arXiv subjects

Antonín Klíč

Publications and source records attributed to Antonín Klíč.

8 recordsLinked to original sources

arXiv2025

Quantum dynamics in symmetry-breaking states of correlated electrons: Antiferromagnetic phase

Symmetry-breaking phases in many-fermion systems are characterized by anomalous functions that represent transient processes during which some properties of free particles, such as spin or charge, are not conserved. Connecting the low-temperature symmetry-breaking phase with the high-temperature one within the Baym-Kadanoff scheme, beyond the static mean-field approximation, remains an unresolved, long-standing challenge. We identify the reason why approximations with critical dynamical fluctuations in the Schwinger-Dyson equation lead to a mismatch in the transition temperatures calculated from the high- and low-temperature phases. We propose a solution to this generic problem by excluding anomalous contributions to response functions that do not obey conservation of excitations in their interactions. We illustrate this behavior using the example of an antiferromagnetic state. We reveal that the spectral function in the antiferromagnetic phase exhibits a double-gap structure at zero temperature when the anomalous self-energy is frequency-dependent.

cond-mat.str-el↗

arXiv2023

Orientational ordering of water molecules confined in beryl: A theoretical study

We present an improved model for studying the interactions between dipole moments of water molecules confined in beryl crystals, inspired by recent NMR experiments. Our model is based on a local crystal potential with dihexagonal symmetry for the rotations of water dipole moments, leading to deflection from the $ab$ hexagonal crystallographic plane. This potential shape has significant implications for dipole ordering, which is linked to the non-zero projection of the dipole moment on the hexagonal $c$ axis. To reveal the tendency toward equilibrium-ordered states, we used a variational mean-field approximation, Monte Carlo simulations, and quantum tunneling. Our analysis reveals three types of equilibrium-ordered states: a purely planar dipole order with an antiparallel arrangement in the adjacent planes, a configuration with deflected dipole moments ordered in antiparallel directions, and a helical structure of the dipoles twisting along the $c$ axis.

cond-mat.mtrl-sci↗

arXiv2022

Zig-zag charged domain walls in ferroelectric PbTiO$_3$

We report a theoretical investigation of a charged 180$^\circ$ domain wall in ferroelectric PbTiO$_3$, compensated by randomly distributed immobile charge defects. For this we utilize atomistic shell-model simulations and continuous phase-field simulations in the framework of the Ginzburg-Landau-Devonshire model. We predict that domain walls form a zig-zag pattern and we discuss its properties in a broad interval of compensation-region widths, ranging from a couple to over a hundred nanometers.

cond-mat.mtrl-sci↗

arXiv2020

Antiferromagnetic fluctuations in the one-dimensional Hubbard model

We study the low-temperature critical behavior of the one-dimensional Hubbard model near half filling caused by enhanced antiferromagnetic fluctuations. We use a mean-field-type approximation with a two-particle self-consistency renormalizing the bare interaction. It allows us to control a transition from high to low temperatures as well as from weak to strong-coupling. We show that there is a crossover temperature $T_{0}= t\exp\{-1/Uρ(0)\}$ for arbitrary interaction $U>0$ and the bare density of states at the Fermi energy $ρ(0)>0$. The solution at lower temperatures goes over to strong coupling and approaches a quantum critical point with the diverging staggered susceptibility and a gap in the excitation spectrum at zero temperature.

cond-mat.str-el↗

arXiv2020

Curie-Weiss susceptibility in strongly correlated electron systems

The genesis of the Curie-Weiss magnetic response observed in most transition metals that are Fermi liquids at low temperatures has been an enigma for decades and has not yet been fully explained from microscopic principles. We show on the single-impurity Anderson model how the quantum dynamics of strong electron correlations leads to the Curie-Weiss magnetic susceptibility sufficiently above the Kondo temperature. Such behavior has not yet been demonstrated and can be observed only when the bare interaction is substantially screened (renormalized) and a balance between quantum and thermal fluctuations is kept. We set quantitative criteria for the existence of the Curie-Weiss law.

cond-mat.str-el↗

arXiv2019

Kondo Temperature and High to Low Temperature Crossover in Quantum Dots

Kondo temperature is standardly defined from the local zero-temperature susceptibility in the regime of strong electron correlations as a new scale controlling the low-temperature asymptotics of thermodynamic quantities. We show by using a two-particle self-consistent theory that the Kondo temperature can be identified as a crossover temperature at which the zero-temperature quantum fluctuations equal the thermal ones in the electron-hole correlation function. The high-temperature Curie-Weiss susceptibility is shown to go over to the Pauli one below the Kondo temperature.

cond-mat.str-el↗

arXiv2019

Strongly correlated electrons: Analytic mean-field theories with two-particle self-consistency

A two-particle self-consistency is rarely part of mean-field theories. It is, however, essential for avoiding spurious critical transitions and unphysical behavior. We present a general scheme for constructing analytically controllable approximations with self-consistent equations for the two-particle vertices based on the parquet equations. We explain in detail how to reduce the full set of parquet equations not to miss quantum criticality in strong coupling. We further introduce a decoupling of convolutions of the dynamical variables in the Bethe-Salpeter equations to make them analytically solvable. We connect the self-energy with the two-particle vertices to satisfy the Ward identity and the Schwinger-Dyson equation. We discuss the role of the one-particle self-consistency in making the approximations reliable in the whole spectrum of the input parameters. We demonstrate the general construction on the simplest static approximation that we apply to the Kondo behavior of the single-impurity Anderson model.

cond-mat.str-el↗

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↗