Search results for "Phase Transition"
showing 10 items of 1281 documents
Phase Transitions in Multicomponent Widom-Rowlinson Models
1995
We use Monte Carlo techniques to study the phase diagram of multicomponent Widom-Rowlinson models on a square lattice: there are M species all with the same fugacity z and a nearest neighbor hard core exclusion between unlike particles. For M between two and six there is a direct transition from the gas phase at z z d (M). For M ≥ 7 there is an intermediate ordered phase in which the even (or odd) sublattice is occupied preferentially by particles chosen at random from any of the species. The existence of such an intermediate phase was proven earlier for M ≥ M 0, M 0 very large. Exact calculations on the Bethe lattice give M0 = 4.
Pressure induced insulator/half-metal/metal transition in a strongly correlatedp-electron system
2012
Mixed-valent Rb${}_{4}$O${}_{6}$ provides an exceptional prototype material for studying the interplay between local correlations (Hubbard $U$) and electron kinetic energy ($W$) in the open $sp$-electron shell. Based on a first-principles calculation we show that depending on $U/W$ ratio, when tuned by external pressure, Rb${}_{4}$O${}_{6}$ exhibits a surprising sequence of phase transitions between strongly correlated antiferromagnetic insulator, ferromagnetic insulator ($U/W\phantom{\rule{-0.16em}{0ex}}\ensuremath{\gg}\phantom{\rule{-0.16em}{0ex}}1$), moderately correlated ferromagnetic half-metal ($U/W\phantom{\rule{-0.16em}{0ex}}\ensuremath{\sim}\phantom{\rule{-0.16em}{0ex}}1$), and fin…
The Ising square lattice in aL�M geometry: A model for the effect of surface steps on phase transitions in adsorbed monolayers
1989
Critical phenomena in adsorbed monolayers on surfaces are influenced by limited substrate homogeneity, such as surface steps. We consider the resulting finite-size and boundary effects in the framework of a lattice gas system with nearest neighbor attraction in aL×M geometry, with two free boundaries of lengthM≫L, and periodic boundary conditions in the other direction (along the direction of the steps). This geometry thus models a “terrace” of the stepped surface, and adatoms adsorbed on neighboring terraces are assumed to be non-interacting. Also the effect of boundary “fields” is considered (describing the effects of missing neighbors and changed binding energy to the substrate near the …
Interplay of order-disorder phenomena and diffusion in rigid binary alloys in the presence of vacancies: Monte Carlo simulations
2006
Transport phenomena are studied for a binary $(AB)$ alloy on a rigid square lattice with nearest-neighbor attraction between unlike particles, assuming a small concentration ${c}_{v}$ of vacancies $V$ being present, to which $A$ $(B)$ particles can jump with rates ${\ensuremath{\Gamma}}_{A}$ $({\ensuremath{\Gamma}}_{B})$ in the case where the nearest-neighbor attractive energy ${ϵ}_{AB}$ is negligible in comparison with the thermal energy ${k}_{B}T$ in the system. This model exhibits a continuous order-disorder transition for concentrations ${c}_{A},{c}_{B}=1\ensuremath{-}{c}_{A}\ensuremath{-}{c}_{V}$ in the range ${c}_{A,1}^{\mathit{crit}}\ensuremath{\leqslant}{c}_{A}\ensuremath{\leqslant}…
Structural and vibrational study of Bi2Se3under high pressure
2011
The structural and vibrational properties of bismuth selenide (Bi${}_{2}$Se${}_{3}$) have been studied by means of x-ray diffraction and Raman scattering measurements up to 20 and 30 GPa, respectively. The measurements have been complemented with ab initio total-energy and lattice dynamics calculations. Our experimental results evidence a phase transition from the low-pressure rhombohedral ($R$-3$m$) phase (\ensuremath{\alpha}-Bi${}_{2}$Se${}_{3}$) with sixfold coordination for Bi to a monoclinic $C$2/$m$ structure (\ensuremath{\beta}-Bi${}_{2}$Se${}_{3}$) with sevenfold coordination for Bi above 10 GPa. The equation of state and the pressure dependence of the lattice parameters and volume …
Realization of a two-dimensional Ising system: Deuterium physisorbed on krypton-preplated graphite
2004
Volumetric adsorption isotherm, calorimetric, and neutron diffraction measurements were used to characterize the quantum system ${\mathrm{D}}_{2}$ coadsorbed on graphite preplated by a monolayer of Kr. From the results obtained by these methods a detailed phase diagram of the complete submonolayer coverage range up to the initial stages of bilayer formation could be constructed. The dominant feature of the phase diagram is a commensurate $(1\ifmmode\times\else\texttimes\fi{}1)[\frac{1}{2}]$ structure, which was determined by neutron diffraction. Three phase transitions of this phase were studied: The order-disorder transition at the critical point which, according to the influence of the co…
Adatom dynamics and diffusion in a model of O/W(110)
1998
We consider adatom dynamics and diffusion in a lattice-gas model of the O/W(110) system under conditions where the adatom interaction effects are important. In particular, we study the behavior of the tracer and collective diffusion coefficients as a function of temperature when crossing over from the high-temperature disordered phase to a low-temperature symmetry broken phase. To this end, we utilize a combined analytical and numerical approach based on the recently developed dynamical mean field theory (DMF) in addition to conventional Monte Carlo simulations. In the case studied here, the origin of the strong temperature dependence of the effective activation barrier ${E}_{A}^{D}$ close …
Oscillation spectra of semilinear photorefractive coherent oscillator with two pump waves
2002
The transition of the single-frequency oscillation of a semilinear photorefractive coherent oscillator for sufficiently large coupling strengths into two-frequency oscillation is predicted and is observed experimentally. The critical value of coupling strength at which the bifurcation occurs is a function of pump-intensity ratio and cavity losses. For certain combinations of these parameters, the critical coupling strength for spectrum bifurcation becomes smaller than the threshold coupling strength: in these cases double-frequency oscillation appears at the threshold. The supercritical bifurcation in the oscillation spectrum is analogous to the second-order phase transition.
Orbital dimerization in NaTiSi2O6: An orbital analogue of the spin-Peierls phase transition
2004
We measure the Raman scattering spectra of NaTiSi2O6, analyze the vibrational properties, and study the origin of the phase transition in this compound. In this quasi-one-dimensional S = 1/2 system we observe anomalous high-temperature phonon broadenings, and large changes of the phonon energies and line-widths across the phase transition temperature of 210 K. These results, combined with theoretical considerations, indicate that the phonon anomalies originate from an orbital order-disorder type of phase transition. We find that the high temperature dynamical Jahn-Teller phase of NaTiSi2O6 exhibits a spontaneous breaking of translational symmetry into a dimerized, Jahn-Teller distorted, orb…
Monte Carlo studies of adsorbed monolayers: Lattice-gas models with translational degrees of freedom
1998
Standard lattice-gas models for the description of the phase behavior of adsorbed monolayers are generalized to ``elastic lattice gases'' which allow for translational degrees of freedom of the adsorbate atoms but have the substrate lattice structure built into the adsorbate-adsorbate interaction. For such models, we derive a simple and efficient grand-canonical Monte Carlo algorithm, which treats the occupied and empty sites in precisely the same way. Using this method, we calculate the phase diagram of a simple model for the adsorption of hydrogen on palladium (100); this model includes only pairwise interactions and exhibits an ordered $c(2\ifmmode\times\else\texttimes\fi{}2)$ structure.…