1,254 research outputs found
Electronic Origin of High Temperature Superconductivity in Single-Layer FeSe Superconductor
The latest discovery of high temperature superconductivity signature in
single-layer FeSe is significant because it is possible to break the
superconducting critical temperature ceiling (maximum Tc~55 K) that has been
stagnant since the discovery of Fe-based superconductivity in 2008. It also
blows the superconductivity community by surprise because such a high Tc is
unexpected in FeSe system with the bulk FeSe exhibiting a Tc at only 8 K at
ambient pressure which can be enhanced to 38 K under high pressure. The Tc is
still unusually high even considering the newly-discovered intercalated FeSe
system A_xFe_{2-y}Se_2 (A=K, Cs, Rb and Tl) with a Tc at 32 K at ambient
pressure and possible Tc near 48 K under high pressure. Particularly
interesting is that such a high temperature superconductivity occurs in a
single-layer FeSe system that is considered as a key building block of the
Fe-based superconductors. Understanding the origin of high temperature
superconductivity in such a strictly two-dimensional FeSe system is crucial to
understanding the superconductivity mechanism in Fe-based superconductors in
particular, and providing key insights on how to achieve high temperature
superconductivity in general. Here we report distinct electronic structure
associated with the single-layer FeSe superconductor. Its Fermi surface
topology is different from other Fe-based superconductors; it consists only of
electron pockets near the zone corner without indication of any Fermi surface
around the zone center. Our observation of large and nearly isotropic
superconducting gap in this strictly two-dimensional system rules out existence
of node in the superconducting gap. These results have provided an unambiguous
case that such a unique electronic structure is favorable for realizing high
temperature superconductivity
Mid-infrared plasmons in scaled graphene nanostructures
Plasmonics takes advantage of the collective response of electrons to
electromagnetic waves, enabling dramatic scaling of optical devices beyond the
diffraction limit. Here, we demonstrate the mid-infrared (4 to 15 microns)
plasmons in deeply scaled graphene nanostructures down to 50 nm, more than 100
times smaller than the on-resonance light wavelength in free space. We reveal,
for the first time, the crucial damping channels of graphene plasmons via its
intrinsic optical phonons and scattering from the edges. A plasmon lifetime of
20 femto-seconds and smaller is observed, when damping through the emission of
an optical phonon is allowed. Furthermore, the surface polar phonons in SiO2
substrate underneath the graphene nanostructures lead to a significantly
modified plasmon dispersion and damping, in contrast to a non-polar
diamond-like-carbon (DLC) substrate. Much reduced damping is realized when the
plasmon resonance frequencies are close to the polar phonon frequencies. Our
study paves the way for applications of graphene in plasmonic waveguides,
modulators and detectors in an unprecedentedly broad wavelength range from
sub-terahertz to mid-infrared.Comment: submitte
Evaluating Depressive Symptoms in Schizophrenia: A Psychometric Comparison of the Calgary Depression Scale for Schizophrenia and the Hamilton Depression Rating Scale
Background: The aim of this study was to compare two measures of depression in patients with schizophrenia and schizophrenia spectrum disorder, including patients with delusional and schizoaffective disorder, to conclude implications for their application. Sampling and Methods: A total of 278 patients were assessed using the Calgary Depression Scale for Schizophrenia (CDSS) and the Hamilton Depression Rating Scale (HAMD-17). The Positive and Negative Syndrome Scale (PANSS) was also applied. At admission and discharge, a principal component analysis was performed with each depression scale. The two depression rating scales were furthermore compared using correlation and regression analyses. Results: Three factors were revealed for the CDSS and HAMD-17 factor component analysis. A very similar item loading was found for the CDSS at admission and discharge, whereas results of the loadings of the HAMD-17 items were less stable. The first two factors of the CDSS revealed correlations with positive, negative and general psychopathology. In contrast, multiple significant correlations were found for the HAMD-17 factors and the PANSS sub-scores. Multiple regression analyses demonstrated that the HAMD-17 accounted more for the positive and negative symptom domains than the CDSS. Conclusions:The present results suggest that compared to the HAMD-17, the CDSS is a more specific instrument to measure depressive symptoms in schizophrenia and schizophrenia spectrum disorder, especially in acutely ill patients. Copyright (c) 2012 S. Karger AG, Base
Two-Particle-Self-Consistent Approach for the Hubbard Model
Even at weak to intermediate coupling, the Hubbard model poses a formidable
challenge. In two dimensions in particular, standard methods such as the Random
Phase Approximation are no longer valid since they predict a finite temperature
antiferromagnetic phase transition prohibited by the Mermin-Wagner theorem. The
Two-Particle-Self-Consistent (TPSC) approach satisfies that theorem as well as
particle conservation, the Pauli principle, the local moment and local charge
sum rules. The self-energy formula does not assume a Migdal theorem. There is
consistency between one- and two-particle quantities. Internal accuracy checks
allow one to test the limits of validity of TPSC. Here I present a pedagogical
review of TPSC along with a short summary of existing results and two case
studies: a) the opening of a pseudogap in two dimensions when the correlation
length is larger than the thermal de Broglie wavelength, and b) the conditions
for the appearance of d-wave superconductivity in the two-dimensional Hubbard
model.Comment: Chapter in "Theoretical methods for Strongly Correlated Systems",
Edited by A. Avella and F. Mancini, Springer Verlag, (2011) 55 pages.
