arXiv subjects

Jiří Hlinka

Publications and source records attributed to Jiří Hlinka.

7 recordsLinked to original sources

arXiv2026

Soft Contrastive Learning for Unsupervised Discovery of Phases of Matter

A machine learning (ML) framework for phase-diagram construction in condensed matter systems with unknown phase structure is presented. The method combines physically motivated descriptors with contrastive representation learning and a two-stage data-generation workflow. First, a sparse set of configurations is generated by Monte Carlo simulated annealing in order to train a contrastive neural network that produces a low-dimensional embedding. Consequently, representative phase configurations are used as initial conditions for gradient-based optimization on a dense parameter grid, enabling efficient generation of refined datasets for high-resolution phase-diagram construction. In order to optimize the workflow, a variant of the method, in which derived descriptors are used as soft labels in the contrastive objective, is examined as well. The framework is demonstrated on a PbZrO3-inspired model potential formulated in terms of coupled order-parameter fields and used as a physically motivated testbed for exploring configuration diversity and phase behavior. The proposed approach provides a physically informed route to ML-assisted phase-diagram construction that integrates data analysis with adaptive dataset refinement.

cond-mat.dis-nn↗

arXiv2025

Pyramidal charged domain walls in ferroelectric BiFeO$_3$

Domain structures play a crucial role in the electric, mechanical and other properties of ferroelectric materials. In this study, we uncover the physical origins of the enigmatic zigzag domain structure in the prototypical multiferroic material BiFeO$_3$. Using phase-field simulations within the Landau-Ginzburg-Devonshire framework, we demonstrate that spatially-homogeneous defect charges result in domain structures that closely resemble those observed experimentally. The acquired understanding of the underlying physics of pyramidal-domain formation may enable the engineering of new materials with self-assembled domain structures exhibiting defined domain periodicity at the nanometre scale, opening avenues for advanced applications.

cond-mat.mtrl-sci↗

arXiv2024

Paradoxical creation of a polydomain pattern by electric field in BaTiO3 crystal

It is known that ferroelectric single crystals can be turned from a polydomain to a monodomain state by the application of an electric field. Here we report an unexpected opposite effect: the formation of through-the-crystal polydomain pattern in a monodomain BaTiO3 crystal in response to the applied electric field favoring the initial orientation of the polarization. The effect is achieved for special electric field direction which equally selects two domain states, which are present in the polydomain pattern. At the formation of the pattern, the new wedge domains propagate from the sides of the sample in the direction transverse to the electric field. The observations are rationalized in terms of a simple analytical model treating energies of competing domain configurations as functions of the electric field. The results of the analytical treatment are supported by phase field modeling.

cond-mat.mtrl-sci↗

arXiv2024

Thermal Stability and Topological Charge Fragmentation in Antiskyrmions of Rhombohedral Barium Titanate

Antiskyrmions, as topological quasi-particles, hold significant promise for spintronics and nanoscale data storage applications. Using molecular dynamics simulations based on effective Hamiltonians, we investigate the thermal stability of antiskyrmion nanodomains in rhombohedral barium titanate. At 1 K, antiskyrmions with a topological charge of -2 emerge as the most stable nanodomain state across all examined diameters. In our systematic study, the most robust antiskyrmion was found to have a diameter of 4 nm, maintaining its original size, shape, and topological charge up to the characteristic temperature T* ~ 85 K. Domains with diameters between 2.8 and approximately 4.5 nm exhibit fragmentation into six topological defects, termed quarks, each carrying a fractional skyrmion charge of -1/3. For domains larger than 4.5 nm, each topological quark splits into two pre-quarks, each with a charge of -1/6. These larger nanodomains demonstrate increased mobility and a growing tendency for shape and skyrmion charge fluctuations. Above the T* temperature, all larger nanodomains gradually shrink to a diameter of about 4 nm before collapsing into a single-domain state. These findings reveal the relatively high stability of antiskyrmions over a broad temperature range, even in the absence of a stabilizing bias field, and emphasize the pivotal role of topological quark dynamics. This establishes barium titanate as a key platform for exploring and applying topological phenomena.

cond-mat.mtrl-sci↗

arXiv2023

Antiskyrmionic ferroelectric medium

Typical magnetic skyrmion is a string of inverted magnetization within a ferromagnet, protected by a sleeve of a vortex-like spin texture, such that its cross-section carries a positive integer topological charge. Some magnets form antiskyrmions, the antiparticle strings which carry a negative topological charge instead. Here we demonstrate that topologically equivalent but purely electric antiskyrmion can exist in a ferroelectric material as well. In particular, our computer experiments reveal that the archetype ferroelectric, barium titanate, can host antiskyrmions. The polarization pattern around their cores reminds ring windings of decorative knots rather than the typical magnetic antiskyrmion texture. We show that the antiskyrmion of barium titanate has just 2-3 nm in diameter, a hexagonal cross-section, and an exotic topological charge of minus two. We deduce that formation of antiskyrmions is favored by a fortunate combination of the moderate anisotropy of the anharmonic electric susceptibility and the characteristic anisotropy of the polarization correlations in barium titanate crystals.

cond-mat.mtrl-sci↗

arXiv2022

Favorable band alignment for photocatalysis at the strontium germanate interface with silicon

Photocatalytic water splitting is a promising strategy for large-scale clean energy production. However, efficient and low-cost solid-state photocatalysts are still lacking. We present here first-principles calculations to investigate the suitability as photocathode of an epitaxial layer of strontium germanate on a Si(100) single crystal. Conduction and valence bands offsets at the interface between these two semiconductors were determined using state-of-the-art approximations of density functional theory for the accurate prediction of band alignments. The resulting type-III band line-up is also confirmed by inspection of the spatially resolved density of states. It is concluded that the electronic structure of the investigated heterostructure is favorable for photocathodic functionality.

cond-mat.mtrl-sci↗

arXiv2017

First-principles based Landau-Devonshire potential for BiFeO$_3$

The work describes a first-principles-based computational strategy for studying structural phase transitions, and in particular, for determination of the so-called Landau-Devonshire potential - the classical zero-temperature limit of the Gibbs energy, expanded in terms of order parameters. It exploits the configuration space attached to the eigenvectors of the modes frozen in the ground state, rather than the space spanned by the unstable modes of the high-symmetry phase, as done usually. This allows us to carefully probe the part of the energy surface in the vicinity of the ground state, which is most relevant for the properties of the ordered phase. We apply this procedure to BiFeO$_3$ and perform ab-initio calculations in order to determine potential energy contributions associated with strain, polarization and oxygen octahedra tilt degrees of freedom, compatible with its two-formula unit cell periodic boundary conditions.

cond-mat.mtrl-sci↗