5,078 research outputs found
Asymmetric Two-component Fermion Systems in Strong Coupling
We study the phase structure of a dilute two-component Fermi system with
attractive interactions as a function of the coupling and the polarization or
number difference between the two components. In weak coupling, a finite number
asymmetry results in phase separation. A mixed phase containing symmetric
superfluid matter and an asymmetric normal phase is favored. With increasing
coupling strength, we show that the stress on the superfluid phase to
accommodate a number asymmetry increases. Near the infinite-scattering length
limit, we calculate the single-particle excitation spectrum and the
ground-state energy at various polarizations. A picture of weakly-interacting
quasi-particles emerges for modest polarizations. In this regime near infinite
scattering length, and for modest polarizations, a homogeneous phase with a
finite population of excited quasi-particle states characterized by a gapless
spectrum should be favored over the phase separated state. These states may be
realized in cold atom experiments.Comment: 4 pages, 3 figur
Photoluminescence measurements of quantum-dot-containing semiconductor microdisk resonators using optical fiber taper waveguides
Fiber taper waveguides are used to improve the efficiency of room temperature
photoluminescence measurements of AlGaAs microdisk resonant cavities with
embedded self-assembled InAs quantum dots. As a near-field collection optic,
the fiber taper improves the collection efficiency from microdisk lasers by a
factor of ~ 15-100 times in comparison to conventional normal incidence
free-space collection techniques. In addition, the fiber taper can serve as a
efficient means for pumping these devices, and initial measurements employing
fiber pumping and collection are presented. Implications of this work towards
chip-based cavity quantum electrodynamics experiments are discussed.Comment: 10 pages, 7 figure
Mesonic Excitations of QGP: Study with an Effective Model
We study the correlations between quark-antiquark pairs in different quantum
number channels in a deconfined plasma by using an effective model of QCD.
Using the three flavour PNJL model, the finite temperature spectral functions
for different mesonic states are studied at zero and nonzero quark chemical
potentials. It is found that in the channel resonance structures survive
above the chiral transition temperature \tc, while the kaonic states seem to
get washed off just above \tc. The sensitivity of the structures to the
anomaly term are carefully investigated.Comment: 15page
Electronic screening and damping in magnetars
We calculate the screening of the ion-ion potential due to electrons in the
presence of a large background magnetic field, at densities of relevance to
neutron star crusts. Using the standard approach to incorporate electron
screening through the one-loop polarization function, we show that the magnetic
field produces important corrections both at short and long distances. In
extreme fields, realized in highly magnetized neutron stars called magnetars,
electrons occupy only the lowest Landau levels in the relatively low density
region of the crust. Here our results show that the screening length for
Coulomb interactions between ions can be smaller than the inter-ion spacing.
More interestingly, we find that the screening is anisotropic and the screened
potential between two static charges exhibits long range Friedel oscillations
parallel to the magnetic field. This long-range oscillatory behavior is likely
to affect the lattice structure of ions, and can possibly create rod-like
structures in the magnetar crusts. We also calculate the imaginary part of the
electron polarization function which determines the spectrum of electron-hole
excitations and plays a role in damping lattice phonon excitations. We
demonstrate that even for modest magnetic fields this damping is highly
anisotropic and will likely lead to anisotropic phonon heat transport in the
outer neutron star crust.Comment: 14 pages, 5 Figure
Concentration of risk measures: A Wasserstein distance approach
Known finite-sample concentration bounds for the Wasserstein distance between
the empirical and true distribution of a random variable are used to derive a
two-sided concentration bound for the error between the true conditional
value-at-risk (CVaR) of a (possibly unbounded) random variable and a standard
estimate of its CVaR computed from an i.i.d. sample. The bound applies under
fairly general assumptions on the random variable, and improves upon previous
bounds which were either one sided, or applied only to bounded random
variables. Specializations of the bound to sub-Gaussian and sub-exponential
random variables are also derived. Using a different proof technique, the