Misprint in Eq.(23) corrected (thanks D. Bergeron
An unusual clinical presentation resembling superior vena cava syndrome post heart surgery
BACKGROUND: An unusual sequence of post operative events heralded by hemodynamic deterioration followed by dyspnea and rapidly progressive dilatation of superficial neck and facial veins, resembling a superior vena cava syndrome, two days post surgical resection of filamentous aortic valve masses, closure of a patent foramen ovale, and performance of a modified Maze procedure for atrial fibrillation in a patient that presented with transient neurologic findings is presented. CASE PRESENTATION: Although both clinical findings and hemodynamic derangements completely resolved following tricuspid valve repair aimed to correct the new onset severe tricuspid regurgitation noted post operatively; a clear mechanism was not readily obvious and diagnostic testing data somewhat conflictive. We present a careful retrospective examination of all clinical data and review possible clinical entities that could have been implicated in this particular case and recognize that transesophageal echocardiographic findings were most useful in identifying the best course of action. CONCLUSION: After reviewing all clinical data and despite the inconclusive nature of test results; the retrospective examination of transesophageal echocardiographic findings proved to be most useful in identifying the best course of action. We postulate that in our case, resolution of the suspected pulmonary embolism with anticoagulation and reestablishment of a normal right ventricular geometry with tricuspid valve repair worked in unison in restoring normal hemodynamics and resolving both dyspnea and venous dilatation
Pubertal development and prostate cancer risk: Mendelian randomization study in a population-based cohort
Background: Epidemiological studies have observed a positive association between an earlier age at sexual development and prostate cancer, but markers of sexual maturation in boys are imprecise and observational estimates are likely to suffer from a degree of uncontrolled confounding. To obtain causal estimates, we examined the role of pubertal development in prostate cancer using genetic polymorphisms associated with Tanner stage in adolescent boys in a Mendelian randomization (MR) approach.Methods: We derived a weighted genetic risk score for pubertal development, combining 13 SNPs associated with male Tanner stage. A higher score indicated a later puberty onset. We examined the association of this score with prostate cancer risk, stage and grade in the UK-based ProtecT case-control study (n = 2,927), and used the PRACTICAL consortium (n = 43,737) as a replication sample.Results: In ProtecT, the puberty genetic score was inversely associated with prostate cancer grade (odds ratio (OR) of high-vs. low-grade cancer, per tertile of the score: 0.76; 95 % CI, 0.64-0.89). In an instrumental variable estimation of the causal OR, later physical development in adolescence (equivalent to a difference of one Tanner stage between pubertal boys of the same age) was associated with a 77 % (95 % CI, 43-91 %) reduced odds of high Gleason prostate cancer. In PRACTICAL, the puberty genetic score was associated with prostate cancer stage (OR of advanced vs. localized cancer, per tertile: 0.95; 95 % CI, 0.91-1.00) and prostate cancer-specific mortality (hazard ratio amongst cases, per tertile: 0.94; 95 % CI, 0.90-0.98), but not with disease grade.Conclusions: Older age at sexual maturation is causally linked to a reduced risk of later prostate cancer, especially aggressive disease
Study of the reaction e^{+}e^{-} -->J/psi\pi^{+}\pi^{-} via initial-state radiation at BaBar
We study the process with
initial-state-radiation events produced at the PEP-II asymmetric-energy
collider. The data were recorded with the BaBar detector at center-of-mass
energies 10.58 and 10.54 GeV, and correspond to an integrated luminosity of 454
. We investigate the mass
distribution in the region from 3.5 to 5.5 . Below 3.7
the signal dominates, and above 4
there is a significant peak due to the Y(4260). A fit to
the data in the range 3.74 -- 5.50 yields a mass value
(stat) (syst) and a width value (stat)(syst) for this state. We do not
confirm the report from the Belle collaboration of a broad structure at 4.01
. In addition, we investigate the system
which results from Y(4260) decay
Blood lipids and prostate cancer: a Mendelian randomization analysis
Genetic risk scores were used as unconfounded instruments for specific lipid traits (Mendelian randomization) to assess whether circulating lipids causally influence prostate cancer risk. Data from 22,249 prostate cancer cases and 22,133 controls from 22 studies within the international PRACTICAL consortium were analyzed. Allele scores based on single nucleotide polymorphisms (SNPs) previously reported to be uniquely associated with each of low-density lipoprotein (LDL), high-density lipoprotein (HDL), and triglyceride (TG) levels, were first validated in an independent dataset, and then entered into logistic regression models to estimate the presence (and direction) of any causal effect of each lipid trait on prostate cancer risk. There was weak evidence for an association between the LDL genetic score and cancer grade: the odds ratio (OR) per genetically instrumented standard deviation (SD) in LDL, comparing high- (≥7 Gleason score) versus low-grade (<7 Gleason score) cancers was 1.50 (95% CI: 0.92, 2.46; P = 0.11). A genetically instrumented SD increase in TGs was weakly associated with stage: the OR for advanced versus localized cancer per unit increase in genetic risk score was 1.68 (95% CI: 0.95, 3.00; P = 0.08). The rs12916-T variant in 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) was inversely associated with prostate cancer (OR: 0.97; 95% CI: 0.94, 1.00; P = 0.03). In conclusion, circulating lipids, instrumented by our genetic risk scores, did not appear to alter prostate cancer risk. We found weak evidence that higher LDL and TG levels increase aggressive prostate cancer risk, and that a variant in HMGCR (that mimics the LDL lowering effect of statin drugs) reduces risk. However, inferences are limited by sample size and evidence of pleiotropy
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