429 research outputs found
A length scale for the superconducting Nernst signal above T in NbSi
We present a study of the Nernst effect in amorphous superconducting thin
films of NbSi. The field dependence of the Nernst coefficient
above T displays two distinct regimes separated by a field scale set by
the Ginzburg-Landau correlation length. A single function , with the
correlation length as its unique argument set either by the zero-field
correlation length (in the low magnetic field limit) or by the magnetic length
(in the opposite limit), describes the Nernst coefficient. We conclude that the
Nernst signal observed on a wide temperature () and field () range is exclusively generated by short-lived Cooper pairs.Comment: 4 pages, 4 figure
Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces
The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide
interfaces becomes superconducting when the carrier density is tuned by gating.
The measured resistance and superfluid density reveal an inhomogeneous
superconductivity resulting from percolation of filamentary structures of
superconducting "puddles" with randomly distributed critical temperatures,
embedded in a non-superconducting matrix. Following the evidence that
superconductivity is related to the appearance of high-mobility carriers, we
model intra-puddle superconductivity by a multi-band system within a weak
coupling BCS scheme. The microscopic parameters, extracted by fitting the
transport data with a percolative model, yield a consistent description of the
dependence of the average intra-puddle critical temperature and superfluid
density on the carrier density.Comment: 7 pages with 3 figures + supplemental material (4 pages and 5
figures
Unusual magneto-transport of YBa2Cu3O7-d films due to the interplay of anisotropy, random disorder and nanoscale periodic pinning
We study the general problem of a manifold of interacting elastic lines whose
spatial correlations are strongly affected by the competition between random
and ordered pinning. This is done through magneto-transport experiments with
YBa2Cu3O7-d thin films that contain a periodic vortex pinning array created via
masked ion irradiation, in addition to the native random pinning. The strong
field-matching effects we observe suggest the prevalence of periodic pinning,
and indicate that at the matching field each vortex line is bound to an
artificial pinning site. However, the vortex-glass transition dimensionality,
quasi-2D instead of the usual 3D, evidences reduced vortex-glass correlations
along the vortex line. This is also supported by an unusual angular dependence
of the magneto-resistance, which greatly differs from that of Bose-glass
systems. A quantitative analysis of the angular magnetoresistance allows us to
link this behaviour to the enhancement of the system anisotropy, a collateral
effect of the ion irradiation
Non-linear characteristics in two-dimensional superconductors: Berezinskii-Kosterlitz-Thouless physics vs inhomogeneity
One of the hallmarks of the Berezinskii-Kosterlitz-Thouless (BKT) transition
in two-dimensional (2D) superconductors is the universal jump of the superfluid
density, that can be indirectly probed via the non-linear exponent of the
current-voltage characteristics. Here, we compare the experimental
measurements of characteristics in two cases, namely NbN thin films and
SrTiO-based interfaces. While the former display a paradigmatic example of
BKT-like non-linear effects, the latter do not seem to justify a BKT analysis.
Rather, the observed characteristics can be well reproduced theoretically
by modelling the effect of mesoscopic inhomogeneity of the superconducting
state. Our results offer an alternative perspective on the spontaneous
fragmentation of the superconducting background in confined 2D systems.Comment: Final version, as publishe
Competition between electron pairing and phase coherence in superconducting interfaces
In LaAlO3/SrTiO3 heterostructures, a gate tunable superconducting electron gas is confined in a quantum well at the interface between two insulating oxides. Remarkably, the gas coexists with both magnetism and strong Rashba spin–orbit coupling. However, both the origin of superconductivity and the nature of the transition to the normal state over the whole doping range remain elusive. Here we use resonant microwave transport to extract the superfluid stiffness and the superconducting gap energy of the LaAlO3/SrTiO3 interface as a function of carrier density. We show that the superconducting phase diagram of this system is controlled by the competition between electron pairing and phase coherence. The analysis of the superfluid density reveals that only a very small fraction of the electrons condenses into the superconducting state. We propose that this corresponds to the weak filling of high- energy dxz/dyz bands in the quantum well, more apt to host superconductivity
Coulomb Explosion and Thermal Spikes
A fast ion penetrating a solid creates a track of excitations. This can
produce displacements seen as an etched track, a process initially used to
detect energetic particles but now used to alter materials. From the seminal
papers by Fleischer et al. [Phys. Rev. 156, 353 (1967)] to the present [C.
Trautmann, S. Klaumunzer and H. Trinkaus, Phys. Rev. Lett. 85, 3648 (2000)],
`Coulomb explosion' and thermal spike models are treated as conflicting models
for describing ion track effects. Here molecular dynamics simulations of
electronic-sputtering, a surface manifestation of ion track formation, show
that `Coulomb explosion' produces a `heat' spike so that these are early and
late aspects of the same process. Therefore, differences in scaling are due to
the use of incomplete spike models.Comment: Submitted to PRL. 4 pages, 3 figures. For related movies see:
http://dirac.ms.virginia.edu/~emb3t/coulomb/coulomb.html PACS added in new
versio
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
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