400 research outputs found
Yet another surprise in the problem of classical diamagnetism
The well known Bohr-van Leeuwen Theorem states that the orbital diamagnetism
of classical charged particles is identically zero in equilibrium. However,
results based on real space-time approach using the classical Langevin equation
predicts non-zero diamagnetism for classical unbounded (finite or infinite)
systems. Here we show that the recently discovered Fluctuation Theorems,
namely, the Jarzynski Equality or the Crooks Fluctuation Theorem surprisingly
predict a free energy that depends on magnetic field as well as on the friction
coefficient, in outright contradiction to the canonical equilibrium results.
However, in the cases where the Langevin approach is consistent with the
equilibrium results, the Fluctuation Theorems lead to results in conformity
with equilibrium statistical mechanics. The latter is demonstrated analytically
through a simple example that has been discussed recently.Comment: 6 pages, 6 figure
Bohr-van Leeuwen theorem and the thermal Casimir effect for conductors
The problem of estimating the thermal corrections to Casimir and
Casimir-Polder interactions in systems involving conducting plates has
attracted considerable attention in the recent literature on dispersion forces.
Alternative theoretical models, based on distinct low-frequency extrapolations
of the plates reflection coefficient for transverse electric (TE) modes,
provide widely different predictions for the magnitude of this correction. In
this paper we examine the most widely used prescriptions for this reflection
coefficient from the point of view of their consistency with the Bohr-van
Leeuwen theorem of classical statistical physics, stating that at thermal
equilibrium transverse electromagnetic fields decouple from matter in the
classical limit. We find that the theorem is satisfied if and only if the TE
reflection coefficient vanishes at zero frequency in the classical limit. This
criterion appears to rule out some of the models that have been considered
recently for describing the thermal correction to the Casimir pressure with
non-magnetic metallic plates.Comment: 12 pages, no figures. Presentation has been significantly improved,
more references included. The new version matches the one accepted for
publication in Phys. Rev.
Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain
The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes
Slow spin relaxation in a highly polarized cooperative paramagnet
We report measurements of the ac susceptibility of the cooperative paramagnet
Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation
maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of
this peak, suggesting very slow spin relaxations are occurring. Measurements on
samples diluted with nonmagnetic Y3+ or Lu3+ and complementary measurements on
pure and diluted Dy2Ti2O7 strongly suggest that the relaxation is associated
with dipolar spin correlations, representing unusual cooperative behavior in a
paramagnetic system.Comment: Accepted for publication in Physical Review Letter
Prediction of genetic values for feed intake from individual body weight gain and total feed intake of the pen
Records of individual feed intake (FI) and BW gain (GN) were obtained from the Germ Plasm Evaluation (GPE) program at US Meat Animal Research Center (USU.S. Meat Animal Research Center). Animals were randomly assigned to pens. Only pens with 6 to 9 steers (n = 289) were used for this study (data set 1). Variance components and genetic parameters were estimated using data set 1. Estimated genetic values (EGV) for FI were calculated by 5 methods using single and 2-trait analyses: 1) individual FI and individual GN, 2) individual FI alone, 3) 2-trait with individual GN but with FI missing, 4) individual GN and pen total FI, and 5) pen total FI alone. Analyses were repeated but with some of the same records assigned artificially to 36 pens of 5 and 4 paternal half sibs per pen (data sets 2 and 3). Models included year as a fixed factor and birth and weaning weights, age on test, and days fed as covariates. Estimates of heritability were 0.42 ± 0.16 and 0.34 ± 0.17 for FI and GN. The estimate of the genetic correlation was 0.57 ± 0.23. Empirical responses to selection were calculated as the average EGV for the top and bottom 10% based on rank for each method but with EGV from method 1 substituted for the EGV on which ranking was based. With data set 1, rank correlations between EGV from method 1 and EGV from methods 2, 3, 4, and 5 were 0.99, 0.53, 0.32, and 0.15, respectively. Empirical responses relative to method 1 agreed with the rank correlations. Accuracy of EGV for method 4 (0.44) was greater than for method 3 (0.35) and for method 5 (0.29). Accuracies for methods 4 and 5 were greater than indicated by empirical responses and correlations with EGV from method 1. Comparisons of the 5 methods were similar for data sets 2 and 3. With data set 2, rank correlations between EGV from method 1 and EGV from methods 3, 4, and 5 were 0.47, 0.64, and 0.62. Average accuracies of 56, 75, and 75% relative to method 1 (0.67) generally agreed with the empirical responses to selection. As expected, accuracy using pen total FI and GN to obtain EGV for FI was greater than using GN alone. With data set 1, empirical response to selection with method 4 was one-third of that for method 1, although average accuracy was 65% of that for method 1. With assignment of 5 paternal half sibs to artificial pens, using pen total FI and individual GN was about 81% as effective for selection as using individual FI and GN to obtain EGV for FI and was substantially more effective than use of GN alone
