arXiv subjects

Sergey Syzranov

Publications and source records attributed to Sergey Syzranov.

14 recordsLinked to original sources

arXiv2026

Spin correlations, low-energy scales, and anisotropy scaling in kagome frustrated magnets

Neutron scattering is central to identifying quantum states of magnetic materials. In the search for quantum spin liquids, broad spectral features of inelastic spectra have been cited as evidence for spinon excitations, but can also arise from magnon excitations excitations in the presence of quenched disorder and strong magnon interactions. We develop a new approach to this problem, based on the adiabatic continuity in the $XXZ$ Heisenberg model on geometrically frustrating (GF) lattices as a function of the model's anisotropy. Using this approach, we identify universal features and energies of finite-temperature spin correlators. Focusing on the kagome lattice, we show that the low-energy spin spectral function contains robust, momentum-independent peaks with frequencies: $\omega_1 \approx 3.4 T^*$ and $\omega_2 \approx 6.3 T^*$, where the ``hidden energy scale'' $T^*$ is the characteristic scale of a low-temperature peak in the heat capacity, at which many GF magnets also display spin-glass freezing. We show that the spectral features at low energies $\omega\lesssim T^*$ arise from single-magnon scattering and identify the magnetizations of the respective excitations. We explore the evolution of the spectral features with temperature and discuss extensions to other GF lattices. Our results provide a sharp spectroscopic criterion for interpreting neutron scattering in kagome and other GF quantum magnets.

cond-mat.str-el↗

arXiv2026

A footprint of zero-point entropy in higher-temperature magnetic thermodynamics

Identifying extensively degenerate zero-temperature states is key in characterizing spin-liquid-candidate materials and spin ices. In experiments, finding zero-point entropy (ZPE) is often attempted by measuring the entropy released by a material when cooled down from very high to very low temperatures. Such investigations are often unreliable and lead to controversial results because accessible temperatures may be insufficient to accurately capture essential low- and high-temperature features of magnetic materials. The purpose of this paper is to point out a simple, easily accessible signature of nonzero ZPE: the Maxwell's relation $\left(\partial S/\partial H\right)_T = \left(\partial M/\partial T\right)_H$ can appear violated if a vanishing ZPE is assumed incorrectly. This relation can further be used for estimating the ZPE. In many materials below characteristic temperatures, the criterion of non-vanishing ZPE has a particularly simple form: $\left(\frac{\partial C}{\partial H}\right)_T\left(\frac{\partial M}{\partial T}\right)_H<0$. We discuss these effects and the ZPE signature in the benchmark test case of the well-studied spin ice $Dy_2Ti_2O_7$.

cond-mat.mes-hall↗

arXiv2025

Low-temperature entropies and possible states in geometrically frustrated magnets

The entropy that an insulating magnetic material releases upon cooling can reveal important information about the properties of spin states in that material. In many geometrically frustrated (GF) magnetic compounds, the heat capacity exhibits a low-temperature peak that comes from the spin states continuously connected to the ground states of classical models, such as the Ising model, on the same GF lattice, which manifests in the amount of entropy associated with this heat-capacity peak. In this work, we simulate numerically the values of entropy released by higher-spin triangular-lattice layered systems and materials on SCGO lattices. We also compare the experimentally measured values of entropy in several strongly GF compounds, $NiGa_2S_4$, $FeAl_2Se_4$ and SCGO/BSZCGO, with possible theoretical values inferred from the classical models to which the quantum states of those materials may be connected. This comparison suggests that the lowest-energy states of higher-spin layered triangular-lattice compounds can be described in terms of doublet states on individual magnetic sites. Our analyses demonstrate how the values of entropy can reveal the structure of low-energy magnetic states in GF compounds and call for more accurate thermodynamic measurement in GF magnetic materials.

cond-mat.str-el↗

arXiv2025

Specific-heat anomaly in frustrated magnets with vacancy defects

Motivated by frustrated magnets and spin-liquid-candidate materials, we study the thermodynamics of a 2D geometrically frustrated magnet with vacancy defects. The presence of vacancies imposes significant constraints on the bulk spins, which freeze some of the degrees of freedom in the system at low temperatures. With increasing temperature, these constraints get relaxed, resulting in an increase in the system's entropy. This leads to the emergence of a peak in the heat capacity $C(T)$ of the magnet at a temperature $T_\text{imp}$ determined by the concentration of the vacancy defects. The entropy associated with this peak comes from the lowest-energy degrees of freedom in the material. To illustrate the emergence of such an anomaly, we compute analytically the heat capacity of the antiferromagnetic (AFM) Ising model on the triangular lattice with vacancy defects. The presence of the vacancy leads to a peak in $C(T)$ at the temperature $T_\text{imp}=-4J/\ln n_\text{imp}$, where $J$ is the AFM coupling between the spins and $n_\text{imp}$ is the fraction of the missing sites.

