mass of a large polaron. The tensor is found to be fully determined by two components: the longitudinal component, corresponding to the case where the force acting on the polaron is parallel to the polaron velocity, and the transverse component, corresponding to the case where the acceleration is perpendicular to the polaron velocity. The components of the polaron inertial mass tensor depend quasirelativistically on the polaron velocity due to the quasirelativistic compression of the polarization field in the direction of motion, which constitutes the effect of spatial dispersion of the lattice polarizability. We derive a formula that approximates the dependence of the components of the polaron mass tensor on all the parameters: the frequency and dispersion of the phonons, the polaron velocity, and the effective dielectric constant. Zh. éksp. Teor. Fiz. 115, 180–186 (January 1999)     

二维链间扩展的极化子动力学研究

基于扩展的SSH模型,研究了有序耦合聚合物链系统中的极化子动力学,包括极化子的形成过程及其在外场下的输运.发现,当聚合物链间的耦合较强时,注入到系统中的电子会诱发二维链间扩展的极化子态,分布在多条聚合物链上.另外,动力学模拟表明,与一维链内定域极化子相比,在相同的电场强度下二维极化子具有更大的运动速度,这与实验结果一致. 关键词： 链间扩展极化子 链间耦合     

Novel states of electrons on a helium film

Three novel states of electrons on a helium film are described. The first is a diplon, an electron trapped by a positive charge located on a substrate beneath the film, the second is a variation of the first leading to the possibility of a two-dimensional metal-insulator transition, and the third is a ripplonic polaron, an electron self-trapped in a surface dimple. From a survey of experimental searches for the ripplonic polaron state I conclude that no polaron transition occurs on films thicker than 14 nm at T>1.5 K.     

A study of two-dimensional magnetic polaron

By using the variational method and anneal simulation, we study in this paper the self-trapped magnetic polaron (STMP) in two-dimensional anti-ferromagnetic material and the bound magnetic polaron (BMP) in ferromagnetic material. Schwinger angular momentum theory is applied to changing the problem into a coupling problem of carriers and two types of Bosons. Our calculation shows that there are single-peak and multi-peak structures in the two-dimensional STMP. For the ferromagnetic material, the properties of the two-dimensional BMP are almost the same as that in one-dimensional case; but for the anti-ferromagnetic material, the two-dimensional STMP structure is much richer than the one-dimensional case.     

Dynamic process of self-trapped polaron in photoexcited SrTiO3

Relaxation process of self-trapped polaron is investigated by a nonadiabatic molecular dynamic method. We show localized disorder due to lattice fluctuations can give rise to a tightly-bound electronic state in ultraviolet illuminated SrTiO₃ crystal. This bound state is actually a self-trapped polaron in accordance with the experimentally observed large Stokes-shift. The formation of the self-trapped polaron is shown to be an ultrafast process.     

Strong Electron-Phonon Interaction Effect in Quantum Dots

Strong-coupling polaron in a parabolic quantum dot is investigated by the Landau-Pekar variational treatment. The polaron binding energy and the average number of virtual phonons around the electron as a function of the effective confinement length of the quantum dot are obtained in Gaussian function approximation. It is shown that both the polaron binding energy and the average number of virtual phonons around the electron decrease with increasing the effective confinement length. The results indicate that the polaronic effects are more pronounced in quantum dots than those in two-dimensional and three-dimensional cases.     

Two-step dissociation of a polaron in conjugated polymers

We have studied the electric-field-driven motion of a polaron by solving the time-dependent Schr\"{o}dinger equation nonadiabatically and the lattice equation of motion simultaneously. It is found that the polaron may experience two sequent transitions under high fields; one is the transition from the subsonic to the supersonic state, and the other from the supersonic to dissociated state. The acoustic mode is decoupled from the charge when the polaron moves at a speed faster than the sound speed, and then the optical mode is decoupled at the second transition to make the polaron dissociate completely.     

On the possibility of electric transport mediated by long living intrinsic localized solectron modes

We consider a polaron Hamiltonian in which not only the lattice and the electron-lattice interactions, but also the electron hopping term is affected by anharmonicity. We find that the one-electron ground states of this system are localized in a wide range of the parameter space. Furthermore, low energy excited states, generated either by additional momenta in the lattice sites or by appropriate initial electron conditions, lead to states constituted by a localized electron density and an associated lattice distortion, which move together through the system, at subsonic or supersonic velocities. Thus we investigate here the localized states above the ground state which correspond to moving electrons. We show that besides the stationary localized electron states (proper polaron states) there exist moving localized solectron states which can be easily excited. The evolution of these localized states suggests their potential as new carriers for fast electric charge transport.