Charge- and spin-density waves in existing superconductors: competition between Cooper pairing and Peierls or excitonic instabilities

This review is an up-to-date snapshot of the current situation in studies of materials with competing electron spectrum instabilities, namely Cooper pairing, on the one hand, and charge-density waves (CDWs) or spin-density waves (SDWs), on the other. The CDW- or SDW-driven instabilities in electron spectra with reduced dimensionalities may result from either the Fermi surface (FS) nesting or the existence of the Van Hove saddle points and lead to the appearance of the Peierls (excitonic) gap on parts of the FS (partial gapping). CDW superconductors include layered dichalcogenides, NbSe3, some organic substances and compounds with A15 and C15 structures among others. A large body of data is presented which may be considered as an evidence that high-T-c oxides also belong to this class of materials. In particular, an interpretation is given for the pseudogap phenomena in cuprates as having the CDW origin. The SDW superconductors include heavy-fermion compounds URU2Si2, UNi2Al3 and UPd2Al3, Cr-Re alloys and a number of organic superconductors. We discuss the experimental information on the existence of CDW and SDW instabilities in a wide range of different superconductors, and assess their contention with the Cooper pairing on the Fermi surface. The main emphasis is placed on the properties of the existing substances with the interplay between superconductivity and CDW- or SDW-waves rather than on theoretically analyzing the possibility of such mixed phases in the framework of idealized microscopic theoretical models. The problem of symmetry for relevant order parameters is also touched upon. The theoretical description of the partially gapped CDW and SDW superconductors is presented based mostly on the phenomenological Bilbro-McMillan model. Various thermodynamical and electrodynamical properties are analyzed in this framework. Much emphasis is given to the nonstationary Josephson effect in tunnel junctions involving CDW or SDW superconductors. A comparison is carried out with the experiment, with special attention paid to high-T-c oxides. (C) 2002 Elsevier Science B.V. All rights reserved.