225 research outputs found
3D Lowest Landau Level Theory Applied to YBCO Magnetization and Specific Heat Data: Implications for the Critical Behavior in the H-T Plane
We study the applicability of magnetization and specific heat equations
derived from a lowest-Landau-level (LLL) calculation, to the high-temperature
superconducting (HTSC) materials of the YBaCuO (YBCO)
family. We find that significant information about these materials can be
obtained from this analysis, even though the three-dimensional LLL functions
are not quite as successful in describing them as the corresponding
two-dimensional functions are in describing data for the more anisotropic HTSC
Bi- and Tl-based materials. The results discussed include scaling fits, an
alternative explanation for data claimed as evidence for a second order flux
lattice melting transition, and reasons why 3DXY scaling may have less
significance than previously believed. We also demonstrate how 3DXY scaling
does not describe the specific heat data of YBCO samples in the critical
region. Throughout the paper, the importance of checking the actual scaling
functions, not merely scaling behavior, is stressed.Comment: RevTeX; 10 double-columned pages with 7 figures embedded. (A total of
10 postscript files for the figures.) Submitted to Physical Review
Extreme Type-II Superconductors in a Magnetic Field: A Theory of Critical Fluctuations
A theory of critical fluctuations in extreme type-II superconductors
subjected to a finite but weak external magnetic field is presented. It is
shown that the standard Ginzburg-Landau representation of this problem can be
recast, with help of a novel mapping, as a theory of a new "superconductor", in
an effective magnetic field whose overall value is zero, consisting of the
original uniform field and a set of neutralizing unit fluxes attached to
fluctuating vortex lines. The long distance behavior is related to
the anisotropic gauge theory in which the original magnetic field plays the
role of "charge". The consequences of this "gauge theory" scenario for the
critical behavior in high temperature superconductors are explored in detail,
with particular emphasis on questions of 3D XY vs. Landau level scaling,
physical nature of the vortex "line liquid" and the true normal state, and
fluctuation thermodynamics and transport. A "minimal" set of requirements for
the theory of vortex-lattice melting in the critical region is also proposed
and discussed.Comment: 28 RevTeX pages, 4 .ps figures; appendix A added, additional
references, streamlined Secs. IV and V in response to referees' comment
Critical Dynamics of a Vortex Loop Model for the Superconducting Transition
We calculate analytically the dynamic critical exponent measured in
Monte Carlo simulations for a vortex loop model of the superconducting
transition, and account for the simulation results. In the weak screening
limit, where magnetic fluctuations are neglected, the dynamic exponent is found
to be . In the perfect screening limit, . We relate
to the actual value of observable in experiments and find that , consistent with some experimental results
Critical scaling of the a.c. conductivity for a superconductor above Tc
We consider the effects of critical superconducting fluctuations on the
scaling of the linear a.c. conductivity, \sigma(\omega), of a bulk
superconductor slightly above Tc in zero applied magnetic field. The dynamic
renormalization- group method is applied to the relaxational time-dependent
Ginzburg-Landau model of superconductivity, with \sigma(\omega) calculated via
the Kubo formula to O(\epsilon^{2}) in the \epsilon = 4 - d expansion. The
critical dynamics are governed by the relaxational XY-model
renormalization-group fixed point. The scaling hypothesis \sigma(\omega) \sim
\xi^{2-d+z} S(\omega \xi^{z}) proposed by Fisher, Fisher and Huse is explicitly
verified, with the dynamic exponent z \approx 2.015, the value expected for the
d=3 relaxational XY-model. The universal scaling function S(y) is computed and
shown to deviate only slightly from its Gaussian form, calculated earlier. The
present theory is compared with experimental measurements of the a.c.
conductivity of YBCO near Tc, and the implications of this theory for such
experiments is discussed.Comment: 16 pages, submitted to Phys. Rev.
Gait parameters and characteristics associated with increased risk of falls in people with dementia: a systematic review
Background: People with dementia fall twice as often and have more serious fall-related injuries than healthy older adults. While gait impairment as a generic term is understood as a fall risk factor in this population, a clear elaboration of the specific components of gait that are associated with falls risk is needed for knowledge translation to clinical practice and the development of fall prevention strategies for people with dementia.
Objective: To review gait parameters and characteristics associated with falls in people with dementia.
Methods: Electronic databases CINAHL, EMBASE, MedLine, PsycINFO, and PubMed were searched (from inception to April 2017) to identify prospective cohort studies evaluating the association between gait and falls in people with dementia.