Chaos and Semiclassical Limit in Quantum Cosmology
In this paper we present a Friedmann-Robertson-Walker cosmological model
conformally coupled to a massive scalar field where the WKB approximation fails
to reproduce the exact solution to the Wheeler-DeWitt equation for large
Universes. The breakdown of the WKB approximation follows the same pattern than
in semiclassical physics of chaotic systems, and it is associated to the
development of small scale structure in the wave function. This result puts in
doubt the ``WKB interpretation'' of Quantum Cosmology.Comment: 14 pages in LaTex (RevTex), 6 figure
Adiabatic orientation of rotating dipole molecules in an external field
The induced polarization of a beam of polar clusters or molecules passing
through an electric or magnetic field region differs from the textbook
Langevin-Debye susceptibility. This distinction, which is important for the
interpretation of deflection and focusing experiments, arises because instead
of acquiring thermal equilibrium in the field region, the beam ensemble
typically enters the field adiabatically, i.e., with a previously fixed
distribution of rotational states. We discuss the orientation of rigid
symmetric-top systems with a body-fixed electric or magnetic dipole moment. The
analytical expression for their "adiabatic-entry" orientation is elucidated and
compared with exact numerical results for a range of parameters. The
differences between the polarization of thermodynamic and "adiabatic-entry"
ensembles, of prolate and oblate tops, and of symmetric-top and linear rotators
are illustrated and identified.Comment: 18 pages, 4 figure
Genetic relationships between sex-specific traits in beef cattle: Mature weight, weight adjusted for body condition score, height and body condition score of cows, and carcass traits of their steer relatives
Data from the first four cycles of the Germplasm Evaluation Program at the U.S. Meat Animal Research Center (USMARC) were used to investigate genetic relationships between mature weight (MW, n = 37,710), mature weight adjusted for body condition score (AMW, n = 37,676), mature height (HT, n = 37,123), and BCS (n = 37,676) from 4- to 8-yr old cows (n = 1,800) and carcass traits (n = 4,027) measured on their crossbred paternal half-sib steers. Covariance components among traits were estimated using REML. Carcass traits were adjusted for age at slaughter. Estimates of heritability for hot carcass weight (HCWT); percentage of retail product; percentage of fat; percentage of bone; longissimus muscle area; fat thickness adjusted visually; estimated kidney, pelvic, and heart fat percentage; marbling score; Warner-Bratzler shear force; and taste panel tenderness measured on steers were moderate to high (0.26 to 0.65), suggesting that selection for carcass and meat traits could be effective. Estimates of heritability for taste panel flavor and taste panel juiciness were low and negligible (0.05 and 0.01, respectively). Estimates of heritability from cow data over all ages and seasons were high for MW, AMW, and HT (0.52, 0.57, 0.71; respectively) and relatively low for BCS (0.16). Pair-wise analyses for each female mature trait with each carcass trait were done with bivariate animal models. Estimates of genetic correlations between cow mature size and carcass composition or meat quality traits, with the exception of HCWT, were relatively low. Selection for cow mature size (weight and/or height) could be effective and would not be expected to result in much, if any, correlated changes in carcass and meat composition traits. However, genetic correlations of cow traits, with the possible exception of BCS, with HCWT may be too large to ignore. Selection for steers with greater HCWT would lead to larger cows
One-Loop Corrections to Bubble Nucleation Rate at Finite Temperature
We present an evaluation of the 1-loop prefactor in the lifetime of a
metastable state which decays at finite temperature by bubble nucleation. Such
a state is considered in one-component phi^4 model in three space dimensions.
The calculation serves as a prototype application of a fast numerical method
for evaluating the functional determinants that appear in semiclassical
approximations.Comment: DO-TH-93/18, 15 pages, 11 Figures available on request, LaTeX, no
macros neede
Superconducting Fluctuations in a Multi-Band 1D Hubbard Model
A renormalization-group and bosonization approach for a multi-band Hubbard
Hamiltonian in one dimension is described. Based on the limit of many bands, it
is argued that this Hamiltonian with bare repulsive electron-electron
interactions is scaled under specific conditions to a model in which
superconducting fluctuations dominate.Comment: 12 pages + 1 fig, Revtex, Preprint - Los Alamo
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