arXiv subjects

C. L. Henley

Publications and source records attributed to C. L. Henley.

47 records · Page 3Linked to original sources

arXiv1999

Nonlinear Measures for Characterizing Rough Surface Morphologies

We develop a new approach to characterizing the morphology of rough surfaces based on the analysis of the scaling properties of contour loops, i.e. loops of constant height. Given a height profile of the surface we perform independent measurements of the fractal dimension of contour loops, and the exponent that characterizes their size distribution. Scaling formulas are derived and used to relate these two geometrical exponents to the roughness exponent of a self-affine surface, thus providing independent measurements of this important quantity. Furthermore, we define the scale dependent curvature and demonstrate that by measuring its third moment departures of the height fluctuations from Gaussian behavior can be ascertained. These nonlinear measures are used to characterize the morphology of computer generated Gaussian rough surfaces, surfaces obtained in numerical simulations of a simple growth model, and surfaces observed by scanning-tunneling-microscopes. For experimentally realized surfaces the self-affine scaling is cut off by a correlation length, and we generalize our theory of contour loops to take this into account.

cond-mat.mtrl-sci↗

arXiv1998

Phonon-phason coupling in icosahedral quasicrystals

From relaxation simulations of decoration-based quasicrystal structure models using microscopically based interatomic pair potentials, we have calculated the (usually neglected) phonon-phason coupling constant. Its sign is opposite for the two alloys studied, i-AlMn and i-(Al,Cu)Li; a dimensionless measure of its magnitude relative to the phonon and phason elastic constants is of order 1/10, suggesting its effects are small but detectable. We also give a criterion for when phonon-phason effects are noticeable in diffuse tails of Bragg peaks.

cond-mat.mtrl-sci↗

arXiv1997

A constrained Potts antiferromagnet model with an interface representation

We define a four-state Potts model ensemble on the square lattice, with the constraints that neighboring spins must have different values, and that no plaquette may contain all four states. The spin configurations may be mapped into those of a 2-dimensional interface in a 2+5 dimensional space. If this interface is in a Gaussian rough phase (as is the case for most other models with such a mapping), then the spin correlations are critical and their exponents can be related to the stiffness governing the interface fluctuations. Results of our Monte Carlo simulations show height fluctuations with an anomalous dependence on wavevector, intermediate between the behaviors expected in a rough phase and in a smooth phase; we argue that the smooth phase (which would imply long-range spin order) is the best interpretation.

cond-mat.stat-mech↗

arXiv1997

Cluster maximization, non-locality, and random tilings

Jeong and Steinhardt (JS) implement local rules by selecting the sub-ensemble of tilings which have the maximum occurrences of a chosen pattern (``cluster'') C It is unknown how to prove that a given C implies a given sub-ensemble; counterexamples given here demonstrate this problem is nonlocal, and show the JS results depend on periodic boundary conditions. Sub-ensembles which are tilings of supertiles are at least as interesting, mathematically and physically, as the ideal cases emphasized by JS; the case C = star-decagon is explored here. Finally, I suggest minimizing the frequency of chosen intersite distances as a more physical alternative to the cluster approach.

cond-mat↗

arXiv1997

New Two-color Dimer Models with Critical Ground States

We define two new models on the square lattice, in which each allowed configuration is a superposition of a covering by ``white'' dimers and one by ``black'' dimers. Each model maps to a solid-on-solid (SOS) model in which the ``height'' field is two dimensional. Measuring the stiffness of the SOS fluctuations in the rough phase provides critical exponents of the dimer models. Using this ``height'' representation, we have performed Monte Carlo simulations. They confirm that each dimer model has critical correlations and belongs to a new universality class. In the "dimer-loop" model (which maps to a loop model) one height component is smooth but has unusual correlated fluctuations; the other height component is rough. In the ``noncrossing-dimer'' model, the heights are rough having two different elastic constants; an unusual form of its elastic theory implies anisotropic critical correlations.

cond-mat↗

arXiv1997

Zero-Temperature Phase Transitions of Antiferromagnetic Ising Model of General Spin on a Triangular Lattice

