113 research outputs found
Bose glass behavior in (YbLu)As representing the randomly diluted quantum spin-1/2 chains
The site-diluted compound (YbLu)As is a scarce
realization of the linear Heisenberg antiferromagnet partitioned into
finite-size segments and is an ideal model compound for studying
field-dependent effects of quenched disorder in the one-dimensional
antiferromagnets. It differentiates from the systems studied so far in two
aspects - the type of randomness and the nature of the energy gap in the pure
sample. We have measured the specific heat of single-crystal
(YbLu)As in magnetic fields up to 19.5 T. The contribution
arising from the magnetic subsystem in an applied magnetic field
perpendicular to the chains is determined. Compared to pure YbAs, for
which indicates a gap opening, for diluted systems a
non-exponential decay is found at low temperatures which is consistent with the
thermodynamic scaling of the specific heat established for a Bose-glass phase.Comment: 8 pages, 17 figures, including supplemental material, accepted for
PRB rapid communicatio
The critical Ising lines of the d=2 Ashkin-Teller model
The universal critical point ratio is exploited to determine positions of
the critical Ising transition lines on the phase diagram of the Ashkin-Teller
(AT) model on the square lattice. A leading-order expansion of the ratio in
the presence of a non-vanishing thermal field is found from finite-size scaling
and the corresponding expression is fitted to the accurate perturbative
transfer-matrix data calculations for the square clusters with
.Comment: RevTex, 4 pages, two figure
Cumulant ratios and their scaling functions for Ising systems in strip geometries
We calculate the fourth-order cumulant ratio (proposed by Binder) for the
two-dimensional Ising model in a strip geometry L x oo. The Density Matrix
Renormalization Group method enables us to consider typical open boundary
conditions up to L=200. Universal scaling functions of the cumulant ratio are
determined for strips with parallel as well as opposing surface fields.Comment: 4 pages, RevTex, one .eps figure; references added, format change
Combinatorics of bicubic maps with hard particles
We present a purely combinatorial solution of the problem of enumerating
planar bicubic maps with hard particles. This is done by use of a bijection
with a particular class of blossom trees with particles, obtained by an
appropriate cutting of the maps. Although these trees have no simple local
characterization, we prove that their enumeration may be performed upon
introducing a larger class of "admissible" trees with possibly doubly-occupied
edges and summing them with appropriate signed weights. The proof relies on an
extension of the cutting procedure allowing for the presence on the maps of
special non-sectile edges. The admissible trees are characterized by simple
local rules, allowing eventually for an exact enumeration of planar bicubic
maps with hard particles. We also discuss generalizations for maps with
particles subject to more general exclusion rules and show how to re-derive the
enumeration of quartic maps with Ising spins in the present framework of
admissible trees. We finally comment on a possible interpretation in terms of
branching processes.Comment: 41 pages, 19 figures, tex, lanlmac, hyperbasics, epsf. Introduction
and discussion/conclusion extended, minor corrections, references adde
Response of a catalytic reaction to periodic variation of the CO pressure: Increased CO_2 production and dynamic phase transition
We present a kinetic Monte Carlo study of the dynamical response of a
Ziff-Gulari-Barshad model for CO oxidation with CO desorption to periodic
variation of the CO presure. We use a square-wave periodic pressure variation
with parameters that can be tuned to enhance the catalytic activity. We produce
evidence that, below a critical value of the desorption rate, the driven system
undergoes a dynamic phase transition between a CO_2 productive phase and a
nonproductive one at a critical value of the period of the pressure
oscillation. At the dynamic phase transition the period-averged CO_2 production
rate is significantly increased and can be used as a dynamic order parameter.
