arXiv subjects

V. A. Abalmasov

Publications and source records attributed to V. A. Abalmasov.

3 recordsLinked to original sources

arXiv2025

Hydrogen modes in KH$_2$PO$_4$ under pressure from ab initio calculation and inelastic neutron scattering

The nature of the phonon triplet in the region of OH-stretching modes in hydrogen-bonded materials is often explained by the interplay of OH-stretching modes and combinations and overtones of OH-bending modes. In order to elucidate the both contributions in KH$_2$PO$_4$ (KDP), we compare the pressure dependence of the OH-bending and stretching modes from ab initio calculation and inelastic neutron scattering (INS) measurements. The ab initio calculation predicts a hardening of OH-bending modes and a softening of OH-stretching modes with pressure. At the same time, INS measurements in the region of OH-stretching modes indicate a hardening of the phonon triplet together with the bending modes. This means that this triplet in INS measurements is mainly due to combinations and overtones of OH-bending modes, while the intensity of OH-stretching modes appears to be relatively low. This conclusion may also apply to other hydrogen-bonded materials.

cond-mat.mtrl-sci↗

arXiv2024

Free energy and metastable states in the square-lattice $J_1$-$J_2$ Ising model

Free energy as a function of polarization is calculated for the square-lattice $J_1$-$J_2$ Ising model for $J_2 < |J_1|/2$ using the random local field approximation (RLFA) and Monte Carlo (MC) simulations. Within RLFA, it reveals a metastable state with zero polarization in the ordered phase. In addition, the Landau free energy calculated within RLFA indicates a geometric slab-droplet phase transition at low temperature, which cannot be predicted by the mean field approximation. In turn, restricted free energy calculations for finite-size samples, exact and using MC simulations, reveal metastable states with a wide range of polarization values, but with only two domains. Taking into account the dependence of the restricted free energy on the nearest-neighbor correlations allows us to identify several more metastable states. The calculations also reveal additional slab-droplet transitions at $J_2 > |J_1|/4$. These findings enrich our knowledge of the $J_1$-$J_2$ Ising model and the RLFA as a useful theoretical tool to study phase transitions in spin systems.

cond-mat.mtrl-sci↗

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↗