1,024 research outputs found
Intersubband plasmons in quasi-one-dimensional electron systems on a liquid helium surface
The collective excitation spectra are studied for a multisubband
quasi-one-dimensional electron gas on the surface of liquid helium. Different
intersubband plasmon modes are identified by calculating the spectral weight
function of the electron gas within a 12 subband model. Strong intersubband
coupling and depolarization shifts are found. When the plasmon energy is close
to the energy differences between two subbands, Landau damping in this finite
temperature system leads to plasmon gaps at small wavevectors.Comment: To be published as a Rapid Communication in Phys. Rev.
Universality of Frequency and Field Scaling of the Conductivity Measured by Ac-Susceptibility of a Ybco-Film
Utilizing a novel and exact inversion scheme, we determine the complex linear
conductivity from the linear magnetic ac-susceptibility
which has been measured from 3\,mHz to 50\,MHz in fields between 0.4\,T and
4\,T applied parallel to the c-axis of a 250\,nm thin disk. The frequency
derivative of the phase and the dynamical scaling of
above and below provide clear evidence for a
continuous phase transition at to a generic superconducting state. Based
on the vortex-glass scaling model, the resulting critical exponents and
are close to those frequently obtained on films by other means and
associated with an 'isotropic' vortex glass. The field effect on
can be related to the increase of the glass coherence length,
.Comment: 8 pages (5 figures upon request), revtex 3.0, APK.94.01.0
The incidence of first stroke in and around pregnancy: A population-based cohort study from Sweden
Introduction:
Research has suggested that delivery is associated with an increased risk of stroke in women; however, there is a lack of contemporary estimates on the incidence of stroke in and after pregnancy compared with the baseline risk in women of childbearing age in Sweden.
Patients and methods:
All women aged 15–49 years with live births/stillbirths in 1992–2011 were identified from the Swedish Medical Birth Registry linked with the National Patient Registry. First stroke during the study period was identified. Incidence rates per 100,000 person-years and adjusted incidence rate ratios (IRRs) were calculated for antepartum, peripartum and early and late postpartum periods, compared with all other available follow-up time (time before pregnancy and after postpartum) using Poisson regression adjusted for maternal age, education attainment and calendar time.
Results:
Of 1,124,541 women, 3094 had a first incident stroke (331 occurred during pregnancy or first 12 weeks postpartum), about half having ischaemic stroke. The incidence was 15.0 per 100,000 person-years (95% confidence interval 14.5–15.6) in non-pregnant time. The incidence was lower antepartum (7.3/100,000 person-years, 6.0–8.9; adjusted IRR = 0.7, 0.5–0.8) but higher peripartum (314.4/100,000 person-years, 247.5–399.5; adjusted IRR = 27.3, 21.4–34.9) and early postpartum (64.0/100,000 person-years, 54.1–75.7; adjusted IRR = 5.5, 4.6–6.6). The increased risk in peripartum was more evident for intracerebral haemorrhage than other types of stroke.
Conclusion:
Overall risk of stroke was low in women of childbearing age, but stroke risk peaks in the peripartum and early postpartum periods. Future work should address factors that contribute to this increased risk in order to develop approaches to attenuate risk
Peak effect and its evolution with defect structure in YBa2Cu3O7-d thin films at microwave frequencies
The vortex dynamics in YBa2Cu3O7-d thin films have been studied at microwave
frequencies. A pronounced peak in the surface resistance, Rs, is observed in
these films at frequencies of 4.88 and 9.55 GHz for magnetic fields varying
from 0.2 to 0.8 T. The peak is associated with an order-disorder transformation
of the flux line lattice as the temperature or field is increased. The
occurrence of the peak in Rs is crucially dependent on the depinning frequency,
wp and on the nature and concentration of growth defects present in these
films. Introduction of artificial defects by swift heavy ion irradiation with
200 MeV Ag ion at a fluence of 4x1010 ions/cm2 enhances wp and suppresses the
peak at 4.88 GHz but the peak at 9.55 GHz remains unaffected. A second peak at
lower temperature has also been observed at 9.55 GHz. This is related to twin
boundaries from angular dependence studies of Rs. Based on the temperature
variation of Rs, vortex phase diagrams have been constructed at 9.55 GHz.Comment: 8 pages, 4 figures Submitted to Physical Review
Melting as a String-Mediated Phase Transition
We present a theory of the melting of elemental solids as a
dislocation-mediated phase transition. We model dislocations near melt as
non-interacting closed strings on a lattice. In this framework we derive simple
expressions for the melting temperature and latent heat of fusion that depend
on the dislocation density at melt. We use experimental data for more than half
the elements in the Periodic Table to determine the dislocation density from
both relations. Melting temperatures yield a dislocation density of (0.61\pm
0.20) b^{-2}, in good agreement with the density obtained from latent heats,
(0.66\pm 0.11) b^{-2}, where b is the length of the smallest
perfect-dislocation Burgers vector. Melting corresponds to the situation where,
on average, half of the atoms are within a dislocation core.Comment: 18 pages, LaTeX, 3 eps figures, to appear in Phys. Rev.
