Constructing a phenomenological theory for La2−x Sr x CuO4 which includes a relation betweenT c and the optical absorption

Summary Relations between many of the results of recent investigations of superconducting doped lanthanium cuprate and a model expression for the dielectric permittivityε(ω, g) are developed.     

On the formation and stability of the Fröhlich bipolaron in two- and three-dimensional systems

Summary Within the framework of Rayleigh-Schr?dinger perturbation theory the bound state of two electrons in polar crystals is investigated in various dimensional systems. It is found that Fr?hlich bipolaron can exist in both two and three dimensions in the strong-coupling region, the binding being stronger in the lower dimension. The stability criterion for the formation of the Fr?hlich bipolaron is estimated to yield better results compared to the values quoted in the literature. The author of this paper has agreed to not receive the proofs for correction     

Longitudinal-optical phonon effects on the binding energy of a hydrogenic impurity in a quantum well structure

Summary The polaron effect on the binding energy of a hydrogenic impurity in a semiconductor quantum well is calculated by a variational approach taking into account the quasi-two-dimensional and the realist structures of systems. For the finite potential barrier, a correction is obtained as a function of the well thickness and presents a maximum; its evolution is analogous to the binding energy.     

Linear screening effects on large bipolarons

Summary The screening effects of the formation of large bipolarons have been studied. It is shown that the dependence of the bipolaron binding energy on the electron density does not change qualitatively by using different dielectric functions for describing electron screening. The same result is found if the electron system is described as a bipolaron gas (charged bosons).     

-wave nonadiabatic superconductivity

The inclusion of nonadiabatic corrections to the electron-phonon interaction leads to a strong momentum dependence in the generalized Eliashberg equations beyond Migdal's limit. For a s-wave symmetry of the order parameter, this induced momentum dependence leads to an enhancement of when small momentum transfer is dominant. Here we study how the d-wave symmetry affects the above behavior. We find that the nonadiabatic corrections depend only weakly on the symmetry of the order parameter provided that only small momentum scatterings are allowed for the electron-phonon interaction. In this situation, We show that also for a d-wave symmetry of the order parameter, the nonadiabatic corrections enhance . We also discuss the possible interplay and crossover between s- and d-wave depending on the material's parameters. Received 12 May 2000     

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     

Stability of low-dimensionally-confined bipolarons

In the limit of strong electron-phonon coupling, we provide a unified insight into the stability criterion for bipolaron formation in low-dimensionally confined media. The model that we use consists of a pair of electrons immersed in a reservoir of bulk LO phonons and confined within an anisotropic parabolic potential box, whose barrier slopes can be tuned arbitrarily from zero to infinity. Thus, encompassing the bulk and all low-dimensional geometric configurations of general interest, we obtain an explicit tracking of the critical ratio of dielectric constants below which bipolarons can exist. Received 15 September 1999 and Received in final form 20 March 2000     

Electron-phonon induced superconductivity in highT c superconductors

It is proposed that phonon induced mechanism is capable of explaining the observed range ofT _c for the newly discovered high temperature superconductor within the Allen and Dynes strong coupling theory.     

Possible mechanism for high-T c superconductivity

Based on recent experiments and numerical simulations on the glass behaviour of the new high-T _c superconductors, a microscopic mechanism is proposed. Disorder is found to play an important role. Pairing of holes and subsequent bose condensation on an antiferromagnetic background is proposed as a basic mechanism for high-T _c superconductivity.     

Quantum adiabatic polarons by translationally invariant perturbation theory

The translationally invariant diagrammatic quantum perturbation theory (TPT) is applied to the polaron problem on the 1D lattice, modeled through the Holstein Hamiltonian with the phonon frequency ω₀, the electron hopping t and the electron-phonon coupling constant g. The self-energy diagrams of the fourth-order in g are calculated exactly for an intermittently added electron, in addition to the previously known second-order term. The corresponding quadratic and quartic corrections to the polaron ground state energy become comparable at t/ω₀>1 for g/ω₀∼(t/ω₀) ^1/4 when the electron self-trapping and translation become adiabatic. The corresponding non adiabatic/adiabatic crossover occurs while the polaron width is large, i.e. the lattice coarsening negligible. This result is extended to the range (t/ω₀)^1/2>g/ω₀>(t/ω₀)^1/4>1 by considering the scaling properties of the high-order self-energy diagrams. It is shown that the polaron ground state energy, its width and the effective mass agree with the results found traditionally from the broken symmetry side, kinematic corrections included. The Landau self-trapping of the electron in the classic self-consistent, localized displacement potential, the restoration of the translational symmetry by the classic translational Goldstone mode and the quantization of the polaronic translational coordinate are thus all encompassed by a quantum theory which is translationally invariant from the outset. This represents the first example, open to various generalizations, of the capability of TPT to hold through the adiabatic symmetry breaking crossover. Plausible arguments are also given that TPT can describe the g/ω₀>(t/ω₀)^1/2 regime of the small polaron with adiabatic or non-adiabatic translation, i.e., that TPT can cover the whole g/ω₀, t/ω₀ parameter space of the Holstein Hamiltonian.     

