arXiv subjects

Bernard Nienhuis

Publications and source records attributed to Bernard Nienhuis.

50 records · Page 3Linked to original sources

arXiv1998

Exact Stationary State for an ASEP with Fully Parallel Dynamics

The exact stationary state of an asymmetric exclusion process with fully parallel dynamics is obtained using the matrix product Ansatz. We give a simple derivation for the deterministic case by a physical interpretation of the dimension of the matrices. We prove the stationarity via a cancellation mechanism and by making use of an explicit representation of the matrix algebra we easily find closed expressions for the correlation functions in the general probabalistic case. Asymptotic expressions, obtained by making use of earlier results, allow us to derive the exact phase diagram.

cond-mat.stat-mech↗

arXiv1998

First and second order transitions in dilute O(n) models

We explore the phase diagram of an O(n) model on the honeycomb lattice with vacancies, using finite-size scaling and transfer-matrix methods. We make use of the loop representation of the O(n) model, so that $n$ is not restricted to positive integers. For low activities of the vacancies, we observe critical points of the known universality class. At high activities the transition becomes first order. For n=0 the model includes an exactly known theta point, used to describe a collapsing polymer in two dimensions. When we vary $n$ from 0 to 1, we observe a tricritical point which interpolates between the universality classes of the theta point and the Ising tricritical point.

cond-mat↗

arXiv1998

Phase Diagram of a Loop on the Square Lattice

The phase diagram of the O(n) model, in particular the special case $n=0$, is studied by means of transfer-matrix calculations on the loop representation of the O(n) model. The model is defined on the square lattice; the loops are allowed to collide at the lattice vertices, but not to intersect. The loop model contains three variable parameters that determine the loop density or temperature, the energy of a bend in a loop, and the interaction energy of colliding loop segments. A finite-size analysis of the transfer-matrix results yields the phase diagram in a special plane of the parameter space. These results confirm the existence of a multicritical point and an Ising-like critical line in the low-temperature O(n) phase.

cond-mat↗

arXiv1997

Bethe Ansatz solution of a decagonal rectangle triangle random tiling

A random tiling of rectangles and triangles displaying a decagonal phase is solved by Bethe Ansatz. Analogously to the solutions of the dodecagonal square triangle and the octagonal rectangle triangle tiling an exact expression for the maximum of the entropy is found.

cond-mat.stat-mech↗

arXiv1997

Scaling Limit of RSOS Models and TBA Equations

We study the scaling limits of the L-state Restricted Solid-on-Solid (RSOS) lattice models and their fusion hierarchies in the off-critical regimes. Starting with the elliptic functional equations of Klumper and Pearce, we derive the Thermodynamic Bethe Ansatz (TBA) equations of Zamolodchikov. Although this systematic approach, in principle, allows TBA equations to be derived for all the excited states we restrict our attention here to the largest eigenvalue or groundstate in Regimes III and IV. In Regime III the TBA equations are massive while in Regime IV there is massless scattering describing the renormalization group flow between distinct A_1^{(1)} coset conformal field theories. Regimes I and II, pertaining to Z_{L-1} parafermions, will be treated in a subsequent paper.

hep-th↗

arXiv1997

Solvable Rectangle Triangle Random Tilings

We show that a rectangle triangle random tiling with a tenfold symmetric phase is solvable by Bethe Ansatz. After the twelvefold square triangle and the eightfold rectangle triangle random tiling, this is the third example of a rectangle triangle tiling which is solvable. A Bethe Ansatz solution provides in principle an accurate estimate of the entropy and phason elastic constants. In the twelvefold and eightfold cases even exact analytic expressions have been obtained from the Bethe Ansatz solution.

cond-mat.stat-mech↗

arXiv1996

On the integrability of the square-triangle random tiling model

It is shown that the square-triangle random tiling model is equivalent to an asymmetric limit of the three colouring model on the honeycomb lattice. The latter model is known to be the O(n) model at T=0 and corresponds to the integrable model connected to the affine $A_2^{(1)}$ Lie algebra. Thus it is shown that the weights of the square-triangle random tiling satisfy the Yang-Baxter equation, albeit in a singular limit of a more general model. The three colouring model for general vertex weights is solved by algebraic Bethe Ansatz.

