2,344 research outputs found
Influence of topological excitations on Shapiro steps and microwave dynamical conductance in bilayer exciton condensates
The quantum Hall state at total filling factor in bilayer systems
realizes an exciton condensate and exhibits a zero-bias tunneling anomaly,
similar to the Josephson effect in the presence of fluctuations. In contrast to
conventional Josephson junctions, no Fraunhofer diffraction pattern has been
observed, due to disorder induced topological defects, so-called merons. We
consider interlayer tunneling in the presence of microwave radiation, and find
Shapiro steps in the tunneling current-voltage characteristic despite the
presence of merons. Moreover, the Josephson oscillations can also be observed
as resonant features in the microwave dynamical conductance
Thermal fluctuations in moderately damped Josephson junctions: Multiple escape and retrapping, switching- and return-current distributions and hysteresis
A crossover at a temperature T* in the temperature dependence of the width s
of the distribution of switching currents of moderately damped Josephson
junctions has been reported in a number of recent publications, with positive
ds/dT and IV characteristics associated with underdamped behaviour for lower
temperatures T<T*, and negative ds/dT and IV characteristics resembling
overdamped behaviour for higher temperatures T>T*. We have investigated in
detail the behaviour of Josephson junctions around the temperature T* by using
Monte Carlo simulations including retrapping from the running state into the
supercurrent state as given by the model of Ben-Jacob et al. We develop
discussion of the important role of multiple escape and retrapping events in
the moderate-damping regime, in particular considering the behaviour in the
region close to T*. We show that the behaviour is more fully understood by
considering two crossover temperatures, and that the shape of the distribution
and s(T) around T*, as well as at lower T<T*, are largely determined by the
shape of the conventional thermally activated switching distribution. We show
that the characteristic temperatures T* are not unique for a particular
Josephson junction, but have some dependence on the ramp rate of the applied
bias current. We also consider hysteresis in moderately damped Josephson
junctions and discuss the less commonly measured distribution of return
currents for a decreasing current ramp. We find that some hysteresis should be
expected to persist above T* and we highlight the importance, even well below
T*, of accounting properly for thermal fluctuations when determining the
damping parameter Q.Comment: Accepted for publication in PR
Correlated metallic two particle bound states in quasiperiodic chains
Single particle states in a chain with quasiperiodic potential show a
metal-insulator transition upon the change of the potential strength. We
consider two particles with local interaction in the single particle insulating
regime. The two particle states change from being localized to delocalized upon
an increase of the interaction strength to a nonperturbative finite value. At
even larger interaction strength the states become localized again. This
transition of two particle bound states into a correlated metal is due to a
resonant mixing of the noninteracting two particle eigenstates. In the
discovered correlated metal states two particles move coherently together
through the whole chain, therefore contributing to a finite conductivity.Comment: 4 pages, 4 figure
Probing the superconducting condensate on a nanometer scale
Superconductivity is a rare example of a quantum system in which the
wavefunction has a macroscopic quantum effect, due to the unique condensate of
electron pairs. The amplitude of the wavefunction is directly related to the
pair density, but both amplitude and phase enter the Josephson current : the
coherent tunneling of pairs between superconductors. Very sensitive devices
exploit the superconducting state, however properties of the {\it condensate}
on the {\it local scale} are largely unknown, for instance, in unconventional
high-T cuprate, multiple gap, and gapless superconductors.
The technique of choice would be Josephson STS, based on Scanning Tunneling
Spectroscopy (STS), where the condensate is {\it directly} probed by measuring
the local Josephson current (JC) between a superconducting tip and sample.
However, Josephson STS is an experimental challenge since it requires stable
superconducting tips, and tunneling conditions close to atomic contact. We
demonstrate how these difficulties can be overcome and present the first
spatial mapping of the JC on the nanometer scale. The case of an MgB film,
subject to a normal magnetic field, is considered.Comment: 7 pages, 6 figure
Evidence of Josephson-coupled superconducting regions at the interfaces of Highly Oriented Pyrolytic Graphite
Transport properties of a few hundreds of nanometers thick (in the graphene
plane direction) lamellae of highly oriented pyrolytic graphite (HOPG) have
been investigated. Current-Voltage characteristics as well as the temperature
dependence of the voltage at different fixed input currents provide evidence
for Josephson-coupled superconducting regions embedded in the internal
two-dimensional interfaces, reaching zero resistance at low enough
temperatures. The overall behavior indicates the existence of superconducting
regions with critical temperatures above 100 K at the internal interfaces of
oriented pyrolytic graphite.Comment: 6 Figures, 5 page
Phase diffusion in graphene-based Josephson junctions
We report on graphene-based Josephson junctions with contacts made from lead.
The high transition temperature of this superconductor allows us to observe the
supercurrent branch at temperatures up to K, at which point we can
detect a small, but non-zero, resistance. We attribute this resistance to the
phase diffusion mechanism, which has not been yet identified in graphene. By
measuring the resistance as a function of temperature and gate voltage, we can
further characterize the nature of electromagnetic environment and dissipation
in our samples.Comment: 4 pages, 3 figures, PR
Current and universal scaling in anomalous transport
Anomalous transport in tilted periodic potentials is investigated within the
framework of the fractional Fokker-Planck dynamics and the underlying
continuous time random walk. The analytical solution for the stationary,
anomalous current is obtained in closed form. We derive a universal scaling law
for anomalous diffusion occurring in tilted periodic potentials. This scaling
relation is corroborated with precise numerical studies covering wide parameter
regimes and different shapes for the periodic potential, being either symmetric
or ratchet-like ones
Mechanism for flux guidance by micrometric antidot arrays in superconducting films
A study of magnetic flux penetration in a superconducting film patterned with
arrays of micron sized antidots (microholes) is reported. Magneto-optical
imaging (MOI) of a YBCO film shaped as a long strip with perpendicular antidot
arrays revealed both strong guidance of flux, and at the same time large
perturbations of the overall flux penetration and flow of current. These
results are compared with a numerical flux creep simulation of a thin
superconductor with the same antidot pattern. To perform calculations on such a
complex geometry, an efficient numerical scheme for handling the boundary
conditions of the antidots and the nonlocal electrodynamics was developed. The
simulations reproduce essentially all features of the MOI results. In addition,
the numerical results give insight into all other key quantities, e.g., the
electrical field, which becomes extremely large in the narrow channels
connecting the antidots.Comment: 8 pages, 7 figure
Local superfluid densities probed via current-induced superconducting phase gradients
We have developed a superconducting phase gradiometer consisting of two
parallel DNA-templated nanowires connecting two thin-film leads. We have ramped
the cross current flowing perpendicular to the nanowires, and observed
oscillations in the lead-to-lead resistance due to cross-current-induced phase
differences. By using this gradiometer we have measured the temperature and
magnetic field dependence of the superfluid density and observed an
amplification of phase gradients caused by elastic vortex displacements. We
examine our data in light of Miller-Bardeen theory of dirty superconductors and
a microscale version of Campbell's model of field penetration.Comment: 5 pages, 6 figure
Detection of Complex Networks Modularity by Dynamical Clustering
Based on cluster de-synchronization properties of phase oscillators, we
introduce an efficient method for the detection and identification of modules
in complex networks. The performance of the algorithm is tested on computer
generated and real-world networks whose modular structure is already known or
has been studied by means of other methods. The algorithm attains a high level
of precision, especially when the modular units are very mixed and hardly
detectable by the other methods, with a computational effort on
a generic graph with nodes and links.Comment: 5 pages, 2 figures. Version accepted for publication on PRE Rapid
Communications: figures changed and text adde
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