A theorist's view of high-T c superconductivity

A brief review is given of the current proposals to explain the observation of superconductivity with high transition temperatures in the copper oxides.     

Polaronic effects on the energy levels of a double donor impurity in quantum wells in the presence of a magnetic field

In the presence of a magnetic field the Hamiltonian of the single or double polaron bound to a helium-type donor impurity in semiconductor quantum wells (QWs) are given in the case of positively charged donor center and neutral donor center. The couplings of an electron and the impurity with various phonon modes are considered. The binding energy of the single and double bound polaron in Al_xlGa _1-xlAs/GaAs/Al_xrGa _1-xrAs QWs are calculated. The results show that for a thin well the cumulative effects of the electron-phonon coupling and the impurity-phonon coupling can contribute appreciably to the binding energy in the case of ionized donor. In the case of neutral donor the contribution of polaronic effects are not very important, however the magnetic field significantly modifies the binding energy of the double donor. The comparison between the binding energies in the case of the impurity placed at the quantum well center and at the quantum well edge is also given. Received 16 February 1999     

Ground state description of bound polarons in parabolic quantum wires

The Feynman-Haken variational path integral theory is, for the first time, generalized to calculate the ground-state energy of an electron coupled simultaneously to a Coulomb potential and to a longitudinal-optical (LO) phonon field in parabolic quantum wires. It is shown that the polaronic correction to the ground-state energy is more sensitive to the electron-phonon coupling constant than the Coulomb binding parameter and monotonically stronger as the effective wire radius decreases. We apply our calculations to several semiconductor quantum wires and find that the polaronic correction can be considerably large. Received 16 November 1998     

Phonon-assisted thermoelectric effects in strongly interacting quantum dot

The joint effects of the electron-phonon interaction and Kondo effect on the charge and heat transport through a single molecule transistor are investigated by applying the improved canonical transformation and noncrossing approximation technique. We find that the electron-phonon interaction decreases the conductance, thermopower and the Wiedemann-Franz law in the Kondo regime due to the splitting and the decreasing of the main Kondo peak. However, the thermoelectric figure of merit achieves enhancement with the electron-phonon coupling strength increasing. In addition, the dip value of the thermopower at the Kondo temperature for the different electron-phonon coupling strength can give a straightforward reliable estimate of the electron-phonon coupling strength.     

One-dimensional moving polarons with extended coherent states

The general properties of one-dimensional large Fr?hlich polarons in motion are investigated with the previous extended coherent states where two-phonon correlations are considered. As a result, the polaron energy, velocity, effective mass, and average number of virtual phonons as a function the polaron total momentum are evaluated in a wide range of the coupling constant. In addition, rich information about virtual phonons emitted by the electron in motion is obtained. More importantly, some intrinsic features of 1D moving polarons are presented for the first time, which may also be suited to moving polarons in more than one dimensions. Received: 23 October 1997 / Revised and Accepted: 27 January 1998     

Polaron effects on excitons in parabolic quantum wells: fractional-dimension variational approach

Polaron effects on excitons in parabolic quantum wells are studied theoretically by using a variational approach with the so-called fractional dimension model. The numerical results for the exciton binding energies and longitudinal-optical phonon contributions in GaAs/Al_0.3Ga_0.7As parabolic quantum well structures are obtained as functions of the well width. It is shown that the exciton binding energies are obviously reduced by the electron (hole)-phonon interaction and the polaron effects are un-negligible. The results demonstrate that the fractional-dimension variational theory is effectual in the investigations of excitonic polaron problems in parabolic quantum wells.     

Topological solitons and zero modes in high-T c superconductivity

The realization of topological solitons, zero modes, and supersymmetry in theCP ₁ nonlinear sigma model, a theory of high-T _c superconductivity, is presented.     

Direct angle resolved photoemission spectroscopy and superconductivity of strained high-T c films

Since 1997 we systematically perform direct angle resolved photoemission spectroscopy (ARPES) on in-situ grown thin (<30 nm) cuprate films. Specifically, we probe low-energy electronic structure and properties of high-T _c superconductors (HTSC) under different degrees of epitaxial (compressive vs. tensile) strain. In overdoped and underdoped in-plane compressed (the strain is induced by the choice of substrate) ≈15 nm thin La_{2 − x} Sr _x CuO₄ (LSCO) films we almost double T _c to 40 K, from 20 K and 24 K, respectively. Yet the Fermi surface (FS) remains essentially two-dimensional. In contrast, ARPES data under tensile strain exhibit the dispersion that is three-dimensional, yet T _c drastically decreases. It seems that the in-plane compressive strain tends to push the apical oxygen far away from the CuO₂ plane, enhances the two-dimensional character of the dispersion and increases T _c, while the tensile strain acts in the opposite direction and the resulting dispersion is three-dimensional. We have established the shape of the FS for both cases, and all our data are consistent with other ongoing studies, like EXAFS. As the actual lattice of cuprates is like a ‘Napoleon-cake’, i.e. rigid CuO₂ planes alternating with softer ‘reservoir’, that distort differently under strain, our data rule out all oversimplified two-dimensional (rigid lattice) mean field models. The work is still in progress on optimized La-doped Bi-2201 films with enhanced T _c.