solv-int↗

arXiv1996

The exact solution of an octagonal rectangle triangle random tiling

We present a detailed calculation of the recently published exact solution of a random tiling model possessing an eight-fold symmetric phase. The solution is obtained using Bethe Ansatz and provides closed expressions for the entropy and phason elastic constants. Qualitatively, this model has the same features as the square-triangle random tiling model. We use the method of P. Kalugin, who solved the Bethe Ansatz equations for the square-triangle tiling, which were found by M. Widom.

solv-int↗

arXiv1996

Scaling Limit of the Ising Model in a Field

The dilute A_3 model is a solvable IRF (interaction round a face) model with three local states and adjacency conditions encoded by the Dynkin diagram of the Lie algebra A_3. It can be regarded as a solvable version of an Ising model at the critical temperature in a magnetic field. One therefore expects the scaling limit to be governed by Zamolodchikov's integrable perturbation of the c=1/2 conformal field theory. Indeed, a recent thermodynamic Bethe Ansatz approach succeeded to unveil the corresponding E_8 structure under certain assumptions on the nature of the Bethe Ansatz solutions. In order to check these conjectures, we perform a detailed numerical investigation of the solutions of the Bethe Ansatz equations for the critical and off-critical model. Scaling functions for the ground-state corrections and for the lowest spectral gaps are obtained, which give very precise numerical results for the lowest mass ratios in the massive scaling limit. While these agree perfectly with the E_8 mass ratios, we observe one state which seems to violate the assumptions underlying the thermodynamic Bethe Ansatz calculation. We also analyze the critical spectrum of the dilute A_3 model, which exhibits massive excitations on top of the massless states of the Ising conformal field theory.

hep-th↗

arXiv1996

Operator Spectrum and Exact Exponents of the Fully Packed Loop Model

We develop a Coulomb gas description of the critical fluctuations in the fully packed loop model on the honeycomb lattice. We identify the complete operator spectrum of this model in terms of electric and magnetic {\em vector}-charges, and we calculate the scaling dimensions of these operators exactly. We also study the geometrical properties of loops in this model, and we derive exact results for the fractal dimension and the loop size distribution function. A review of the many different representations of this model that have recently appeared in the literature, is given.

cond-mat↗

arXiv1996

Exact Solution of an Octagonal Random Tiling Model

We consider the two-dimensional random tiling model introduced by Cockayne, i.e. the ensemble of all possible coverings of the plane without gaps or overlaps with squares and various hexagons. At the appropriate relative densities the correlations have eight-fold rotational symmetry. We reformulate the model in terms of a random tiling ensemble with identical rectangles and isosceles triangles. The partition function of this model can be calculated by diagonalizing a transfer matrix using the Bethe Ansatz (BA). The BA equations can be solved providing {\em exact} values of the entropy and elastic constants.

solv-int↗

arXiv1995

Scaling Properties of the Ising Model in a Field

The dilute A_3 model is a solvable IRF (interaction-round-a-face) model with three local states and adjacency conditions encoded by the Dynkin diagram of the Lie algebra A_3. It can be regarded as a solvable version of a critical Ising model in a magnetic field. One therefore expects the scaling limit to be governed by Zamolodchikov's integrable perturbation of the c=1/2 conformal field theory. We perform a detailed numerical investigation of the solutions of the Bethe ansatz equation for the off-critical model. Our results agree perfectly with the predicted values for the lowest masses of the stable particles and support the assumptions on the nature of the Bethe ansatz solutions which enter crucially in a recent thermodynamic Bethe ansatz calculation of the factorized scattering matrix.

hep-th↗

arXiv1993

Solvable lattice models labelled by Dynkin diagrams

An equivalence between generalised restricted solid-on-solid (RSOS) models, associated with sets of graphs, and multi-colour loop models is established. As an application we consider solvable loop models and in this way obtain new solvable families of critical RSOS models. These families can all be classified by the Dynkin diagrams of the simply-laced Lie algebras. For one of the RSOS models, labelled by the Lie algebra pair (A$_L$,A$_L$) and related to the C$_2^{(1)}$ vertex model, we give an off-critical extension, which breaks the Z$_2$ symmetry of the Dynkin diagrams.

hep-th↗

arXiv1993

A Solvable Model for Intersecting Loops

We show that some models with non-local (and non-localizable) interactions have a property, called quasi-locality, which allows for the definition of a transfer matrix. We give the Yang-Baxter equation as a sufficient condition for the existence of a family of commuting transfer matrices and solve them for a loop model with intersections. This solvable model is then analyzed in some detail and its applications to a Lorentz gas are briefly discussed.

hep-th↗