178 research outputs found
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
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
Evidence for pseudogap and phase-coherence gap separation by Andreev reflection experiments in Au/La_{2-x}Sr_{x}CuO_4 point-contact junctions
We present new Au/La_{2-x}Sr_{x}CuO_{4} (LSCO) point-contact conductance
measures as a function of voltage and temperature in samples with 0.08 <= x <=
0.2. Andreev reflection features disappear at about the bulk Tc, giving no
evidence of gap for T > Tc. The fit of the normalized conductance at any T < Tc
supports a (s + d)-wave symmetry of the gap, whose dominant low-T s component
follows the Tc(x) curve in contrast with recent angle-resolved photoemission
spectroscopy and quasiparticle tunneling data. These results prove the
separation between pseudogap and phase-coherence superconducting gap in LSCO at
x <= 0.2.Comment: 4 pages, 4 eps figures, 1 table (RevTeX). Labels added to Fig. 1;
Fig. 3 resized; references added; short discussion about ballistic contact
regime adde
Doping and critical-temperature dependence of the energy gaps in Ba(Fe_{1-x}Co_x)_2As_2 thin films
The dependence of the superconducting gaps in epitaxial
Ba(Fe_{1-x}Co_{x})_2As_2 thin films on the nominal doping x (0.04 \leq x \leq
0.15) was studied by means of point-contact Andreev-reflection spectroscopy.
The normalized conductance curves were well fitted by using the 2D
Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps -- although
the possible presence of gap anisotropies cannot be completely excluded. The
amplitudes of the two gaps \Delta_{S} and \Delta_{L} show similar monotonic
trends as a function of the local critical temperature T_{c}^{A} (measured in
the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x
is well correlated to the trend of the critical temperature, i.e. to the shape
of the superconducting region in the phase diagram. When analyzed within a
simple three-band Eliashberg model, this trend turns out to be compatible with
a mechanism of superconducting coupling mediated by spin fluctuations, whose
characteristic energy scales with T_{c} according to the empirical law
\Omega_{0}= 4.65*k_{B}*T_{c}, and with a total electron-boson coupling strength
\lambda_{tot}= 2.22 for x \leq 0.10 (i.e. up to optimal doping) that slightly
decreases to \lambda_{tot}= 1.82 in the overdoped samples (x = 0.15).Comment: 8 pages, 5 color figure
Resistivity in Co-doped Ba-122: comparison of thin films and single crystals
The temperature dependence of the resistivity of epitaxial
Ba(Fe_(1-x)Co_x)2As2 thin films (with nominal doping x = 0.08, 0.10 and 0.15)
has been analyzed and compared with analogous measurements on single crystals
taken from literature. The rho(T) of thin films looks different from that of
single crystals, even when the cobalt content is the same. All rho(T) curves
can be fitted by considering an effective two-band model (with holes and
electrons bands) in which the electrons are more strongly coupled with the
bosons (spin fluctuations) than holes, while the effect of impurities is mainly
concentrated in the hole band. Within this model the mediating boson has the
same characteristic energy in single crystals and thin films, but the shape of
the transport spectral function at low energy has to be very different, leading
to a "hardening" of the electron-boson spectral function in thin films,
associated with the strain induced by the substrate.Comment: 13 pages, 4 figure
Proximity Eliashberg theory of electrostatic field-effect-doping in superconducting films
We calculate the effect of a static electric field on the critical
temperature of a s-wave one band superconductor in the framework of proximity
effect Eliashberg theory. In the weak electrostatic field limit the theory has
no free parameters while, in general, the only free parameter is the thickness
of the surface layer where the electric field acts. We conclude that the best
situation for increasing the critical temperature is to have a very thin film
of a superconducting material with a strong increase of electron-phonon (boson)
constant upon charging.Comment: 9 pages, 5 figure
Point-contact Andreev-reflection spectroscopy in segregation-free Mg_{1-x}Al_{x}B_2 single crystals up to x=0.32
We present new results of point-contact Andreev-reflection (PCAR)
spectroscopy in single-phase Mg_{1-x}Al_{x}B_{2} single crystals with x up to
0.32. Fitting the conductance curves of our point contacts with the two-band
Blonder-Tinkham-Klapwijk model allowed us to extract the gap amplitudes
Delta_{sigma} and Delta_{pi}. The gap values agree rather well with other PCAR
results in Al-doped crystals and polycrystals up to x=0.2 reported in
literature, and extend them to higher Al contents. In the low-doping regime,
however, we observed an increase in the small gap Delta_{pi} on increasing x
(or decreasing the local critical temperature of the junctions, T_{c}^{A})
which is not as clearly found in other samples. On further decreasing T_{c}^{A}
below 30 K, both the gaps decrease and, up to the highest doping level x=0.32
and down to T_{c}^{A}= 12 K, no gap merging is observed. A detailed analysis of
the data within the two-band Eliashberg theory shows that this gap trend can be
explained as being mainly due to the band filling and to an increase in the
interband scattering which is necessary to account for the increase in
Delta_{pi} at low Al contents (x < 0.1). We suggest to interpret the following
decrease of Delta_{pi} for T_{c}^{A} < 30 K as being governed by the onset of
inhomogeneity and disorder in the Al distribution that partly mask the
intrinsic effects of doping and is not taken into account in standard
theoretical approaches.Comment: 22 pages, 9 eps figures, Elsevier style. Theoretical details added in
appendix. Characterization of crystals include
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
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