326 research outputs found
Intrinsic Josephson Effect and Violation of the Josephson Relation in Layered Superconductors
Equations describing the resistive state of a layered superconductor with
anisotropic pairing are derived. The similarity with a stack of Josephson
junctions is found at small voltages only, when current density in the
direction perpendicular to the layers can be interpreted as a sum of the
Josephson superconducting, the Ohmic dissipative and the interference currents.
In the spatially uniform state differential conductivity at higher voltages
becomes negative. Nonuniformity of the current distribution generates the
branch imbalance and violates the Josephson relation between frequency and
voltage.Comment: 11 pages, no figures, revtex, to be published in Phys. Rev. Let
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
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
Another deep dimming of the classical T Tauri star RW Aur A
Context. RW Aur A is a classical T Tauri star (CTTS) with an unusually rich
emission line spectrum. In 2014 the star faded by ~ 3 magnitudes in the V band
and went into a long-lasting minimum. In 2010 the star suffered from a similar
fading, although less deep. These events in RW Aur A are very unusual among the
CTTS, and have been attributed to occultations by passing dust clouds. Aims. We
want to find out if any spectral changes took place after the last fading of RW
Aur A with the intention to gather more information on the occulting body and
the cause of the phenomenon. Methods. We collected spectra of the two
components of RW Aur. Photometry was made before and during the minimum.
Results. The overall spectral signatures reflecting emission from accretion
flows from disk to star did not change after the fading. However, blue-shifted
absorption components related to the stellar wind had increased in strength in
certain resonance lines, and the profiles and strengths, but not fluxes, of
forbidden lines had become drastically different. Conclusions. The extinction
through the obscuring cloud is grey indicating the presence of large dust
grains. At the same time, there are no traces of related absorbing gas. The
cloud occults the star and the interior part of the stellar wind, but not the
wind/jet further out. The dimming in 2014 was not accompanied by changes in the
accretion flows at the stellar surface. There is evidence that the structure
and velocity pattern of the stellar wind did change significantly. The dimmings
could be related to passing condensations in a tidally disrupted disk, as
proposed earlier, but we also speculate that large dust grains have been
stirred up from the inclined disk into the line-of-sight through the
interaction with an enhanced wind.Comment: 5 pages, 5 figures, Accepted for publication in A&
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
Low-temperature conductivity of quasi-one-dimensional conductors: Luttinger liquid stabilized by impurities
A new non-Fermi-liquid state of quasi-one-dimensional conductors is suggested
in which electronic system exists in a form of collection of bounded Luttinger
liquids stabilized by impurities. This state is shown to be stable towards
interchain electron hopping at low temperatures. Electronic spectrum of the
system contains zero modes and collective excitations of the bounded Luttinger
liquids in the segments between impurities. Zero modes give rise to randomly
distributed localized electronic levels, and long-range interaction generates
the Coulomb gap in the density of states at the Fermi energy. Mechanism of
conductivity at low temperatures is phonon-assisted hopping via zero-mode
states. At higher voltages the excitations of Luttinger liquid are involved in
electron transport, and conductivity obeys power-law dependence on voltage. The
results provide a qualitative explanation for recent experimental data for
NbSe3 and TaS3 crystals.Comment: 12 pages, 1 figur
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
Reflectivity and Microwave Absorption in Crystals with Alternating Intrinsic Josephson Junctions
We compute the frequency and magnetic field dependencies of the reflectivity
in layered superconductors with two alternating intrinsic Josephson
junctions with different critical current densities and quasiparticle
conductivities for the electric field polarized along the c-axis. The parameter
describing the electronic compressibility of the layers and the charge
coupling of neighboring junctions was extracted for the
SmLaSrCuO superconductor from two independent
optical measurements, the fit of the loss function at zero magnetic
field and the magnetic field dependence of the peak positions in .
The experiments are consistent with a free electron value for near
the Josephson plasma frequencies.Comment: 4 pages, 4 postscript figures, misprints in table correcte
Semiclassical theory of shot noise in disordered SN contacts
We present a semiclassical theory of shot noise in diffusive superconductor -
normal metal contacts. At subgap voltages, we reproduce the doubling of shot
noise with respect to conventional normal-metal contacts, which is interpreted
in terms of an energy balance of electrons. Above the gap, the voltage
dependence of the noise crosses over to the standard one with a
voltage-independent excess noise. The semiclassical description of noise leads
to correlations between currents at different electrodes of multiterminal SN
contacts which are always of fermionic type, i.e. negative. Using a quantum
extension of the Boltzmann - Langevin method, we reproduce the peculiarity of
noise at the Josephson frequency and obtain an analytical frequency dependence
of noise at above-gap voltages.Comment: 4 pages RevTeX, 1 eps figur
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