211 research outputs found
Mechanism of thermally activated c-axis dissipation in layered High-T superconductors at high fields
We propose a simple model which explains experimental behavior of -axis
resistivity in layered High-T superconductors at high fields in a limited
temperature range. It is generally accepted that the in-plane dissipation at
low temperatures is caused by small concentration of mobile pancake vortices
whose diffusive motion is thermally activated. We demonstrate that in such
situation a finite conductivity appears also in -direction due to the phase
slips between the planes caused by the mobile pancakes. The model gives
universal relation between the components of conductivity which is in good
agreement with experimental data.Comment: RevTeX, 4 pages, 2 Postscript figure
Absence of the Transition into Abrikosov Vortex State of Two-Dimensional Type-II Superconductor with Weak Pinning
The resistive properties of thin amorphous NbO_{x} films with weak pinning
were investigated experimentally above and below the second critical field
H_{c2}. As opposed to bulk type II superconductors with weak pinning where a
sharp change of resistive properties at the transition into the Abrikosov state
is observed at H_{c4}, some percent below H_{c2} (V.A.Marchenko and
A.V.Nikulov, 1981), no qualitative change of resistive properties is observed
down to a very low magnetic field, H_{c4} < 0.006 H_{c2}, in thin films with
weak pinning. The smooth dependencies of the resistivity observed in these
films can be described by paraconductivity theory both above and below H_{c2}.
This means that the fluctuation superconducting state without phase coherence
remains appreciably below H_{c2} in the two-dimensional superconductor with
weak pinning. The difference the H_{c4}/H_{c2} values, i.e. position of the
transition into the Abrikosov state, in three- and two-dimensional
superconductors conforms to the Maki-Takayama result 1971 year according to
which the Abrikosov solution 1957 year is valid only for a superconductor with
finite dimensions. Because of the fluctuation this solution obtained in the
mean field approximation is not valid in a relatively narrow region below
H_{c2} for bulk superconductors with real dimensions and much below H_{c2} for
thin films with real dimensions. The superconducting state without phase
coherence should not be identified with the mythical vortex liquid because the
vortex, as a singularity in superconducting state with phase coherence, can not
exist without phase coherence.Comment: 4 pages, 4 figure
Direct and crossed effects of somatosensory electrical stimulation on motor learning and neuronal plasticity in humans
Purpose Sensory input can modify voluntary motor function. We examined whether somatosensory electrical stimulation (SES) added to motor practice (MP) could augment motor learning, interlimb transfer, and whether physiological changes in neuronal excitability underlie these changes. Methods Participants (18-30 years, n = 31) received MP, SES, MP + SES, or a control intervention. Visuomotor practice included 300 trials for 25 min with the right-dominant wrist and SES consisted of weak electrical stimulation of the radial and median nerves above the elbow. Single-and double-pulse transcranial magnetic stimulation (TMS) metrics were measured in the intervention and nonintervention extensor carpi radialis. Results There was 27 % motor learning and 9 % (both p <0.001) interlimb transfer in all groups but SES added to MP did not augment learning and transfer. Corticospinal excitability increased after MP and SES when measured at rest but it increased after MP and decreased after SES when measured during contraction. No changes occurred in intra-cortical inhibition and facilitation. MP did not affect the TMS metrics in the transfer hand. In contrast, corticospinal excitability strongly increased after SES with MP + SES showing sharply opposite of these effects. Conclusion Motor practice and SES each can produce motor learning and interlimb transfer and are likely to be mediated by different mechanisms. The results provide insight into the physiological mechanisms underlying the effects of MP and SES on motor learning and cortical plasticity and show that these mechanisms are likely to be different for the trained and stimulated motor cortex and the non-trained and non-stimulated motor cortex
Order in driven vortex lattices in superconducting Nb films with nanostructured pinning potentials
Driven vortex lattices have been studied in a material with strong pinning,
such as Nb films. Samples in which natural random pinning coexists with
artificial ordered arrays of defects (submicrometric Ni dots) have been
fabricated with different geometries (square, triangular and rectangular).
