794 research outputs found
Probing multiband superconductivity by point-contact spectroscopy
Point-contact spectroscopy was originally developed for the determination of
the electron-phonon spectral function in normal metals. However, in the past 20
years it has become an important tool in the investigation of superconductors.
As a matter of fact, point contacts between a normal metal and a superconductor
can provide information on the amplitude and symmetry of the energy gap that,
in the superconducting state, opens up at the Fermi level. In this paper we
review the experimental and theoretical aspects of point-contact spectroscopy
in superconductors, and we give an experimental survey of the most recent
applications of this technique to anisotropic and multiband superconductors.Comment: 38 pages, 28 eps figures. To appear as a Topical Review in
Superconductor Science and Technolog
Intrinsic surface depression of the order parameter under mixed (s+id)-wave pair symmetry and its effect on the critical current of high-Tc SIS Josephson junctions
An intrinsic gap depression at the Superconductor-Insulator interface due to
the very short value of the coherence length in High-Tc Superconductors [HTSs]
is considered, in the framework of a mixed (s+id)-wave pair symmetry for the
order parameter ranging from pure s to pure d-wave. This gap depression acts as
the main physical agent causing the relevant reduction of IcRn(T) values with
respect to BCS expectations in HTS SIS Josephson junctions. Good agreement with
various experimental data is obtained with both pure s-wave and pure d-wave
symmetries of the order parameter, but with amounts of gap depression depending
on the pair symmetry adopted. Regardless of the pair symmetry considered, these
results prove the importance of the surface order-parameter depression in the
correct interpretation of the Ic(T)Rn(T) data in HTS SIS junctions. In a case
of planar YBCO-based junction the use of the de Gennes condition allowed us to
tentatively obtain an upper limit for the amount of d-wave present in the order
parameter of YBCO.Comment: 11 pages REVTeX file, 6 PostScript figures, to be published in J.
Superconductivit
Point contact spectroscopy in Fe-based superconductors: recent advancements and future challenges
Iron-based superconductors (FeSC) present an unprecedented variety of
features both in the superconducting and in the normal state. Different
families differ in the value of the critical temperature, in the shape of the
Fermi surface, in the existence or absence of quasi-nesting conditions, in the
range of doping in which the antiferromagnetic (AFM) and the superconducting
phase coexist and in the structure of the order parameter in the reciprocal
space, and so on. In this paper the most important results of point-contact
spectroscopy (PCS) in Fe-based superconductors are reviewed, and the most
recent advances are described with the aim to discuss the future perspectives
and challenges of this spectroscopic technique in the characterization of the
superconducting properties of these complex compounds. One of the main
challenges, faced so far only by a few researchers in the PCS field, is to
fully explore the phase diagram of these materials, as a function of doping or
pressure, to understand the interplay between superconductivity and magnetism,
the effect of intrinsic or extrinsic inhomogeneities, the role of spin
fluctuations (SFs) in the pairing, the symmetry and the structure of the order
parameter(s).Comment: 12 pages, 4 figure
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.
Breakdown of Migdal's theorem and intensity of electron-phonon coupling in high-Tc superconductors
In this article we quantify the possible effects of the breakdown of Migdal's
theorem on the electron-phonon (e-p) coupling constant, on the critical
temperature and on the superconducting gap by examining different kinds of
superconducting materials either with low and high critical temperature. We use
the theoretical approach developed by Grimaldi, Pietronero and Strassler [PRB
52, 10516 & 10530 (1995)] on experimental data taken both from literature and
from our recent break-junction tunneling experiments in Bi_2Sr_2CaCu_2O_{8+x}
(BSCCO) [Physica C 275, 162 (1997) and cond-mat/9612220]. The results show that
a large violation of the Migdal's theorem (as in BSCCO) yields to a large
increase of the observed e-p coupling constant and, in a first approximation
and by direct solution of the Eliashberg equations, to large but different
increases of Tc and gap. The same theory gives no modifications when applied to
low-Tc conventional superconductors. In BSCCO, the renormalized values of the
e-p coupling constant and of the ratio 2Delta/kTc (1.85 and 4.98, respectively)
are compatible with a strong-coupling electron-phonon origin of
superconductivity.Comment: 11 pages REVTeX file, 3 PostScript figures, to be published in Phys.
Rev. B (Rapid Commun.
Anomalous screening of an electrostatic field at the surface of niobium nitride
The interaction between an electric field and the electric charges in a
material is described by electrostatic screening, which in metallic systems is
commonly thought to be confined within a distance of the order of the
Thomas-Fermi length. The validity of this picture, which holds for surface
charges up to , has been recently questioned by
several experimental results when dealing with larger surface charges, such as
those routinely achieved via the ionic gating technique. Whether these results
can be accounted for in a purely electrostatic picture is still debated. In
this work, we tackle this issue by calculating the spatial dependence of the
charge carrier density in thin slabs of niobium nitride via an ab initio
density functional theory approach in the field-effect transistor
configuration. We find that perturbations induced by surface charges are mainly screened within the first layer, while
those induced by larger surface charges can
penetrate over multiple atomic layers, in reasonable agreement with the
available experimental data. Furthermore, we show that a significant
contribution to the screening of large fields is associated not only to the
accumulation layer of the induced charge carriers at the surface, but also to
the polarization of the pre-existing charge density of the undoped system.Comment: 8 pages, 4 figure
3D-melting features of the irreversibility line in overdoped BiSrCuO at ultra-low temperature and high magnetic field
We have measured the irreversible magnetization of an overdoped
BiSrCuO single crystal up to B=28 T and down to T=60 mK, and
extracted the irreversibility line : the data can be
interpreted in the whole temperature range as a 3D-anisotropic vortex lattice
melting line with Lindemann number . We also briefly discuss
the applicability of alternative models such as 2D- and quantum melting, and
the connection with magnetoresistance experiments.Comment: M2S-HTSC-VI Conference paper (2 pages, 1 figure), using Elsevier
style espcrc2.st
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
Independent determination of the two gaps by directional point-contact spectroscopy in MgB_2 single crystals
Directional point-contact spectroscopy measurements were performed for the
first time in state-of-the-art MgB_2 single crystals. The selective suppression
of the superconductivity in the "pi" band by means of a suitable magnetic field
allowed separating the partial contribution of each band to the total
point-contact conductance. By fitting the partial conductance curves
sigma_sigma(V) and sigma_pi(V), we got an independent determination of the two
gaps, Delta_sigma and Delta_pi, with a strong reduction of the experimental
uncertainty. Their temperature dependence was found to agree well with the
predictions of the two-band models for MgB_2.Comment: 6 pages, 4 eps figures. References added, abstract rewritten, text
slightly changed. Proceedings of the BOROMAG Conference, June 17-19, Genoa,
Ital
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