11,369 research outputs found
Dynamical tunneling in optical cavities
The lifetime of whispering gallery modes in a dielectric cavity with a
metallic inclusion is shown to fluctuate by orders of magnitude when size and
location of the inclusion are varied. We ascribe these fluctuations to
tunneling transitions between resonances quantized in different regions of
phase space. This interpretation is confirmed by a comparison of the classical
phase space structure with the Husimi distribution of the resonant modes. A
model Hamiltonian is introduced that describes the phenomenon and shows that it
can be expected in a more general class of systems.Comment: 8 pages LaTeX with 5 postscript figure
Elementary Excitations of Heisenberg Ferrimagnetic Spin Chains
We numerically investigate elementary excitations of the Heisenberg
alternating-spin chains with two kinds of spins 1 and 1/2 antiferromagnetically
coupled to each other. Employing a recently developed efficient Monte Carlo
technique as well as an exact diagonalization method, we verify the spin-wave
argument that the model exhibits two distinct excitations from the ground state
which are gapless and gapped. The gapless branch shows a quadratic dispersion
in the small-momentum region, which is of ferromagnetic type. With the
intention of elucidating the physical mechanism of both excitations, we make a
perturbation approach from the decoupled-dimer limit. The gapless branch is
directly related to spin 1's, while the gapped branch originates from
cooperation of the two kinds of spins.Comment: 7 pages, 7 Postscript figures, RevTe
Hidden Order and Dimerization Transition in Chains
We study ground state properties of the quantum antiferromagnetic chain
with a bond alternation H = \sum_{j} [ 1 + \delta (-1)^j ] \mbox{\boldmath
$S$}_{j} \cdot \mbox{\boldmath $S$}_{j+1} by a Quantum Monte Carlo
calculation. We find that the hidden symmetry is broken for
while it is unbroken in the other regions. This confirms
the successive dimerization transitions first predicted by Affleck and Haldane.
Our result shows that these transitions can be understood in terms of the
hidden symmetry breaking, as was discussed using the
Valence-Bond-Solid states. Furthermore, we find that the behavior of the
generalized string correlation is qualitatively very similar to that in the
Valence-Bond-Solid states, including the location of zeroes as a function of
the angle parameter.Comment: 3 pages (LaTex with jpsj-style files
(ftp://ftp.u-tokyo.ac.jp/pub/SOCIETY/JPSJ)) and 1 Postscript figur
Nuclear Magnetic Relaxation in the Haldane-Gap Antiferromagnet Ni(C_2_H_8_N_2_)_2_NO_2_(ClO_4_)
A new theory is proposed to interpret nuclear spin-lattice relaxation-time
(T_1_) measurements on the spin-1 quasi-one-dimensional Heisenberg
antiferromagnet Ni(C_2_H_8_N_2_)_2_NO_2_(ClO_4_) (NENP). While Sagi and Affleck
pioneeringly discussed this subject in terms of field-theoretical languages,
there is no theoretical attempt yet to explicitly simulate the novel
observations of 1/T_1_ reported by Fujiwara et al.. By means of modified spin
waves, we solve the minimum of 1/T_1_ as a function of an applied field,
pending for the past decade.Comment: to be published in J. Phys. Soc. Jpn. 73, No. 4 (2004
Absence of string order in the anisotropic S=2 Heisenberg antiferromagnet
We study an AFM Heisenberg S=2 quantum spin chain at T=0 with both
interaction and on-site anisotropy, H = \sum_{i}
{1/2}(S^{+}_{i}S^{-}_{i+1}+S^{-}_{i}S^{+}_{i+1})
+J^{z}S^{z}_{i}S^{z}_{i+1}+D(S^{z}_{i})^{2}. Contradictory scenarios exist for
the S=2 anisotropic phase diagram, implying different mechanisms of the
emergence of the classical limit. One main AKLT-based scenario predicts the
emergence of a cascade of phase transitions not seen in the S=1 case. Another
scenario is in favor of an almost classical phase diagram for S=2; the S=1 case
then is very special with its dominant quantum effects. Numerical studies have
not been conclusive. Using the DMRG, the existence of hidden topological order
in the anisotropic S=2 chain is examined, as it distinguishes between the
proposed scenarios. We show that the topological order is zero in the
