560 research outputs found
Magnus force and acoustic Stewart-Tolman effect in type II superconductors
At zero magnetic field we have observed an electromagnetic radiation from
superconductors subjected by a transverse elastic wave. This radiation has an
inertial origin, and is a manifestation of the acoustic Stewart-Tolman effect.
The effect is used for implementing a method of measurement of an effective
Magnus force in type II superconductors. The method does not require the flux
flow regime and allows to investigate this force for almost the whole range of
the existence of the mixed state. We have studied behavior of the gyroscopic
force in nonmagnetic borocarbides and Nb. It is found that in borocarbides the
sign of the gyroscopic force in the mixed state is the same as in the normal
state, and its value (counted for one vortex of unit length) has only a weak
dependence on the magnetic field. In Nb the change of sign of the gyroscopic
force under the transition from the normal to the mixed state is observed.Comment: 4 pages, 5 figure
Ballistic magnon transport and phonon scattering in the antiferromagnet NdCuO
The thermal conductivity of the antiferromagnet NdCuO was measured
down to 50 mK. Using the spin-flop transition to switch on and off the acoustic
Nd magnons, we can reliably separate the magnon and phonon contributions to
heat transport. We find that magnons travel ballistically below 0.5 K, with a
thermal conductivity growing as , from which we extract their velocity. We
show that the rate of scattering of acoustic magnons by phonons grows as ,
and the scattering of phonons by magnons peaks at twice the average Nd magnon
frequency.Comment: 4 pages, 3 figures, one figure modifie
Low-temperature phonon thermal conductivity of cuprate single crystals
The effect of sample size and surface roughness on the phonon thermal
conductivity of NdCuO single crystals was studied down to 50
mK. At 0.5 K, is proportional to , where is the
cross-sectional area of the sample. This demonstrates that is
dominated by boundary scattering below 0.5 K or so. However, the expected
dependence of is not observed down to 50 mK. Upon roughing the
surfaces, the dependence is restored, showing that departures from
are due to specular reflection of phonons off the mirror-like sample surfaces.
We propose an empirical power law fit, to (where
) in cuprate single crystals. Using this method, we show that
recent thermal conductivity studies of Zn doping in YBaCuO
re-affirm the universal heat conductivity of d-wave quasiparticles at .Comment: 4 pages, 4 figure
Stationary waves in a superfluid exciton gas in quantum Hall bilayers
Stationary waves in a superfluid magnetoexciton gas in nu = 1 quantum Hall
bilayers are considered. The waves are induced by counter-propagating
electrical currents that flow in a system with a point obstacle. It is shown
that stationary waves can emerge only in imbalanced bilayers in a certain
diapason of currents. It is found that the stationary wave pattern is modified
qualitatively under a variation of the ratio of the interlayer distance to the
magnetic length d/l. The advantages of use graphene-dielectric-graphene
sandwiches for the observation of stationary waves are discussed. We determine
the range of parameters (the dielectric constant of the layer that separates
two graphene layers and the ratio d/l) for which the state with superfluid
magnetoexcitons can be realized in such sandwiches. Typical stationary wave
patterns are presented as density plotsComment: 17 pages, 8 figure
Nonlinear absorption of surface acoustic waves by composite fermions
Absorption of surface acoustic waves by a two-dimensional electron gas in a
perpendicular magnetic field is considered. The structure of such system at the
filling factor close to 1/2 can be understood as a gas of {\em composite
fermions}. It is shown that the absorption at can be strongly
nonlinear, while small deviation form 1/2 will restore the linear absorption.
Study of nonlinear absorption allows one to determine the force acting upon the
composite fermions from the acoustic wave at turning points of their
trajectories.Comment: 7 pages, 1 figure, submitted to Europhysics letter
Exciton Condensation and Perfect Coulomb Drag
Coulomb drag is a process whereby the repulsive interactions between
electrons in spatially separated conductors enable a current flowing in one of
the conductors to induce a voltage drop in the other. If the second conductor
is part of a closed circuit, a net current will flow in that circuit. The drag
current is typically much smaller than the drive current owing to the heavy
screening of the Coulomb interaction. There are, however, rare situations in
which strong electronic correlations exist between the two conductors. For
example, bilayer two-dimensional electron systems can support an exciton
condensate consisting of electrons in one layer tightly bound to holes in the
other. One thus expects "perfect" drag; a transport current of electrons driven
through one layer is accompanied by an equal one of holes in the other. (The
electrical currents are therefore opposite in sign.) Here we demonstrate just
this effect, taking care to ensure that the electron-hole pairs dominate the
transport and that tunneling of charge between the layers is negligible.Comment: 12 pages, 4 figure
Direct observation of micron-scale ordered structure in a two-dimensional electron system
We have applied a novel scanned probe method to directly resolve the interior
structure of a GaAs/AlGaAs two-dimensional electron system in a tunneling
geometry. We find that the application of a perpendicular magnetic field can
induce surprising density modulations that are not static as a function of the
field. Near six and four filled Landau levels, stripe-like structures emerge
with a characteristic wave length ~2 microns. Present theories do not account
for ordered density modulations on this length scale.Comment: 5 pages, 4 figures. To appear in Phys. Rev.
Density Induced Interchange of Anisotropy Axes at Half-Filled High Landau Levels
We observe density induced 90 rotations of the anisotropy axes in
transport measurements at half-filled high Landau levels in the two dimensional
electron system, where stripe states are proposed (=9/2, 11/2, etc). Using
a field effect transistor, we find the transition density to be
cm at =9/2. Hysteresis is observed in the
vicinity of the transition. We construct a phase boundary in the filling
factor-magnetic field plane in the regime . An in-plane magnetic
field applied along either anisotropy axis always stabilizes the low density
orientation of the stripes.Comment: 1 revtex file, 3 eps figure
Fetal membranes as a source of stem cells
ABSTRACT In recent years, a constant growth of knowledge and clinical applications of stem cells have been observed. Mesenchymal stromal cells, also described as mesenchymal stem cells (MSCs) represent a particular cell type for research and therapy because of their ability to differentiate into mesodermal lineage cells. The most investigated source of MSCs is bone marrow (BM). Yet, collection of BM is an invasive procedure associated with significant discomfort to the patient. The procedure results in a relatively low number of these cells, which can decrease with donor´s age. Therefore, it seems to be very important to find other sources of mesenchymal stem cells nowadays. A human placenta, which is routinely discarded postpartum, in spite of its natural aging process, is still a rich source of stem cells capable to proliferate and in vitro differentiate in many directions. Besides homing and differentiation in the area of injury, MSCs there elicit strong paracrine effects stimulating the processes of repair. In this review, we focus on the biology, characteristics and potential clinical applications of cells derived from human fetal membranes: amnion and chorion
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