Properties of the π\pi state induced by impurities in a d-wave superconductor

We study the properties of a quantum impurity embedded in a superconducting host of a d-wave symmetry. The superconductor is described by the extended negative $U$ Hubbard model while the impurity introduces a repulsive interaction to the system around the central site. We discuss the influence of this repulsion on the local properties (density of states, electron pairing and spontaneous current around the impurity) of the superconductor. We show the condition of $\pi$ - like behaviour, defined as two subsystems having a phase difference of $\pi$, in the system by using a proper combination of attractive pairing bond interaction and repulsive one between electrons located at the impurity and nearest neighbour sites.Comment: 2 pages, 1 figur

Superconducting Pairing Amplitude and Local Density of States in Presence of Repulsive Centers

We study the properties of superconductor in presence of a finite concentration of repulsive centers. The superconductor is described by the negative $U$ Hubbard model while repulsive centers are treated as randomly distributed impurities with repulsive interaction. Analyzing the paring potential and local density of states at impurity sites we find a wide range of the system parameters where the $\pi$ - like state could possibly be realized. Comparison of our results to the single repulsive center case is also given.Comment: 2 pages, 2 figures, Physica B in pres

Spatial fluctuations of the pairing potential in disordered superconductors

We study the effect of site diagonal, non-magnetic, disorder on the a pairing amplitude in an extended Hubbard model with the intersite attraction. Analyzing fluctuations of a pairing potential we discuss the instability of mixed solutions, 's+d' and 's+id', in presence of disorder. The influence of disorder on extended s- and d-wave superconductors appear to be comparable but in certain regions of the phase diagram, even weak disorder can change the symmetry of the order parameter.Comment: 14 pages, 7 figures, Physica C (accepted

Specific Heat of Disordered Superconductors Induced by Negative Centers

We show that superconductors with inhomogeneous order parameters can show similar features as anisotropic ones. In this paper we study the low temperature specific heat dependence in such a system and we show that the disorder associated with randomly distributed attractive centers convert the BCS temperature exponential behaviour into a power law formula. To describe superconductivity we used a random version of the negative U Hubbard model, while the disorder was treated by means of the Coherent Potential Approximation (CPA).Comment: 2 pages, 3 figures, to apear in Journal of Magnetism and Magnetic Material