1,605 research outputs found
Lifshitz Transition in Underdoped Cuprates
Recent studies show that quantum oscillations thought to be associated with a
density wave reconstructed Fermi surface disappear at a critical value of the
doping for YBa2Cu3O6+y, and the cyclotron mass diverges as the critical value
is approached from the high doping side. We argue that the phenomenon is due to
a Lifshitz transition where the pockets giving rise to the quantum oscillations
connect to form an open (quasi-1d) Fermi surface. The estimated critical doping
is close to that found by experiment, and the theory predicts a logarithmic
divergence of the cyclotron mass with a coefficient comparable to that observed
in experiment.Comment: 4 pages, 4 figure
Antiphase Stripe Order as the Origin of Electron Pockets Observed in 1/8-Hole-Doped Cuprates
Recent quantum oscillation measurements on underdoped cuprates are shown to
be consistent with the predictions of a mean field theory of the 1/8 magnetic
antiphase stripe order proposed to occur in high- cuprates. In particular,
for intermediate values of the stripe order parameter, the magneto-transport is
found to be dominated by an electron pocket
OTV bearing deflection investigation
The primary goal of the Bearing Deflectometer Investigation was to gain experience in the use of fiber optic displacement probe technology for bearing health monitoring in a liquid hydrogen turbo pump. The work specified in this Task Order was conducted in conjunction with Air Force Rocket Propulsion Laboratory Contract F04611-86-C-0010. APD conducted the analysis and design coordination to provide a displacement probe design compatible with the XLR-134 liquid hydrogen turbo pump assembly (TPA). Specifications and requirements of the bearing deflectometer were established working with Mechanical Technology Instruments, Inc. (MTI). The TPA design accommodated positioning of the probe to measure outer race cyclic deflections of the pump inlet bearing. The fiber optic sensor was installed as required in the TPA and sensor output was recorded during the TPA testing. Data review indicated that no bearing deflection signature could be differentiated from the inherent system noise. Alternate sensor installations were not investigated, but might yield different results
On the Bilayer Coupling in the Yttrium-Barium Family of High Temperature Superconductors
We present and solve a model for the susceptibility of two CuO2 planes
coupled by an interplane coupling J_perp and use the results to analyze a
recent "cross-relaxation" NMR experiment on Y2Ba4Cu7O15. We deduce that in this
material the product of J_perp and the maximum value of the in-plane
susceptibility chi_max varies from approximately 0.2 at T = 200 K to 0.4 at T =
120 K and that this implies the existence of a temperature dependent in-plane
spin correlation length. Using estimates of chi_max from the literature we find
5 meV < J_perp < 20 meV. We discuss the relation of the NMR results to neutron
scattering results which have been claimed to imply that in YBa2Cu3O_{6+x} the
two planes of a bilayer are perfectly anticorrelated. We also propose that the
recently observed 41 meV excitation in YBa2Cu3O7 is an exciton pulled down
below the superconducting gap by J_perp.Comment: 11 pages, 3 postscript figures (uuencoded and compressed
Resistive Anomalies at Ferromagnetic Transitions Revisited: the case of SrRuO_3
We show that recent resistivity data on SrRuO_3 for T->T_c are consistent
with conventional theory when corrections to scaling are included and a small
shift in T_c is allowed.Comment: 2 pages, 1 figure; revte
Kepler and the Kuiper Belt
The proposed field-of-view of the Kepler mission is at an ecliptic latitude
of ~55 degrees, where the surface density of scattered Kuiper Belt Objects
(KBOs) is a few percent that in the ecliptic plane. The rate of occultations of
Kepler target stars by scattered KBOs with radii r>10km is ~10^-6 to 10^-4 per
star per year, where the uncertainty reflects the current ignorance of the
thickness of the scattered KBO disk and the faint-end slope of their magnitude
distribution. These occultation events will last only ~0.1% of the planned
t_exp=15 minute integration time, and thus will appear as single data points
that deviate by tiny amounts. However, given the target photometric accuracy of
Kepler, these deviations will nevertheless be highly significant, with typical
signal-to-noise ratios of ~10. I estimate that 1-20 of the 10^5 main-sequence
stars in Kepler's field-of-view will exhibit detectable occultations during its
four-year mission. For unresolved events, the signal-to-noise of individual
occultations scales as t_exp^{-1/2}, and the minimum detectable radius could be
decreased by an order of magnitude to ~1 km by searching the individual
3-second readouts for occultations. I propose a number of methods by which
occultation events may be differentiated from systematic effects. Kepler should
measure or significantly constrain the frequency of highly-inclined, ~10
km-sized KBOs.Comment: 5 pages, 1 figure. No changes. Accepted to ApJ, to appear in the
August 1, 2004 issue (v610
Double-stranded RNA elements associated with the MVX disease of Agaricus bisporus
Double-stranded RNA (dsRNA) has been isolated from Agaricus bisporus fruit bodies exhibiting a wide range of disease symptoms. The symptoms which occurred singularly or in combination included; bare cropping areas on commercial beds (primordia disruption), crop delay, premature veil opening, off- or brown-coloured mushrooms, sporophore malformations and loss of crop yield. All symptoms were associated with loss of yield and/or product quality. Collectively, these symptoms are described as mushroom virus X (MVX) disease. The dsRNA titre was much lower than that previously encountered with the La France viral disease of mushrooms and a modified cellulose CF11 protocol was used for their detection. A broad survey of cultivated mushrooms from the British industry identified dsRNA elements ranging between 640 bp and 20.2 kbp; the majority have not previously been described in A. bisporus. 26 dsRNA elements were identified with a maximum of 17, apparently non-encapsidated dsRNA elements, in any one sample. Three dsRNAs (16.2, 9.4 and 2.4 kbp) were routinely found in mushrooms asymptomatic for MVX. Previously, La France disease was effectively contained and controlled by minimising the on-farm production and spread of basidiospores. Our on-farm observations suggest that MVX could be spread by infected spores and/or mycelial fragments
On the optical conductivity of Electron-Doped Cuprates I: Mott Physics
The doping and temperature dependent conductivity of electron-doped cuprates
is analysed. The variation of kinetic energy with doping is shown to imply that
the materials are approximately as strongly correlated as the hole-doped
materials. The optical spectrum is fit to a quasiparticle scattering model;
while the model fits the optical data well, gross inconsistencies with
photoemission data are found, implying the presence of a large, strongly doping
dependent Landau parameter
Ground state properties and dynamics of the bilayer t-J model
We present an exact diagonalization study of bilayer clusters of t-J model.
Our results indicate a crossover between two markedly different regimes which
occurs when the ratio J_perp/J between inter-layer and intra-layer exchange
constants increases: for small J_perp/J the data suggest the development of 3D
antiferromagnetic correlations without appreciable degradation of the
intra-layer spin order and the d_(x2-y2) hole pairs within the planes persist.
For larger values of J_perp/J local singlets along the inter-layer bonds
dominate, leading to an almost complete suppression of the intra-layer spin
correlation and the breaking of the intra-layer pairs. The ground state with
two holes in this regime has s-like symmetry. The data suggest that the
crossover may occur for values of J_perp/J as small as 0.2. We present data for
static spin correlations, spin gap, and electron momentum distribution and
spectral function of the `inter-layer RVB state' realized for large J_perp/J.
The latter deviates from the single layer ground state, making it an
implausible candidate for modelling high-temperature superconductors.Comment: Revtex-file, 6 PRB pages, figures appended as uu-encoded postscript.
Hardcopies of figures (or the entire manuscript) can be obtained by e-mailing
to: [email protected]
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