Phase transition of the compressible ising lattice

It is shown that the presence of harmonic lattice vibrations in a spin-$\text{1}\text{2}$ Ising system leads, in the mean field approximation, to transitions of first or second order, according to whether the pressure lies below or above a critical value. The results are being discussed in relation to experiments on the order-disorder transition in NH₄Cl.     

Soliton lattice to metal: A first order phase transition

$\text{In-situ}$ measurements of the electron spin resonance of [Na⁺_y (CH)^?y]_x during electrochemical doping are reported. The susceptibility and the esr linewidth exhibit step-like increases, with hysteresis, as functions of the chemical potential. These magnetic data, when combined with electrochemical and structural results on the same system, imply a first order phase transition from soliton lattice to metal.     

First order magnetic phase transition in GeCo2O4

The antiferromagnetic compound GeCo₂O₄ exhibits a magnetic phase transition characterized by thermal hysteresis of the susceptibility versus temperature curve and by a diffuse neutron scattering with a small correlation length. The data are compared to the expected first-order phase transition of the n ≥ 4 component vector models.     

First order phase transition in V3Si

Thermal expansion measurements on a single crystal of V₃Si for the first time give evidence that the low temperature structural transition is of first order. The associated relative volume change is of the order of 10^-5.     

Gap solitons in an anharmonic diatomic lattice

Localized stationary modes with frequency in the forbidden region of the elastic vibrations of a diatomic anharmonic lattice are studied by the coupled-wave method. The mechanisms responsible for the brightening of the lattice in the cases of narrow and wide forbidden frequency bands are determined and an analytical expression is found for the stationary gap solitons. Fiz. Tverd. Tela (St. Petersburg) 39, 158–162 (January 1997)     

Magnetic quantum phase transition in an anisotropic kondo lattice

The quantum phase transition between paramagnetic and antiferromagnetic phases of the Kondo lattice model with Ising anisotropy in the intersite exchange is studied within extended dynamical mean-field theory. Nonperturbative numerical solutions at zero temperature point to a continuous transition for both two- and three-dimensional magnetism. In the former case, the transition is associated with critical local physics, characterized by a vanishing Kondo scale and by an anomalous exponent in the dynamics close in value to that measured in heavy-fermion CeCu5.9Au0.1.     

First order antiferromagnetic phase transition in MnS2

The antiferromagnetic structure of MnS₂ has been re-examined by neutron diffraction from a single crystal. The phase transition has been found to be of the first order without any detectable thermal hysteresis. Application of uniaxial stress tends to convert the first order transition into a second order one.     

First order phase transition in the frustrated triangular antiferromagnet CsNiCl3

By means of high-resolution ultrasonic velocity measurements, as a function of temperature and magnetic field, the nature of the different low temperatures magnetic phase transitions observed for the quasi-one-dimensional compound CsNiCl3 is established. Special attention has been devoted to the field-induced 120 degrees phase transition above the multicritical point in the H-T phase diagram where the elastic constant C44 reveals a steplike variation and hysteresis effects. These results represent the first experimental evidence that the 120 degrees phase transition is weakly first order and contradict the popular notion of new universality classes for chiral systems.     

First order phase transition of expanding matter and its fragmentation

Based on an expanding matter model using Lennard-Jones potential, the instability of the system is examined. The instantaneous pressure, temperature and density fluctuation are calculated as a function of the density. The system undergoes the first order phase transition when the expanding velocity is slow. Although the system behaves dynamically for faster expansion, a kind of criticality seems to play an important role even in this case.     

First order high-spin/low-spin phase transition induced by acoustic-phonons

A new interpretation of light-induced magnetization changes of a magnetic semiconductor, manganese arsenide (MnAs), observed by the authors of references [1,2], is proposed in this paper. Contrary to references [1,2], where the results of experiments were interpreted as the observation of light-induced phase transition, here we propose a completely different approach. It suggests that at least far from the vicinity of T _c, there are no real magnetization changes as in case of phase transition, but there are changes of the magnetic flux threading the MnAs-sample. These changes are due to non-equilibrium light-induced diamagnetic moments of quasi-free electrons of narrow d-subbands of the MnAs-conduction band. The other aspects of the experiments of [1,2] are also discussed and some similarity between this effect and the orbital diamgnetism due to persistent currents in mesoscopic structures is emphasised. Received 7 November 2000     

