Possible alterations of the gravitational field in a superconductor

In this paper I calculate the possible alteration of the gravitational field in a superconductor by using the time-dependent Ginzburg-Landau equations (TDGL). I compare the behaviour of a high-Tc superconductor (HTCS) like YBa_2Cu_3O_7 (YBCO) with a classical low-Tc superconductor (LTCS) like Pb. Finally, I discuss what values of the parameters characterizing a superconductor can enhance the reduction of gravitational field.Comment: LaTex2e, 10 pages, 3 figure

Pressure dependence of critical temperature in MgB2 and two bands Eliashberg theory

The variation of the superconducting critical temperature Tc as a function of the pressure p in the magnesium diboride MgB2 has been studied in the framework of two-bands Eliashberg theory and traditional phonon coupling mechanism. I have solved the two-bands Eliashberg equations using first-principle calculations or simple assumptions for the variation, with the pressure, of the relevant physical quantities. I have found that the experimental Tc versus p curve can be fitted very well and information can be obtained on the dependence of the electron-phonon interaction matrix by pressure. The pressure dependence of the superconductive gaps Delta{sigma} and Delta{pi} is also predicted.Comment: 6 pages, 7 figure

Strong-coupling properties of unbalanced Eliashberg superconductors

In this paper we investigate the thermodynamical properties of ``unbalanced'' superconductors, namely, systems where the electron-boson coupling $\lambda$ is different in the self-energy and in the Cooper channels. This situation is encountered in a variety of situation, as for instance in d-wave superconductors. Quite interesting is the case where the pairing in the self-energy is smaller than the one in the gap equation. In this case we predict a finite critical value $\lambda_c$ where the superconducting critical temperature $T_c$ diverges but the zero temperature gap is still finite. The specific heat, magnetic critical field and the penetration depth are also evaluated.Comment: 9 Revtex pages, 7 eps figures include

Eliashberg theory of a multiband non-phononic spin glass superconductor

I solved the Eliashberg equations for multiband non-phononic $s\pm$ wave spin glass superconductor and I calculated the temperature dependence of the gaps and superfluid density that reveals unusual behavior as non monotonic temperature dependence and reentrant superconductivity. For particular values of input parameters the phase diagram is still more complex with two different ranges of temperature where the superconductivity appears

Real-Axis Solution of Eliashberg Equations in Various Order-Parameter Symmetries and Tunneling Conductance of Optimally-Doped HTSC

In the present work we calculate the theoretical tunneling conductance curves of SIN junctions involving high-Tc superconductors, for different possible symmetries of the order parameter (s, d, s+id, s+d, anisotropic s and extended s). To do so, we solve the real-axis Eliashberg equations in the case of an half-filled infinite band. We show that some of the peculiar characteristics of HTSC tunneling curves (dip and hump at eV > Delta, broadening of the gap peak, zero bias and so on) can be explained in the framework of the Migdal-Eliashberg theory. The theoretical dI/dV curves calculated for the different symmetries at T=4 K are then compared to various experimental tunneling data obtained in optimally-doped BSCCO, TBCO, HBCO, LSCO and YBCO single crystals. To best fit the experimental data, the scattering by non-magnetic impurities has to be taken into account, thus limiting the sensitivity of this procedure in determining the exact gap symmetry of these materials. Finally, the effect of the temperature on the theoretical tunneling conductance is also discussed and the curves obtained at T=2 K are compared to those given by the analytical continuation of the imaginary-axis solution.Comment: 6 pages, 3 figures, Proceedings of SATT10 Conference, to be published in Int. J. Mod. Phys.

Tunneling conductance of SIN junctions with different gap symmetries and non-magnetic impurities by direct solution of real-axis Eliashberg equations

We theoretically investigate the effect of various symmetries of the superconducting order parameter Delta(omega) on the normalized tunneling conductance of SIN junctions by directly solving the real-axis Eliashberg equations (EEs) for a half-filled infinite band, with the simplifying assumption mu*=0. We analyze six different symmetries of the order parameter: s, d, s+id, s+d, extended s and anisotropic s, by assuming that the spectral function alpha^{2}F(Omega) contains an isotropic part alpha^{2}F(Omega)_{is} and an anisotropic one, alpha^{2}F(Omega)_{an}, such that alpha^{2}F(Omega)_{an} = g alpha^{2}F(Omega)_{is}, where g is a constant. We compare the resulting conductance curves at T=2 K to those obtained by analytical continuation of the imaginary-axis solution of the EEs, and we show that the agreement is not equally good for all symmetries. Then, we discuss the effect of non-magnetic impurities on the theoretical tunneling conductance curves at T=4 K for all the symmetries considered. Finally, as an example, we apply our calculations to the case of optimally-doped high-T_{c} superconductors (HTSC). Surprisingly, although the possibility of explaining the very complex phenomenology of HTSC is probably beyond the limits of the Eliashberg theory, the comparison of the theoretical curves calculated at T=4 K with the experimental ones obtained in various optimally-doped copper-oxides gives fairly good results.Comment: 11 pages, 11 figures, 2 tables. References added, figs. 6,7,10 and 11 changed, text change

Standard Behaviour of Bi2Sr2CaCu2O8+δ Overdoped

I calculated the critical temperature and superconducting gap in the framework of one band d wave Eliashberg theory with only one free parameter in order to reproduce the experimental data relative to Bi2Sr2CaCu2O8+d (BSCCO) in the overdoped regime. The theoretical calculations are in excellent agreement with the experimental data and indicate that cuprates in the overdoped regime are well described by standard d-wave Eliashberg theory with coupling provided by antiferromagnetic spin fluctuations