arXiv subjects

V. E. Valiulin

Publications and source records attributed to V. E. Valiulin.

10 recordsLinked to original sources

arXiv2026

Altermagnetism without a long-range order

The Kugel-Khomskii spin-pseudospin model, originally developed for transition-metal compounds with orbital degrees of freedom, has recently been reinterpreted in the context of altermagnetism. In this work, we theoretically investigate the emergence of altermagnetic behavior in the absence of long-range magnetic or orbital order. Using the rotation-invariant Green's function method for the SU(2) x SU(2) symmetric model on a square lattice and on a linear chain, we analyze spin-spin and spin-pseudospin correlation functions, excitation spectra, heat capacity, and susceptibilities. We show that beyond a critical intersubsystem exchange Kc(T), a composite state arises with nonzero spin-pseudospin correlations, even though the average spin and pseudospin at each site are zero. The excitation spectrum splits into acoustic and optical branches, with nodal lines along qx = qy - a direct signature of altermagnetic symmetry. A peak in heat capacity and a jump in susceptibility are observed at the phase boundary. In 1D, the phase boundary is nonmonotonic and demonstrates reentrant transition. These results establish the concept of an "altermagnetic paramagnet" or "altermagnetic liquid" without long-range order, relevant for low-dimensional and strongly fluctuating systems.

cond-mat.str-el↗

arXiv2026

Machine learning potential as a guide for eutectic in ultra-refractory multicomponent ceramics

The experimental determination of eutectic points is a long-established and widely used technique, but it is generally only practical for systems with relatively low melting points. Many modern, promising materials, however, are ultra-refractory, with melting points exceeding 3000 K. For these systems, conventional melting experiments become prohibitively expensive and technically challenging. Advanced AI modeling can serve as a powerful precursor to guide successful experimentation in such cases. This work proposes a novel criterion for determining the eutectic point concentration in ultra-refractory alloys. The approach is verified using the Ti-B-C system - the most thoroughly studied three-component refractory system to date. The core of the algorithm is a machine-learning interatomic potential, based on a neural network, which achieves accuracy comparable to ab initio methods. Crucially, the algorithm operates effectively in the liquid phase, eliminating the need for information about the solid alloy's crystalline structure to estimate eutectic points.

cond-mat.dis-nn↗

arXiv2026

Spherically symmetric approaches in the theoretical study of low-dimensional magnets

The main ideas and some of the most important results of the spherically symmetric self-consistent approach and a number of related theoretical algorithms are presented. These methods make it possible to study low-dimensional Heisenberg-type spin models, including frustrated ones, with careful consideration of the theoretic (Mermin-Wagner and Marshall) theorems, as well as the site spin constraint. Thus, the difficulties that may arise in the traditional analysis of low-dimensional magnetic systems are avoided. The approach can also be applied to the spin-pseudospin model, and is also embedded in more complex constructions when considering spin models with free carriers, such as the basic and three-band Hubbard models, t-J and s-d models, and the Kondo lattice.

cond-mat.str-el↗

arXiv2022

Time-dependent exchange creates the time-frustrated state of matter

Magnetic systems governed by exchange interactions between magnetic moments harbor frustration that leads to ground state degeneracy and results in the new topological state often referred to as a frustrated state of matter (FSM). The frustration in the commonly discussed magnetic systems has a spatial origin. Here we demonstrate that an array of nanomagnets coupled by the real retarded exchange interactions develops a new state of matter, time frustrated matter (TFM). In a spin system with the time-dependent retarded exchange interaction, a single spin-flip influences other spins not instantly but after some delay. This implies that the sign of the exchange interaction changes, leading to either ferro- or antiferromagnetic interaction, depends on time. As a result, the system's temporal evolution is essentially non-Markovian. The emerging competition between different magnetic orders leads to a new kind of time-core frustration. To establish this paradigmatic shift, we focus on the exemplary system, a granular multiferroic, where the exchange transferring medium has a pronounced frequency dispersion and hence develops the TFM.

cond-mat.str-el↗

arXiv2022

The resistance of quantum entanglement to temperature in the Kugel-Khomskii model

The Kugel--Khomskii model with entangled spin and orbital degrees of freedom is a good testing ground for many important features in quantum information processing, such as robust gaps in the entanglement spectra. Here, we demonstrate that the entanglement can be also robust under effect of temperature within a wide range of parameters. It is shown, in particular, that the temperature dependence of entanglement often exhibits a nonmonotonic behavior. Namely, there turn out to be ranges of the model parameters, where entanglement is absent at zero temperature, but then, with an increase in temperature, it appears, passes through a maximum, and again vanishes.

quant-ph↗

arXiv2020

Quantum entanglement, local indicators and effect of external fields in the Kugel-Khomskii model

