1,247 research outputs found
Viscoelastic Behavior of Solid He
Over the last five years several experimental groups have reported anomalies
in the temperature dependence of the period and amplitude of a torsional
oscillator containing solid He. We model these experiments by assuming that
He is a viscoelastic solid--a solid with frequency dependent internal
friction. We find that while our model can provide a quantitative account of
the dissipation observed in the torsional oscillator experiments, it only
accounts for about 10% of the observed period shift, leaving open the
possibility that the remaining period shift is due to the onset of
superfluidity in the sample.Comment: 4 pages, 3 figure
Cross-Modal Health State Estimation
Individuals create and consume more diverse data about themselves today than
any time in history. Sources of this data include wearable devices, images,
social media, geospatial information and more. A tremendous opportunity rests
within cross-modal data analysis that leverages existing domain knowledge
methods to understand and guide human health. Especially in chronic diseases,
current medical practice uses a combination of sparse hospital based biological
metrics (blood tests, expensive imaging, etc.) to understand the evolving
health status of an individual. Future health systems must integrate data
created at the individual level to better understand health status perpetually,
especially in a cybernetic framework. In this work we fuse multiple user
created and open source data streams along with established biomedical domain
knowledge to give two types of quantitative state estimates of cardiovascular
health. First, we use wearable devices to calculate cardiorespiratory fitness
(CRF), a known quantitative leading predictor of heart disease which is not
routinely collected in clinical settings. Second, we estimate inherent genetic
traits, living environmental risks, circadian rhythm, and biological metrics
from a diverse dataset. Our experimental results on 24 subjects demonstrate how
multi-modal data can provide personalized health insight. Understanding the
dynamic nature of health status will pave the way for better health based
recommendation engines, better clinical decision making and positive lifestyle
changes.Comment: Accepted to ACM Multimedia 2018 Conference - Brave New Ideas, Seoul,
Korea, ACM ISBN 978-1-4503-5665-7/18/1
Anisotropic states of two-dimensional electrons in high magnetic fields
We study the collective states formed by two-dimensional electrons in Landau
levels of index near half-filling. By numerically solving the
self-consistent Hartree-Fock (HF) equations for a set of oblique
two-dimensional lattices, we find that the stripe state is an anisotropic
Wigner crystal (AWC), and determine its precise structure for varying values of
the filling factor. Calculating the elastic energy, we find that the shear
modulus of the AWC is small but finite (nonzero) within the HF approximation.
This implies, in particular, that the long-wavelength magnetophonon mode in the
stripe state vanishes like as in an ordinary Wigner crystal, and not
like as was found in previous studies where the energy of shear
deformations was neglected.Comment: minor corrections; 5 pages, 4 figures; version to be published in
Physical Review Letter
Nucleation and Growth of the Superconducting Phase in the Presence of a Current
We study the localized stationary solutions of the one-dimensional
time-dependent Ginzburg-Landau equations in the presence of a current. These
threshold perturbations separate undercritical perturbations which return to
the normal phase from overcritical perturbations which lead to the
superconducting phase. Careful numerical work in the small-current limit shows
that the amplitude of these solutions is exponentially small in the current; we
provide an approximate analysis which captures this behavior. As the current is
increased toward the stall current J*, the width of these solutions diverges
resulting in widely separated normal-superconducting interfaces. We map out
numerically the dependence of J* on u (a parameter characterizing the material)
and use asymptotic analysis to derive the behaviors for large u (J* ~ u^-1/4)
and small u (J -> J_c, the critical deparing current), which agree with the
numerical work in these regimes. For currents other than J* the interface
moves, and in this case we study the interface velocity as a function of u and
J. We find that the velocities are bounded both as J -> 0 and as J -> J_c,
contrary to previous claims.Comment: 13 pages, 10 figures, Revte
A Self-Consistent Microscopic Theory of Surface Superconductivity
The electronic structure of the superconducting surface sheath in a type-II