thermodynamical limit in all disordered phases, in particular in the new phase
interposed between the Haldane and large- phases. This excludes the
AKLT-model based scenario in favor of an almost classical phase diagram for the
spin chains.Comment: 9 pages, 9 eps figures, uses RevTeX, submitted to PR
Theoretical Analysis of the "Double-q" Magnetic Structure of CeAl2
A model involving competing short-range isotropic Heisenberg interactions is
developed to explain the "double-q" magnetic structure of CeAl. For
suitably chosen interactions, terms in the Landau expansion quadratic in the
order parameters explain the condensation of incommensurate order at
wavevectors in the star of (1/2 , 1/2 , 1/2), where
is the cubic lattice constant. We show that the fourth order terms in the
Landau expansion lead to the formation of the so-called "double-q" magnetic
structure in which long-range order develops simultaneously at two
symmetry-related wavevectors, in striking agreement with the magnetic structure
determinations. Based on the value of the ordering temperature and of the
Curie-Weiss of the susceptibility, we estimate that the nearest
neighbor interaction is ferromagnetic, with K and the
next-nearest neighbor interaction is antiferromagnetic with K.
We also briefly comment on the analogous phenomenon seen in the similar system
TmS.Comment: 22 pages, 6 figure
Mode-coupling theory for multiple-time correlation functions of tagged particle densities and dynamical filters designed for glassy systems
The theoretical framework for higher-order correlation functions involving
multiple times and multiple points in a classical, many-body system developed
by Van Zon and Schofield [Phys. Rev. E 65, 011106 (2002)] is extended here to
include tagged particle densities. Such densities have found an intriguing
application as proposed measures of dynamical heterogeneities in structural
glasses. The theoretical formalism is based upon projection operator techniques
which are used to isolate the slow time evolution of dynamical variables by
expanding the slowly-evolving component of arbitrary variables in an infinite
basis composed of the products of slow variables of the system. The resulting
formally exact mode-coupling expressions for multiple-point and multiple-time
correlation functions are made tractable by applying the so-called N-ordering
method. This theory is used to derive for moderate densities the leading mode
coupling expressions for indicators of relaxation type and domain relaxation,
which use dynamical filters that lead to multiple-time correlations of a tagged
particle density. The mode coupling expressions for higher order correlation
functions are also succesfully tested against simulations of a hard sphere
fluid at relatively low density.Comment: 15 pages, 2 figure
Electronic structure, magnetism, and disorder in the Heusler compound CoTiSn
Polycrystalline samples of the half-metallic ferromagnet Heusler compound
CoTiSn have been prepared and studied using bulk techniques (X-ray
diffraction and magnetization) as well as local probes (Sn M\"ossbauer
spectroscopy and Co nuclear magnetic resonance spectroscopy) in order to
determine how disorder affects half-metallic behavior and also, to establish
the joint use of M\"ossbauer and NMR spectroscopies as a quantitative probe of
local ion ordering in these compounds. Additionally, density functional
electronic structure calculations on ordered and partially disordered
CoTiSn compounds have been carried out at a number of different levels of
theory in order to simultaneously understand how the particular choice of DFT
scheme as well as disorder affect the computed magnetization. Our studies
suggest that a sample which seems well-ordered by X-ray diffraction and
magnetization measurements can possess up to 10% of antisite (Co/Ti)
disordering. Computations similarly suggest that even 12.5% antisite Co/Ti
disorder does not destroy the half-metallic character of this material.
However, the use of an appropriate level of non-local DFT is crucial.Comment: 11 pages and 5 figure
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