The lattice dynamics of an anharmonic crystal

The theory of the physical properties of an anharmonic crystal is discussed by using the thermodynamic Green's functions for the phonons. A perturbation procedure is developed to obtain the Green's functions and it is shown that for some purposes a quasi-harmonic approximation is useful, in which the frequencies of the normal modes are those determined by infra-red or neutron spectrometry. The thermodynamic, elastic, dielectric and scattering properties of an anharmonic crystal are discussed in terms of the Green's functions, and detailed expressions are given for the more important contributions. Detailed numerical calculations are presented of the thermal expansion, dielectric properties and shapes of some of the inelastically scattered neutron groups, for sodium iodide and potassium bromide. The calculations, which give reasonable agreement with experiment, show that even at quite low temperatures, the lifetimes of some of the normal modes can be quite short. By using the quasi-harmonic approximation it is shown that the large temperature dependence of the normal modes in a ferroelectric crystal can be treated adequately.     

First order phase transition in the system of two coupled fields with different transition temperatures

It is shown that for the case of a strong field coupling in the system of two fields the low temperature phase may be stable in the fluctuation region of the high temperature phase.     

First order transition to oscillation death through an environment

The emergence of dynamical abrupt transitions for the first time in an ensemble of identical limit-cycle and chaotic oscillators coupled via a common environment is reported. The transition from the oscillatory state to the death state and vice versa, in these networks of oscillators are found not only discontinuous as well as irreversible in the parameter space. This first order phase transition in these systems is termed as Explosive Death. The occurrence of such transition is studied in details by using an appropriate order parameter for both limit-cycle and chaotic oscillators, in particular, Stuart–Landau and Rössler oscillators. The backward transition point for this phenomenon is obtained analytically using linear stability analysis and is found to be consistent with the numerical results.     

First order phase transitions in the antiferromagnetic Ising model on a pure Husimi lattice

The antiferromagnetic spin-1/2 Ising model on the pure Husimi lattice with three sites in the elementary polygon (p=3$p=3$) and the coordination number z=6$z=6$ is investigated which represents the simplest approximation of the antiferromagnetic Ising model on the regular triangular lattice which takes into account effects of geometric frustration. The region of parameters is found in which two physical phases coexist. In addition, the existence of the first order phase transitions between these two coexisting phases is demonstrated and investigated in detail. A detailed analysis of the magnetization properties of the model is performed and the existence of the magnetization plateaus for low temperatures is shown. All possible ground states of the model are found and discussed.     

First order phase transitions in lattice and continuous systems: Extension of Pirogov-Sinai theory

We generalize the notion of ground states in the Pirogov-Sinai theory of first order phase transitions at low temperatures, applicable to lattice systems with a finite number of periodic ground states to that of restricted ensembles with equal free energies. A restricted ensemble is a Gibbs ensemble, i.e. equilibrium probability measure, on a restricted set of configurations in the phase space of the system. When a restricted ensemble contains only one configuration it coincides with a ground state. In the more general case the entropy is also important.An example of a system we can treat by our methods is theq-state Potts model where we prove that forq sufficiently large there exists a temperature at which the system coexists inq+1 phases;q-ordered phases are small modifications of theq perfectly ordered ground states and one disordered phase which is a modification of the restricted ensemble consisting of all perfectly disordered (neighboring sites must have different spins) configurations. The free energy thus consists entirely of energy in the firstq-restricted ensembles and of entropy in the last one.Our main motivation for this work is to develop a rigorous theory for phase transitions in continuum fluids in which there is no symmetry between the phases, e.g. the liquid-vapour phase transition. The present work goes a certain way in that direction.Supported in part by NSF Grant N^r DMR81-14726-02     

First order phase transition in the plane rotator ferromagnetic model in two dimensions

We show that the two-dimensional isotropic ferromagnetic rotator model exhibits a first order phase transition if the interaction decays asr ^– with 2<<4.Work supported by Fonds National Suisse de la Recherche Scientifique.     

First order β, γ phase transition in NH4Br

The course of the order parameter around the β, γ phase transition (235 K) in the NH₄Br has been closely observed by means of linear optical birefringence. The experimental data were fitted in the critical region T > 0.9T₀ by the Landau theory and a simple power law. Both relations reveal a first order transition, with the jump height in the birefringence amounting to Δn(T₀)/?Δn(120 K) = 0.145.