The optical absorption of polarons at rest at zero temperature is calculated starting from the Feynman-Hellwarth-Iddings-Platzman (FHIP) theory of the impedance.The results are compared with the results of theories whose physical interpretation is clearer [weak-coupling theory of Gurevich, Lang, and Firsov (GLF) and product-ansatz strong-coupling theory of Kartheuser, Evrard, and Devreese (KED)] in order. to obtain a better understanding of the FHIP approximation.We are particularly interested in the possible role of lattice relaxation [leading to relaxed excited states {RES)j in the optical absorption process. If the FHIP perturbation method were used to expand the conductivity {this would be the normal procedure), essentially Franck-Condon transitions would be found in the spectrum, and lattice relaxation would be absent. In this case the results do not fit with the product ansatz and provide merely the asymptotic limit & 0, where 0' is the electron-phonon coupling constant. If, however, the impedance function rather than the conductivity is expanded (as preferred by FHIP for intuitive reasons, without further justification) more reliable results for the optical absorption appear. For G' &5, intense absorption peaks now occur, which presumably correspond to transitions to RES, and the results are in qualitative agreement with the predictions of the product-ansatz treatment in this coupling region. Also in the limit e -0 the correct behavior is found.For 3 & e & 5, the interpretation of the results is somewhat delicate but the possibility that RES contribute to the oscillator strength as soon as 0: &3 should be considered. The results so obtained for the optical absorption seem reliable at all &. This provides an indirect justification for the expansion of Z{Q) rather than 1/Z{Q) in FHIP theory and a confirmation of the qualitative strong-coupling predictions of KED. The present study indicates that optical absorption peaks due to free polarons should be observable experimentally in crystals for which a &1.

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Recent experimental data on the optical conductivity of niobium doped SrTiO3 are interpreted in terms of a gas of large polarons with effective coupling constant α ef f ≈ 2. The theoretical approach takes into account many-body effects, the electron-phonon interaction with multiple LOphonon branches, and the degeneracy and the anisotropy of the Ti t2g conduction band. Based on the Fröhlich interaction, the many-body large-polaron theory provides an interpretation for the essential characteristics, except -interestingly -for the unexpectedly large intensity of a peak at ∼ 130 meV, of the observed optical conductivity spectra of SrTi1−xNbxO3 without any adjustment of material parameters.

A thermodynamically stable vortex-antivortex pattern has been revealed in mesoscopic type I superconducting triangles, contrary to type II superconductors where similar patterns are unstable. The stable vortex-antivortex "molecule" appears due to the interplay between two factors: a repulsive vortex-antivortex interaction in type I superconductors and the vortex confinement in the triangle.PACS numbers: 74.60. Ec; 74.55.+h; 74.20.De Symmetrically-confined vortex matter in superconductors, superfluids and Bose-Einstein condensates offers unique possibilities to study the interplay between the C ∞ symmetry of the magnetic field and the discrete symmetry of the boundary conditions. More specifically, superconductivity in mesoscopic equilateral triangles, squares etc. in the presence of a magnetic field nucleates by conserving the imposed symmetry (C 3 , C 4 ) of the boundary conditions [1] and the applied vorticity. As a result, in an equilateral triangle, for example, in an applied magnetic field H generating two flux quanta, 2Φ 0 , superconductivity appears as the C 3 -symmetric combination 3Φ 0 − Φ 0 (further on denoted as "3 − 1") of three vortices and one antivortex in the center. These symmetry-induced antivortices can be important not only for superconductors but also for symmetrically confined superfluids and Bose-Einstein condensates. Since the order parameter patterns reported in Refs. [1] have been obtained in the framework of the linearized Ginzburg-Landau (GL) theory, this approach is valid only close to the nucleation line T c (H). Can then these novel symmetry-induced vortex-antivortex patterns survive deep in the superconducting state? Several attempts have been already made to answer this crucial question. In the limit of an extreme type II superconductor (κ ≫ 1), it has been shown that for a thin-film square, a configuration of one antivortex in the center and four vortices on the diagonals of the square is unstable away from the phase boundary [2]. According to the analysis based on the coupled nonlinear GL equations, the vortex-antivortex pairs are unstable and no antivortices appear spontaneously at the T, H points far away from the T c (H) line [3]. Possible scenarios of penetration of a vortex into a mesoscopic superconducting triangle with increasing magnetic field have been studied in Ref. [4]. Two different states were considered: a single vortex state and a state in the form of a symmetric combination of three vortices and an antivortex with vorticity L av = −2 ("3 − 2" combination). The calculations [4] have shown that while a single vortex enters the triangle through a midpoint of one side, the "3 − 2" combination turns out to be energetically favorable when the vortices are close to the center of the triangle. Equilibrium is achieved when a single vortex is in the center of the triangle. When approaching the phase boundary, the free energy of a single-vortex state tends to coincide with the free energy of the "3 − 2" combination [4], thus confirming conclusions [1,2] that formation ...

