1,723 research outputs found
Electron-phonon coupling in the conventional superconductor YNiBC at high phonon energies studied by time-of-flight neutron spectroscopy
We report an inelastic neutron scattering investigation of phonons with
energies up to 159 meV in the conventional superconductor YNiBC. Using
the SWEEP mode, a newly developed time-of-flight technique involving the
continuous rotation of a single crystal specimen, allowed us to measure a four
dimensional volume in (Q,E) space and, thus, determine the dispersion surface
and linewidths of the (~ 102 meV) and (~ 159 meV) type phonon
modes for the whole Brillouin zone. Despite of having linewidths of , modes do not strongly contribute to the total electron-phonon
coupling constant . However, experimental linewidths show a remarkable
agreement with ab-initio calculations over the complete phonon energy range
demonstrating the accuracy of such calculations in a rare comparison to a
comprehensive experimental data set.Comment: accepted for publication in PR
Magnetic excitations in underdoped Ba(Fe1-xCox)2As2 with x=0.047
The magnetic excitations in the paramagnetic-tetragonal phase of underdoped
Ba(Fe0.953Co0.047)2As2, as measured by inelastic neutron scattering, can be
well described by a phenomenological model with purely diffusive spin dynamics.
At low energies, the spectrum around the magnetic ordering vector Q_AFM
consists of a single peak with elliptical shape in momentum space. At high
energies, this inelastic peak is split into two peaks across the direction
perpendicular to Q_AFM. We use our fittings to argue that such a splitting is
not due to incommensurability or propagating spin-wave excitations, but is
rather a consequence of the anisotropies in the Landau damping and in the
magnetic correlation length, both of which are allowed by the tetragonal
symmetry of the system. We also measure the magnetic spectrum deep inside the
magnetically-ordered phase, and find that it is remarkably similar to the
spectrum of the paramagnetic phase, revealing the strongly overdamped character
of the magnetic excitations.Comment: 12 pages, 7 figure
Carrier-mediated ferromagnetic ordering in Mn ion-implanted p+GaAs:C
Highly p-type GaAs:C was ion-implanted with Mn at differing doses to produce
Mn concentrations in the 1 - 5 at.% range. In comparison to LT-GaAs and
n+GaAs:Si samples implanted under the same conditions, transport and magnetic
properties show marked differences. Transport measurements show anomalies,
consistent with observed magnetic properties and with epi- LT-(Ga,Mn)As, as
well as the extraordinary Hall Effect up to the observed magnetic ordering
temperature (T_C). Mn ion-implanted p+GaAs:C with as-grown carrier
concentrations > 10^20 cm^-3 show remanent magnetization up to 280 K
Pseudo-Goldstone magnons in the frustrated S=3/2 Heisenberg helimagnet ZnCr2Se4 with a pyrochlore magnetic sublattice
Low-energy spin excitations in any long-range ordered magnetic system in the
absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating
from the ordering wave vectors. In helimagnets, these modes hybridize into the
so-called helimagnon excitations. Here we employ neutron spectroscopy supported
by theoretical calculations to investigate the magnetic excitation spectrum of
the isotropic Heisenberg helimagnet ZnCr2Se4 with a cubic spinel structure, in
which spin-3/2 magnetic Cr3+ ions are arranged in a geometrically frustrated
pyrochlore sublattice. Apart from the conventional Goldstone mode emanating
from the (0 0 q) ordering vector, low-energy magnetic excitations in the
single-domain proper-screw spiral phase show soft helimagnon modes with a small
energy gap of ~0.17 meV, emerging from two orthogonal wave vectors (q 0 0) and
(0 q 0) where no magnetic Bragg peaks are present. We term them
pseudo-Goldstone magnons, as they appear gapless within linear spin-wave theory
and only acquire a finite gap due to higher-order quantum-fluctuation
corrections. Our results are likely universal for a broad class of symmetric
helimagnets, opening up a new way of studying weak magnon-magnon interactions
with accessible spectroscopic methods.Comment: V3: Final version to be published in Phys. Rev.
Binary Black Hole Mergers in the First Advanced LIGO Observing Run
The first observational run of the Advanced LIGO detectors, from September 12, 2015 to January 19, 2016, saw the first detections of gravitational waves from binary black hole mergers. In this paper, we present full results from a search for binary black hole merger signals with total masses up to 100M⊙ and detailed implications from our observations of these systems. Our search, based on general-relativistic models of gravitational-wave signals from binary black hole systems, unambiguously identified two signals, GW150914 and GW151226, with a significance of greater than 5σ over the observing period. It also identified a third possible signal, LVT151012, with substantially lower significance and with an 87% probability of being of astrophysical origin. We provide detailed estimates of the parameters of the observed systems. Both GW150914 and GW151226 provide an unprecedented opportunity to study the two-body motion of a compact-object binary in the large velocity, highly nonlinear regime. We do not observe any deviations from general relativity, and we place improved empirical bounds on several highorder post-Newtonian coefficients. From our observations, we infer stellar-mass binary black hole merger rates lying in the range 9–240 Gpc−3 yr−1. These observations are beginning to inform astrophysical predictions of binary black hole formation rates and indicate that future observing runs of the Advanced detector network will yield many more gravitational-wave detections
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Doping and isolation of GaN, InGaN and InAlN using ion implantation
Both n- and p-type doping have been achieved in GaN using Si{sup +} or Mg{sup +}/P{sup +} implantation, respectively, followed annealing at {ge} 1050{degrees}C. Using proximity rapid thermal annealing (10sec) the GaN surface retains both smooth morphology and its original stoichiometry. Variable temperature Hall measurements reveal approximate energy levels of 62meV for the implanted Si and 171meV for the Mg, which are similar to their values in epitaxially grown GaN. Implant isolation of both n- and p-type GaN, and n-type In{sub 0.75}Al{sub 0.25}N with multiple energy inert species (e.g. N{sup +} or F{sup +}) produces high resistivity ({ge}10{sup 8}{omega}/{open_square}) after subsequent annealing in the range 600-700{degrees}C. Smaller increases in sheet resistance are observed for In{sub x}Ga{sup 1-x}N (x=0.33-0.75) under the same conditions due to the smaller energy bandgaps and the shallower energy levels of the damage-related states controlling the resistivity
Flexible production and entry: institutional, technological, and organizational determinants
Academics, the media, and policymakers have all raised concerns about the implications of human workers being replaced by machines or software. Few have discussed the implications of the reverse: firms’ ability to replace capital with workers. We show that this flexibility can help new firms overcome uncertainty and increase entrepreneurial entry. We develop a simple real options model where permissive labor regulations allow firms to take advantage of capital-labor substitutability by replacing ‘rigid’ capital with ‘flexible’ labor. The model highlights institutional, technological, and organizational preconditions to using this flexibility. Using a large and comprehensive dataset on entry by standalone firms and group affiliates, we provide evidence in support of the model
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