Vortices on Higher Genus Surfaces
We consider the topological interactions of vortices on general surfaces. If
the genus of the surface is greater than zero, the handles can carry magnetic
flux. The classical state of the vortices and the handles can be described by a
mapping from the fundamental group to the unbroken gauge group. The allowed
configurations must satisfy a relation induced by the fundamental group. Upon
quantization, the handles can carry ``Cheshire charge.'' The motion of the
vortices can be described by the braid group of the surface. How the motion of
the vortices affects the state is analyzed in detail.Comment: 28 pages with 10 figures; uses phyzzx and psfig; Caltech preprint
CALT-68-187
Imprints of the Quantum World in Classical Mechanics
The imprints left by quantum mechanics in classical (Hamiltonian) mechanics
are much more numerous than is usually believed. We show Using no physical
hypotheses) that the Schroedinger equation for a nonrelativistic system of
spinless particles is a classical equation which is equivalent to Hamilton's
equations.Comment: Paper submitted to Foundations of Physic
Ab initio Calculations of Multilayer Relaxations of Stepped Cu Surfaces
We present trends in the multilayer relaxations of several vicinals of
Cu(100) and Cu(111) of varying terrace widths and geometry. The electronic
structure calculations are based on density functional theory in the local
density approximation with norm-conserving, non-local pseudopotentials in the
mixed basis representation. While relaxations continue for several layers, the
major effect concentrates near the step and corner atoms. On all surfaces the
step atoms contract inwards, in agreement with experimental findings.
Additionally, the corner atoms move outwards and the atoms in the adjacent
chain undergo large inward relaxation. Correspondingly, the largest contraction
(4%) is in the bond length between the step atom and its bulk nearest neighbor
(BNN), while that between the corner atom and BNN is somewhat enlarged. The
surface atoms also display changes in registry of upto 1.5%. Our results are in
general in good agreement with LEED data including the controversial case of
Cu(511). Subtle differences are found with results obtained from semi-empirical
potentials.Comment: 21 pages and 3 figure
Finite temperature phase diagram of spin-1/2 bosons in two-dimensional optical lattice
We study a two-species bosonic Hubbard model on a two-dimensional square
lattice by means of quantum Monte Carlo simulations and focus on finite
temperature effects. We show in two different cases, ferro- and
antiferromagnetic spin-spin interactions, that the phase diagram is composed of
solid Mott phases, liquid phases and superfluid phases. In the
antiferromagnetic case, the superfluid (SF) is polarized while the Mott
insulator (MI) and normal Bose liquid (NBL) phases are not. On the other hand,
in the ferromagnetic case, none of the phases is polarized. The
superfluid-liquid transition is of the Berezinsky-Kosterlitz-Thouless type
whereas the solid-liquid passage is a crossover.Comment: 9 pages, 13 figure
Dynamic Vortex Phases and Pinning in Superconductors with Twin Boundaries
We investigate the pinning and driven dynamics of vortices interacting with
twin boundaries using large scale molecular dynamics simulations on samples
with near one million pinning sites. For low applied driving forces, the vortex
lattice orients itself parallel to the twin boundary and we observe the
creation of a flux gradient and vortex free region near the edges of the twin
boundary. For increasing drive, we find evidence for several distinct dynamical
flow phases which we characterize by the density of defects in the vortex
lattice, the microscopic vortex flow patterns, and orientation of the vortex
lattice. We show that these different dynamical phases can be directly related
to microscopically measurable voltage - current V(I) curves and voltage noise.
By conducting a series of simulations for various twin boundary parameters we
derive several vortex dynamic phase diagrams.Comment: 5 figures, to appear in Phys. Rev.
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