results are extended to the class of spectral risk measures having a bounded
risk spectrum. A similar procedure is followed to derive concentration bounds
for the error between the true and estimated Cumulative Prospect Theory (CPT)
value of a random variable, in cases where the random variable is bounded or
sub-Gaussian. These bounds are shown to match a known bound in the bounded
case, and improve upon the known bound in the sub-Gaussian case. The usefulness
of the bounds is illustrated through an algorithm, and corresponding regret
bound for a stochastic bandit problem, where the underlying risk measure to be
optimized is CVaR
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Top-Gated Chemical Vapor Deposited Mos2 Field-Effect Transistors On Si3N4 Substrates
We report the electrical characteristics of chemical vapor deposited (CVD) monolayer molybdenum disulfide (MoS2) top-gated field-effect transistors (FETs) on silicon nitride (Si3N4) substrates. We show that Si3N4 substrates offer comparable electrical performance to thermally grown SiO2 substrates for MoS2 FETs, offering an attractive passivating substrate for transition-metal dichalcogenides (TMD) with a smooth surface morphology. Single-crystal MoS2 grains are grown via vapor transport process using solid precursors directly on low pressure CVD Si3N4, eliminating the need for transfer processes which degrade electrical performance. Monolayer top-gated MoS2 FETs with Al2O3 gate dielectric on Si3N4 achieve a room temperature mobility of 24 cm(2)/V s with I-on/I-off current ratios exceeding 10(7). Using HfO2 as a gate dielectric, monolayer top-gated CVD MoS2 FETs on Si3N4 achieve current densities of 55 mu A/mu m and a transconductance of 6.12 mu S/mu m at V-tg of -5V and V-ds of 2V. We observe an increase in mobility at lower temperatures, indicating phonon scattering may dominate over charged impurity scattering in our devices. Our results show that Si3N4 is an attractive alternative to thermally grown SiO2 substrate for TMD FETs. (C) 2015 AIP Publishing LLC.STTR programNSF NASCENT ERCArmy Research Office under STTR W911NF-14-P-0030Microelectronics Research Cente
Isospin asymmetry and type-I superconductivity in neutron star matter
It has been argued by Buckley et. al.(Phys. Rev. Lett. 92, 151102, 2004) that
nuclear matter is a type-I rather than a type-II superconductor. The suggested
mechanism is a strong interaction between neutron and proton Cooper pairs,
which arises from an assumed U(2) symmetry of the effective potential, which is
supposed to originate in isospin symmetry of the underlying nuclear
interactions. To test this claim, we perform an explicit mean-field calculation
of the effective potential of the Cooper pairs in a model with a simple
four-point pairing interaction. In the neutron star context, matter is very
neutron rich with less than 10% protons, so there is no neutron-proton pairing.
We find that under these conditions our model shows no interaction between
proton Cooper pairs and neutron Cooper pairs at the mean-field level. We
estimate the leading contribution beyond mean field and find that it is is
small and attractive at weak coupling.Comment: 7 pages, 2 figure
A Mean Field Analysis of Pairing in Asymmetric Fermi Systems at Finite Temperature
We study the phase diagram of a two component Fermi system with a weak
attractive interaction. Our analysis includes the leading order Hartree energy
shifts and pairing correlations at finite temperature and chemical potential
difference between the two fermion species. We show that in an asymmetric
system, the Hartree shift to the single particle energies are important for the
phase competition between normal and superfluid phase and can change the phase
transition curve qualitatively. At large asymmetry we find that a novel but
somewhat fragile superfluid state can be favored due to finite temperature
effects. We also investigate the transition between the normal phase and an
inhomogeneous superfluid phase to study how gradient instabilities evolve with
temperature and asymmetry. Finally, we adopt our analysis to study the density
profiles of similar asymmetric Fermi systems that are being observed in cold
atom experiments.Comment: 17 pages, 7 figure
Clostridium difficile infection in the United States: A national study assessing preventive practices used and perceptions of practice evidence
We surveyed 571 US hospitals about practices used to prevent Clostridium difficile infection (CDI). Most hospitals reported regularly using key CDI prevention practices, and perceived their strength of evidence as high. The largest discrepancy between regular use and perceived evidence strength occurred with antimicrobial stewardship programs.Infect. Control Hosp. Epidemiol. 2015;36(8):969–971</jats:p
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