Frozen electron phase in layered conductors and HTS

The ground state and superconducting properties of a layered sd-electron system formed by “s-layers” of wide-band s-electrons and “d-layers” of narrow-band d-electrons are investigated. It is shown that over a wide range of electron densities of state (including “metallic” values), the long-range (weak screening) character of Coulomb forces between electrons associated with the layered nature of the system leads to the formation of a new state in d-layers, viz., the “frozen” electron phase having an incommensurate structure in general and differing qualitatively from a Wigner crystal. The spectrum of elementary excitations of such an “electron glass” (“EG-excitations”), which were found to be electron transitions in spatially localized two-level system, is studied. It is shown that the Coulomb sd-interaction, which causes a virtual exchange by EG-excitations between s-electrons, leads to effective ss-attraction and to superconductivity of purely “Coulomb” origin. Expressions are obtained for the gap and superconducting transition temperature Tc. It is shown that Tc can attain values of the order of the d-band width. Peculiarities of the gap structure associated with the layered nature of the sd-system and the specific features of the EG-excitation spectrum are studied.

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