Smeared Phase Transition Research Articles

Magnetic properties of La0.70Sr0.30MnO2.85 anion-deficient manganite under hydrostatic pressure

The magnetic properties of La0.70Sr0.30MnO2.85 anion-deficient manganite are studied experimentally under hydrostatic pressure. The results show that, in the whole pressure range under investigation (0–1 GPa), the sample is a spin glass with a smeared phase transition to the paramagnetic state. It is found that the spin glass state arises from the frustration of the exchange coupling of the ferromagnetic clusters embedded in the antiferromagnetic matrix. The fraction of the sample volume occupied by the ferromagnetic phase is found to be V fer ∼ 13%. Under hydrostatic pressure, the freezing temperature T f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T MO increases at a rate of 12.90 K/GPa. In addition, the ferromagnetic part of the sample increases by ΔV fer ∼ 5%. The enhancement of the ferromagnetic properties of La0.70Sr0.30MnO2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit-cell parameters.

Dynamics at a smeared phase transition

We investigate the effects of rare regions on the dynamics of Ising magnets with planar defects, i.e., disorder perfectly correlated in two dimensions. In these systems, the magnetic phase transition is smeared because static long-range order can develop on isolated rare regions. We first study an infinite-range model by numerically solving local dynamic mean-field equations. Then we use extremal statistics and scaling arguments to discuss the dynamics beyond mean-field theory. In the tail region of the smeared transition the dynamics is even slower than in a conventional Griffiths phase: the spin autocorrelation function decays like a stretched exponential at intermediate times before approaching the exponentially small equilibrium value following a power law at late times.

Monte Carlo simulations of the smeared phase transition in a contact process with extended defects

We study the nonequilibrium phase transition in a contact process with extended quenched defects by means of Monte-Carlo simulations. We find that the spatial disorder correlations dramatically increase the effects of the impurities. As a result, the sharp phase transition is completely destroyed by smearing. This is caused by effects similar to but stronger than the usual Griffiths phenomena, viz., rare strongly coupled spatial regions can undergo the phase transition independently from the bulk system. We determine both the stationary density in the vicinity of the smeared transition and its time evolution, and we compare the simulation results to a recent theory based on extremal statistics.

The action of intense nonresonance laser radiation on the phase equilibrium of systems with coupled states*

This paper analyzes the state of a bound two-electron formation (a bipolaron) in an electromagnetic radiation field that is not considered weak. By using the Kramers-Henneberger unitary transformation, the external, rapidly oscillating action is transferred to the argument of the potential energy. It is shown that increasing the external field intensity produces a monotonic decrease of the dissociation energy of a bound two-electron formation and reduces the number of its vibrational states. It is established that the external field produces attenuation of the electron-phonon interaction. The degree of dissociation of the bipolaron is computed as a function of the external field intensity, the temperature, and the concentration. The transition between the bound two-particle state and the single-particle state is treated as a smeared phase transition. © 2004 Optical Society of America

Smeared phase transition in a three-dimensional Ising model with planar defects: Monte Carlo simulations

We present results of large-scale Monte Carlo simulations for a three-dimensional Ising model with short range interactions and planar defects, i.e., disorder perfectly correlated in two dimensions. We show that the phase transition in this system is smeared, i.e., there is no single critical temperature, but different parts of the system order at different temperatures. This is caused by effects similar to but stronger than Griffiths phenomena. In an infinite-size sample there is an exponentially small but finite probability to find an arbitrary large region devoid of impurities. Such a rare region can develop true long-range order while the bulk system is still in the disordered phase. We compute the thermodynamic magnetization and its finite-size effects, the local magnetization, and the probability distribution of the ordering temperatures for different samples. Our Monte-Carlo results are in good agreement with a recent theory based on extremal statistics.

Critical scattering of polarized neutrons in the invarFe65Ni35alloy

The magnetic phase transition in the invar Fe65Ni35 alloy has been investigated by small-angle polarizedneutron scattering ~SAPNS! along with both three-dimensional neutron depolarization and thermal-expansion measurements. The data give evidence for a smeared phase transition. A spatial distribution in the Curie temperature TC is introduced to explain the experimental data. The local TC variations are described by the standard deviation of the transition temperature DTC and by the characteristic length of the TC variations, R0. The deduced parameters are ^TC&548560.5 K, DTC512.560.2 K, and R0’300 nm. The critical SAPNS experiment was performed in a special inclined geometry (H W is inclined to the wave vector k ). Two different contributions to the critical scattering were analyzed in the magnetic-field range ~1‐1000 G! and the temperature range ( TC60.1TC) of interest. First, the two-spin-correlation function was studied. Second, the dynamical three-spin-correlation function, or the so-called dynamical chirality of the system, was investigated by extracting the asymmetric part of the polarization-dependent scattering. We distinguish two contributions in the chiral scattering, which stem from the spin waves in the large magnetic inhomogeneities and from the three-spincorrelation function of the critical fluctuations. The data are interpreted in terms of the static and dynamic scaling theory accounting for the concept of local TC variations.

