426 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
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
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
Dzyaloshinsky-Moriya Spin Canting in the LTT Phase of La2-x-yEuySrxCuO4
The Cu spin magnetism in La2-x-yEuySrxCuO4 (x<=0.17; y<=0.2) has been studied
by means of magnetization measurements up to 14 T. Our results clearly show
that in the antiferromagnetic phase Dzyaloshinsky-Moriya (DM)superexchange
causes Cu spin canting not only in the LTO phase but also in the LTLO and LTT
phases. In La1.8Eu0.2CuO4 the canted DM-moment is about 50% larger than in pure
La2CuO4 which we attribute to the larger octahedral tilt angle. We also find
clear evidence that the size of the DM-moment does not change significantly at
the structural transition at T_LT from LTO to LTLO and LTT. The most important
change induced by the transition is a significant reduction of the magnetic
coupling between the CuO2 planes. As a consequence, the spin-flip transition of
the canted Cu spins which is observed in the LTO phase for magnetic field
perpendicular to the CuO2 planes disappears in the LTT phase. The shape of the
magnetization curves changes from the well known spin-flip type to a
weak-ferromagnet type. However, no spontaneous weak ferromagnetism is observed
even at very low temperatures, which seems to indicate that the interlayer
decoupling in our samples is not perfect. Nonetheless, a small fraction (<15%)
of the DM-moments can be remanently magnetized throughout the entire
antiferromagnetically ordered LTT/LTLO phase, i.e. for T<T_LT and x<0.02. It
appears that the remanent DM-moment is perpendicular to the CuO2 planes. For
magnetic field parallel to the CuO2 planes we find that the critical field of
the spin-flop transition decreases in the LTLO phase, which might indicate a
competition between different in-plane anisotropies. To study the Cu spin
magnetism in La2-x-yEuySrxCuO4, a careful analysis of the Van Vleck
paramagnetism of the Eu3+ ions was performed.Comment: 22 pages, 27 figure
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
Classical Langevin dynamics of a charged particle moving on a sphere and diamagnetism: A surprise
It is generally known that the orbital diamagnetism of a classical system of
charged particles in thermal equilibrium is identically zero -- the Bohr-van
Leeuwen theorem. Physically, this null result derives from the exact
cancellation of the orbital diamagnetic moment associated with the complete
cyclotron orbits of the charged particles by the paramagnetic moment subtended
by the incomplete orbits skipping the boundary in the opposite sense. Motivated
by this crucial, but subtle role of the boundary, we have simulated here the
case of a finite but \emph{unbounded} system, namely that of a charged particle
moving on the surface of a sphere in the presence of an externally applied
uniform magnetic field. Following a real space-time approach based on the
classical Langevin equation, we have computed the orbital magnetic moment which
now indeed turns out to be non-zero, and has the diamagnetic sign. To the best
of our knowledge, this is the first report of the possibility of finite
classical diamagnetism in principle, and it is due to the avoided cancellation.Comment: Accepted for publication in EP
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
Thermal Casimir Force between Magnetic Materials
We investigate the Casimir pressure between two parallel plates made of
magnetic materials at nonzero temperature. It is shown that for real
magnetodielectric materials only the magnetic properties of ferromagnets can
influence the Casimir pressure. This influence is accomplished through the
contribution of the zero-frequency term of the Lifshitz formula. The
possibility of the Casimir repulsion through the vacuum gap is analyzed
depending on the model used for the description of the dielectric properties of
the metal plates.Comment: 9 pages, 3 figures. Contribution to the Proceedings of QFEXT09,
Norman, OK, September 21-25, 200
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