11,619 research outputs found
An equations-of-motion approach to quantum mechanics: application to a model phase transition
We present a generalized equations-of-motion method that efficiently
calculates energy spectra and matrix elements for algebraic models. The method
is applied to a 5-dimensional quartic oscillator that exhibits a quantum phase
transition between vibrational and rotational phases. For certain parameters,
10 by 10 matrices give better results than obtained by diagonalising 1000 by
1000 matrices.Comment: 4 pages, 1 figur
Genetic heterogeneity of residual variance in broiler chickens
Aims were to estimate the extent of genetic
heterogeneity in environmental variance. Data comprised 99â535 records of
35-day body weights from broiler chickens reared in a controlled
environment. Residual variance within dam families was estimated using
ASREML, after fitting fixed effects such as genetic groups and hatches, for
each of 377 genetically contemporary sires with a large number of progeny
(100 males or females each). Residual variance was computed separately
for male and female offspring, and after correction for sampling, strong
evidence for heterogeneity was found, the standard deviation between sires
in within variance amounting to 15â18% of its mean. Reanalysis using
log-transformed data gave similar results, and elimination of 2â3% of
outlier data reduced the heterogeneity but it was still over 10%. The
correlation between estimates for males and females was low, however. The
correlation between sire effects on progeny mean and residual variance for
body weight was small and negative (-0.1). Using a data set bigger than any
yet presented and on a trait measurable in both sexes, this study has shown
evidence for heterogeneity in the residual variance, which could not be
explained by segregation of major genes unless very few determined the
trait
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Testing a phantom eye under various signal-to-noise ratio conditions using eleven different OCT devices
We compared eleven OCT devices in their ability to quantify retinal layer thicknesses under different signal-strength conditions, using a commercially available phantom eye. We analyzed a medium-intensity 50 ”m layer in an identical manner for all devices, using the provided log-scale images and a reconstructed linear-scale tissue reflectivity metric. Thickness measurements were highly comparable when the data were analyzed in an identical manner. With optimal signal strength, the thickness of the 50 ”m layer was overestimated by a mean of 4.3 ”m in the log-scale images and of 2.7 ”m in the linear-scale images
Experimental Determination of the Lorenz Number in Cu0.01Bi2Te2.7Se0.3 and Bi0.88Sb0.12
Nanostructuring has been shown to be an effective approach to reduce the
lattice thermal conductivity and improve the thermoelectric figure of merit.
Because the experimentally measured thermal conductivity includes contributions
from both carriers and phonons, separating out the phonon contribution has been
difficult and is mostly based on estimating the electronic contributions using
the Wiedemann-Franz law. In this paper, an experimental method to directly
measure electronic contributions to the thermal conductivity is presented and
applied to Cu0.01Bi2Te2.7Se0.3, [Cu0.01Bi2Te2.7Se0.3]0.98Ni0.02, and
Bi0.88Sb0.12. By measuring the thermal conductivity under magnetic field,
electronic contributions to thermal conductivity can be extracted, leading to
knowledge of the Lorenz number in thermoelectric materials
The erosion/corrosion of small superalloy turbine rotors operating in the effluent of a PFB coal combustor
Superalloy turbine rotors in a single stage turbine with 6 percent partial admittance were operated in the effluent of a pressurized fluidized bed coal combustor for up to 164 hours. Total mass flow was 300 kg/hr and average particulate loadings ranged from 600 to 2800 ppm for several coal/sorbent combinations. A 5.5 atm turbine inlet gas pressure and inlet gas temperatures from 700 to 800 C yielded absolute gas velocities at the stator exit of about 500 m/s. The angular rotation speed (40,000 rpm) of the six inch diameter rotors was equivalent to a tip speed of about 300 m/s, and average gas velocities relative to the rotating surface ranged from 260 to 330 m/s at mean radius. The rotor erosion pattern reflects heavy particle separation with severe (5 to 500 cm/yr) erosion at the leading edge, pressure side center, and suction side trailing edge at the tip. The erosion distribution pattern provides a spectrum of erosion/oxidation/deposition as a function of blade position. This spectrum includes enhanced oxidation (10 to 100 x air), mixed oxides in exposed depletion zones, sulfur rich oxides in deposition zones, and rugged areas of erosive oxide removal
An exactly solvable model of a superconducting to rotational phase transition
We consider a many-fermion model which exhibits a transition from a
superconducting to a rotational phase with variation of a parameter in its
Hamiltonian. The model has analytical solutions in its two limits due to the
presence of dynamical symmetries. However, the symmetries are basically
incompatible with one another; no simple solution exists in intermediate
