Nonlocal composite spin-lattice polarons in high temperature superconductors

The nonlocal nature of the polaron formation in t - t '- t' - J model is studied in large lattices up to 64 sites by developing a new numerical method. We show that the effect of longer-range hoppings t' and t' is a large anisotropy of the electron-phonon interaction (EPI) leading to a completely different influence of EPI on the nodal and antinodal points in agreement with the experiments. Furthermore, nonlocal EPI preserves polaron's quantum motion, which destroys the antiferromagnetic order effectively, even in the strong coupling regime, although the quasiparticle weight in angle-resolved-photoemission spectroscopy is strongly suppressed.     

Optically induced polarons in bose-einstein condensates: monitoring composite quasiparticle decay

Nonresonant light scattering off atomic Bose-Einstein condensates is predicted to give rise to hitherto unexplored composite quasiparticles: unstable polarons, i.e., local "impurities" dressed by virtual phonons. Optical monitoring of their spontaneous decay can display either Zeno or anti-Zeno deviations from the golden rule, and thereby probe the temporal correlations of elementary excitations in the condensates.     

The screened polarons

We present a theory of polarons incorporating the screening of the Coulomb interaction, and we apply this theory to the case of anisotropic ionic crystals as the perovskites. We show that the “screened polarons" cannot be treated individually: all the polarons present in the material are coupled via the screening. We also show that, in the frame of this theory of large-scale polarons, the bipolarons are excluded and replaced by pairs of polarons; we propose to associate the pseudogap experimentally observed in perovskites with the binding energy of these pairs. Finally we suggest that the existence of the polarons pairs poses in new terms the problem of a polaronic theory of superconductivity. Received 9 December 1998 and Received in final form 2 March 2000     

Theory of surface polarons

We investigate the possibility that an electron may be trapped at the surface of an ionic crystal by the electron-surface optical phonon interaction. For a model Hamiltonian used earlier in a discussion of the inelastic scattering of low energy electrons by surface optical phonons, we find that for all values of the polaron coupling constant α, this trapping does occur. We refer to these surface states as surface polaron states. By the use of a variational procedure, we calculate the surface polaron binding energy and effective mass as a function of α. The present treatment does not include the coupling of the electron to bulk LO phonons.     

Extended particles and polarons

A coherent-state variational method that describes particles as spacially extended objects is applied to the problem of an electron in a polar crystal. Accuracy is obtained for both strong and weak coupling by determining the variational parameters after the realization of translational invariance.     

Optical spectra of strong-coupling polarons

The photodissociation spectra of Landau-Pekar polarons are calculated using the theory of quantum coherent states. It is shown that the number of phonons emitted in one dissociation event can differ and that their energy is equal to the doubled polaron binding energy E _p only on the average. It is established that the absorption spectrum is a superposition of bands corresponding to different numbers of phonons emitted during the dissociation of one polaron and that the half-width of each of the bands is much greater than the distance between the bands (which is equal to the phonon energy ?θ). Therefore, the absorption spectrum looks like a very wide unstructured band with the low-frequency edge lying at E _p + ?θ, a maximum at an energy of about 5E _p (for the band carrier mass equal to (1–3)m _e ), and the half-width being of the order of the energy corresponding to the maximum.     

Optical absorption of piezoelectric polarons

The optical absorption of piezoelectric polarons is calculated for all values of the electron-phonon coupling. The spectrum shows a series of peaks due to internal transitions of the polaron superposed on a background resulting from emission and absorption of acoustical phonons.     

Energy levels of bound polarons

A new variational wave function to describe the ground state and the excited states of a bound polaron is proposed. It is of the form

$\text{|Ψ〉 = c|O〉|ø}{}_{\mathrm{n}}\text{〉 +}\text{∑}\text{g}{}_{\mathrm{k}}{}^1\text{V}{}_{\mathrm{k}}{}^1\text{(e}{}^{\mathrm{<mtext>i</mtext><mtext>k·r</mtext>}}\text{? ρ}{}_{\mathrm{k}}{}^1\text{)a}{}_{\mathrm{k}}{}^{\mathrm{+}}\text{|O〉|ø}{}_{\mathrm{n}}$

. It is argued that this form is reasonable for all electron—phonon coupling α and all strengths β of the Coulomb potential. Numerical and analytical results are derived for the energy of the ground state and compared to existing results. Results for the energy of the lowest p-type excited state of the bound polaron are obtained.     

