26,883 research outputs found
Magnetic correlations of the quasi-one-dimensional half-integer spin-chain antiferromagnets SrVO ( = Co, Mn)
Magnetic correlations of two iso-structural quasi-one-dimensional (1D)
antiferromagnetic spin-chain compounds SrVO ( = Co, Mn) have
been investigated by magnetization and powder neutron diffraction. Two
different collinear antiferromagnetic (AFM) structures, characterized by the
propagation vectors, = (0 0 1) and = (0 0 0), have been found below
5.2 K and 42.2 K for the Co- and Mn-compounds, respectively. For
the Mn-compound, AFM chains (along the axis) order ferromagnetically within
the plane, whereas, for the Co-compound, AFM chains order
ferro-/antiferromagnetically along the direction. The critical exponent
study confirms that the Co- and Mn-compounds belong to the Ising and Heisenberg
universality classes, respectively. For both compounds, short-range spin-spin
correlations are present over a wide temperature range above . The reduced
ordered moments at base temperature (1.5 K) indicate the presence of quantum
fluctuations in both compounds due to the quasi-1D magnetic interactions.Comment: 14 pages, 10 figures, 9 table
Noisy Classical Field Theories with Two Coupled Fields: Dependence of Escape Rates on Relative Field Stiffnesses
Exit times for stochastic Ginzburg-Landau classical field theories with two
or more coupled classical fields depend on the interval length on which the
fields are defined, the potential in which the fields deterministically evolve,
and the relative stiffness of the fields themselves. The latter is of
particular importance in that physical applications will generally require
different relative stiffnesses, but the effect of varying field stiffnesses has
not heretofore been studied. In this paper, we explore the complete phase
diagram of escape times as they depend on the various problem parameters. In
addition to finding a transition in escape rates as the relative stiffness
varies, we also observe a critical slowing down of the string method algorithm
as criticality is approached.Comment: 16 pages, 10 figure
Positron-inert gas differential elastic scattering
Measurements are being made in a crossed beam experiment of the relative elastic differential cross section (DCS) for 5 to 300 eV positrons scattering from inert gas atoms (He, Ne, Ar, Kr, and Xe) in the angular range from 30 to 134 deg. Results obtained at energies around the positronium (Ps) formation threshold provide evidence that Ps formation and possibly other inelastic channels have an effect on the elastic scattering channel
The Order of Phase Transitions in Barrier Crossing
A spatially extended classical system with metastable states subject to weak
spatiotemporal noise can exhibit a transition in its activation behavior when
one or more external parameters are varied. Depending on the potential, the
transition can be first or second-order, but there exists no systematic theory
of the relation between the order of the transition and the shape of the
potential barrier. In this paper, we address that question in detail for a
general class of systems whose order parameter is describable by a classical
field that can vary both in space and time, and whose zero-noise dynamics are
governed by a smooth polynomial potential. We show that a quartic potential
barrier can only have second-order transitions, confirming an earlier
conjecture [1]. We then derive, through a combination of analytical and
numerical arguments, both necessary conditions and sufficient conditions to
have a first-order vs. a second-order transition in noise-induced activation
behavior, for a large class of systems with smooth polynomial potentials of
arbitrary order. We find in particular that the order of the transition is
especially sensitive to the potential behavior near the top of the barrier.Comment: 8 pages, 6 figures with extended introduction and discussion; version
accepted for publication by Phys. Rev.
New Limits on Local Lorentz Invariance in Mercury and Cesium
We report new bounds on Local Lorentz Invariance (LLI) violation in Cs and
Hg. The limits are obtained through the observation of the the spin- precession
frequencies of 199Hg and 133Cs atoms in their ground states as a function of
the orientation of an applied magnetic field with respect to the fixed stars.
We measure the amplitudes of the dipole couplings to a preferred direction in
the equatorial plane to be 19(11) nHz for Hg and 9(5) microHz for Cs. The upper
bounds established here improve upon previous bounds by about a factor of four.
The improvement is primarily due to mounting the apparatus on a rotating table.
