301 research outputs found
Subharmonic gap structure in short ballistic graphene junctions
We present a theoretical analysis of the current-voltage characteristics of a
ballistic superconductor-normal-superconductor (SNS) junction, in which a strip
of graphene is coupled to two superconducting electrodes. We focus in the
short-junction regime, where the length of the strip is much smaller than
superconducting coherence length. We show that the differential conductance
exhibits a very rich subharmonic gap structure which can be modulated by means
of a gate voltage. On approaching the Dirac point the conductance normalized by
the normal-state conductance is identical to that of a short diffusive SNS
junction.Comment: revtex4, 4 pages, 4 figure
On Effect of Equilibrium Fluctuations on Superfluid Density in Layered Superconductors
We calculate suppression of inter- and intralayer superconducting currents
due to equilibrium phase fluctuations and find that, in contrast to a recent
prediction, the effect of thermal fluctuations cannot account for linear
temperature dependence of the superfluid density in high-Tc superconductors at
low temperatures. Quantum fluctuations are found to dominate over thermal
fluctuations at low temperatures due to hardening of their spectrum caused by
the Josephson plasma resonance. Near Tc sizeable thermal fluctuations are found
to suppress the critical current in the stack direction stronger, than in the
direction along the layers. Fluctuations of quasiparticle branch imbalance make
the spectral density of voltage fluctuations at small frequencies non zero, in
contrast to what may be expected from a naive interpretation of Nyquist
formula.Comment: 5 pages, LaTeX, RevTeX, Submitted to PR
Linear response and collective oscillations in superconductors with d-wave pairing
Simple and physically transparent equations for the linear response of
layered superconductors with d-wave symmetry of the order parameter are derived
by means of the quasiclassic kinetic theory of superconductivity. Responses to
solenoidal and potential electric fields have different frequency dependencies.
The conductivity describing the response to the solenoidal field is limited by
the momentum relaxation, like in a normal metal. The response to the potential
electric field depends, in addition, on the branch imbalance relaxation rate.
The damping of plasma oscillations of superconducting electrons is determined
by dielectric relaxation and is small. Relaxation of branch imbalance
determined by elastic scattering is large enough to make the Carlson-Goldman
mode in d-wave superconductors overdamped.Comment: 11 pages, latex, no figures, submitted to Physical Review
AC Josephson Effect Induced by Spin Injection
Pure spin currents can be injected and detected in conductors via
ferromagnetic contacts. We consider the case when the conductors become
superconducting. A DC pure spin current flowing in one superconducting wire
towards another superconductor via a ferromagnet contact induces AC voltage
oscillations caused by Josephson tunneling of condensate electrons.
Quasiparticles simultaneously counterflow resulting in zero total electric
current through the contact. The Josephson oscillations can be accompanied by
Carlson-Goldman collective modes leading to a resonance in the voltage
oscillation amplitude.Comment: 5 page
Convective Term and Transversely Driven Charge-Density Waves
We derive the convective terms in the damping which determine the structure
of the moving charge-density wave (CDW), and study the effect of a current
flowing transverse to conducting chains on the CDW dynamics along the chains.
In contrast to a recent prediction we find that the effect is orders of
magnitude smaller, and that contributions from transverse currents of electron-
and hole-like quasiparticles to the force exerted on the CDW along the chains
act in the opposite directions. We discuss recent experimental verification of
the effect and demonstrate experimentally that geometry effects might mimic the
transverse current effect.Comment: RevTeX, 9 pages, 1 figure, accepted for publications in PR
Evidence for Two Time Scales in Long SNS Junctions
We use microwave excitation to elucidate the dynamics of long superconductor
/ normal metal / superconductor Josephson junctions. By varying the excitation
frequency in the range 10 MHz - 40 GHz, we observe that the critical and
retrapping currents, deduced from the dc voltage vs. dc current characteristics
of the junction, are set by two different time scales. The critical current
increases when the ac frequency is larger than the inverse diffusion time in
the normal metal, whereas the retrapping current is strongly modified when the
excitation frequency is above the electron-phonon rate in the normal metal.
