2,458 research outputs found
Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBaCuO Superconductor
We report planar tunneling measurements on thin films of
YBaCuO at various doping levels under magnetic fields. By
choosing a special setup configuration, we have probed a field induced energy
scale that dominates in the vicinity of a node of the d-wave superconducting
order parameter. We found a high doping sensitivity for this energy scale. At
Optimum doping this energy scale is in agreement with an induced
order parameter. We found that it can be followed down to low fields at optimum
doping, but not away from it.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
Quantum Effects and Broken Symmetries in Frustrated Antiferromagnets
We investigate the interplay between frustration and zero-point quantum
fluctuations in the ground state of the triangular and Heisenberg
antiferromagnets, using finite-size spin-wave theory, exact diagonalization,
and quantum Monte Carlo methods. In the triangular Heisenberg antiferromagnet,
by performing a systematic size-scaling analysis, we have obtained strong
evidences for a gapless spectrum and a finite value of the thermodynamic order
parameter, thus confirming the existence of long-range N\'eel order.The good
agreement between the finite-size spin-wave results and the exact and quantum
Monte Carlo data also supports the reliability of the spin-wave expansion to
describe both the ground state and the low-energy spin excitations of the
triangular Heisenberg antiferromagnet. In the Heisenberg model, our
results indicate the opening of a finite gap in the thermodynamic excitation
spectrum at , marking the melting of the antiferromagnetic
N\'eel order and the onset of a non-magnetic ground state. In order to
characterize the nature of the latter quantum-disordered phase we have computed
the susceptibilities for the most important crystal symmetry breaking
operators. In the ordered phase the effectiveness of the spin-wave theory in
reproducing the low-energy excitation spectrum suggests that the uniform spin
susceptibility of the model is very close to the linear spin-wave prediction.Comment: Review article, 44 pages, 18 figures. See also PRL 87, 097201 (2001
Transition from a mixed to a pure d-wave symmetry in superconducting optimally doped YBaCuO thin films under applied fields
We have probed the Landau levels of nodal quasi-particles by tunneling along
a nodal direction of (110) oriented YBaCuO thin films with a
magnetic field applied perpendicular to the planes, and parallel to the
film's surface. In optimally doped films and at low temperature, finite energy
nodal states are clearly observed in films thinner than the London penetration
depth. Above a well defined temperature the order parameter reverts to a pure
\emph{d}-wave symmetry.Comment: 4 pages, 4 figure
Remarkable change of tunneling conductance in YBCO films in fields up to 32.4T
We studied the tunneling density of states in YBCO films under strong
currents flowing along node directions. The currents were induced by fields of
up to 32.4T parallel to the film surface and perpendicular to the
planes. We observed a remarkable change in the tunneling conductance at high
fields where the gap-like feature shifts discontinuously from 15meV to a lower
bias of 11meV, becoming more pronounced as the field increases. The effect
takes place in increasing fields around 9T and the transition back to the
initial state occurs around 5T in decreasing fields. We argue that this
transition is driven by surface currents induced by the applied magnetic field.Comment: 4 pages, 7 figure
Coexistence of a triplet nodal order-parameter and a singlet order-parameter at the interfaces of ferromagnet-superconductor Co/CoO/In junctions
We present differential conductance measurements of Cobalt / Cobalt-Oxide /
Indium planar junctions, 500nm x 500nm in size. The junctions span a wide range
of barriers, from very low to a tunnel barrier. The characteristic conductance
of all the junctions show a V-shape structure at low bias instead of the
U-shape characteristic of a s-wave order parameter. The bias of the conductance
peaks is, for all junctions, larger than the gap of indium. Both properties
exclude pure s-wave pairing. The data is well fitted by a model that assumes
the coexistence of s-wave singlet and equal spin p-wave triplet fluids. We find
that the values of the s-wave and p-wave gaps follow the BCS temperature
dependance and that the amplitude of the s-wave fluid increases with the
barrier strength.Comment: 5 pages, Accepted to Phys. Rev.
Characteristic energies, transition temperatures, and switching effects in clean SNS graphene nanostructures
We study proximity effects in clean nanoscale superconductor-normal
metal-superconductor (SNS) graphene heterostructures using a
self-consistent numerical solution to the continuum Dirac Bogoliubov-de Gennes
(DBdG) equations. We obtain results for the pair amplitude and the local
density of states (DOS), as a function of doping and of the geometrical
parameters determining the width of the structures. The superconducting
correlations are found to penetrate the normal graphene layers even when there
is extreme mismatch in the normal and superconducting doping levels, where
specular Andreev reflection dominates. The local DOS exhibits peculiar
features, which we discuss, arising from the Dirac cone dispersion relation and
from the interplay between the superconducting and Thouless energy scales. The
corresponding characteristic energies emerge in the form of resonant peaks in
the local DOS, that depend strongly on the doping level, as does the energy
gap, which declines sharply as the relative difference in doping between the S
and N regions is reduced. We also linearize the DBdG equations and develop an
essentially analytical method that determines the critical temperature of
an \sns nanostructure self-consistently. We find that for S regions that occupy
a fraction of the coherence length, can undergo substantial variations as
a function of the relative doping. At finite temperatures and by manipulating
the doping levels, the self consistent pair amplitudes reveal dramatic
transitions between a superconducting and resistive normal state of the
structure. Such behavior suggests the possibility of using the proposed system
as a carbon-based superconducting switch, turning superconductivity on or off
by tuning the relative doping levels.Comment: 13 pages, figures include
Generic Finite Size Enhancement of Pairing in Mesoscopic Fermi Systems
The finite size dependent enhancement of pairing in mesoscopic Fermi systems
is studied under the assumption that the BCS approach is valid and that the two
body force is size independent. Different systems are investigated such as
superconducting metallic grains and films as well atomic nuclei. It is shown
that the finite size enhancement of pairing in these systems is in part due to
the presence of a surface which accounts quite well for the data of nuclei and
explains a good fraction of the enhancement in Al grains.Comment: Updated version 17/02/0
Proximity Effect Enhancement Induced by Roughness of SN Interface
Critical temperature reduction is considered for a thin film of
a layered superconductor (S) with a rough surface covered by a thick layer of a
normal metal (N). The roughness of the SN interface increases the penetration
of electrons from the normal metal into the superconductor and leads to an
enhancement of the proximity effect. The value of induced by the
roughness of the SN interface can be much higher than for a film
with a plain surface for an extremely anisotropic layered superconductor with
the coherence lengths .Comment: 2 page
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