759 research outputs found
Isotope effects and possible pairing mechanism in optimally doped cuprate superconductors
We have studied the oxygen-isotope effects on T_{c} and in-plane penetration
depth \lambda_{ab}(0) in an optimally doped 3-layer cuprate
Bi_{1.6}Pb_{0.4}Sr_{2}Ca_{2}Cu_{3}O_{10+y} (T_{c} \sim 107 K). We find a small
oxygen-isotope effect on T_{c} (\alpha_{O} = 0.019), and a substantial effect
on \lambda_{ab} (0) (\Delta \lambda_{ab} (0)/\lambda_{ab} (0) = 2.5\pm0.5%).
The present results along with the previously observed isotope effects in
single-layer and double-layer cuprates indicate that the isotope exponent
\alpha_{O} in optimally doped cuprates is small while the isotope effect on the
in-plane effective supercarrier mass is substantial and nearly independent of
the number of the CuO_{2} layers. A plausible pairing mechanism is proposed to
explain the isotope effects, high-T_{c} superconductivity and tunneling spectra
in a consistent way.Comment: 5 pages, 4 figure
Large oxygen-isotope effect in Sr_{0.4}K_{0.6}BiO_{3}: Evidence for phonon-mediated superconductivity
Oxygen-isotope effect has been investigated in a recently discovered
superconductor Sr_{0.4}K_{0.6}BiO_{3}. This compound has a distorted perovskite
structure and becomes superconducting at about 12 K. Upon replacing ^{16}O with
^{18}O by 60-80%, the T_c of the sample is shifted down by 0.32-0.50 K,
corresponding to an isotope exponent of alpha_{O} = 0.40(5). This isotope
exponent is very close to that for a similar bismuthate superconductor
Ba_{1-x}K_{x}BiO_{3} with T_c = 30 K. The very distinctive doping and T_c
dependencies of alpha_{O} observed in bismuthates and cuprates suggest that
bismuthates should belong to conventional phonon-mediated superconductors while
cuprates might be unconventional supercondutors.Comment: 9 pages, 5 figure
Thermal Hadron Production in High Energy Heavy Ion Collisions
We provide a method to test if hadrons produced in high energy heavy ion
collisions were emitted at freeze-out from an equilibrium hadron gas. Our
considerations are based on an ideal gas at fixed temperature , baryon
number density , and vanishing total strangeness. The constituents of this
gas are all hadron resonances up to a mass of 2 GeV; they are taken to decay
according to the experimentally observed branching ratios. The ratios of the
various resulting hadron production rates are tabulated as functions of
and . These tables can be used for the equilibration analysis of any heavy
ion data; we illustrate this for some specific cases.Comment: 12 pages (not included :13 figures + tables) report CERN-TH 6523/92
and Bielefeld preprint BI-TP 92/0
Digital Cognitive Behavioral Therapy for Insomnia in Women With Chronic Migraines
Objective/Background: Insomnia commonly co-occurs with chronic migraines (CM). Non-pharmacological treatments for insomnia in CM patients remain understudied. This is a proof-of-concept study, which aims to evaluate the feasibility, acceptability, and preliminary efficacy of a digital cognitive behavioral therapy for insomnia (dCBT-I) for individuals with CM and insomnia (CM-I) in the United States. Methods: We recruited 42 females with CM-I symptoms from a U.S.-based observational cohort and from the general population via advertisements. Within a multiple baseline design, participants were randomized to receive dCBT-I after 2, 4, or 6 weeks of completing baseline sleep diaries. DCBT-I was scrutinized against benchmarks for completion rates (≥90% to complete dCBT-I), acceptability (≥80% to find dCBT-I acceptable), and posttreatment changes in insomnia symptoms (≥50% indicating a clinically relevant improvement in their insomnia symptoms). As a secondary measure, we also reported percentage of individuals reverting to episodic migraines. Results: Out of 42 randomized, 35 (83.3%) completed dCBT-I within the 12 weeks provided. Of these completers, 33 (94.3%) reported being satisfied (n = 16) or very satisfied (n = 17) with treatment. Additionally, 65.7% of completers responded to treatment as per universally accepted criteria for insomnia. Lastly, 34% of completers reverted from CM to episodic migraine. Conclusion: This study provides evidence for the feasibility and acceptability of dCBT-I in patients with CM-I complaints. Effects of improving insomnia and migraines were suggested. These results indicate that a randomized controlled trial is needed to determine the efficacy of dCBT-I in CM patients
Worldsheet Instantons and a Null String Limit of Born-Infeld Theory
For a superstring theory in four spacetime dimensions, we propose a
modification of the Born-Infeld action that possesses a well-defined
tensionless limit. We interpret this as describing the effective target space
dynamics of null strings on a D3-brane. We argue that such a modification can
be induced by nonperturbative contributions from instantons in the worldsheet
sigma-model describing string propagation on the brane.Comment: 11 pages; Comments and references adde
Strange hadron matter and SU(3) symmetry
We calculate saturation curves for strange hadron matter using recently
constructed baryon-baryon potentials which are constrained by SU(3) symmetry.
