Fluctuating order parameter in doped cuprate superconductors

We discuss static fluctuations of the d-wave superconducting order parameter $\Delta$ in CuO$_2$ planes, due to quasiparticle scattering by charged dopants. The analysis of two-particle anomalous Green functions at $T = 0$ permits to estimate the mean-square fluctuation $\delta^2 = - ^2$, averaged in random dopant configurations, to the lowest order in doping level $c$. Since $\Delta$ is found to saturate with growing doping level while $\delta$ remains to grow, this can explain the collapse of $T_c$ at overdoping. Also we consider the spatial correlations $$ for order parameter in different points of the plane.Comment: RevTex4, 3 pages, to be published in Proceedings of New3SC-4 International Conference, San Diego, California, January 15-21, 200

Superconducting junctions from non-superconducting doped CuO2_2 layers

The theoretical approach proposed recently for description of redistribution of electronic charge in multilayered selectively doped systems is modified for a system with finite number of layers. A special attention is payed to the case of a finite heterostructure made of copper-oxide layers which are all non-superconducting (including non-conducting) because of doping levels being beyond the well-known characteristic interval for superconductivity. Specific finite structures and doping configurations are proposed to obtain atomically thin superconducting heterojunctions of different compositions.Comment: 4 pages, 3 figures, two bibliography references were update

Boundary Friction on Molecular Lubricants: Rolling Mode?

A theoretical model is proposed for low temperature friction between two smooth rigid solid surfaces separated by lubricant molecules, admitting their deformations and rotations. Appearance of different modes of energy dissipation (by ''rocking'' or ''rolling'' of lubricants) at slow relative displacement of the surfaces is shown to be accompanied by the stick-and-slip features and reveals a non-monotonic (mean) friction force {\it vs} external loadComment: revtex4, 4 pages, 5 figure

Group expansions for impurities in superconductors

A new method is proposed for practical calculation of the effective interaction between impurity scatterers in superconductors, based on algebraic properties of related Nambu matrices for Green functions. In particular, we show that the density of states within the s-wave gap can have a non-zero contribution (impossible either in Born and in T-matrix approximation) from non-magnetic impurities with concentration $c \ll 1$, beginning from $\sim c^{3}$ order.Comment: 5 pages, 1 figur

On the theory of high-TcT_\text{c} superconductivity of doped cuprates

A theoretical analysis is presented on possible effects of disorder by dopants in high-temperature superconducting cuprate perovskites, to define their basic spectra of spin and electronic excitations, and the subsequent observable properties, especially doping dependence of superconducting order parameter. The central point in the proposed physical picture is formation of specific impurity subband within the insulating bandgap of initial undoped material, serving as a source for the system metallization and further transition into superconducting state with anisotropic order parameter.Comment: 20 pages, 11 figure

Low-temperature orientational order and possible domain structures in C(_{60}) fullerite

Based on a simple model for ordering of hexagons on square planar lattice, an attempt has been made to consider possible structure of C(_{60}) fullerite in its low temperature phase. It is shown that hexagons, imitating fullerens oriented along (C_{3}) axes of \emph{sc} lattice, can be ordered into an ideal structure with four non-equivalent molecules in unit cell. Then the energy degeneracy for each hexagon rotations by (\pi /3) around its (C_{3}) axis leaves the translational and orientational order in this structure, but leads to a random distribution of (\pi /3) rotations and hence to {}``averaged{}'' unit cell with two molecules. However the most relevant structural defects are not these intrinsic \char`\"{}misorientations\char`\"{} but certain walls between the domains with different sequencies of the above-mentioned two (non-ideal) sublattices. Numeric estimates have been made for the anisotropic inter-molecular potential showing that the anisotropy is noticeably smaller for molecules in walls than in domains