663 research outputs found
Vanishing of phase coherence in underdoped Bi_2Sr_2CaCu_2O_8+d
Coherent time-domain spectroscopy is used to measure the screening and
dissipation of high-frequency electromagnetic fields in a set of underdoped
Bi_2Sr_2CaCu_2O_8+d thin films. The measurements provide direct evidence for a
phase-fluctuation driven transition from the superconductor to normal state,
with dynamics described well by the Berezinskii-Kosterlitz-Thouless theory of
vortex-pair unbinding.Comment: Nature, Vol. 398, 18 March 1999, pg. 221 4 pages with 4 included
figure
Continuous Hawking-Page transitions in Einstein-scalar gravity
We investigate continuous Hawking-Page transitions in Einstein's gravity
coupled to a scalar field with an arbitrary potential in the weak gravity
limit. We show that this is only possible in a singular limit where the
black-hole horizon marginally traps a curvature singularity. Depending on the
subleading terms in the potential, a rich variety of continuous phase
transitions arise. Our examples include second and higher order, including the
Berezinskii-Kosterlitz-Thouless type. In the case when the scalar is dilaton,
the condition for a continuous phase transition lead to (asymptotically)
linear-dilaton background. We obtain the scaling laws of thermodynamic
functions, as well as the viscosity coefficients near the transition. In the
limit of weak gravitational interactions, the bulk viscosity asymptotes to a
universal constant, independent of the details of the scalar potential. As a
byproduct of our analysis we obtain a one-parameter family of kink solutions in
arbitrary dimension d that interpolate between AdS near the boundary and
linear-dilaton background in the deep interior. The continuous Hawking-Page
transitions found here serve as holographic models for normal-to superfluid
transitions.Comment: 35 pages + appendice
Electron spin-flip correlations due to nuclear dynamics in driven GaAs double dots
We present experimental data and associated theory for correlations in a series of experiments involving repeated Landau-Zener sweeps through the crossing point of a singlet state and a spin-aligned triplet state in a GaAs double quantum dot containing two conduction electrons, which are loaded in the singlet state before each sweep, and the final spin is recorded after each sweep. The experiments reported here measure correlations on time scales from 4
μ
s
to 2 ms. When the magnetic field is aligned in a direction such that spin-orbit coupling cannot cause spin flips, the correlation spectrum has prominent peaks centered at zero frequency and at the differences of the Larmor frequencies of the nuclei, on top of a frequency-independent background. When the spin-orbit field is relevant, there are additional peaks, centered at the frequencies of the individual species. A theoretical model which neglects the effects of high-frequency charge noise correctly predicts the positions of the observed peaks, and gives a reasonably accurate prediction of the size of the frequency-independent background, but gives peak areas that are larger than the observed areas by a factor of 2 or more. The observed peak widths are roughly consistent with predictions based on nuclear dephasing times of the order of 60
μ
s
. However, there is extra weight at the lowest observed frequencies, which suggests the existence of residual correlations on the scale of 2 ms. We speculate on the source of these discrepancies
Contemporary management of atrial fibrillation: what can clinical registries tell us about stroke prevention and current therapeutic approaches?
Charge Induced Vortex Lattice Instability
It has been predicted that superconducting vortices should be electrically
charged and that this effect is particularly enhanced for, high temperature
superconductors.\cite{kho95,bla96} Hall effect\cite{hag91} and nuclear magnetic
resonance (NMR) experiments\cite{kum01} suggest the existence of vortex
charging, but the effects are small and the interpretation controversial. Here
we show that the Abrikosov vortex lattice, characteristic of the mixed state of
superconductors, will become unstable at sufficiently high magnetic field if
there is charge trapped on the vortex core. Our NMR measurements of the
magnetic fields generated by vortices in BiSrCaCuO
single crystals\cite{che07} provide evidence for an electrostatically driven
vortex lattice reconstruction with the magnitude of charge on each vortex
pancake of x, depending on doping, in line
with theoretical estimates.\cite{kho95,kna05}Comment: to appear in Nature Physics; 6 pages, 7 figure
Spin- and energy relaxation of hot electrons at GaAs surfaces
The mechanisms for spin relaxation in semiconductors are reviewed, and the
mechanism prevalent in p-doped semiconductors, namely spin relaxation due to
the electron-hole exchange interaction, is presented in some depth. It is shown
that the solution of Boltzmann-type kinetic equations allows one to obtain
quantitative results for spin relaxation in semiconductors that go beyond the
original Bir-Aronov-Pikus relaxation-rate approximation. Experimental results
using surface sensitive two-photon photoemission techniques show that the spin
relaxation-time of electrons in p-doped GaAs at a semiconductor/metal surface
is several times longer than the corresponding bulk spin relaxation-times. A
theoretical explanation of these results in terms of the reduced density of
holes in the band-bending region at the surface is presented.Comment: 33 pages, 12 figures; earlier submission replaced by corrected and
expanded version; eps figures now included in the tex
Presymptomatic risk assessment for chronic non-communicable diseases
The prevalence of common chronic non-communicable diseases (CNCDs) far
overshadows the prevalence of both monogenic and infectious diseases combined.
