15,802 research outputs found
Thermally-Assisted Current-Driven Domain Wall Motion
Starting from the stochastic Landau-Lifschitz-Gilbert equation, we derive
Langevin equations that describe the nonzero-temperature dynamics of a rigid
domain wall. We derive an expression for the average drift velocity of the
domain wall as a function of the applied current, and find qualitative
agreement with recent magnetic semiconductor experiments. Our model implies
that at any nonzero temperature the average domain-wall velocity initially
varies linearly with current, even in the absence of non-adiabatic spin
torques.Comment: 4 pages, 2 figure
The Renormalization Group and the Superconducting Susceptibility of a Fermi Liquid
A free Fermi gas has, famously, a superconducting susceptibility that
diverges logarithmically at zero temperature. In this paper we ask whether this
is still true for a Fermi liquid and find that the answer is that it does {\it
not}. From the perspective of the renormalization group for interacting
fermions, the question arises because a repulsive interaction in the Cooper
channel is a marginally irrelevant operator at the Fermi liquid fixed point and
thus is also expected to infect various physical quantities with logarithms.
Somewhat surprisingly, at least from the renormalization group viewpoint, the
result for the superconducting susceptibility is that two logarithms are not
better than one. In the course of this investigation we derive a
Callan-Symanzik equation for the repulsive Fermi liquid using the
momentum-shell renormalization group, and use it to compute the long-wavelength
behavior of the superconducting correlation function in the emergent low-energy
theory. We expect this technique to be of broader interest.Comment: 9 pages, 2 figure
Radicals in carbonaceous residue deposited on mordenite from methanol
It is shown that control of the degree of coking can lead to the observation of hyperfine structures in the carbonaceous residues deposited from methanol over mordenite (H-MOR) at temperatures relevant to the conversion of methanol to hydrocarbons. EPR measurements of the catalyst samples at various times on stream have been recorded, with a rich hyperfine splitting pattern observed in the early stages of the reaction. Interpretation of the EPR data with the aid of density functional theoretical calculations has afforded the first definitive assignment of the radical cations formed in high temperature coke. The results detail a shortlist of six species: 2,3/2,6/2,7-dimethylnaphthalenium, 2,3,6-trimethylnaphthalenium, 2,3,6,7-tetramethylnaphthalenium, and anthracenium radical cations whose proton hyperfine splitting profiles match the experimental spectra; 2,3,6,7-tetramethylnaphthalenium showed the best agreement. The observation of these particular isomers of polymethylnaphthalene suggest the formation of more highly branched polyaromatic species is less likely within the confines of the H-MOR 12-membered ring channel. These radicals formed when the catalyst is active may constitute key intermediates in the conversion of methanol to light olefins
Field Theory of Propagating Reaction-Diffusion Fronts
The problem of velocity selection of reaction-diffusion fronts has been
widely investigated. While the mean field limit results are well known
theoretically, there is a lack of analytic progress in those cases in which
fluctuations are to be taken into account. Here, we construct an analytic
theory connecting the first principles of the reaction-diffusion process to an
effective equation of motion via field-theoretic arguments, and we arrive at
the results already confirmed by numerical simulations
Absence of vortex condensation in a two dimensional fermionic XY model
Motivated by a puzzle in the study of two dimensional lattice Quantum
Electrodynamics with staggered fermions, we construct a two dimensional
fermionic model with a global U(1) symmetry. Our model can be mapped into a
model of closed packed dimers and plaquettes. Although the model has the same
symmetries as the XY model, we show numerically that the model lacks the well
known Kosterlitz-Thouless phase transition. The model is always in the gapless
phase showing the absence of a phase with vortex condensation. In other words
the low energy physics is described by a non-compact U(1) field theory. We show
that by introducing an even number of layers one can introduce vortex
condensation within the model and thus also induce a KT transition.Comment: 5 pages, 5 figure
A renormalized large-n solution of the U(n) x U(n) linear sigma model in the broken symmetry phase
Dyson-Schwinger equations for the U(n) x U(n) symmetric matrix sigma model
reformulated with two auxiliary fields in a background breaking the symmetry to
U(n) are studied in the so-called bare vertex approximation. A large n solution
is constructed under the supplementary assumption so that the scalar components
are much heavier than the pseudoscalars. The renormalizability of the solution
is investigated by explicit construction of the counterterms.Comment: RevTeX4, 14 pages, 2 figures. Version published in Phys. Rev.
Mean-Field and Anomalous Behavior on a Small-World Network
We use scaling results to identify the crossover to mean-field behavior of
equilibrium statistical mechanics models on a variant of the small world
network. The results are generalizable to a wide-range of equilibrium systems.
Anomalous scaling is found in the width of the mean-field region, as well as in
the mean-field amplitudes. Finally, we consider non-equilibrium processes.Comment: 4 pages, 0 figures; reference adde
Behavior of the anomalous correlation function in uniform 2D Bose gas
We investigate the behavior of the anomalous correlation function in two
dimensional Bose gas. In the local case, we find that this quantity has a
finite value in the limit of weak interactions at zero temperature. The effects
of the anomalous density on some thermodynamic quantities are also considered.
These effects can modify in particular the chemical potential, the ground sate
energy, the depletion and the superfluid fraction. Our predictions are in good
agreement with recent analytical and numerical calculations. We show also that
the anomalous density presents a significant importance compared to the
non-condensed one at zero temperature. The single-particle anomalous
correlation function is expressed in two dimensional homogenous Bose gases by
using the density-phase fluctuation. We then confirm that the anomalous average
accompanies in analogous manner the true condensate at zero temperature while
it does not exist at finite temperature.Comment: 15 pages, 3 figure
Gauged supersymmetries in Yang-Mills theory
In this paper we show that Yang-Mills theory in the
Curci-Ferrari-Delbourgo-Jarvis gauge admits some up to now unknown local linear
Ward identities. These identities imply some non-renormalization theorems with
practical simplifications for perturbation theory. We show in particular that
all renormalization factors can be extracted from two-point functions. The Ward
identities are shown to be related to supergauge transformations in the
superfield formalism for Yang-Mills theory. The case of non-zero Curci-Ferrari
mass is also addressed.Comment: 11 pages. Minor changes. Some added reference
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