All possible interaction channels within the baryon octet (consisting of ,
, , and ) are considered. It is found that a small
fraction in nuclear matter slightly increases binding, but that
larger fractions () rapidly cause a decrease. Charge-neutral
systems, with equal densities for nucleons and cascades, are
only very weakly bound. The dependence of the binding energies on the
strangeness per baryon, , is predicted for various and
systems. The implications of our results in
relativistic heavy-ion collisions and the core of a dense star are discussed.
We also discuss the differences between our results and previous hadron matter
calculations.Comment: 14 pages RevTeX, 7 postscript figure
A numerical study of multi-soliton configurations in a doped antiferromagnetic Mott insulator
We evaluate from first principles the self-consistent Hartree-Fock energies
for multi-soliton configurations in a doped, spin-1/2, antiferromagnetic Mott
insulator on a two-dimensional square lattice. We find that nearest-neighbor
Coulomb repulsion stabilizes a regime of charged meron-antimeron vortex soliton
pairs over a region of doping from 0.05 to 0.4 holes per site for intermediate
coupling 3 < U/t <8. This stabilization is mediated through the generation of
``spin-flux'' in the mean-field antiferromagnetic (AFM) background. Holes
cloaked by a meron-vortex in the spin-flux AFM background are charged bosons.
Our static Hartree-Fock calculations provide an upper bound on the energy of a
finite density of charged vortices. This upper bound is lower than the energy
of the corresponding charged stripe configurations. A finite density of charge
carrying vortices is shown to produce a large number of unoccupied electronic
levels in the Mott-Hubbard charge transfer gap. These levels lead to
significant band tailing and a broad mid-infrared band in the optical
absorption spectrum as observed experimentally. At very low doping (below 0.05)
the doping charges create extremely tightly bound meron-antimeron pairs or even
isolated conventional spin-polarons, whereas for very high doping (above 0.4)
the spin background itself becomes unstable to formation of a conventional
Fermi liquid and the spin-flux mean-field is energetically unfavorable. Our
results point to the predominance of a quantum liquid of charged, bosonic,
vortex solitons at intermediate coupling and intermediate doping
concentrations.Comment: 12 pages, 25 figures; added references, modified/eliminated some
figure
Isotope Effect for the Penetration Depth in Superconductors
We show that various factors can lead to an isotopic dependence of the
penetration depth . Non-adiabaticity (Jahn-Teller crossing) leads to
the isotope effect of the charge carrier concentration and, consequently,
of in doped superconductors such as the cuprates. A general equation
relating the isotope coefficients of and of is presented for
London superconductors. We further show that the presence of magnetic
impurities or a proximity contact also lead to an isotopic dependence of
; the isotope coefficient turns out to be temperature dependent,
, in these cases. The existence of the isotope effect for the
penetration depth is predicted for conventional as well as for high-temperature
superconductors. Various experiments are proposed and/or discussed.Comment: 11 pages, 8 figures, accepted for publication in Phys. Rev.
Open chromatin profiling in adipose tissue marks genomic regions with functional roles in cardiometabolic traits
Identifying the regulatory mechanisms of genome-wide association study (GWAS) loci affecting adipose tissue has been restricted due to limited characterization of adipose transcriptional regulatory elements. We profiled chromatin accessibility in three frozen human subcutaneous adipose tissue needle biopsies and preadipocytes and adipocytes from the Simpson Golabi-Behmel Syndrome (SGBS) cell strain using an assay for transposase-accessible chromatin (ATAC-seq). We identified 68,571 representative accessible chromatin regions (peaks) across adipose tissue samples (FDR, 5%). GWAS loci for eight cardiometabolic traits were enriched in these peaks (P, 0.005), with the strongest enrichment for waist-hip ratio. Of 110 recently described cardiometabolic GWAS loci colocalized with adipose tissue eQTLs, 59 loci had one or more variants overlapping an adipose tissue peak. Annotated variants at the SNX10 waist-hip ratio locus and the ATP2A1-SH2B1 body mass index locus showed allelic differences in regulatory assays. These adipose tissue accessible chromatin regions elucidate genetic variants that may alter adipose tissue function to impact cardiometabolic traits
Au+Au Reactions at the AGS: Experiments E866 and E917
Particle production and correlation functions from Au+Au reactions have been
measured as a function of both beam energy (2-10.7AGeV) and impact parameter.
These results are used to probe the dynamics of heavy-ion reactions, confront
hadronic models over a wide range of conditions and to search for the onset of
new phenomena.Comment: 12 pages, 14 figures, Talk presented at Quark Matter '9
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