Therefore the critical and retrapping currents are associated with elastic and
inelastic scattering, respectively
Field Dependence of the Josephson Plasma Resonance in Layered Superconductors with Alternating Junctions
The Josephson plasma resonance in layered superconductors with alternating
critical current densities is investigated in a low perpendicular magnetic
field. In the vortex solid phase the current densities and the squared bare
plasma frequencies decrease linearly with the magnetic field. Taking into
account the coupling due to charge fluctuations on the layers, we extract from
recent optical data for SmLa_{1-x} Sr_x CuO_{4-delta} the Josephson penetration
length lambda_{ab} approximately 1100 A parallel to the layers at T=10 K.Comment: 5 pages, 6 eps-figures, final version with minor misprints correcte
Josephson Plasma Resonance as a Structural Probe of Vortex Liquid
Recent developments of the Josephson plasma resonance and transport c-axis
measurements in layered high T superconductors allow to probe Josephson
coupling in a wide range of the vortex phase diagram. We derive a relation
between the field dependent Josephson coupling energy and the density
correlation function of the vortex liquid. This relation provides a unique
opportunity to extract the density correlation function of pancake vortices
from the dependence of the plasma resonance on the -component of the
magnetic field at a fixed -axis component.Comment: 4 pages, 1 fugure, accepted to Phys. Rev. Let
The Monitor Project: Stellar rotation at 13~Myr: I. A photometric monitoring survey of the young open cluster h~Per
We aim at constraining the angular momentum evolution of low mass stars by
measuring their rotation rates when they begin to evolve freely towards the
ZAMS, i.e. after the disk accretion phase has stopped. We conducted a
multi-site photometric monitoring of the young open cluster h Persei that has
an age of ~13 Myr. The observations were done in the I-band using 4 different
telescopes and the variability study is sensitive to periods from less than 0.2
day to 20 days. Rotation periods are derived for 586 candidate cluster members
over the mass range 0.4<=M/Msun<=1.4. The rotation period distribution
indicates a sligthly higher fraction of fast rotators for the lower mass
objects, although the lower and upper envelopes of the rotation period
distribution, located respectively at ~0.2-0.3d and ~10d, are remarkably flat
over the whole mass range. We combine this period distribution with previous
results obtained in younger and older clusters to model the angular momentum
evolution of low mass stars during the PMS. The h Per cluster provides the
first statistically robust estimate of the rotational period distribution of
solar-type and lower mass stars at the end of the PMS accretion phase (>10
Myr). The results are consistent with models that assume significant
core-envelope decoupling during the angular momentum evolution to the ZAMS.Comment: 39 pages, 19 figures, light curves in appendix, 1 long tabl
Equilibrium Low Temperature Heat Capacity of the Spin Density Wave compound (TMTTF)2 Br: effect of a Magnetic Field
We have investigated the effect of the magnetic field (B) on the very
low-temperature equilibrium heat capacity ceq of the quasi-1 D organic compound
(TMTTF)2Br, characterized by a commensurate Spin Density Wave (SDW) ground
state. Below 1K, ceq is dominated by a Schottky-like contribution, very
sensitive to the experimental time scale, a property that we have previously
measured in numerous DW compounds. Under applied field (in the range 0.2- 7 T),
the equilibrium dynamics, and hence ceq extracted from the time constant,
increases enormously. For B = 2-3 T, ceq varies like B2, in agreement with a
magnetic Zeeman coupling. Another specific property, common to other
Charge/Spin density wave (DW) compounds, is the occurrence of metastable
branches in ceq, induced at very low temperature by the field exceeding a
critical value. These effects are discussed within a generalization to SDWs in
a magnetic field of the available Larkin-Ovchinnikov local model of strong
pinning. A limitation of the model when compared to experiments is pointed out.Comment: 10 pages, 11 figure
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