155 research outputs found
Magnetic Instability in Strongly Correlated Superconductors
Recently a new phenomenological Hamiltonian has been proposed to describe the
superconducting cuprates. This so-called Gossamer Hamiltonian is an apt model
for a superconductor with strong on-site Coulomb repulsion betweenthe
electrons. It is shown that as one approaches half-filling the Gossamer
superconductor, and hence the superconducting state, with strong repulsion is
unstable toward an antiferromagnetic insulator an can undergo a quantum phase
transition to such an insulator if one increases the on-site Coulomb repulsion
Semiconductor Surface Studies
Contains reports on two research projects.Joint Services Electronics Program (Contract DAAB07-76-C-1400)U.S. Navy-Office of Naval Research (Contract N00014-77-C-0132
Fabric Parameters and Pesticide Characteristics That Impact on Dermal Exposure of Applicators
Fabric functional finish and formulation of pesticides are factors that contribute to pesticide wicking, wetting, and penetration. Fluorocarbon soil-repellent finishes inhibit contamination of the fabric and of sentinel pads. An undergarment layer offers better protection than does a single layer. Spun-bonded olefin offers protection of the same magnitude as soil-repellent finishes. Methyl parathion residues after laundering were similar for the unfinished fabric, the durable-press finished fabric, and the soil-repellent finished fabric, but the initial contamination of the soil-repellent finished fabric was only 20~ of that of the other two fabrics
Survival Rates Indicate that Correlations Between Community-Weighted Mean Traits and Environments can be Unreliable Estimates of the Adaptive Value of Traits
Correlations between community-weighted mean (CWM) traits and environmental gradients are often assumed to quantify the adaptive value of traits. We tested this assumption by comparing these correlations with models of survival probability using 46 perennial species from long-term permanent plots in pine forests of Arizona. Survival was modeled as a function of trait-by-environment interactions, plant size, climatic variation, and neighborhood competition. The effect of traits on survival depended on the environmental conditions, but the two statistical approaches were inconsistent. For example, CWM specific leaf area (SLA) and soil fertility were uncorrelated. However, survival was highest for species with low SLA in infertile soil, a result which agreed with expectations derived from the physiological tradeoff underpinning leaf economic theory. CWM trait-environment relationships were unreliable estimates of how traits affected survival, and should only be used in predictive models when there is empirical support for an evolutionary tradeoff that affects vital rates
Interstellar Comets from Post-Main Sequence Systems as Tracers of Extrasolar Oort Clouds
Interstellar small bodies are unique probes into the histories of
exoplanetary systems. One hypothesized class of interlopers are "Jurads,"
exo-comets released into the Milky Way during the post-main sequence as the
thermally-pulsing asymptotic giant branch (AGB) host stars lose mass. In this
study, we assess the prospects for the Legacy Survey of Space and Time (LSST)
to detect a Jurad and examine whether such an interloper would be
observationally distinguishable from exo-comets ejected during the (pre-)main
sequence. Using analytic and numerical methods, we estimate the fraction of
exo-Oort Cloud objects that are released from 1-8 solar mass stars during
post-main sequence evolution. We quantify the extent to which small bodies are
altered by the increased luminosity and stellar outflows during the AGB,
finding that some Jurads may lack hypervolatiles and that stellar winds could
deposit dust that covers the entire exo-comet surface. Next, we construct
models of the interstellar small body reservoir for various size-frequency
distribution slopes, characteristic sizes, and the total mass sequestered in
the minor planets of exo-Oort Clouds. Even with the LSST's increased search
volume compared to contemporary surveys, we find that detecting a Jurad is
unlikely but not infeasible given the current understanding of (exo)planet
formation.Comment: 28 pages, 13 figures; accepted to PS
A High Stellar Obliquity in the WASP-7 Exoplanetary System
We measure a tilt of 86+-6 deg between the sky projections of the rotation
axis of the WASP-7 star, and the orbital axis of its close-in giant planet.
This measurement is based on observations of the Rossiter-McLaughlin (RM)
effect with the Planet Finder Spectrograph on the Magellan II telescope. The
result conforms with the previously noted pattern among hot-Jupiter hosts,
namely, that the hosts lacking thick convective envelopes have high
obliquities. Because the planet's trajectory crosses a wide range of stellar
latitudes, observations of the RM effect can in principle reveal the stellar
differential rotation profile; however, with the present data the signal of
differential rotation could not be detected. The host star is found to exhibit
radial-velocity noise (``stellar jitter') with an amplitude of ~30m/s over a
timescale of days.Comment: ApJ accepted, 9 pages, 9 figure
Automorphic Equivalence within Gapped Phases of Quantum Lattice Systems
Gapped ground states of quantum spin systems have been referred to in the
physics literature as being `in the same phase' if there exists a family of
Hamiltonians H(s), with finite range interactions depending continuously on , such that for each , H(s) has a non-vanishing gap above its
ground state and with the two initial states being the ground states of H(0)
and H(1), respectively. In this work, we give precise conditions under which
any two gapped ground states of a given quantum spin system that 'belong to the
same phase' are automorphically equivalent and show that this equivalence can
be implemented as a flow generated by an -dependent interaction which decays
faster than any power law (in fact, almost exponentially). The flow is
constructed using Hastings' 'quasi-adiabatic evolution' technique, of which we
give a proof extended to infinite-dimensional Hilbert spaces. In addition, we
derive a general result about the locality properties of the effect of
perturbations of the dynamics for quantum systems with a quasi-local structure
and prove that the flow, which we call the {\em spectral flow}, connecting the
gapped ground states in the same phase, satisfies a Lieb-Robinson bound. As a
result, we obtain that, in the thermodynamic limit, the spectral flow converges
to a co-cycle of automorphisms of the algebra of quasi-local observables of the
infinite spin system. This proves that the ground state phase structure is
preserved along the curve of models .Comment: Updated acknowledgments and new email address of S
Semiconductor Surface Studies
Contains reports on two research projects.Joint Services Electronics Program (Contract DAAG29-78-C-0020)U.S. Navy - Office of Naval Research (Contract N00014-77-C-0132
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