Magnetic polarons in semimagnetic-semiconductor-based heterostructures

Specific features in the behavior of localized magnetic polarons formed under optical excitation in heterostructures based on semimagnetic semiconductors are considered. These features are due to the strong anisotropy of the hole g factor in low-dimensional systems based on zincblende crystals. The anisotropy is due to the strong spin-orbit coupling in the valence band which, in quantum confinement conditions, results in quadrupole splitting of the hole spin levels. The g factor anisotropy manifests itself in a strong anisotropy of the magnetic and magneto-optical characteristics of localized magnetic polarons. Fiz. Tverd. Tela (St. Petersburg) 40, 800–802 (May 1998)     

钙钛矿锰氧化物中的极化子研究

钙钛矿锰氧化物(以下简称锰氧化物,如La_1-xSr_xMnO₃等,x为掺杂浓度)因其优异的电、磁性质受到人们广泛的关注,但是对于其材料内部载流子性质的认识至今仍没有统一定论.本文基于锰氧化物内Mn—O链的特点,建立一维紧束缚模型,对锰氧化物载流子的性质展开研究.发现在掺杂浓度x=0.5时,系统处于铁磁态,自旋能级完全劈裂,价带和导带之间存在带隙,所有电子态呈现扩展行为.进一步掺杂,将出现局域电子态,同时伴随着晶格的局域畸变,形成所谓的极化子.伴随着极化子的出现,带隙中出现极化子深能级.极化子携带的电荷量越多,形成的晶格缺陷越深,局域能级也越深.当极化子的电荷量继续增加时,极化子解离,载流子倾向于形成能量更低的正反"孤子"对.     

Infrared absorption by free polarons

Expressions are obtained for the coefficient of absorption of a weak electromagnetic wave by free carriers in polar semiconductors with interaction of polarons with acoustic and optical phonons. The appearance in the absorption coefficient of Feynman variational parameters leads, in general, to a change in the frequency dependence. The results are applicable for the frequency range (), where () is the mean reciprocal relaxation time.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 76–80, July, 1980.     

Ionic polarons in solid electrolytes

Small polaron theory is used to calculate the activation energy of cationic hopping in solid electrolytes. The numerical results show a small activation energy for materials which have forms which are good super ionic conductors, and large for those which have not.     

Squeezed polarons in one dimension

Starting from a simple one-dimensional model for many-electron and phonon interacting systems we have shown that in order to arrive at a stable minimum of the total energy the ordinary polaronic state must be replaced by the squeezed polaronic state in which the phonon subsystem is in the two-phonon coherent state.     

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     

Revisiting the theory of large polarons

We extend the theory of large polarons in the frame of a semi classical model. We analyse two physical effects in which the discreteness of the periodic lattice plays a major role: i) In addition to standard Holstein polarons, we show the existence of braggons which are polarons whose energy lies in a lattice gap. According to the type of gap considered, some of the them are ordinary solitons, some others are vector-solitons. ii) The pinning (or Peierls Nabarro) effect of the lattice on the polarons. We show that the coupling of the electrons with acoustic phonons is able to produce polarons or bipolarons in crystallographic situations where the coupling with optical phonons does'nt. The existence of multidimensional polarons and bipolarons is investigated, and we find interesting possibilities in dimension D=2. The binding energy and effective mass of the polarons are determined in the frame of the model, and their dynamical stability is analysed. We also show the existence, for D=1, of tripolarons in which two energy levels are occupied in the potential well created by the electron-lattice interaction. Finally the conditions of validity of the semi-classical approximation are given.     

有机高分子中光生极化子的物理机制

有机聚合物中光生载流子（荷电极化子）产生的物理机制是一个非常重要的物理问题，但一直存在争议，文章介绍了作者对此问题的最新研究结果：荷电极化子和中性激子一样，是由光激发直接产生的，而不是激子解体的结果，其生成时间在100fs的量级．荷电极化子的量子产率约为25％，并且与激发能量无关，与实验观测的结果一致．     

Existence criteria for Landau-Pekar polarons

We build the distribution function for a system with coexisting self-localized and delocalized fermions. The distribution function is used to study the behavior of the chemical potential of the carriers in such a system, which is found to differ substantially from the behavior of the chemical potential in a system of delocalized fermions. We also find that as the temperature changes, isostructural first-order phase transitions can emerge in the system of self-localized and delocalized fermions. These transitions, for which changes in the state of the macroscopic number of particles are responsible, manifest themselves in the electrical conductivity, in the contribution of carriers to the specific heat, and in the optical properties of such systems. Formulas are derived that approximate the dependence of the temperature of such a phase transition on the binding energy of the self-localized states of carriers and on the maximum group velocity of phonons participating in the formation of such states. Finally, we show that the special features of the behavior of the chemical potential of the carriers in a system with carrier self-localization lead to the possibility of Bose condensation in a system where bipolaron states are metastable. Zh. éksp. Teor. Fiz. 116, 1386–1397 (October 1999)