situations. Exact (numerical) solutions are possible and enable one to study
the behavior of competing but incompatible symmetries and the phase transitions
that result in a semirealistic situation. The results are remarkably simple and
shed light on the nature of phase transitions.Comment: 11 pages including 1 figur
Thermoelectricity in Nanowires: A Generic Model
By employing a Boltzmann transport equation and using an energy and size
dependent relaxation time () approximation (RTA), we evaluate
self-consistently the thermoelectric figure-of-merit of a quantum wire
with rectangular cross-section. The inferred shows abrupt enhancement in
comparison to its counterparts in bulk systems. Still, the estimated for
the representative BiTe nanowires and its dependence on wire parameters
deviate considerably from those predicted by the existing RTA models with a
constant . In addition, we address contribution of the higher energy
subbands to the transport phenomena, the effect of chemical potential tuning on
, and correlation of with quantum size effects (QSEs). The obtained
results are of general validity for a wide class of systems and may prove
useful in the ongoing development of the modern thermoelectric applications.Comment: 15 pages, 6 figures; Dedicated to the memory of Amirkhan Qezell
Vector coherent state representations, induced representations, and geometric quantization: II. Vector coherent state representations
It is shown here and in the preceeding paper (quant-ph/0201129) that vector
coherent state theory, the theory of induced representations, and geometric
quantization provide alternative but equivalent quantizations of an algebraic
model. The relationships are useful because some constructions are simpler and
more natural from one perspective than another. More importantly, each approach
suggests ways of generalizing its counterparts. In this paper, we focus on the
construction of quantum models for algebraic systems with intrinsic degrees of
freedom. Semi-classical partial quantizations, for which only the intrinsic
degrees of freedom are quantized, arise naturally out of this construction. The
quantization of the SU(3) and rigid rotor models are considered as examples.Comment: 31 pages, part 2 of two papers, published versio
Richardson-Gaudin integrability in the contraction limit of the quasispin
Background: The reduced, level-independent, Bardeen-Cooper-Schrieffer
Hamiltonian is exactly diagonalizable by means of a Bethe Ansatz wavefunction,
provided the free variables in the Ansatz are the solutions of the set of
Richardson-Gaudin equations. On the one side, the Bethe Ansatz is a simple
product state of generalised pair operators. On the other hand, the
Richardson-Gaudin equations are strongly coupled in a non-linear way, making
them prone to singularities. Unfortunately, it is non-trivial to give a clear
physical interpretation to the Richardson-Gaudin variables because no physical
operator is directly related to the individual variables. Purpose: The purpose
of this paper is to shed more light on the critical behavior of the
Richardson-Gaudin equations, and how this is related to the product wave
structure of the Bethe Ansatz. Method: A pseudo-deformation of the quasi-spin
algebra is introduced, leading towards a Heisenberg-Weyl algebra in the
contraction limit of the deformation parameter. This enables an adiabatic
connection of the exact Bethe Ansatz eigenstates with pure bosonic multiphonon
states. The physical interpretation of this approach is an adiabatic
suppression of the Pauli exclusion principle. Results: The method is applied to
a so-called "picket-fence" model for the BCS Hamiltonian, displaying a typical
critical behavior in the Richardson-Gaudin variables. It was observed that the
associated bosonic multiphonon states change collective nature at the critical
interaction strengths of the Richardson-Gaudin equations. Conclusions: The
Pauli exclusion principle is the main responsible for the critical behavior of
the Richardson-Gaudin equations, which can be suppressed by means of a pseudo
deformation of the quasispin algebra.Comment: PACS 02.30.Ik, 21.10.Re, 21.60.Ce, 74.20.F
Thermoelectric and Seebeck coefficients of granular metals
In this work we present a detailed study and derivation of the thermopower
and thermoelectric coefficient of nano-granular metals at large tunneling
conductance between the grains, g_T>> 1. An important criterion for the
performance of a thermoelectric device is the thermodynamic figure of merit
which is derived using the kinetic coefficients of granular metals. All results
are valid at intermediate temperatures, E_c>>T/g_T>\delta, where \delta is the
mean energy level spacing for a single grain and E_c its charging energy. We
show that the electron-electron interaction leads to an increase of the
thermopower with decreasing grain size and discuss our results in the light of
future generation thermoelectric materials for low temperature applications.
The behavior of the figure of merit depending on system parameters like grain
size, tunneling conductance, and temperature is presented.Comment: 27 pages, 10 figures, revtex
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