25,765 research outputs found
Spin-density wave Fermi surface reconstruction in underdoped YBa2Cu3O6+x
We consider the reconstruction expected for the Fermi surface of underdoped
YBa2Cu3O6+x in the case of a collinear spin-density wave with a characteristic
vector Q=(pi[1+/-2 delta],pi), assuming an incommensurability delta~0.06
similar to that found in recent neutron scattering experiments. A Fermi surface
possibly consistent with the multiple observed quantum oscillation frequencies
is obtained. From the low band masses expected using this model as compared
with experiment, a uniform enhancement of the quasiparticle effective mass over
the Fermi surface by a factor of ~7 is indicated. Further predictions of the
Fermi surface topology are made, which may potentially be tested by experiment
to indicate the relevance of this model to underdoped YBa2Cu3O6+x.Comment:
A new quantum fluid at high magnetic fields in the marginal charge-density-wave system -(BEDT-TTF)Hg(SCN) (where ~K and Rb)
Single crystals of the organic charge-transfer salts
-(BEDT-TTF)Hg(SCN) have been studied using Hall-potential
measurements (K) and magnetization experiments ( = K, Rb). The data show
that two types of screening currents occur within the high-field,
low-temperature CDW phases of these salts in response to time-dependent
magnetic fields. The first, which gives rise to the induced Hall potential, is
a free current (), present at the surface of the sample.
The time constant for the decay of these currents is much longer than that
expected from the sample resistivity. The second component of the current
appears to be magnetic (), in that it is a microscopic,
quasi-orbital effect; it is evenly distributed within the bulk of the sample
upon saturation. To explain these data, we propose a simple model invoking a
new type of quantum fluid comprising a CDW coexisting with a two-dimensional
Fermi-surface pocket which describes the two types of current. The model and
data are able to account for the body of previous experimental data which had
generated apparently contradictory interpretations in terms of the quantum Hall
effect or superconductivity.Comment: 13 pages, 11 figure
A model for time-dependent grain boundary diffusion of ions and electrons through a film or scale, with an application to alumina
A model for ionic and electronic grain boundary transport through thin films,
scales or membranes with columnar grain structure is introduced. The grain
structure is idealized as a lattice of identical hexagonal cells - a honeycomb
pattern. Reactions with the environment constitute the boundary conditions and
drive the transport between the surfaces. Time-dependent simulations solving
the Poisson equation self-consistently with the Nernst-Planck flux equations
for the mobile species are performed. In the resulting Poisson-Nernst-Planck
system of equations, the electrostatic potential is obtained from the Poisson
equation in its integral form by summation. The model is used to interpret
alumina membrane oxygen permeation experiments, in which different oxygen gas
pressures are applied at opposite membrane surfaces and the resulting flux of
oxygen molecules through the membrane is measured. Simulation results involving
four mobile species, charged aluminum and oxygen vacancies, electrons, and
holes, provide a complete description of the measurements and insight into the
microscopic processes underpinning the oxygen permeation of the membrane. Most
notably, the hypothesized transition between p-type and n-type ionic
conductivity of the alumina grain boundaries as a function of the applied
oxygen gas pressure is observed in the simulations. The range of validity of a
simple analytic model for the oxygen permeation rate, similar to the Wagner
theory of metal oxidation, is quantified by comparison to the numeric
simulations. The three-dimensional model we develop here is readily adaptable
to problems such as transport in a solid state electrode, or corrosion scale
growth
EXIST: Mission Design Concept and Technology Program
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed very large
area coded aperture telescope array, incorporating 8m^2 of pixellated Cd-Zn-Te
(CZT) detectors, to conduct a full-sky imaging and temporal hard x-ray (10-600
keV) survey each 95min orbit. With a sensitivity (5sigma, 1yr) of ~0.05mCrab
(10-150 keV), it will extend the ROSAT soft x-ray (0.5-2.5keV) and proposed
ROSITA medium x-ray (2-10 keV) surveys into the hard x-ray band and enable
identification and study of sources ~10-20X fainter than with the ~15-100keV
survey planned for the upcoming Swift mission. At ~100-600 keV, the ~1mCrab
sensitivity is 300X that achieved in the only previous (HEAO-A4, non-imaging)
all-sky survey. EXIST will address a broad range of key science objectives:
from obscured AGN and surveys for black holes on all scales, which constrain
the accretion history of the universe, to the highest sensitivity and
resolution studies of gamma-ray bursts it will conduct as the Next Generation
Gamma-Ray Burst mission. We summarize the science objectives and mission
drivers, and the results of a mission design study for implementation as a free
flyer mission, with Delta IV launch. Key issues affecting the telescope and
detector design are discussed, and a summary of some of the current design
concepts being studied in support of EXIST is presented for the wide-field but
high resolution coded aperture imaging and very large area array of imaging CZT
detectors. Overall mission design is summarized, and technology development
needs and a development program are outlined which would enable the launch of
EXIST by the end of the decade, as recommended by the NAS/NRC Decadal Survey.Comment: 14 pages, 8 figures, 2 tables. PDF file only. Presented at SPIE (Aug.