superconductor in magnetic fields is calculated
self-consistently using the Bogoliubov-de Gennes equations. We find that the
pair potential exhibits pronounced Friedel oscillations near the
surface, in marked contrast with the results of Ginzburg-Landau theory. The
role of magnetic edge states is emphasized. The local density of states near
the surface shows a significant depletion near the Fermi energy due to the
development of local superconducting order. We suggest that this structure
could be unveiled by scanning-tunneling microscopy studies performed near the
edge of a superconducting sample.Comment: 12 pages, Revtex 3.0, 3 postscript figures appende
Velocity Selection for Propagating Fronts in Superconductors
Using the time-dependent Ginzburg-Landau equations we study the propagation
of planar fronts in superconductors, which would appear after a quench to zero
applied magnetic field. Our numerical solutions show that the fronts propagate
at a unique speed which is controlled by the amount of magnetic flux trapped in
the front. For small flux the speed can be determined from the linear marginal
stability hypothesis, while for large flux the speed may be calculated using
matched asymptotic expansions. At a special point the order parameter and
vector potential are dual, leading to an exact solution which is used as the
starting point for a perturbative analysis.Comment: 4 pages, 2 figures; submitted to Phys. Rev. Letter
Dislocation-induced superfluidity in a model supersolid
Motivated by recent experiments on the supersolid behavior of He, we
study the effect of an edge dislocation in promoting superfluidity in a Bose
crystal. Using Landau theory, we couple the elastic strain field of the
dislocation to the superfluid density, and use a linear analysis to show that
superfluidity nucleates on the dislocation before occurring in the bulk of the
solid. Moving beyond the linear analysis, we develop a systematic perturbation
theory in the weakly nonlinear regime, and use this method to integrate out
transverse degrees of freedom and derive a one-dimensional Landau equation for
the superfluid order parameter. We then extend our analysis to a network of
dislocation lines, and derive an XY model for the dislocation network by
integrating over fluctuations in the order parameter. Our results show that the
ordering temperature for the network has a sensitive dependence on the
dislocation density, consistent with numerous experiments that find a clear
connection between the sample quality and the supersolid response.Comment: 10 pages, 6 figure
Flux penetration in slab shaped Type-I superconductors
We study the problem of flux penetration into type--I superconductors with
high demagnetization factor (slab geometry).Assuming that the interface between
the normal and superconducting regions is sharp, that flux diffuses rapidly in
the normal regions, and that thermal effects are negligible, we analyze the
process by which flux invades the sample as the applied field is increased
slowly from zero.We find that flux does not penetrate gradually.Rather there is
an instability in the process and the flux penetrates from the boundary in a
series of bursts, accompanied by the formation of isolated droplets of the
normal phase, leading to a multiply connected flux domain structure similar to
that seen in experiments.Comment: 4 pages, 2 figures, Fig 2.(b) available upon request from the
authors, email - [email protected]
Superheating fields of superconductors: Asymptotic analysis and numerical results
The superheated Meissner state in type-I superconductors is studied both
analytically and numerically within the framework of Ginzburg-Landau theory.
Using the method of matched asymptotic expansions we have developed a
systematic expansion for the solutions of the Ginzburg-Landau equations in the
limit of small , and have determined the maximum superheating field
for the existence of the metastable, superheated Meissner state as
an expansion in powers of . Our numerical solutions of these
equations agree quite well with the asymptotic solutions for . The
same asymptotic methods are also used to study the stability of the solutions,
as well as a modified version of the Ginzburg-Landau equations which
incorporates nonlocal electrodynamics. Finally, we compare our numerical
results for the superheating field for large- against recent asymptotic
results for large-, and again find a close agreement. Our results
demonstrate the efficacy of the method of matched asymptotic expansions for
dealing with problems in inhomogeneous superconductivity involving boundary
layers.Comment: 14 pages, 8 uuencoded figures, Revtex 3.
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