224 research outputs found
Vortex-glass phases in type-II superconductors
A review is given on the theory of vortex-glass phases in impure type-II
superconductors in an external field. We begin with a brief discussion of the
effects of thermal fluctuations on the spontaneously broken U(1) and
translation symmetries, on the global phase diagram and on the critical
behaviour. Introducing disorder we restrict ourselves to the experimentally
most relevant case of weak uncorrelated randomness which is known to destroy
the long-ranged translational order of the Abrikosov lattice in three
dimensions. Elucidating possible residual glassy ordered phases, we distinguish
betwee positional and phase-coherent vortex glasses. The discussion of elastic
vortex glasses, in two and three dimensions occupy the main part of our review.
In particular, in three dimensions there exists an elastic vortex-glass phase
which still shows quasi-long-range translational order: the `Bragg glass'. It
is shown that this phase is stable with respect to the formation of
dislocations for intermediate fields. Preliminary results suggest that the
Bragg-glass phase may not show phase-coherent vortex-glass order. The latter is
expected to occur in systems with weak disorder only in higher dimensions. We
further demonstrate that the linear resistivity vanishes in the vortex-glass
phase. The vortex-glass transition is studied in detail for a superconducting
film in a parallel field. Finally, we review some recent developments
concerning driven vortex-line lattices moving in a random environment.Comment: 133 pages Latex with figures. accepted for publication in Adv. Phy
High resolution ultrasonography of peripheral nerves: measurements on 14 nerve segments in 56 healthy subjects and reliability assessments.
Purpose: The aim of this study was to assess different
aspects of reliability in high-resolution ultrasonography
(HRUS) of the peripheral nerves
and to establish reference values for the most frequently
examined nerve segments.
Materials and Methods: A nerve size parameter,
the cross-sectional area (CSA) of the C5, C6 and
C7 cervical roots, the median, ulnar, radial, superficial
radial, peroneal, tibial, and the sural nerves
was measured using HRUS at a total of 14 predefined
anatomical sites in two different cohorts of Results: The mean CSA of the 14 nerve segments
ranged from 2 to 10mm2. The intra-rater, interrater
and inter-equipment reliability was high
with intraclass correlation coefficients of 0.93,
0.98, and 0.86, respectively. The CSA values
showed no consistent correlation with age,
height, and body weight, but males had significantly
larger values than females for nerve segments
on the armafter correcting for age,weight
and height in multivariate analysis. CSA values
did not differ when two independent cohorts
were compared.
Conclusion: Peripheral nerve ultrasonography is a
reliable and reproducible diagnostic method in
the hands of experienced examiners. Normal values
for several upper and lower extremity nerves
are provided by our study.
healthy subjects (n = 56), and the inter-rater, intra-
rater and inter-equipment reliability of measurements
was assessed
Non-universal ordering of spin and charge in stripe phases
We study the interplay of topological excitations in stripe phases: charge
dislocations, charge loops, and spin vortices. In two dimensions these defects
interact logarithmically on large distances. Using a renormalization-group
analysis in the Coulomb gas representation of these defects, we calculate the
phase diagram and the critical properties of the transitions. Depending on the
interaction parameters, spin and charge order can disappear at a single
transition or in a sequence of two transitions (spin-charge separation). These
transitions are non-universal with continuously varying critical exponents. We
also determine the nature of the points where three phases coexist.Comment: 4 pages, 3 figure
Nonequilibrium dislocation dynamics and instability of driven vortex lattices in two dimensions
We consider dislocations in a vortex lattice that is driven in a
two-dimensional superconductor with random impurities. The structure and
dynamics of dislocations is studied in this genuine nonequilibrium situation on
the basis of a coarse-grained equation of motion for the displacement field.
