1,507 research outputs found
Structure and composition of the superconducting phase in alkali iron selenide KFeSe
We use neutron diffraction to study the temperature evolution of the average
structure and local lattice distortions in insulating and superconducting
potassium iron selenide KFeSe. In the high temperature
paramagnetic state, both materials have a single phase with crystal structure
similar to that of the BaFeAs family of iron pnictides. While the
insulating KFeSe forms a iron
vacancy ordered block antiferromagnetic (AF) structure at low-temperature, the
superconducting compounds spontaneously phase separate into an insulating part
with iron vacancy order and a superconducting phase
with chemical composition of KFeSe and BaFeAs structure.
Therefore, superconductivity in alkaline iron selenides arises from alkali
deficient KFeSe in the matrix of the insulating block AF phase.Comment: 10 pages, 5 figure
Cover slip external cavity diode laser
The design of a 671 nm diode laser with a mode-hop-free tuning range of 40
GHz is described. This long tuning range is achieved by simultaneously ramping
the external cavity length with the laser injection current. The external
cavity consists of a microscope cover slip mounted on piezoelectric actuators.
In such a configuration the laser output pointing remains fixed, independent of
its frequency. Using a diode with an output power of 5-7 mW, the laser
linewidth was found to be smaller than 30 MHz. This cover slip cavity and
feedforward laser current control system is simple, economical, robust, and
easy to use for spectroscopy, as we demonstrate with lithium vapor and lithium
atom beam experiments.Comment: 7 pages, 6 figures, submitted to Review of Scientific Instruments
7/29/0
Electron doping evolution of the magnetic excitations in NaFeCoAs
We use time-of-flight (ToF) inelastic neutron scattering (INS) spectroscopy
to investigate the doping dependence of magnetic excitations across the phase
diagram of NaFeCoAs with and .
The effect of electron-doping by partially substituting Fe by Co is to form
resonances that couple with superconductivity, broaden and suppress low energy
( meV) spin excitations compared with spin waves in undoped NaFeAs.
However, high energy ( meV) spin excitations are weakly Co-doping
dependent. Integration of the local spin dynamic susceptibility
of NaFeCoAs reveals a total
fluctuating moment of 3.6 /Fe and a small but systematic reduction
with electron doping. The presence of a large spin gap in the Co-overdoped
nonsuperconducting NaFeCoAs suggests that Fermi surface
nesting is responsible for low-energy spin excitations. These results parallel
Ni-doping evolution of spin excitations in BaFeNiAs, confirming
the notion that low-energy spin excitations coupling with itinerant electrons
are important for superconductivity, while weakly doping dependent high-energy
spin excitations result from localized moments.Comment: 14 pages, 16 figure
What have we already learned from the CMB?
The COBE satellite, and the DMR experiment in particular, was extraordinarily
successful. However, the DMR results were announced about 7 years ago, during
which time a great deal more has been learned about anisotropies in the Cosmic
Microwave Background (CMB). The CMB experiments currently being designed and
built, including long-duration balloons, interferometers, and two space
missions, promise to address several fundamental cosmological issues. We
present our evaluation of what we already know, what we are beginning to learn
now, and what the future may bring.Comment: 20 pages, 3 figures. Changes to match version accepted by PAS
Sex Differences in rt-PA Utilization at Hospitals Treating Stroke: The National Inpatient Sample.
BACKGROUND AND PURPOSE: Sex and race disparities in recombinant tissue plasminogen activator (rt-PA) use have been reported. We sought to explore sex and race differences in the utilization of rt-PA at primary stroke centers (PSCs) compared to non-PSCs across the US.
METHODS: Data from the National (Nationwide) Inpatient Sample (NIS) 2004-2010 was utilized to assess sex differences in treatment for ischemic stroke in PSCs compared to non-PSCs.
RESULTS: There were 304,152 hospitalizations with a primary diagnosis of ischemic stroke between 2004 and 2010 in the analysis: 75,160 (24.7%) patients were evaluated at a PSC. A little over half of the patients evaluated at PSCs were female (53.8%). A lower proportion of women than men received rt-PA at both PSCs (6.8 vs. 7.5%, p \u3c 0.001) and non-PSCs (2.3 vs. 2.8%, p \u3c 0.001). After adjustment for potential confounders the odds of being treated with rt-PA remained lower for women regardless of presentation to a PSC (OR 0.87, 95% CI 0.81-0.94) or non-PSC (OR 0.88, 95% CI 0.82-0.94). After stratifying by sex and race, the lowest absolute treatment rates were observed in black women (4.4% at PSC, 1.9% at non-PSC). The odds of treatment, relative to white men, was however lowest for white women (PSC OR = 0.85, 95% CI 0.78-0.93; non-PSC OR = 0.80, 95% CI 0.75-0.85). In the multivariable model, sex did not modify the effect of PSC certification on rt-PA utilization (p-value for interaction = 0.58).
CONCLUSION: Women are less likely to receive rt-PA than men at both PSCs and non-PSCs. Absolute treatment rates are lowest in black women, although the relative difference in men and women was greatest for white women
Electron doping evolution of the neutron spin resonance in NaFeCoAs
Neutron spin resonance, a collective magnetic excitation coupled to
superconductivity, is one of the most prominent features shared by a broad
family of unconventional superconductors including copper oxides, iron
pnictides, and heavy fermions. In this work, we study the doping evolution of
the resonances in NaFeCoAs covering the entire superconducting
dome. For the underdoped compositions, two resonance modes coexist. As doping
increases, the low-energy resonance gradually loses its spectral weight to the
high-energy one but remains at the same energy. By contrast, in the overdoped
regime we only find one single resonance, which acquires a broader width in
both energy and momentum, but retains approximately the same peak position even
when drops by nearly a half compared to optimal doping. These results
suggest that the energy of the resonance in electron overdoped
NaFeCoAs is neither simply proportional to nor the
superconducting gap, but is controlled by the multi-orbital character of the
system and doped impurity scattering effect.Comment: accepted by PR
Nonlinear Scattering of a Bose-Einstein Condensate on a Rectangular Barrier
We consider the nonlinear scattering and transmission of an atom laser, or
Bose-Einstein condensate (BEC) on a finite rectangular potential barrier. The
nonlinearity inherent in this problem leads to several new physical features
beyond the well-known picture from single-particle quantum mechanics. We find
numerical evidence for a denumerably infinite string of bifurcations in the
transmission resonances as a function of nonlinearity and chemical potential,
when the potential barrier is wide compared to the wavelength of oscillations
in the condensate. Near the bifurcations, we observe extended regions of
near-perfect resonance, in which the barrier is effectively invisible to the
BEC. Unlike in the linear case, it is mainly the barrier width, not the height,
that controls the transmission behavior. We show that the potential barrier can
be used to create and localize a dark soliton or dark soliton train from a
phonon-like standing wave.Comment: 15 pages, 15 figures, new version includes clarification of
definition of transmission coefficient in general nonlinear vs. linear cas
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