We map the ground-state ensemble of antiferromagnetic Ising model of spin-S on a triangular lattice to an interface model whose entropic fluctuations are proposed to be described by an effective Gaussian free energy, which enables us to calculate the critical exponents of various operators in terms of the stiffness constant of the interface. Monte Carlo simulations for the ground-state ensemble utilizing this interfacial representation are performed to study both the dynamical and the static properties of the model. This method yields more accurate numerical results for the critical exponents. By varying the spin magnitude in the model, we find that the model exhibits three phases with a Kosterlitz-Thouless phase transition at 3/2<S_{KT}<2 and a locking phase transition at 5/2 < S_L \leq 3. The phase diagram at finite temperatures is also discussed.

cond-mat↗

arXiv1997

Exact realization of SO(5) symmetry in extended Hubbard models

Zhang recently conjectured an approximate SO(5) symmetry relating antiferromagnetic and superconducting states in high-T_c cuprates. Here, an exact SO(5) symmetry is implemented in a generalized Hubbard model (with long-range interactions) on a lattice. The possible relation to a more realistic extended Hubbard Hamiltonian is discussed.

cond-mat.str-el↗

arXiv1997

Relaxation time for a dimer covering with height representation

This paper considers the Monte Carlo dynamics of random dimer coverings of the square lattice, which can be mapped to a rough interface model. Two kinds of slow modes are identified, associated respectively with long-wavelength fluctuations of the interface height, and with slow drift (in time) of the system-wide mean height. Within a continuum theory, the longest relaxation time for either kind of mode scales as the system size N. For the real, discrete model, an exact lower bound of O(N) is placed on the relaxation time, using variational eigenfunctions corresponding to the two kinds of continuum modes.

cond-mat↗

arXiv1995

Kac-Moody Symmetries of Critical Ground States

The symmetries of critical ground states of two-dimensional lattice models are investigated. We show how mapping a critical ground state to a model of a rough interface can be used to identify the chiral symmetry algebra of the conformal field theory that describes its scaling limit. This is demonstrated in the case of the six-vertex model, the three-coloring model on the honeycomb lattice, and the four-coloring model on the square lattice. These models are critical and they are described in the continuum by conformal field theories whose symmetry algebras are the $su(2)_{k=1}$, $su(3)_{k=1}$, and the $su(4)_{k=1}$ Kac-Moody algebra, respectively. Our approach is based on the Frenkel--Kac--Segal vertex operator construction of level one Kac--Moody algebras.

cond-mat↗

arXiv1995

Spin Glass and Antiferromagnetic Behaviour in a Diluted fcc Antiferromagnet

We report on a Monte Carlo study of a diluted Ising antiferromagnet on a fcc lattice. This is a typical model example of a highly frustrated antiferromagnet, and we ask, whether sufficient random dilution of spins does produce a spin glass phase. Our data strongly indicate the existence of a spin glass transition for spin--concentration $p<0.75$: We find a divergent spin glass susceptibility and a divergent spin glass correlation length, whereas the antiferromagnetic correlation length saturates in this regime. Furthermore, we find a first order phase transition to an antiferromagnet for $1\ge p>0.85$, which becomes continuous in the range $0.85>p>0.75$. Finite size scaling is employed to obtain critical exponents. We compare our results with experimental systems as diluted frustrated antiferromagnets as ${\rm Zn_{1-p}Mn_{p}Te}$.

cond-mat↗

arXiv1995

Phason elasticity of a three-dimensional quasicrystal: transfer-matrix method

We introduce a new transfer matrix method for calculating the thermodynamic properties of random-tiling models of quasicrystals in any number of dimensions, and describe how it may be used to calculate the phason elastic properties of these models, which are related to experimental measurables such as phason Debye-Waller factors, and diffuse scattering wings near Bragg peaks. We apply our method to the canonical-cell model of the icosahedral phase, making use of results from a previously-presented calculation in which the possible structures for this model under specific periodic boundary conditions were cataloged using a computational technique. We give results for the configurational entropy density and the two fundamental elastic constants for a range of system sizes. The method is general enough allow a similar calculation to be performed for any other random tiling model.

cond-mat↗