New bounds are established on several terms in the standard model extension
including the first bounds on the spin-couplings of the neutron and proton to
the z direction, <7e-30 GeV and <7e-29 GeV, respectively.Comment: 17 pages, 6 figure
Photonic band mixing in linear chains of optically coupled micro-spheres
The paper deals with optical excitations arising in a one-dimensional chain
of identical spheres due optical coupling of whispering gallery modes (WGM).
The band structure of these excitations depends significantly on the
inter-mixing between WGMs characterized by different values of angular quantum
number, . We develop a general theory of the photonic band structure of
these excitations taking these effects into account and applied it to several
cases of recent experimental interest. In the case of bands originating from
WQMs with the angular quantum number of the same parity, the calculated
dispersion laws are in good qualitative agreement with recent experiment
results. Bands resulting from hybridization of excitations resulting from
whispering gallery modes with different parity of exhibits anomalous
dispersion properties characterized by a gap in the allowed values of
\emph{wave numbers} and divergence of group velocity.Comment: RevTex, 28 pages, 7 Figure
Practical Bayesian Modeling and Inference for Massive Spatial Datasets On Modest Computing Environments
With continued advances in Geographic Information Systems and related
computational technologies, statisticians are often required to analyze very
large spatial datasets. This has generated substantial interest over the last
decade, already too vast to be summarized here, in scalable methodologies for
analyzing large spatial datasets. Scalable spatial process models have been
found especially attractive due to their richness and flexibility and,
particularly so in the Bayesian paradigm, due to their presence in hierarchical
model settings. However, the vast majority of research articles present in this
domain have been geared toward innovative theory or more complex model
development. Very limited attention has been accorded to approaches for easily
implementable scalable hierarchical models for the practicing scientist or
spatial analyst. This article is submitted to the Practice section of the
journal with the aim of developing massively scalable Bayesian approaches that
can rapidly deliver Bayesian inference on spatial process that are practically
indistinguishable from inference obtained using more expensive alternatives. A
key emphasis is on implementation within very standard (modest) computing
environments (e.g., a standard desktop or laptop) using easily available
statistical software packages without requiring message-parsing interfaces or
parallel programming paradigms. Key insights are offered regarding assumptions
and approximations concerning practical efficiency.Comment: 20 pages, 4 figures, 2 table
Breakdown of Strong-Coupling Perturbation Theory in Doped Mott Insulators
We show that doped Mott insulators, such as the copper-oxide superconductors,
are asymptotically slaved in that the quasiparticle weight, , near
half-filling depends critically on the existence of the high energy scale set
by the upper Hubbard band. In particular, near half filling, the following
dichotomy arises: when the high energy scale is integrated out but Z=0
in the thermodynamic limit when it is retained. Slavery to the high energy
scale arises from quantum interference between electronic excitations across
the Mott gap. Broad spectral features seen in photoemission in the normal state
of the cuprates are argued to arise from high energy slavery.Comment: Published versio
Nontangential limits and Fatou-type theorems on post-critically finite self-similar sets
In this paper we study the boundary limit properties of harmonic functions on
, the solutions to the Poisson equation where is a p.c.f. set
and its Laplacian given by a regular harmonic structure. In
particular, we prove the existence of nontangential limits of the corresponding
Poisson integrals, and the analogous results of the classical Fatou theorems
for bounded and nontangentially bounded harmonic functions.Comment: 22 page
Scattering of positrons and electrons by alkali atoms
Absolute total scattering cross sections (Q sub T's) were measured for positrons and electrons colliding with sodium, potassium, and rubidium in the 1 to 102 eV range, using the same apparatus and experimental approach (a beam transmission technique) for both projectiles. The present results for positron-sodium and -rubidium collisions represent the first Q sub T measurements reported for these collision systems. Features which distinguish the present comparisons between positron- and electron-alkali atom Q sub T's from those for other atoms and molecules (room-temperature gases) which have been used as targets for positrons and electrons are the proximity of the corresponding positron- and electron-alkali atom Q sub T's over the entire energy range of overlap, with an indication of a merging or near-merging of the corresponding positron and electron Q sub T's near (and above) the relatively low energy of about 40 eV, and a general tendency for the positron-alkali atom Q sub T's to be higher than the corresponding electron values as the projectile energy is decreased below about 40 eV
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