We perform a finite-size scaling analysis that indicates the existence of
power-law singularities for the order parameter and its fluctuations, yielding
estimated critical exponent ratios and . These exponent ratios, together with theoretical symmetry
arguments and numerical data for the fourth-order cumulant associated with the
transition, give reasonable support for the hypothesis that the observed
nonequilibrium dynamic phase transition is in the same universality class as
the two-dimensional equilibrium Ising model.Comment: 18 pages, 10 figures, accepted in Physical Review
Two dimensional Ising spin glasses with non-zero ordering temperatures
We demonstrate numerically that for Ising spins on square lattices with
ferromagnetic second neighbour interactions and random near neighbour
interactions, two dimensional Ising spin glass order with a non-zero freezing
temperature can occur. We compare some of the physical properties of these spin
glasses with those of standard spin glasses in higher dimensions.Comment: 9 page latex file and 9 ps figures. To appear in Phys. Rev. Let
Critical behavior of hard-core lattice gases: Wang-Landau sampling with adaptive windows
Critical properties of lattice gases with nearest-neighbor exclusion are
investigated via the adaptive-window Wang-Landau algorithm on the square and
simple cubic lattices, for which the model is known to exhibit an Ising-like
phase transition. We study the particle density, order parameter,
compressibility, Binder cumulant and susceptibility. Our results show that it
is possible to estimate critical exponents using Wang-Landau sampling with
adaptive windows. Finite-size-scaling analysis leads to results in fair
agreement with exact values (in two dimensions) and numerical estimates (in
three dimensions).Comment: 20 pages, 11 figure
Series expansions from the corner transfer matrix renormalization group method: the hard squares model
The corner transfer matrix renormalization group method is an efficient
method for evaluating physical quantities in statistical mechanical models. It
originates from Baxter's corner transfer matrix equations and method, and was
developed by Nishino and Okunishi in 1996. In this paper, we review and adapt
this method, previously used for numerical calculations, to derive series
expansions. We use this to calculate 92 terms of the partition function of the
hard squares model. We also examine the claim that the method is subexponential
in the number of generated terms and briefly analyse the resulting series.Comment: 10 figure
the role of soluble and insoluble fibers during fermentation of Chicory root pulp
This thesis was aimed at understanding the in vitro fermentability of soluble and insoluble fibers in chicory root pulp (CRP). First, CRP and ensiled chicory root pulp (ECRP) were characterized for cell wall polysaccharides (CWPs). Both CRP and ECRP were rich in CWPs (56-58 w/w (%)) and had rather similar sugar compositions. The CWPs consist of 62 % pectin, 11% hemicellulose and 27% cellulose. Pectin and xyloglucan were acetylated and the rhamnogalacturonan-I segments of pectin were branched mostly with arabinan. Compared to CRP, ECRP has four times more soluble pectin. In vitrofermentability in a batch model for 24 h using human faecal inoculum, showed that fibers in both CRP (51% carbohydrate utilisation) and ECRP (59% carbohydrate utilisation) were fermentable, especially pectin (80-87%). The increased levels of soluble pectin (arabinan, homogalacturonan and galactan) and the hypothesized open cell wall structure in ECRP contributed to a quicker fermentation and a higher level of carbohydrate utilization compared to CRP. In contrast to batch fermentation, fermentation in the dynamic TNO In vitro model of the colon (TIM-2) was rapid (57% carbohydrate utilisation in 2 h). ECRP carbohydrates (85%) were less fermented in 24 h compared to CRP carbohydrates (92%) due to lower utilisation of ECRP insoluble fibers than CRP insoluble fibers. It was hypothesized that soluble fibers that are readily fermentable and dominantly present in ECRP, programmed the microbiota in TIM-2 to fully adapt to these soluble fibers. After their utilization, the microbiota was not able to adapt towards the fermentation of insoluble fibers. Analysis of enzyme activities during batch fermentation of CRP showed increased levels of arabinofuranosidase, β-galactosidase, endo-arabinanase, endo-galactanase, exo-polygalacturonase, pectin de-esterifying enzymes and endo-polygalacturonase. They synergistically contributed to degrading pectin in CRP from 12 to 24 h of fermentation.</p
- …