The presence of dislocations leads to a characteristic anisotropic distortion
of the vortex density that is controlled by a Kardar-Parisi-Zhang nonlinearity
in the coarse-grained equation of motion. This nonlinearity also implies a
screening of the interaction between dislocations and thereby an instability of
the vortex lattice to the proliferation of free dislocations.Comment: published version with minor correction
Nonequilibrium Phase Transitions of Vortex Matter in Three-Dimensional Layered Superconductors
Large-scale simulations on three-dimensional (3D) frustrated anisotropic XY
model have been performed to study the nonequilibrium phase transitions of
vortex matter in weak random pinning potential in layered superconductors. The
first-order phase transition from the moving Bragg glass to the moving smectic
is clarified, based on thermodynamic quantities. A washboard noise is observed
in the moving Bragg glass in 3D simulations for the first time. It is found
that the activation of the vortex loops play the dominant role in the dynamical
melting at high drive.Comment: 3 pages,5 figure
Dynamic transition in driven vortices across the peak effect in superconductors
We study the zero-temperature dynamic transition from the disordered flow to
an ordered flow state in driven vortices in type-II superconductors. The
transition current is marked by a sharp kink in the
characteristic with a concomitant large increase in the defect concentration.
On increasing magnetic field , the follows the behaviour of the
critical current . Specifically, in the peak effect regime
increases rapidly along with . We also discuss the effect of varying
disorder strength on .Comment: 4 pages, 4 figure
Hall noise and transverse freezing in driven vortex lattices
We study driven vortices lattices in superconducting thin films. Above the
critical force we find two dynamical phase transitions at and
, which could be observed in simultaneous noise measurements of the
longitudinal and the Hall voltage. At there is a transition from plastic
flow to smectic flow where the voltage noise is isotropic (Hall noise =
longitudinal noise) and there is a peak in the differential resistance. At
there is a sharp transition to a frozen transverse solid where the Hall
noise falls down abruptly and vortex motion is localized in the transverse
direction.Comment: 4 pages, 3 figure
Quantum teleportation using active feed-forward between two Canary Islands
Quantum teleportation [1] is a quintessential prerequisite of many quantum
information processing protocols [2-4]. By using quantum teleportation, one can
circumvent the no-cloning theorem [5] and faithfully transfer unknown quantum
states to a party whose location is even unknown over arbitrary distances. Ever
since the first experimental demonstrations of quantum teleportation of
independent qubits [6] and of squeezed states [7], researchers have
progressively extended the communication distance in teleportation, usually
without active feed-forward of the classical Bell-state measurement result
which is an essential ingredient in future applications such as communication
between quantum computers. Here we report the first long-distance quantum
teleportation experiment with active feed-forward in real time. The experiment
employed two optical links, quantum and classical, over 143 km free space
between the two Canary Islands of La Palma and Tenerife. To achieve this, the
experiment had to employ novel techniques such as a frequency-uncorrelated
polarization-entangled photon pair source, ultra-low-noise single-photon
detectors, and entanglement-assisted clock synchronization. The average
teleported state fidelity was well beyond the classical limit of 2/3.
Furthermore, we confirmed the quality of the quantum teleportation procedure
(without feed-forward) by complete quantum process tomography. Our experiment
confirms the maturity and applicability of the involved technologies in
real-world scenarios, and is a milestone towards future satellite-based quantum
teleportation
XY models with disorder and symmetry-breaking fields in two dimensions
The combined effect of disorder and symmetry-breaking fields on the
two-dimensional XY model is examined. The study includes disorder in the
interaction among spins in the form of random phase shifts as well as disorder
in the local orientation of the field. The phase diagrams are determined and
the properties of the various phases and phase transitions are calculated. We
use a renormalization group approach in the Coulomb gas representation of the
model. Our results differ from those obtained for special cases in previous
works. In particular, we find a changed topology of the phase diagram that is
composed of phases with long-range order, quasi-long-range order, and
short-range order. The discrepancies can be ascribed to a breakdown of the
fugacity expansion in the Coulomb gas representation.
Implications for physical systems such as planar Josephson junctions and the
faceting of crystal surfaces are discussed.Comment: 17 pages Latex with 5 eps figures, change: acknowledgment extende
- …