Fifth training course in the physics of correlated electron systems and high-Tc superconductors
580(2001); http://dx.doi.org/10.1063/1.1398135View Description Hide Description
The observation of high-temperature superconductivity in complex layered cuprates by Bednorz and Müller must now rate as one of the greatest experimental discoveries of the last century. Identifying and understanding the microscopic origin of high-temperature superconductivity stands as one of the greatest theoretical challenges of this century. These lectures describe an approach, based on the extension of the BCS theory to the strong-coupling regime with small polarons and bipolarons. We discuss the canonical Migdal-Eliashberg theory of strongly coupled electrons and phonons (or any bosons) and its breakdown in the strong-coupling regime. Then, starting from the dynamic properties of a single polaron, we show how the multi-polaron problem is reduced to a weakly interacting charged Bose gas. Finally we discuss a few recent applications of the bipolaron theory to cuprates, in particular the essential interaction in oxides, the parameter-free expression for the superconducting critical temperature, upper critical field, symmetry of the order parameter, the London penetration depth, vortex structure, theory of tunnelling and Andreev reflection, normal and superconducting gaps, angle-resolved photoemission, and stripes.
580(2001); http://dx.doi.org/10.1063/1.1398136View Description Hide Description
The principles of the type II superconductor behavior are briefly and simply introduced and are applied to particular cases, such as the magnetic field dependence of pinning mechanisms in thin films, the melting of the vortex lattice in Nb/CuMn multilayers and the transport properties of High Temperature Superconductor based superlattices. The common behaviors of these three systems are pointed out, in view of a complete and deeper comprehension of the superconducting phenomena observed in the new family of the oxide superconductors.
580(2001); http://dx.doi.org/10.1063/1.1398137View Description Hide Description
A microscopical theory of spin-fluctuation superconducting pairing in systems with strong electron correlations as high-temperature superconductors is formulated. Basic models with strong electron correlations: model, Hubbard and models are considered. The Dyson equations for the matrix Green functions in the Nambu notation for the Hubbard operators are derived. The equations are solved in the self-consistent Born approximation and quasi-particle spectra and superconducting are calculated. By comparison the results for the Hubbard and models it is shown that the antiferromagnetic superexchange interaction acting in a broad energy region with negligible retardation effects gives the major contribution to the superconducting pairing. We argue that spin-fluctuations which have been observed in many experiments in cuprates are the driving force for superconducting pairing in high-temperature superconductors.
580(2001); http://dx.doi.org/10.1063/1.1398138View Description Hide Description
In these Lectures we discuss some aspects of theory of strongly correlated low-dimensional systems. In and two dimensions availability of non-perturbative techniques makes it possible to obtain solutions of non-trivial models. One of such techniques is called bosonization. In this course we discuss a generalization of this technique for systems with isotopic symmetry.