2002) and to appear in Proc. SPIE, vol. 485
Confinement-induced Berry phase and helicity-dependent photocurrents
The photocurrent in an optically active metal is known to contain a component
that switches sign with the helicity of the incident radiation. At low
frequencies, this current depends on the orbital Berry phase of the Bloch
electrons via the "anomalous velocity" of Karplus and Luttinger. We consider
quantum wells in which the parent material, such as GaAs, is not optically
active and the relevant Berry phase only arises as a result of quantum
confinement. Using an envelope approximation that is supported by numerical
tight-binding results, it is shown that the Berry phase contribution is
determined for realistic wells by a cubic Berry phase intrinsic to the bulk
material, the well width, and the well direction. These results for the
magnitude of the Berry-phase effect suggest that it may already have been
observed in quantum well experiments.Comment: 4 pages, 2 figure
Study of Chromium-Frit-Type Coatings for High-Temperature Protection of Molybdenum
The achievement of more compact and efficient power plants for aircraft is dependent, among other factors, on the perfection of heat-resisting materials that are superior to those in current use. Molybdenum is one of the high-melting metals (melting point, 4750 F). It is fairly abundant and also can be worked into many of the shapes required in modern power plants. To permit its widespread use at elevated temperatures, however, some means must first be found to prevent its rapid oxidation. The application of a protective coating is one method that might be used to achieve this goal. In the present work, a number of chromium-frit-type coatings were studied. These were bonded to molybdenum specimens by firing in controlled atmospheres to temperatures in the range of 2400 to 2700 F
Interface hole-doping in cuprate-titanate superlattices
The electronic structure of interfaces between YBaCuO and
SrTiO is studied using local spin density approximation (LSDA) with
intra-atomic Coulomb repulsion (LSDA+U). We find a metallic state in
cuprate/titanate heterostructures with the hole carriers concentrated
substantially in the CuO-layers and in the first interface TiO and SrO
planes. This effective interface doping appears due to the polarity of
interfaces, caused by the first incomplete copper oxide unit cell.
Interface-induced high pre-doping of CuO-layers is a key mechanism
controlling the superconducting properties in engineered field-effect devices
realized on the basis of cuprate/titanate superlattices.Comment: 5 pages, 5 figure
Electron-phonon coupling and electron self-energy in electron-doped graphene: calculation of angular resolved photoemission spectra
We obtain analytical expressions for the electron self-energy and the
electron-phonon coupling in electron-doped graphene using electron-phonon
matrix elements extracted from density functional theory simulations. From the
electron self-energies we calculate angle resolved photoemission spectra. We
demonstrate that the measured kink at eV from the Fermi level is
actually composed of two features, one at eV due to the
twofold degenerate E mode, and a second one at eV due to
the A mode. The electron-phonon coupling extracted from the kink
observed in ARPES experiments is roughly a factor of 5.5 larger than the
calculated one. This disagreement can only be partially reconciled by the
inclusion of resolution effects. Indeed we show that a finite resolution
increases the apparent electron-phonon coupling by underestimating the
renormalization of the electron velocity at energies larger than the kinks
positions. The discrepancy between theory and experiments is thus reduced to a
factor of 2.2. From the linewidth of the calculated ARPES spectra we
obtain the electron relaxation time. A comparison with available experimental
data in graphene shows that the electron relaxation time detected in ARPES is
almost two orders of magnitudes smaller than what measured by other
experimental techniques.Comment: 9 pages, 7 figures, see also Matteo Calandra and Francesco Mauri,
arXiv:0707.149
Detector and Telescope Development for ProtoEXIST and Fine Beam Measurements of Spectral Response of CZT Detectors
We outline our plan to develop ProtoEXIST, a balloon-borne prototype
experiment for the Energetic X-ray Imaging Survey Telescope (EXIST) for the
Black Hole Finder Probe. EXIST will consist of multiple wide-field hard X-ray
coded-aperture telescopes. The current design of the EXIST mission employs two
types of telescope systems: high energy telescopes (HETs) using CZT detectors,
and low energy telescopes (LETs) using Si detectors. With ProtoEXIST, we will
develop and demonstrate the technologies required for the EXIST HETs. As part
of our development efforts, we also present recent laboratory measurements of
the spectral response and efficiency variation of imaging CZT detectors on a
fine scale (~0.5 mm). The preliminary results confirm the need for multi-pixel
readouts and small inter-pixel gaps to achieve uniform spectral response and
high detection efficiency across detectors.Comment: 9 pages, 12 figures, 1 table, appears in SPIE 2005 proceedings (5898:
UV, X-ray, and Gamma-ray Space Instrumentation for Astronomy XIV
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