6,126 research outputs found
A very high accuracy potential energy surface for H3
An exact quantum Monte Carlo (EQMC) method was used to calculate the potential energy surface (PES) for the ground electronic state of H3 over a grid of about 76000 nuclear geometries. The absolute abinitio statistical or sampling error of the calculation was ±0.01 kcal mol^-1 for energies (V) smaller than 3 eV. This PES was fitted by a three-dimensional cubic spline method and the fitting accuracy was determined from a set of 3684 randomly selected nuclear geometries not used in the fitting. For the range V3 eV the rms fitting error was ±0.010 kcal mol^-1, and the absolute value of the corresponding maximum error was 0.018 kcal mol^-1. This fitted EQMC PES is an order of magnitude more accurate than the best PES previously obtained for this system. Detailed comparisons are made with previous PESs, for the more dynamically important nuclear configurations
Second Law Violations in Lovelock Gravity for Black Hole Mergers
We study the classical second law of black hole thermodynamics, for Lovelock
theories (other than General Relativity), in arbitrary dimensions. Using the
standard formula for black hole entropy, we construct scenarios involving the
merger of two black holes in which the entropy instantaneously decreases. Our
construction involves a Kaluza-Klein compactification down to a dimension in
which one of the Lovelock terms is topological. We discuss some open issues in
the definition of the second law which might be used to compensate this entropy
decrease.Comment: 15 pages, 1 figure, v2 Title change & minor revisions to match
published version, v3 fixed accidental deletion of author name
Queen control of a key life-history event in a eusocial insect
In eusocial insects, inclusive fitness theory predicts potential queen–worker conflict over the timing of events in colony life history. Whether queens or workers control the timing of these events is poorly understood. In the bumble-bee Bombus terrestris, queens exhibit a ‘switch point’ in which they switch from laying diploid eggs yielding females (workers and new queens) to laying haploid eggs yielding males. By rearing foundress queens whose worker offspring were removed as pupae and sexing their eggs using microsatellite genotyping, we found that queens kept in the complete absence of adult workers still exhibit a switch point. Moreover, the timing of their switch points relative to the start of egg-laying did not differ significantly from that of queens allowed to produce normal colonies. The finding that bumble-bee queens can express the switch point in the absence of workers experimentally demonstrates queen control of a key life-history event in eusocial insects. In addition, we found no evidence that workers affect the timing of the switch point either directly or indirectly via providing cues to queens, suggesting that workers do not fully express their interests in queen–worker conflicts over colony life history
Overcoming status quo bias in the human brain
Humans often accept the status quo when faced with conflicting choice alternatives. However, it is unknown how neural pathways connecting cognition with action modulate this status quo acceptance. Here we developed a visual detection task in which subjects tended to favor the default when making difficult, but not easy, decisions. This bias was suboptimal in that more errors were made when the default was accepted. A selective increase in subthalamic nucleus (STN) activity was found when the status quo was rejected in the face of heightened decision difficulty. Analysis of effective connectivity showed that inferior frontal cortex, a region more active for difficult decisions, exerted an enhanced modulatory influence on the STN during switches away from the status quo. These data suggest that the neural circuits required to initiate controlled, nondefault actions are similar to those previously shown to mediate outright response suppression. We conclude that specific prefrontal-basal ganglia dynamics are involved in rejecting the default, a mechanism that may be important in a range of difficult choice scenarios
Magnetization dynamics: path-integral formalism for the stochastic Landau-Lifshitz-Gilbert equation
We construct a path-integral representation of the generating functional for
the dissipative dynamics of a classical magnetic moment as described by the
stochastic generalization of the Landau-Lifshitz-Gilbert equation proposed by
Brown, with the possible addition of spin-torque terms. In the process of
constructing this functional in the Cartesian coordinate system, we critically
revisit this stochastic equation. We present it in a form that accommodates for
any discretization scheme thanks to the inclusion of a drift term. The
generalized equation ensures the conservation of the magnetization modulus and
the approach to the Gibbs-Boltzmann equilibrium in the absence of non-potential
and time-dependent forces. The drift term vanishes only if the mid-point
Stratonovich prescription is used. We next reset the problem in the more
natural spherical coordinate system. We show that the noise transforms
non-trivially to spherical coordinates acquiring a non-vanishing mean value in
this coordinate system, a fact that has been often overlooked in the
literature. We next construct the generating functional formalism in this
system of coordinates for any discretization prescription. The functional
formalism in Cartesian or spherical coordinates should serve as a starting
point to study different aspects of the out-of-equilibrium dynamics of magnets.
Extensions to colored noise, micro-magnetism and disordered problems are
straightforward.Comment: 47 pages + appendix, published versio
Observation of collapse of pseudospin order in bilayer quantum Hall ferromagnets
The Hartree-Fock paradigm of bilayer quantum Hall states with finite
tunneling at filling factor =1 has full pseudospin ferromagnetic order
with all the electrons in the lowest symmetric Landau level. Inelastic light
scattering measurements of low energy spin excitations reveal major departures
from the paradigm at relatively large tunneling gaps. The results indicate the
emergence of a novel correlated quantum Hall state at =1 characterized by
reduced pseudospin order. Marked anomalies occur in spin excitations when
pseudospin polarization collapses by application of in-plane magnetic fields.Comment: ReVTeX4, 4 pages, 3 EPS figure
Open ocean carbon monoxide photo-production
Sunlight-initiated photolysis of chromophoric dissolved organic matter (CDOM) is the dominant source of carbon monoxide (CO) in the open-ocean. A modelling study was conducted to constrain this source. Spectral solar irradiance was obtained from two models (GCSOLAR and SMARTS2). Water-column CDOM and total light absorption were modelled using spectra collected along a Meridional transect of the Atlantic ocean using a 200-cm pathlength liquid waveguide UV-visible spectrophotometer. Apparent quantum yields for the production of CO (AQYCO) from CDOM were obtained from a parameterisation describing the relationship between CDOM light absorption coefficient and AQYCO and the CDOM spectra collected. The sensitivity of predicted rates to variations in model parameters (solar irradiance, cloud cover, surface-water reflectance, CDOM and whole water light absorbance, and AQYCO was assessed. The model\u27s best estimate of open-ocean CO photoproduction was 47 +/- 7 Tg CO-C yr-1, with lower and upper limits of 38 and 84 Tg CO-C yr-1, as indicated by sensitivity analysis considering variations in AQYs, CDOM absorbance, and spectral irradiance. These results represent significant constraint of open-ocean CO photoproduction at the lower limit of previous estimates. Based on these results, and their extrapolation to total photochemical organic carbon mineralisation, we recommend a downsizing of the role of photochemistry in the open-ocean carbon cycle.
(c) 2006 Elsevier Ltd. All rights reserved
Instrumental Effects in a Retarding Field Energy Analyzer
Measurements of the well known electron impact spectra of helium show that a retarding field energy analyzer can give spurious responses; these can be minimized but not eliminated by careful adjustments of the operating potentials. Probable sources of the trouble are discussed
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