Superlight bipolarons and a checkerboard d-wave condensate in cuprates
arXiv:cond-mat/0306649 · doi:10.1023/B:JOSC.0000011840.24281.eb
Abstract
The seminal work by Bardeen, Cooper and Schrieffer taken further by Eliashberg to the intermediate coupling solved the problem of conventional superconductors about half a century ago. The Froehlich and Jahn-Teller electron-phonon interactions were identified as an essential piece of physics in all novel superconductors. The BCS theory provides a qualitatively correct description of some of them like magnesium diborade and doped fullerenes (if the polaron formation is taken into account). However, cuprates remain a problem. Here I show that the bipolaron extension of the BCS theory to the strong-coupling regime could be a solution. Low-energy physics in this regime is that of small 'superlight' bipolarons, which are real-space mobile bosonic pairs dressed by phonons. The symmetry and space modulations of the order parameter are explained in the framework of the bipolaron theory. A d-wave Bose-Einstein condensate of bipolarons reveals itself as a checkerboard modulation of the hole density and of the gap below Tc.
5 pages, 1 figure, invited paper at the International Conference on 'Dynamic Inhomogenities in Complex Oxides' (June 2003, Bled, Slovenia)