176 research outputs found
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.
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
Superconductivity of RbC: breakdown of the Migdal-Eliashberg theory
In this paper, through an exhaustive analysis within the Migdal-Eliashberg
theory, we show the incompatibility of experimental data of RbC with
the basic assumptions of the standard theory of superconductivity. For
different models of the electron-phonon spectral function
we solve numerically the Eliashberg equations to find which values of the
electron-phonon coupling , of the logarithmic phonon frequency
and of the Coulomb pseudopotential reproduce the
experimental data of RbC. We find that the solutions are essentially
independent of the particular shape of and that, to explain
the experimental data of RbC, one has to resort to extremely large
couplings: . This results differs from the usual partial
analyses reported up to now and we claim that this value exceeds the maximum
allowed compatible with the crystal lattice stability. Moreover, we
show quantitatively that the obtained values of and
strongly violate Migdal's theorem and consequently are incompatible with the
Migdal-Eliashberg theory. One has therefore to consider the generalization of
the theory of superconductivity in the nonadiabatic regime to account for the
experimental properties of fullerides.Comment: 9 pages, 8 eps figure encloses, epjb style, to appear on Eur. Phys.
J.
Possible explanation of electric-field-doped C60 phenomenology in the framework of Eliashberg theory
In a recent paper [J. H. Schön, Ch. Kloc, R. C. Haddon, and B. Batlogg, Nature (London) 408, 549 (2000)] a large increase in the superconducting critical temperature was observed in C60 doped with holes by application of a high electric field. We demonstrate that the measured Tc versus doping curves can be explained by solving the (four) s-wave Eliashberg equations in the case of a finite, non-half-filled energy band. In order to reproduce the experimental data, we assume a Coulomb pseudopotential depending on the filling in a very simple and plausible way. Reasonable values of the physical parameters involved are obtained. The application of the same approach to other experimental data [J. H. Schön, Ch. Kloc, and B. Batlogg, Science 293, 2432 (2001)] on electric field-doped, lattice-expanded C60 single crystals (Tc = 117 K in the hole-doped case) gives equally good results and sets a theoretical limit to the linear increase of Tc at the increase of the lattice spacing
Point-contact spectroscopy in MgB2 single crystals in magnetic field
We present the results of a spectroscopic study of state-of-the-art MgB2
single crystals, carried out by using a modified point-contact technique. The
use of single crystals allowed us to obtain point contacts with current
injection either parallel or perpendicular to the ab planes. The effect of
magnetic fields up to 9 T on the conductance spectra of these contacts is here
thoroughly studied, for both B parallel and perpendicular to the ab planes. The
complete thermal evolution of the upper critical field of the "pi" band is
determined for the first time, and quantitative information about the upper
critical field of the "sigma" band and its anisotropy is obtained. Finally, by
exploiting the different effect of a magnetic field applied parallel to the ab
planes on the two band systems, the partial contributions of the "sigma" and
"pi" bands to the total conductance are obtained separately. Fitting each of
them with the standard BTK model yields a great reduction of the uncertainty on
Delta_sigma and Delta_pi, whose complete temperature dependence is obtained
with the greatest accuracy.Comment: 8 pages, 7 eps figure
Direct evidence for two-band superconductivity in MgB_2 single crystals from directional point-contact spectroscopy in magnetic fields
We present the results of the first directional point-contact spectroscopy
experiments in high-quality MgB_2 single crystals. Due to the directionality of
the current injection into the samples, the application of a magnetic field
allowed us to separate the contributions of the sigma and pi bands to the total
conductance of our point contacts. By using this technique, we were able to
obtain the temperature dependency of each gap independent of the other. The
consequent, strong reduction of the error on the value of the gap amplitude as
function of temperature allows a stricter test of the predictions of the
two-band model for MgB_2.Comment: 4 pages, 5 eps figures. References added, Fig.1 changed, some text
adde
Coexistence of two order parameters and a pseudogaplike feature in the iron-based superconductor LaFeAsO_(1-x)F_x
The nature and value of the order parameters (OPs) in the superconducting
Fe-based oxypnictides REFeAsO_(1-x)F_x (RE = rare earth) are a matter of
intense debate, also connected to the pairing mechanism which is probably
unconventional. Point-contact Andreev-reflection experiments on
LaFeAsO_(1-x)F_x gave us direct evidence of three energy scales in the
superconducting state: a nodeless superconducting OP, Delta1 = 2.8-4.6 meV,
which scales with the local Tc of the contact; a larger unconventional OP that
gives conductance peaks at 9.8-12 meV, apparently closes below Tc and decreases
on increasing the Tc of the contact; a pseudogaplike feature (i.e. a depression
in the conductance around zero bias), that survives in the normal state up to
T* ~ 140 K (close to the Neel temperature of the undoped compound), which we
associate to antiferromagnetic spin fluctuations (AF SF) coexisting with
superconductivity. These findings point toward a complex, unconventional nature
of superconductivity in LaFeAsO_(1-x)F_x.Comment: 19 pages, 12 figures - one figure and some insets added, minor
changes to the tex
Josephson effect in MgB_2 break junctions
We present the first observation of the DC and AC Josephson effect in MgB_2
break junctions. The junctions, obtained at 4.2 K in high-quality, high-density
polycrystalline metallic MgB_2 samples, show a non-hysteretic DC Josephson
effect. By irradiating the junctions with microwaves we observe clear Shapiro
steps spaced by the ideal value. The temperature dependence of the
DC Josephson current and the dependence of the height of the steps on the
microwave power are obtained. These results are a direct prove for the
existence of pairs with charge 2e in this new metallic superconductor and give
evidence of the superconductor-normal metal-superconductor weak link character
of these junctions.Comment: 4 RevTEX pages, 4 eps figure
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