503 research outputs found
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 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 where the superconducting critical
temperature 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 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
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