Three different dynamic regimes are found: for low vortex velocities, there is
a plastic regime in which random defects frustrate the effect of the ordered
array; then, for vortex velocities in the range 1-100 m/s, there is a sudden
increase in the interaction between the vortex lattice and the ordered dot
array, independent on the geometry. This effect is associated to the onset of
quasi long range order in the vortex lattice leading to an increase in the
overlap between the vortex lattice and the magnetic dots array. Finally, at
larger velocities the ordered array-vortex lattice interaction is suppresed
again, in agreement with the behavior found in numerical simulations.Comment: 8 text pages + 4 figure
Existence of the Abrikosov vortex state in two-dimensional type-II superconductors without pinning
Theory alternative to the vortex lattice melting theories is advertised. The
vortex lattice melting theories are science fiction cond-mat/9811051 because
the Abrikosov state is not the vortex lattice with crystalline long-range
order. Since the fluctuation correction to the Abrikosov solution is infinite
in the thermodynamic limit (K.Maki and H.Takayama, 1972) any fluctuation theory
of the mixed state should consider a superconductor with finite sizes. Such
nonperturbative theory for the easiest case of two-dimensional superconductor
in the lowest Landau level approximation is presented in this work. The
thermodynamic averages of the spatial average order parameter and of the
Abrikosov parameter are calculated. It is shown that the position
H_{c4} of the transition into the Abrikosov state (i.e. in the mixed state with
long-range phase coherence) depends strongly on sizes of two-dimensional
superconductor. Fluctuations eliminate the Abrikosov vortex state in a wide
region of the mixed state of thin films with real sizes and without pinning
disorders, i.e. H_{c4} << H_{c2}. The latter has experimental corroboration in
Phys.Rev.Lett. 75, 2586 (1995).Comment: 4 pages, 0 figure
Characterization of human mesenchymal stem cells from Ewing sarcoma patients. Pathogenetic implications
Ewing Sarcoma (EWS) is a mesenchymal-derived tumor that generally arises in bone and soft tissue. Intensive research regarding the pathogenesis of EWS has been insufficient to pinpoint the early events of Ewing sarcomagenesis. However, the Mesenchymal Stem Cell (MSC) is currently accepted as the most probable cell of origin
Self-organized current transport through low angle grain boundaries in YBaCuO thin films, studied magnetometrically
The critical current density flowing across low angle grain boundaries in
YBaCuO thin films has been studied magnetometrically.
Films (200 nm thickness) were deposited on SrTiO bicrystal substrates
containing a single [001] tilt boundary, with angles of 2, 3, 5, and 7 degrees,
and the films were patterned into rings. Their magnetic moments were measured
in applied magnetic fields up to 30 kOe at temperatures of 5 - 95 K; current
densities of rings with or without grain boundaries were obtained from a
modified critical state model. For rings containing 5 and 7 degree boundaries,
the magnetic response depends strongly on the field history, which arises in
large part from self-field effects acting on the grain boundary.Comment: 8 pages, including 7 figure
Instabilities and disorder-driven first-order transition of the vortex lattice
Transport studies in a Corbino disk geometry suggest that the Bragg glass
phase undergoes a first-order transition into a disordered solid. This
transition shows a sharp reentrant behavior at low fields. In contrast, in the
conventional strip configuration, the phase transition is obscured by the
injection of the disordered vortices through the sample edges, which results in
the commonly observed vortex instabilities and smearing of the peak effect in
NbSe2 crystals. These features are found to be absent in the Corbino geometry,
in which the circulating vortices do not cross the sample edges.Comment: 12 pages 3 figures. Accepted for publication in Physical Review
Letter
Moving glass theory of driven lattices with disorder
We study periodic structures, such as vortex lattices, moving in a random
potential. As predicted in [T. Giamarchi, P. Le Doussal Phys. Rev. Lett. 76
3408 (1996)] the periodicity in the direction transverse to motion leads to a
new class of driven systems: the Moving Glasses. We analyse using several RG
techniques the properties at T=0 and : (i) decay of translational long
range order (ii) particles flow along static channels (iii) the channel pattern
is highly correlated (iv) barriers to transverse motion. We demonstrate the
existence of the ``transverse critical force'' at T=0. A ``static random
force'' is shown to be generated by motion. Displacements grow logarithmically
in and algebraically in . The persistence of quasi long range
translational order in at weak disorder, or large velocity leads to
predict a topologically ordered ``Moving Bragg Glass''. This state continues
the static Bragg glass and is stable at , with non linear transverse
response and linear asymptotic behavior. In , or in at intermediate
disorder, another moving glass exist (the Moving Transverse Glass) with smectic
quasi order in the transverse direction. A phase diagram in force and
disorder for static and moving structures is proposed. For correlated disorder
we predict a ``moving Bose glass'' state with anisotropic transverse Meissner
effect and transverse pinning. We discuss experimental consequences such as
anomalous Hall effect in Wigner crystal and transverse critical current in
vortex lattice.Comment: 74 pages, 27 figures, RevTe
Dynamic ordering and frustration of confined vortex rows studied by mode-locking experiments
The flow properties of confined vortex matter driven through disordered
mesoscopic channels are investigated by mode locking (ML) experiments. The
observed ML effects allow to trace the evolution of both the structure and the
number of confined rows and their match to the channel width as function of
magnetic field. From a detailed analysis of the ML behavior for the case of
3-rows we obtain ({\it i}) the pinning frequency , ({\it ii}) the onset
frequency for ML ( ordering velocity) and ({\it iii}) the
fraction of coherently moving 3-row regions in the channel. The
field dependence of these quantities shows that, at matching, where is
maximum, the pinning strength is small and the ordering velocity is low, while
at mismatch, where is small, both the pinning force and the ordering
velocity are enhanced. Further, we find that , consistent
with the dynamic ordering theory of Koshelev and Vinokur. The microscopic
nature of the flow and the ordering phenomena will also be discussed.Comment: 10 pages, 7 figure, submitted to PRB. Discussion has been improved
and a figure has been adde
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