Specific features in the ferroelectric properties of Sr0.75Ba0.25Nb2O6 crystals

The dielectric properties of strontium-barium niobate crystals with a strontium concentration of 0.75, which corresponds to the most smeared ferroelectric phase transition, were measured. The dielectric hysteresis loops and the polarization relaxation are shown to exhibit clearly pronounced features characteristic of crystals with structural disorder and a broad distribution of a random internal electric field. The measurements were performed in dc, slowly varying (quasi-static), and pulsed electric fields. The experimental data obtained are used to construct the distribution function for the coercive field in the crystal bulk.

Phase transition and random-field induced domain wall response of SrTi O 3

The dielectric permittivity e′ - i\(\) of SrTi 18O 3 (STO18) is studied under a dc electric field E as a function of the temperature, T. In e′vs.T, a double-peak is found when 0 < E < 30 KV/m. While the peak at high-T is attributed to the smeared ferroelectric phase transition, the low-T one is induced by domain wall motion. The transverse Ising model including an external homogeneous and quenched random-fields is successfully used to describe both the smeared phase transition and the domain wall response in the low-T domain state. The calculations are in good agreement with the experimental results.

Dielectric and photoelectric properties of Pb(Mg1/3Ta2/3O3 crystals

The temperature dependence of the dielectric constant ɛ and the spectral dependence of the photoconductivity of Pb(Mg1/3Ta2/3)O3 crystals are investigated. The width of the band gap (3.4 eV) is determined. It is found that above the temperature of the maximum of the dielectric constant, its reciprocal 1/ɛ varies with temperature first quadratically and then according to a linear law, as is characteristic of ferroelectrics with a smeared phase transition. The smearing parameter of the transition estimated from the experimental data is close to the value calculated assuming the absence of long-range order in the arrangement of the Mg and Ta ions.

Relaxation effects in perovskite ferroelectric ceramics with smeared phase transition

In solid solutions with a perovskite structure PbZrO3-K0.5Bi0.5TiO3 and PbZrO3 — PbTiO3 the behaviours of low-frequency internal friction, dielectric permittivity and dielectric losses have been investigated at a change of temperature, bias electric field, as well as at isothermal annealing of samples. In structures, close to morphotropic phase transition, anomalous thermal hysteresis of dielectric losses, which is characterised by a long-term relaxation of metastable states is found out.

Equilibrium statistics of the one-dimensional Ising model for a solid solution of limited size

We treat the one-dimensional Ising model of a binary solution closed in a ring. We analyze the criteria for two-phase behavior of the model system. Bimodality of the distribution function of the atoms over clusters was used as one of the criteria. The effect of the size and dimensionality of the model on the form of the thermodynamic functions is traced. It is shown that in the one-dimensional Ising model of limited size there is a configurational change which can be treated as a smeared phase transition. The results of the present work obtained for a small one-dimensional model agree well with the results for two-dimensional small systems and are analogous to the behavior of three-dimensional systems.

Phase transitions for gallium microparticles in a porous glass

Studies of sound velocity and absorption supplemented with studies of heat capacity were carried out for the first time in the melting-freezing phase transition temperature range for Ga microparticles in a porous glass. The pronounced asymmetry of temperature behavior in cooling-heating processes was obtained, which was interpreted as the first order phase transition at cooling and the smeared phase transition at heating.

Pyroelectric properties and applications of piezoelectric PLZT ceramics

Abstract Pyroelectric and dielectric properties of piezoelectric ceramics have been studied in the broad temperature range 293–483 K. The effect of gamma (γ)-irradiation on the pyroelectric coefficient and figure of merit of piezoceramic samples has been examined for the smeared phase transition region. The results obtained are discussed in the framework of the model of “quasiferroelectricity.” It is shown that PLZT-6 type piezoceramics produce stable performance pyroelectric devices when exposed to large γ-irradiation doses.

Electrical resistivity of an exactly solvable random model exhibiting a smeared phase transition

The effect of impurities on the critical behaviour of the electrical resistivity due to spin-electron scattering is investigated within the context of the random mean spherical model, an exactly solvable model with one-dimensional disorder, exhibiting a smeared phase transition due to Griffiths singularities. It is shown that the singular part of the temperature derivative of the resistivity will have the same behaviour as the specific heat at the transition point, a relationship that is not self-evident for random systems.