40,568 research outputs found
Field-Induced Magnetic Order and Simultaneous Lattice Deformation in TlCuCl3
We report the results of Cu and Cl nuclear magnetic resonance experiments
(NMR) and thermal expansion measurements in magnetic fields in the coupled
dimer spin system TlCuCl3. We found that the field-induced antiferromagnetic
transition as confirmed by the splitting of NMR lines is slightly
discontinuous. The abrupt change of the electric field gradient at the Cl
sites, as well as the sizable change of the lattice constants, across the phase
boundary indicate that the magnetic order is accompanied by simultaneous
lattice deformation.Comment: 4 pages, 5 figure
The magnetization process of the spin-one triangular-lattice Heisenberg antiferromagnet
We apply the coupled cluster method and exact diagonalzation to study the
uniform susceptibility and the ground-state magnetization curve of the
triangular-lattice spin-1 Heisenberg antiferromagnet. Comparing our theoretical
data for the magnetization curve with recent measurements on the s=1 triangular
lattice antiferromagnet Ba3NiSb2O9 we find a very good agreement.Comment: 2 pages, 3 figure
Vacuum Energy Density for Massless Scalar Fields in Flat Homogeneous Spacetime Manifolds with Nontrivial Topology
Although the observed universe appears to be geometrically flat, it could
have one of 18 global topologies. A constant-time slice of the spacetime
manifold could be a torus, Mobius strip, Klein bottle, or others. This global
topology of the universe imposes boundary conditions on quantum fields and
affects the vacuum energy density via Casimir effect. In a spacetime with such
a nontrivial topology, the vacuum energy density is shifted from its value in a
simply-connected spacetime. In this paper, the vacuum expectation value of the
stress-energy tensor for a massless scalar field is calculated in all 17
multiply-connected, flat and homogeneous spacetimes with different global
topologies. It is found that the vacuum energy density is lowered relative to
the Minkowski vacuum level in all spacetimes and that the stress-energy tensor
becomes position-dependent in spacetimes that involve reflections and
rotations.Comment: 25 pages, 11 figure
DNA in nanopore-counterion condensation and coion depletion
Molecular dynamics simulations are used to study the equilibrium distribution
of monovalent ions in a nanopore connecting two water reservoirs separated by a
membrane, both for the empty pore and that with a single stranded DNA molecule
inside. In the presence of DNA, the counterions condense on the stretched
macromolecule effectively neutralizing it, and nearly complete depletion of
coions from the pore is observed. The implications of our results for
experiments on DNA translocation through alpha-hemolysin nanopores are
discussed.Comment: 8 pages, 2 figure
Holography and Cosmological Singularities
Certain null singularities in ten dimensional supergravity have natural
holographic duals in terms of Matrix Theory and generalizations of the AdS/CFT
correspondence. In many situations the holographic duals appear to be well
defined in regions where the supergravity develops singularities. We describe
some recent progress in this area.Comment: Anomaly equation corrected. References adde
The Orbit and Position of the X-ray Pulsar XTE J1855-026 - an Eclipsing Supergiant System
A pulse timing orbit has been obtained for the X-ray binary XTE J1855-026
using observations made with the Proportional Counter Array on board the Rossi
X-ray Timing Explorer. The mass function obtained of ~16Mo together with the
detection of an extended near-total eclipse confirm that the primary star is a
supergiant as predicted. The orbital eccentricity is found to be very low with
a best fit value of 0.04 +/- 0.02. The orbital period is also refined to be
6.0724 +/- 0.0009 days using an improved and extended light curve obtained with
RXTE's All Sky Monitor. Observations with the ASCA satellite provide an
improved source location of R.A. = 18h 55m 31.3s}, decl. = -02o 36' 24.0"
(2000) with an estimated systematic uncertainty of less than 12". A
serendipitous new source, AX J1855.4-0232, was also discovered during the ASCA
observations.Comment: Accepted for publication in the Astrophysical Journa
Cohomology of Lie superalgebras and of their generalizations
The cohomology groups of Lie superalgebras and, more generally, of color Lie
algebras, are introduced and investigated. The main emphasis is on the case
where the module of coefficients is non-trivial. Two general propositions are
proved, which help to calculate the cohomology groups. Several examples are
included to show the peculiarities of the super case. For L = sl(1|2), the
cohomology groups H^1(L,V) and H^2(L,V), with V a finite-dimensional simple
graded L-module, are determined, and the result is used to show that
H^2(L,U(L)) (with U(L) the enveloping algebra of L) is trivial. This implies
that the superalgebra U(L) does not admit of any non-trivial formal
deformations (in the sense of Gerstenhaber). Garland's theory of universal
central extensions of Lie algebras is generalized to the case of color Lie
algebras.Comment: 50 pages, Latex, no figures. In the revised version the proof of
Lemma 5.1 is greatly simplified, some references are added, and a pertinent
result on sl(m|1) is announced. To appear in the Journal of Mathematical
Physic
Maximum likelihood estimates of pairwise rearrangement distances
Accurate estimation of evolutionary distances between taxa is important for
many phylogenetic reconstruction methods. In the case of bacteria, distances
can be estimated using a range of different evolutionary models, from single
nucleotide polymorphisms to large-scale genome rearrangements. In the case of
sequence evolution models (such as the Jukes-Cantor model and associated
metric) have been used to correct pairwise distances. Similar correction
methods for genome rearrangement processes are required to improve inference.
Current attempts at correction fall into 3 categories: Empirical computational
studies, Bayesian/MCMC approaches, and combinatorial approaches. Here we
introduce a maximum likelihood estimator for the inversion distance between a
pair of genomes, using the group-theoretic approach to modelling inversions
introduced recently. This MLE functions as a corrected distance: in particular,
we show that because of the way sequences of inversions interact with each
other, it is quite possible for minimal distance and MLE distance to
differently order the distances of two genomes from a third. This has obvious
implications for the use of minimal distance in phylogeny reconstruction. The
work also tackles the above problem allowing free rotation of the genome.
Generally a frame of reference is locked, and all computation made accordingly.
This work incorporates the action of the dihedral group so that distance
estimates are free from any a priori frame of reference.Comment: 21 pages, 7 figures. To appear in the Journal of Theoretical Biolog
Intrinsic Josephson Effect in the Layered Two-dimensional t-J Model
The intrinsic Josephson effect in the high-Tc superconductors is studied
using the layered two-dimensional t-J model. The d.c.Josephson current which
flows perpendicular to the t-J planes is obtained within the mean-field
approximation and the Gutzwiller approximation. We find that the Josephson
current has its maximum near the optimum doping region as a function of the
doping rate.Comment: 4 pages, 3 figure
Electrophoresis of a rod macroion under polyelectrolyte salt: Is mobility reversed for DNA?
By molecular dynamics simulation, we study the charge inversion phenomenon of
a rod macroion in the presence of polyelectrolyte counterions. We simulate
electrophoresis of the macroion under an applied electric field. When both
counterions and coions are polyelectrolytes, charge inversion occurs if the
line charge density of the counterions is larger than that of the coions. For
the macroion of surface charge density equal to that of the DNA, the reversed
mobility is realized either with adsorption of the multivalent counterion
polyelectrolyte or the combination of electrostatics and other mechanisms
including the short-range attraction potential or the mechanical twining of
polyelectrolyte around the rod axis.Comment: 8 pages, 5 figures, Applied Statistical Physics of Molecular
Engineering (Mexico, 2003). Journal of Physics: Condensed Matters, in press
(2004). Journal of Physics: Condensed Matters, in press (2004
- …