Physics

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-Tc 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 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-Tc oxides.

The superconducting order parameter Δ and energy gap Δg are calculated taking into account the pair-breaking inelastic quasiparticle scattering by thermal Bose excitations, e.g. phonons. The treatment is self-consistent because the scattering amplitude depends on Δ. The superconducting transition for any strength of the inelastic scattering is the phase transition of the first kind and the dependences Δ(T) and Δg(T) tend to rectangular curves that agree well with tunnel, point-contact, infrared and Raman spectroscopy measurements for high-Tc oxides. On the basis of the developed theory, the critical Josephson tunnel current, ultrasound attenuation, electron thermal conductivity, and nuclear spin-lattice relaxation rate Rs in the superconducting state are calculated. All of these quantities are shown to decrease abruptly below Tc. In particular, the Hebel-Slichter peak in Rs(T) disappears for strong enough inelastic scattering, although the energy gap in the quasiparticle spectrum is retained.

Quasiparticle contribution to heat carriers relaxation time in DyBa 2 Cu 3 O 7−x from heat diffusivity measurements. S. Dorbolo(+) and M. Ausloos( †) Abstract It is shown that the controversy on phonons or electrons being the most influenced heat carriers below the critical temperature of high-T c superconductors can be resolved. Electrical and thermal properties of the same DyBa 2 Cu 3 O 7−x monodomain have been measured for two highly different oxygenation levels. While the oxygenated sample DyBa 2 Cu 3 O 7 has very good superconducting properties (T c = 90 K), the DyBa 2 Cu 3 O 6.3 sample exhibits an insulator behavior. A careful comparison between measurements of the thermal diffusivity of both samples allows us to extract the electronic contribution. This contribution to the relaxation time of heat carriers is shown to be large below T c and more sensitive to the superconducting state than the phonon contribution. P.A.C.S. : 60 +w, 17.15 jf, 44.90 +c Part of this work has been financially supported by the ARC 94-99/174 contract of the Ministry of Higher Education and Scientific Research through the University of Liège Research Council. S. Dorbolo has benefited from a FRIA research fellowship. We would like also to thank Prof. H.W. Vanderschueren for the use of MIEL equipments. We want also to thank Prof. J.P. Maneval (ENS, Paris) and Pr. E. Silva (U. Roma 3, Roma) and referees for their comments. e-mail: (+) [email protected] ( †) [email protected] 9