1,925 research outputs found
Pulsed high-power arc heater with improved cathode and triggering mechanism
System employs pulsed, constricted arc heater capable of multi-MW power, permitting quasi-stationary flow conditions during latter half of pulse of about 5 msec. System description is given
Delayed response of a fermion-pair condensate to a modulation of the interaction strength
The effect of a sinusoidal modulation of the interaction strength on a
fermion-pair condensate is analytically studied. The system is described by a
generalization of the coupled fermion-boson model that incorporates a
time-dependent intermode coupling induced via a magnetic Feshbach resonance.
Nontrivial effects are shown to emerge depending on the relative magnitude of
the modulation period and the relaxation time of the condensate. Specifically,
a nonadiabatic modulation drives the system out of thermal equilibrium: the
external field induces a variation of the quasiparticle energies, and, in turn,
a disequilibrium of the associated populations. The subsequent relaxation
process is studied and an analytical description of the gap dynamics is
obtained. Recent experimental findings are explained: the delay observed in the
response to the applied field is understood as a temperature effect linked to
the condensate relaxation time.Comment: 6 page
Long-Range Order of Vortex Lattices Pinned by Point Defects in Layered Superconductors
How the vortex lattice orders at long range in a layered superconductor with
weak point pinning centers is studied through a duality analysis of the
corresponding frustrated XY model. Vortex-glass order emerges out of the vortex
liquid across a macroscopic number of weakly coupled layers in perpendicular
magnetic field as the system cools down. Further, the naive magnetic-field
scale determined by the Josephson coupling between adjacent layers is found to
serve as an upperbound for the stability of any possible conventional vortex
lattice phase at low temperature in the extreme type-II limit.Comment: 13 pgs., 1 table, published versio
Quantum spin configurations in Tb2Ti2O7
Low energy collective angular momentum states of the Tb3+ ions in Tb2Ti2O7
are classified according to the irreducible representations of the octahedral
point group. Degeneracy lifting due to the exchange interaction is discussed.
Diffuse neutron scattering intensity patterns are calculated for each
collective angular momentum state and the ground state is inferred by comparing
to experiment.Comment: 5 pages, 1 colour figure. Slight corrections and additions to text
and figur
Crossed conductance in FSF double junctions: role of out-of-equilibrium populations
We discuss a model of Ferromagnet / Superconductor / Ferromagnet (FSF) double
junction in which the quasiparticles are not in equilibrium with the condensate
in a region of the superconductor containing the two FS contacts. The role of
geometry is discussed, as well as the role of a small residual density of
states within the superconducting gap, that allows a sequential tunneling
crossed current. With elastic quasiparticle transport and the geometry with
lateral contacts, the crossed conductances in the sequential tunneling channel
are almost equal in the normal and superconducting phases, if the distance
between the FS interfaces is sufficiently small. The sequential tunneling and
spatially separated processes (the so-called crossed Andreev reflection and
elastic cotunneling processes) lead to different signs of the crossed current
in the antiparallel alignment for tunnel interfaces.Comment: 8 pages, 4 figure
Vortex Molecular Crystal and Vortex Plastic Crystal States in Honeycomb and Kagome Pinning Arrays
Using numerical simulations, we investigate vortex configurations and pinning
in superconductors with honeycomb and kagome pinning arrays. We find that a
variety of novel vortex crystal states can be stabilized at integer and
fractional matching field densities. The honeycomb and kagome pinning arrays
produce considerably more pronounced commensuration peaks in the critical
depinning force than triangular pinning arrays, and also cause additional peaks
at noninteger matching fields where a portion of the vortices are located in
the large interstitial regions of the pinning lattices. For the honeycomb
pinning array, we find matching effects of equal strength at most fillings
B/B_\phi=n/2 for n>2, where n is an integer, in agreement with recent
experiments. For kagome pinning arrays, pronounced matching effects generally
occur at B/B_\phi=n/3 for n>3, while for triangular pinning arrays pronounced
matching effects are observed only at integer fillings B/B_\phi=n. At the
noninteger matching field peaks in the honeycomb and kagome pinning arrays, the
interstitial vortices are arranged in dimer, trimer, and higher order n-mer
states that have an overall orientational order. We call these n-mer states
"vortex molecular crystals" and "vortex plastic crystals" since they are
similar to the states recently observed in colloidal molecular crystal systems.
We argue that the vortex molecular crystals have properties in common with
certain spin systems such as Ising and n-state Potts models. We show that
kagome and honeycomb pinning arrays can be useful for increasing the critical
current above that of purely triangular pinning arrays.Comment: 19 pages, 22 postscript figures. Version to appear in Phys. Rev.
Non-Gaussian fluctuations of mesoscopic persistent currents
The persistent current in an ensemble of normal-metal rings shows Gaussian
distributed sample-to-sample fluctuations with non-Gaussian corrections, which
are precursors of the transition into the Anderson localized regime. We here
report a calculation of the leading non-Gaussian correction to the current
autocorrelation function, which is of third order in the current. Although the
third-order correlation function is small, inversely proportional to the
dimensionless conductance of the ring, the mere fact that it is nonzero is
remarkable, since it is an odd moment of the current distribution.Comment: 4+ pages, 2 figure
Influence of Charge and Energy Imbalances on the Tunneling Current through a Superconductor-Normal Metal Junction
We consider quasiparticle charge and energy imbalances in a thin
superconductor weakly coupled with two normal-metal electrodes via tunnel
junctions at low temperatures. Charge and energy imbalances, which can be
created by injecting quasiparticles at one junction, induce excess tunneling
current at the other junction. We numerically obtain
as a function of the bias voltage across the detection junction.
We show that at the zero bias voltage is purely determined by the
charge imbalance, while the energy imbalance causes a nontrivial -dependence of . The obtained voltage-current characteristics
qualitatively agree with the experimental result by R. Yagi [Phys. Rev. B {\bf
73} (2006) 134507].Comment: 10 pages, 5 figure
Observation of large and oscillations in a proximity dc superconducting quantum interference device
We have measured the magnetoresistance of a dc superconducting quantum
interference device in the form of an interrupted mesoscopic normal-metal loop
in contact with two superconducting electrodes. Below the transition
temperature of the superconducting electrodes, large periodic
magnetoresistance oscillations are observed. By adding a small dc bias to the
ac measurement current, oscillations can be produced. Lowering the
temperature further leads to even larger oscillations, and eventually to sharp
switching from the superconducting state to the normal state. This
flux-dependent resistance could be utilized to make highly sensitive flux
detector.Comment: One pdf file, 4 pages, 4 figure. For figure 1, a smaller file is
uploade
Spherical agglomeration of superconducting and normal microparticles with and without applied electric field
It was reported by R. Tao and coworkers that in the presence of a strong
electric field superconducting microparticles assemble into balls of
macroscopic dimensions. Such a finding has potentially important implications
for the understanding of the fundamental physics of superconductors. However,
we report here the results of experimental studies showing that (i) ball
formation also occurs in the absence of an applied electric field, (ii) the
phenomenon also occurs at temperatures above the superconducting transition
temperature, and (iii) it can also occur for non-superconducting materials.
Possible origins of the phenomenon are discussed.Comment: Small changes in response to referee's comments. To be published in
Phys. Rev.
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