36,656 research outputs found
Quantum tunneling of the Neel vector in antiferromagnetic [3 x 3] grid molecules
Based on numerical calculations it is shown that the antiferromagnetic grid
molecule Mn-[3 x 3] is a very promising candidate to experimentally detect the
phenomenon of quantum tunneling of the Neel vector.Comment: 4 pages, 3 figures, REVTEX 4, to appear in PR
Correlation function algebra for inhomogeneous fluids
We consider variational (density functional) models of fluids confined in
parallel-plate geometries (with walls situated in the planes z=0 and z=L
respectively) and focus on the structure of the pair correlation function
G(r_1,r_2). We show that for local variational models there exist two
non-trivial identities relating both the transverse Fourier transform G(z_\mu,
z_\nu;q) and the zeroth moment G_0(z_\mu,z_\nu) at different positions z_1, z_2
and z_3. These relations form an algebra which severely restricts the possible
form of the function G_0(z_\mu,z_\nu). For the common situations in which the
equilibrium one-body (magnetization/number density) profile m_0(z) exhibits an
odd or even reflection symmetry in the z=L/2 plane the algebra simplifies
considerably and is used to relate the correlation function to the finite-size
excess free-energy \gamma(L). We rederive non-trivial scaling expressions for
the finite-size contribution to the free-energy at bulk criticality and for
systems where large scale interfacial fluctuations are present. Extensions to
non-planar geometries are also considered.Comment: 15 pages, RevTex, 4 eps figures. To appear in J.Phys.Condens.Matte
Condensation Transitions in a One-Dimensional Zero-Range Process with a Single Defect Site
Condensation occurs in nonequilibrium steady states when a finite fraction of
particles in the system occupies a single lattice site. We study condensation
transitions in a one-dimensional zero-range process with a single defect site.
The system is analysed in the grand canonical and canonical ensembles and the
two are contrasted. Two distinct condensation mechanisms are found in the grand
canonical ensemble. Discrepancies between the infinite and large but finite
systems' particle current versus particle density diagrams are investigated and
an explanation for how the finite current goes above a maximum value predicted
for infinite systems is found in the canonical ensemble.Comment: 18 pages, 4 figures, revtex
Spontaneous Jamming in One-Dimensional Systems
We study the phenomenon of jamming in driven diffusive systems. We introduce
a simple microscopic model in which jamming of a conserved driven species is
mediated by the presence of a non-conserved quantity, causing an effective long
range interaction of the driven species. We study the model analytically and
numerically, providing strong evidence that jamming occurs; however, this
proceeds via a strict phase transition (with spontaneous symmetry breaking)
only in a prescribed limit. Outside this limit, the nearby transition
(characterised by an essential singularity) induces sharp crossovers and
transient coarsening phenomena. We discuss the relevance of the model to two
physical situations: the clustering of buses, and the clogging of a suspension
forced along a pipe.Comment: 8 pages, 4 figures, uses epsfig. Submitted to Europhysics Letter
The VLT-FLAMES Tarantula Survey
We present a number of notable results from the VLT-FLAMES Tarantula Survey
(VFTS), an ESO Large Program during which we obtained multi-epoch
medium-resolution optical spectroscopy of a very large sample of over 800
massive stars in the 30 Doradus region of the Large Magellanic Cloud (LMC).
This unprecedented data-set has enabled us to address some key questions
regarding atmospheres and winds, as well as the evolution of (very) massive
stars. Here we focus on O-type runaways, the width of the main sequence, and
the mass-loss rates for (very) massive stars. We also provide indications for
the presence of a top-heavy initial mass function (IMF) in 30 Dor.Comment: 7 Figures, 8 pages. Invited talk: IAUS 329: "The Lives and
Death-Throes of Massive Stars
Phase Transition in Two Species Zero-Range Process
We study a zero-range process with two species of interacting particles. We
show that the steady state assumes a simple factorised form, provided the
dynamics satisfy certain conditions, which we derive. The steady state exhibits
a new mechanism of condensation transition wherein one species induces the
condensation of the other. We study this mechanism for a specific choice of
dynamics.Comment: 8 pages, 3 figure
Theoretical and numerical studies of chemisorption on a line with precursor layer diffusion
We consider a model for random deposition of monomers on a line with
extrinsic precursor states. As the adsorbate coverage increases, the system
develops non-trivial correlations due to the diffusion mediated deposition
mechanism. In a numeric simulation, we study various quantities describing the
evolution of the island structure. We propose a simple, self-consistent theory
which incorporates pair correlations. The results for the correlations, island
density number, average island size and probabilities of island nucleation,
growth and coagulation show good agreement with the simulation data.Comment: 17 pages(LaTeX), 11 figures(1 PS file, uuencoded), submmited to Phys.
Rev.
Geometrical families of mechanically stable granular packings
We enumerate and classify nearly all of the possible mechanically stable (MS)
packings of bidipserse mixtures of frictionless disks in small sheared systems.
We find that MS packings form continuous geometrical families, where each
family is defined by its particular network of particle contacts. We also
monitor the dynamics of MS packings along geometrical families by applying
quasistatic simple shear strain at zero pressure. For small numbers of
particles (N < 16), we find that the dynamics is deterministic and highly
contracting. That is, if the system is initialized in a MS packing at a given
shear strain, it will quickly lock into a periodic orbit at subsequent shear
strain, and therefore sample only a very small fraction of the possible MS
packings in steady state. In studies with N>16, we observe an increase in the
period and random splittings of the trajectories caused by bifurcations in
configuration space. We argue that the ratio of the splitting and contraction
rates in large systems will determine the distribution of MS-packing
geometrical families visited in steady-state. This work is part of our
long-term research program to develop a master-equation formalism to describe
macroscopic slowly driven granular systems in terms of collections of small
subsystems.Comment: 18 pages, 23 figures, 5 table
Factorised Steady States in Mass Transport Models
We study a class of mass transport models where mass is transported in a
preferred direction around a one-dimensional periodic lattice and is globally
conserved. The model encompasses both discrete and continuous masses and
parallel and random sequential dynamics and includes models such as the
Zero-range process and Asymmetric random average process as special cases. We
derive a necessary and sufficient condition for the steady state to factorise,
which takes a rather simple form.Comment: 6 page
3D wedge filling and 2D random-bond wetting
Fluids adsorbed in 3D wedges are shown to exhibit two types of continuous
interfacial unbinding corresponding to critical and tricritical filling
respectively. Analytic solution of an effective interfacial model based on the
transfer-matrix formalism allows us to obtain the asymptotic probability
distribution functions for the interfacial height when criticality and
tricriticality are approached. Generalised random walk arguments show that, for
systems with short-ranged forces, the critical singularities at these
transitions are related to 2D complete and critical wetting with random bond
disorder respectively.Comment: 7 pages, 3 figures, accepted for publication in Europhysics Letter
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