All CNCDs, also called complex genetic diseases, have a heritable genetic
component that can be used for pre-symptomatic risk assessment. Common single
nucleotide polymorphisms (SNPs) that tag risk haplotypes across the genome
currently account for a non-trivial portion of the germ-line genetic risk and
we will likely continue to identify the remaining missing heritability in the
form of rare variants, copy number variants and epigenetic modifications. Here,
we describe a novel measure for calculating the lifetime risk of a disease,
called the genetic composite index (GCI), and demonstrate its predictive value
as a clinical classifier. The GCI only considers summary statistics of the
effects of genetic variation and hence does not require the results of
large-scale studies simultaneously assessing multiple risk factors. Combining
GCI scores with environmental risk information provides an additional tool for
clinical decision-making. The GCI can be populated with heritable risk
information of any type, and thus represents a framework for CNCD
pre-symptomatic risk assessment that can be populated as additional risk
information is identified through next-generation technologies.Comment: Plos ONE paper. Previous version was withdrawn to be updated by the
journal's pdf versio
Informative noncompliance in endpoint trials
Noncompliance with study medications is an important issue in the design of endpoint clinical trials. Including noncompliant patient data in an intention-to-treat analysis could seriously decrease study power. Standard methods for calculating sample size account for noncompliance, but all assume that noncompliance is noninformative, i.e., that the risk of discontinuation is independent of the risk of experiencing a study endpoint. Using data from several published clinical trials (OPTIMAAL, LIFE, RENAAL, SOLVD-Prevention and SOLVD-Treatment), we demonstrate that this assumption is often untrue, and we discuss the effect of informative noncompliance on power and sample size
Photoemission "experiments" on holographic superconductors
We study the effects of a superconducting condensate on holographic Fermi
surfaces. With a suitable coupling between the fermion and the condensate,
there are stable quasiparticles with a gap. We find some similarities with the
phenomenology of the cuprates: in systems whose normal state is a non-Fermi
liquid with no stable quasiparticles, a stable quasiparticle peak appears in
the condensed phase.Comment: 14 pages, 13 figures; v2: typos corrected and some clarification
adde
Primary peritoneal and ovarian cancers: an epidemiological comparative analysis
We performed case–control analyses using data from the North Carolina Ovarian Cancer Study to determine risk factors that distinguish primary peritoneal cancer (PPC) from epithelial ovarian cancer (EOC). Our risk factor analyses were restricted to invasive serous cancers including 495 EOC cases, 62 PPC cases and 1,086 control women. Logistic regression analyses were used to calculate adjusted odds ratios and 95% confidence intervals for risk factor associations. Although many case–control associations for the invasive serous PPC cases were similar to those of the invasive serous EOC cases, some differences were observed including a twofold increase in risk of invasive serous PPC in women who were ≥35 years at last pregnancy, whereas a decreased risk was observed for invasive serous EOC risk. We could not confirm a previous report of an association between tubal ligation and PPC, a factor consistently associated with a decreased risk of EOC. The difference in the risk factor associations between invasive serous PPC and EOC cancers suggests divergent molecular development of peritoneal and ovarian cancers. A larger study to determine risk factors for invasive serous PPC is warranted
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