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The effect of weak anisotropic (momentum-dependent) impurity scattering in unconventional superconductors has been investigated. It is shown that the anisotropic scattering can lead either to a small reduction or a small enhancement of the isotropic pair-breaking effect. The influence of the anisotropy of the scattering potential becomes significant for the order parameters with large Fermi surface average values. In that case an unexpected enhancement (up to 10%) of the critical temperature over the critical temperature in the absence of impurities is predicted for a small impurity concentration., Comment: 12 pages, RevTeX, 3 PostScript figures

Effect of J term in the t-t′-J model on the Fermi surface, density of states and the Drude plasma frequency have been studied. The J term is treated within the uniform bond theory of Affleck and Marston [ Phys. Rev. B39 , 11538 (1989)]. Inclusion of the J term succeeds in removing the Brinkman-Rice divergence in the density of states at Fermi energy but fails to obtain an insulating state at zero doping. We have also shown that an exact treatment of the J term does indeed lead to an insulating state at half-filling.

The superconducting density of states ϱ S (ω) and the tunneling differential conductivity σ ns (V) for an antiferromagnetic superconductor (AFSC) are calculated both in the paramagnetic and in the antiferromagnetic phases. Our model for AFSC includes the effects of the molecular field, impurities and elastic scattering from spin-fluctuations. The experimental data on σ ns (V) for TbMO 6 S 8 is understood reasonably well.

The dc-conductivity, plasma frequency and the specific heat are calculated as a function of doping for La 2−χ Sr χ CuO 4 and related compounds. There is good quantitative agreement with experimental data. The volume included by the Fermi surface obeys Luttinger Theorem. The calculations are based on quasiparticles whose energy spectrum has tight binding form renormalized by strong correlations and originating from the t - t ′- J model. We have used the relaxation time approximation.

We show the possibility of the formation of two-phonon bound states which give rise to a localized phonon mode like the one discovered recently in the compound Sm 0.75 Y 0.25 S. A crucial point in our model is to take account of an indirect two-phonon interaction due to local lattice-distortions around localized electrons interacting over nearest neighbor sites.

The superconducting order parameter Δ( T ) and the maximum Josephson tunneling current J s ( T ) for an antiferromagnetic superconductor (AFS) are calculated both in the paramagnetic and the antiferromagnetic phases. The effects of the molecular field, impurities and the elastic scattering from spin-fluctuations have been included. The enhancement in J s ( T ) and Δ( T ) by the antiferromagnetic (AF) ordering observed in SmRh 4 B 4 by Vaglio et al. [1] can be understood with a reasonable choice of parameters. We also predict depression in J s ( T ) and Δ( T ) by AF ordering in some materials.

We study the stability criterion for the critical point of a transition of the second kind in the T-H Q plane of an antiferromagnetic superconductor AFS (T is temperature and H Q is the staggered molecular field). The AFS is described by using the mean-field model given by Nass, Levin and Grest and we assume a one dimensional electron band. Some properties of the system near the critical point are also investigated.

We propose a new pairing mechanism for high- T c superconductivity in doped La 2 CuO 4 compounds. In this mechanism, the attractive interaction between the electrons in a Cooper pair is induced through holes which are spinless neutral bosons. The basic requirements of the model are the existence of CuO 2 planes containing a number of holes and a large intrasite Coulomb repulsion U . We find superconducting state with s -wave symmetry. For small values of the hole concentration δ, T c increases with an increase in δ and t (transfer integral). With δ = 0.15 and t = 1 eV, we obtain T c ∼ 75 K . We expect a maximum in the T c vs δ curve for some value of δ.

Using Anderson model in the non-magnetic limit, we have calculated the jump in specific heat at T c , (Δ C ), for superconducting virtual bound state alloys. It is found that the normalized (Δ C ) vs normalized T c curves deviate considerably from the BCS value. A more significant result of the present study is that the initial slope of such a curve has a maximum value of 3.638. It is the highest value reported in literature for the impurity problem and is much larger than obtained even in a large pair breaking situation like Kondo effect (2.481).

The problem of localized single-particle excitations for the geometry of a superconducting sphere embedded in another superconductor has been investigated. Assuming constant order parameters in both materials Bogoliubov equations are solved for the s-state. The dependence of the excitation energy on the particle size and the ratio of the order parameters is shown.