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

The link between s and d components of electron boson coupling constants in one band d wave Eliashberg theory for high Tc superconductors

The phenomenology of overdoped high Tc uperconductors can be described by a one band d wave Eliashberg theory where the mechanism of superconducting coupling is mediated by antiferromagnetic spin fl uctuations and whose characteristic energy Ω0 scales with Tc according to the empirical law Ω0 = 5.8 kBTc. This model presents universal characteristics that are independent of the critical temperature such as the link between the s and d components of electron boson coupling constants and the invariance of the ratio 2Δ/kBTc. This situation arises from the particular structure of Eliashberg's equations which, despite being non-linear equations, present solutions with these simple propertie

The Phenomenology of Iron Pnictides Superconductors Explained in the Framework of -Wave Three-Band Eliashberg Theory

Thes-wave three-band Eliashberg theory can simultaneously reproduce the experimental critical temperatures and the gap values of the superconducting materials , and as exponent of the more important families of iron pnictides. In this model the dominant role is played by interband interactions and the order parameter undergoes a sign reversal between hole and electron bands (-wave symmetry). The values of all the gaps (with the exact experimental critical temperature) can be obtained by using high values of the electron-boson coupling constants and small typical boson energies (in agreement with experiments)

Superconductive critical temperature of Pb/Ag heterostructures

Recent experimental data (H. Nam et al., Phys. Rev. B 100, 094512 (2019)) of critical temperature and gaps measured on superconductor/normal metal heterostructure Pb/Ag, epitaxially grown, shown a interesting not usual behaviour. The critical temperature decreases strongly but, despite the large differences in the lattice constants and electronic densities of states in the separate components, this heterostructure shows a spatially constant superconducting gap. In the paper it is demonstrated that the proximity Eliashberg equations, whit no free parameters, cannot explain the dependence of critical temperature from the rate $d_{Pb}/d_{Ag}$. However it is sufficient to assume that the density of states at the Fermi level of silver is equal to that of lead in a layer adjacent to the separation interface, presumably of a thickness less than the coherence length of the lead, to perfectly explain the decrease in the critical temperature and the gap value in function of the rate $d_{Pb}/d_{Ag}$ always without free parameters.Comment: 7 pages, 2 figure

Effects of magnetic impurities on two-band superconductor

I calculated the effect of magnetic impurities on the normal and superconductive properties of a multiband s-wave superconductor by direct solution of the two-band Eliashberg equations. In this way I determined the critical temperature, the values of the superconductive gaps, the shape of the superconductive density of states and other physical quantities that depend on the concentration of magnetic impurities. I found that the gaps and the penetration lengths display an unusual behaviour as a function of temperature. I examine the possibility that the presence of a negative induced gap raises the critical temperature.Comment: 4 pages, 5 figures, paper presented to STRIPES06, december 2006, Rom

Possible d+id scenario in La_{2-x}Sr_{x}CuO_4 by point-contact measurements

We analyze the results of point-contact measurements in La_{2-x}Sr_{x}CuO_{4} (LSCO) previously reported as a clear evidence of the separation between gap and pseudogap in this copper oxide. Here we show that, in addition to this, the conductance curves of our point-contact junctions -- showing clear Andreev reflection features -- can be interpreted as supporting a nodeless d_{x^2-y^2}+id_{xy}-wave symmetry of the gap in LSCO. The results of our analysis, in particular the doping dependence of the subdominant d_{xy} gap component, are discussed and compared to the predictions of different theoretical models.Comment: 6 pages, 4 eps figures, presented at SATT11 Conference (Vietri sul Mare, March 2002). To appear in Int. J. Mod. Phy