The interface superconductivity in LaAlO 3-SrTiO 3 heterostructures reveals a nonmonotonic behavior of the critical temperature as a function of the two-dimensional density of charge carriers. We develop a theoretical description of interface superconductivity in strongly polar heterostructures, based on the dielectric function formalism. The density dependence of the critical temperature is calculated, accounting for all phonon branches including different types of optical (interface and half-space) and acoustic phonons. The longitudinal-optic-and acoustic-phonon mediated electron-electron interaction is shown to be the dominating mechanism governing the superconducting phase transition in the heterostructure.

Scaling relations are presented which allow one to obtain static and dynamical physical quantities of the two-dimensional (2D) polaron out of known results for the 3D polaron. These scaling relations are exact within second-order perturbation theory and are approximately valid (on a l%%uo level) for higher orders in the electron-phonon coupling constant. A formal generalization of these scaling relations to the n-dimensional polaron is given.From the 1950's on the polaron problem has been an attractive field of research. In recent years there has been a renewed interest, both from theoreticians and experimentalists, in the concept of polarons. Besides its intrinsic interest in the area of solid-state physics, the polaron problem has become a reference problem for testing various methods of approximation because it represents a comparatively simple, physically realistic example of the interaction of a nonrelativistic particle with a quantized field. During the years the polaron problem has become a "classic" problem in quantum field theory.Polarons are quasiparticles consisting of an electron (or a hole) which is surrounded by its polarization cloud. The electron is moving in a polar semiconductor or an ionic crystal. The physical properties of polarons in three-dimensional systems have been studied' experimentally and theoretically ever since Landau introduced the concept. Theoretically a variety of techniques have been used from which the Feynman path-integral method turns out to be the most successful one.With the advent of new fabrication techniques, like, e.g. , molecular-beam epitaxy, metal-organic chemicalvapor deposition etc. , it has become possible to grow structures in which the electrons are localized in space in one or two directions. This leads to two (2D) or even one-dimensional (1D) electron systems and, consequently, if these systems are made out of polar semiconductors, one has 2D or 1D polarons. Over the last few years 2D polarons have been studied by several experimental and theoretical groups.In actual systems the polaron will not be exactly two dimensional; it is more justified to speak about quasitwo-dimensional polarons. The thickness of the 2D electron layer perpendicular to the layer is nonzero.In the present paper we are interested in ideal 2D systems. One-polaron properties will be studied, and we will neglect the many-electron nature of the system. The calculated polaron quantities are expected to result in an upper bound for the practical polaron eAect which will be measured in real systems. Screening and nonzero layer width vary from one system to another and are not universal. Therefore the following assumptions will be I' made: we consider a strict 2D model, neglect interface phonons and screening, and assume that the Frohlich continuum polaron model is valid. This will allow us to provide a unifying and comprehensive picture of the properties of 2D polarons where the energy scale is set by the bulk-LO-phonon frequency.A series of scaling relations will be developed which will allo...

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