Results: Increased double support time variability, use of mobility aids, walking outdoors, higher scores on the Unified Parkinson’s Disease Rating Scale, and lower average walking bouts were associated with elevated risk of any fall. Increased double support time and step length variability were associated with recurrent falls. The reviewed articles do not support using the Performance Oriented Mobility Assessment and the Timed Up-and-Go tests to predict any fall in this population. There is limited research on the use of dual-task gait assessments for predicting falls in people with dementia.
Conclusion: This systematic review shows the specific spatiotemporal gait parameters and features that are associated with falls in people with dementia. Future research is recommended to focus on developing specialized treatment methods for these specific gait impairments in this patient population
Scaling critical behavior of superconductors at zero magnetic field
We consider the scaling behavior in the critical domain of superconductors at
zero external magnetic field. The first part of the paper is concerned with the
Ginzburg-Landau model in the zero magnetic field Meissner phase. We discuss the
scaling behavior of the superfluid density and we give an alternative proof of
Josephson's relation for a charged superfluid. This proof is obtained as a
consequence of an exact renormalization group equation for the photon mass. We
obtain Josephson's relation directly in the form , that
is, we do not need to assume that the hyperscaling relation holds. Next, we
give an interpretation of a recent experiment performed in thin films of
. We argue that the measured mean field like
behavior of the penetration depth exponent is possibly associated with a
non-trivial critical behavior and we predict the exponents and
for the correlation lenght and specific heat, respectively. In the
second part of the paper we discuss the scaling behavior in the continuum dual
Ginzburg-Landau model. After reviewing lattice duality in the Ginzburg-Landau
model, we discuss the continuum dual version by considering a family of
scalings characterized by a parameter introduced such that
, where is the bare mass of the magnetic
induction field. We discuss the difficulties in identifying the renormalized
magnetic induction mass with the photon mass. We show that the only way to have
a critical regime with is having , that
is, with having the scaling behavior of the renormalized photon mass.Comment: RevTex, 15 pages, no figures; the subsection III-C has been removed
due to a mistak
Anomalous dimensions and phase transitions in superconductors
The anomalous scaling in the Ginzburg-Landau model for the superconducting
phase transition is studied. It is argued that the negative sign of the
exponent is a consequence of a special singular behavior in momentum space. The
negative sign of comes from the divergence of the critical correlation
function at finite distances. This behavior implies the existence of a Lifshitz
point in the phase diagram. The anomalous scaling of the vector potential is
also discussed. It is shown that the anomalous dimension of the vector
potential has important consequences for the critical dynamics in
superconductors. The frequency-dependent conductivity is shown to obey the
scaling . The prediction is
obtained from existing Monte Carlo data.Comment: RevTex, 20 pages, no figures; small changes; version accepted in PR
Systematics of two-component superconductivity in from microwave measurements of high quality single crystals
Systematic microwave surface impedance measurements of YBCO single crystals
grown in crucibles reveal new properties that are not directly seen
in similar measurements of other YBCO samples. Two key observations obtained
from complex conductivity are: a new normal conductivity peak at around 80K and
additional pairing below 65K. High pressure oxygenation of one of the crystals
still yields the same results ruling out any effect of macroscopic segregation
of O-deficient regions. A single complex order parameter cannot describe these
data, and the results suggest at least two superconducting components.
Comparisons with model calculations done for various decoupled two-component
scenarios (i.e. s+d, d+d) are presented. Systematics of three single crystals
show that the 80K quasiparticle peak is correlated with the normal state
inelastic scattering rate. Close to Tc, the data follow a mean-field behavior.
Overall, our results strongly suggest the presence of multiple pairing
temperature and energy scales in .Comment: 14 pages, 2-column, Revtex, 5 embedded postscript figures, uses
graphicx. Postscript version also available at
http://sagar.physics.neu.edu/preprints.htm
Nature of the Low Field Transition in the Mixed State of High Temperature Superconductors
We have numerically studied the statics and dynamics of a model
three-dimensional vortex lattice at low magnetic fields. For the statics we use
a frustrated 3D XY model on a stacked triangular lattice. We model the dynamics
as a coupled network of overdamped resistively-shunted Josephson junctions with
Langevin noise. At low fields, there is a weakly first-order phase transition,
at which the vortex lattice melts into a line liquid. Phase coherence parallel
to the field persists until a sharp crossover, conceivably a phase transition,
near which develops at the same temperature as an infinite
vortex tangle. The calculated flux flow resistivity in various geometries near
closely resembles experiment. The local density of field induced
vortices increases sharply near , corresponding to the experimentally
observed magnetization jump. We discuss the nature of a possible transition or
crossover at (B) which is distinct from flux lattice melting.Comment: Updated references. 46 pages including low quality 25 eps figures.
Contact [email protected] or visit
http://www.physics.ohio-state.edu:80/~ryu/ for better figures and additional
movie files from simulations. To be published in Physical Review B1 01Jun9
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