Using the exact diagonalization technique, we determine the energy spectrum and wave functions for finite chains described by the two-spin (Kugel--Khomskii) model with different types of intersubsystem exchange terms. The found solutions provide a possibility to address the problem of quantum entanglement inherent to this class of models. We put the main emphasis on the calculations of the concurrence treated as an adequate numerical measure of the entanglement. We also analyze the behavior of two-site correlation functions considered as a local indicator of entanglement. We construct the phase diagrams of the models involving the regions of nonzero entanglement. The pronounced effect of external fields, conjugated to both spin variables on the regions with entanglement, could both enhance and weaken the entanglement depending on the parameters of the models.

cond-mat.str-el↗

arXiv2019

Magnetic spiral order in the square-lattice spin system (CuBr)Sr$_{2}$Nb$_{3}$O$_{10}$

We address quantum spin helical states in the strongly frustrated Heisenberg model. Contrary to conventional Dzyaloshinskii-Moriya approach we show that such states appear without central symmetry breaking. As an example, we demonstrate that the magnetic and thermodynamic properties of the quasi-two-dimensional square-lattice compound $\mathrm{(CuBr)Sr_{2}Nb_{3}O_{10}}$ can be interpreted within 2D $S = 1/2$ $J_1-J_2-J_3$ Heisenberg model. In this compound neutron experiment indicates helical spin order while central symmetry does hold.

cond-mat.mtrl-sci↗

arXiv2019

Continuous transformation between ferro and antiferro circular structures in $J_1-J_2-J_3$ frustrated Heisenberg model

Frustrated magnetic compounds, in particular low-dimensional, are topical research due to persistent uncover of novel nontrivial quantum states and potential applications. The problem of this field is that many important results are scattered over the localized parameter ranges, while areas in between still contain hidden interesting effects. We consider $J_1-J_2-J_3$ Heisenberg model on the square lattice and use the spherically symmetric self-consistent approach for spin-spin Green's functions in "quasielastic" approximation. We have found a new local order in spin liquids: antiferromagnetic isotropical helices. On the structure factor we see circular concentric dispersionless structures, while on any radial direction the excitation spectrum has "roton" minima. That implies nontrivial magnetic excitations and consequences in magnetic susceptibility and thermodynamics. On the $J_1-J_2-J_3$ exchange parameters globe we discover a crossover between antiferromagnetic-like local order and ferromagnetic-like; we find stripe-like order in the middle. In fact, our "quasielastic" approach allows investigation of the whole $J_1-J_2-J_3$ globe.

cond-mat.dis-nn↗

arXiv2019

Thermodynamics of symmetric spin--orbital model: One- and two-dimensional cases

The specific heat and susceptibilities for the two- and one-dimensional spin--orbital models are calculated in the framework of a spherically symmetric self-consistent approach at different temperatures and relations between the parameters of the system. It is shown that even in the absence of the long-range spin and orbital order, the system exhibits the features in the behavior of thermodynamic characteristics, which are typical of those manifesting themselves at phase transitions. Such features are attributed to the quantum entanglement of the coupled spin and orbital degrees of freedom.

cond-mat.str-el↗

arXiv2015

Thermodynamic properties of the 2D frustrated Heisenberg model for the entire $J_{1}-J_{2}$ circle

Using the spherically symmetric self-consistent Green's function method, we consider thermodynamic properties of the $S=1/2$ $J_1$-$J_2$ Heisenberg model on the 2D square lattice. We calculate the temperature dependence of the spin-spin correlation functions $c_{\mathbf{r}}=\langle S_{\mathbf{0}}^{z}S_{\mathbf{r}}^{z}\rangle $, the gaps in the spin excitation spectrum, the energy $E$ and the heat capacity $C_{V}$ for the whole $J_{1}$--$J_{2}$-circle, i.e. for arbitrary $φ$, $J_1=cos(φ)$, $J_2=sin(φ)$. Due to low dimension there is no long-range order at $T\neq 0$, but the short-range holds the memory of the parent zero-temperature ordered phase (antiferromagnetic, stripe or ferromagnetic). $E(φ)$ and $C_{V}(φ)$ demonstrate extrema "above" the long-range ordered phases and in the regions of rapid short-range rearranging. Tracts of $c_{\mathbf{r}}(φ)$ lines have several nodes leading to nonmonotonic $c_{\mathbf{r}}(T)$ dependence. For any fixed $φ$ the heat capacity $C_{V}(T)$ always has maximum, tending to zero at $T\rightarrow 0$, in the narrow vicinity of $φ= 155^{\circ}$ it exhibits an additional frustration-induced low-temperature maximum. We have also found the nonmonotonic behaviour of the spin gaps at $φ=270^{\circ}\pm 0$ and exponentially small antiferromagnetic gap up to ($T\lesssim 0.5$) for $φ\gtrsim 270^{\circ}$.

cond-mat.str-el↗