arXiv subjects

B. E. Vugmeister

Publications and source records attributed to B. E. Vugmeister.

7 recordsLinked to original sources

arXiv2023

Metastable states in the J1-J2 Ising model

We study the J1-J2 Ising model on the square lattice using the random local field approximation (RLFA) and Monte Carlo (MC) simulations for various values of the ratio p = J2/|J1| with antiferromagnetic coupling J2, ensuring spin frustration. RLFA predicts metastable states with zero order parameter (polarization) at low temperature for p \in (0,1). This is supported by our MC simulations, in which the system relaxes into metastable states with not only zero, but also with arbitrary polarization, depending on its initial value, external field, and temperature. We support our findings by calculating the energy barriers of these states at the level of individual spin flips relevant to the MC calculation. We discuss experimental conditions and compounds appropriate for experimental verification of our predictions.

cond-mat.mtrl-sci↗

arXiv2000

Comment on: "Phenomenological model of Dynamic Nonlinear Response in Relaxor Ferroelectrics

In a recent paper (A.E. Glazounov and A.K. Tagantsev, Phys.Rev. Lett. 85, 2192 (2000)) it has been proposed a phenomenological model of the dynamic non-linear response in relaxor ferroelectrics based on the averaging of the Landau-Khalatnikov equation over the distribution of the relaxation times. The purpose of the present comment is to clarify the physical assumptions involved in the Glazounov-Tagantsev approach and show that models based on the Landau-Khalatnikov equation can not describe properly the dynamic response in relaxor ferroelectrics.

cond-mat↗

arXiv1999

Coexistence of the Critical Slowing Down and Glassy Freezing in Relaxor Ferroelectrics

We have developed a dynamical model for the dielectric response in relaxor ferroelectrics which explicitly takes into account the coexistence of the critical slowing down and glassy freezing. The application of the model to the experiment in PMN allowed for the reconstruction of the nonequilibrium spin glass state order parameter and its comparison with the results of recent NMR experiment (Blinc et al., Phys. Rev. Lett. 83, No. 2 (1999)). It is shown that the degree of the local freezing is rather small even at temperatures where the field-cooled permittivity exceeds the frequency dependent permittivity by an order of magnitude. This observation indicates the significant role of the critical slowing down (accompanying the glass freezing) in the system dynamics. Also the theory predicts an important interrelationship between the frequency dependent permittivity and the zero-field-cooled permittivity, which proved to be consistent with the experiment in PMN (A. Levstik et. al., Phys. Rev. B 57, 11204 (1998)).

cond-mat.mtrl-sci↗

arXiv1997

Echo in Optical Lattices: Stimulated Revival of Breathing Oscillations

We analyze a stimulated revival (echo) effect for the breathing modes of the atomic oscillations in optical lattices. The effect arises from the dephasing due to the weak anharmonicity being partly reversed in time by means of additional parametric excitation of the optical lattice. The shape of the echo response is obtained by numerically simulating the equation of motion for the atoms with subsequent averaging over the thermal initial conditions. A qualitative analysis of the phenomenon shows that the suggested echo mechanism combines the features of both spin and phonon echoes.

cond-mat↗

arXiv1997

Dynamics of interacting clusters and dielectric response of relaxor ferroelectrics

The dielectric response in relaxor ferroelectrics is analyzed in the framework a model for the polarization dynamics in the presence of polar clusters. We associate the origin of polar clusters with the atoms displaced from their centro-symmerical positions even above $T_c$. Their collective hopping in multi-well potentials induced by disorder is analogous with the situation in glasses. The theory explicitly takes into account the distribution of cluster reorientation frequencies and the effect of cluster-cluster interactions in highly polarizable crystals, which we describe in terms of the local field distribution function. The dielectric constant is obtained from an integral master equation for the polarization dynamics in the presence of a time dependent electric field. The theory is applied for the analysis of the shape of the frequency dependent permittivity in the typical relaxor ferrolectrics $PST$ as a function of temperature. The comparison of the theory with experiment shows that in contrast to earlier assumptions, the observed Vogel-Fulcher dependence of the permittivity maximum is a consequence of the Vogel-Fulcher temperature dependence of the cluster reorientation frequency.

cond-mat↗

arXiv1997

Nonstationary Optimal Paths and Tails of Prehistory Probability Density in Multistable Stochastic Systems

The tails of prehistory probability density in nonlinear multistable stochastic systems driven by white Gaussian noise, which has been a subject of recent study, are analyzed by employing the concepts of nonstationary optimal fluctuations. Results of numerical simulations evidence that the prehistory probability density is non-Gaussian and highly asymmetrical that is an essential feature of noise driven fluctuations in nonlinear systems. We show also that in systems with the detail balance the prehistory probability density is the conventional transition probability that obeys the backward Kolmogorov equation.

cond-mat.stat-mech↗

arXiv1997

Logarithmic susceptibility and optimal control of large fluctuations

We analyze the probabilities of large infrequent fluctuations in systems driven by external fields. In a broad range of the field magnitudes, the logarithm of the fluctuation probability is linear in the field magnitude, and the response can be characterized by a logarithmic susceptibility. This susceptibility is used to analyze optimal control of large fluctuations. For nonadiabatic driving, the activation energies for nucleation and for escape of a Brownian particle display singular behavior as a function of the field shape.

cond-mat.stat-mech↗