cond-mat.dis-nn↗

arXiv2024

Short-range order and hidden energy scale in geometrically frustrated magnets

In geometrically frustrated (GF) magnets, conventional long-range order is suppressed due to the presence of primitive triangular structural units, and the nature of the ensuing ground state remains elusive. One class of candidate states, extensively sought in experiments and vigorously studied theoretically, is the quantum spin liquid (QSL), a magnetically-disordered state in which all spins participate in a quantum-coherent many-body state. Randomly located impurities, present in all materials, may prevent QSL formation and instead lead to the formation of a spin-glass state. In this article, we review available data on the specific heat, magnetic susceptibility, and neutron scattering in GF materials. Such data show that a pure GF magnet possesses a characteristic ``hidden energy scale'' significantly exceeded by the other microscopic energy scales in the material. When cooled down to a temperature below the hidden energy scale, a GF material develops significant short-range order that dominates its properties and, in particular, dictates the spin-glass transition temperature for experimentally accessible impurity densities. We review the manifestations of short-range order in the commonly observed thermodynamics quantities in GF materials, possible scenarios for the hidden energy scale, and related open questions.

cond-mat.str-el↗

arXiv2024

Origin of the hidden energy scale and the $f$-ratio in geometrically frustrated magnets

Sufficiently clean geometrically frustrated (GF) magnets are the largest class of candidate materials that may host quantum spin liquids (QSLs). Some of them have been shown to exhibit spin-glass freezing, potentially precluding QSLs, at the "hidden energy scale", which is significantly lower than the microscopic energy scale of spin interactions. Here, we investigate the origin of the hidden energy scale and its relationship to the $f$-ratio, the figure of merit for the degree of frustration in GF magnetic materials. The available experimental and numerical data provide evidence that GF magnets display, universally, two distinct temperature scales in the specific heat, the lowest of which is of the order of the hidden energy scale $T^*$. We argue that this scale is determined by non-magnetic excitations, similar to spin exchanges in chains of spins. The collective entropy of such excitations matches the entropy of the ground states of the Ising model on the same lattice, which provides a way to verify the proposed scenario in experiment. We demonstrate that in the presence of quenched disorder, a broad class of materials exhibits spin-glass freezing at temperatures of order $T^*$, in accordance with experimental observations. As $T^*$ is a property of the clean GF medium, it leads to a constraint on the $f$-ratio.

cond-mat.str-el↗

arXiv2024

Quasispins of vacancy defects and their interactions in disordered antiferromagnets

Vacancy defects in disordered magnetic materials are known to act as effective spins, ``quasispins'', in response to an external magnetic field. In the dilute limit, the contributions of such ``quasispins'' to the magnetic susceptibility $\chi_\text{vac}(T)\propto N_\text{vac}/T$ are singular in the limit of low temperatures $T$ and match those of free spins. With increasing the density of vacancies, their interactions may become essential. Motivated by frustrated and quasi-one-dimensional magnetic materials, we study analytically quasispins and their interactions in a generic system that has short-range antiferromagnetic order and lacks long-range order. We predict that if the vacancy defect does not disrupt the short-range antiferromagnetic order around it, the quasispin value matches the value of spins of the magnetic atoms in the material, and the correlators of the quasispins of different vacancies match the spin-spin correlators in the vacancy-free material. We confirm our conclusions by exact calculations for Ising chains with nearest-neighbour and next-to-nearest-neighbour interactions. We also compute the first virial correction to the susceptibility of a magnetic material due to the interactions of vacancy quasispins.

cond-mat.dis-nn↗

arXiv2023

Weyl excitations via helicon-phonon mixing in conducting materials

Quasiparticles with Weyl dispersion can display an abundance of novel topological, thermodynamic and transport phenomena, which is why novel Weyl materials and platforms for Weyl physics are being intensively looked for in electronic, magnetic, photonic and acoustic systems. We demonstrate that conducting materials in magnetic fields generically host Weyl excitations due to the hybridisation of phonons with helicons, collective neutral modes of electrons interacting with electromagnetic waves propagating in the material. Such Weyl excitations are, in general, created by the interactions of helicons with longitudinal acoustic phonons. An additional type of Weyl excitation in polar crystals comes from the interaction between helicons and longitudinal optical phonons. Such excitations can be detected in X-ray and Raman scattering experiments. The existence of the Weyl excitations involving optical phonons in the bulk of the materials also leads to the formation of topologically protected surface arc states that can be detected via surface plasmon resonance.

cond-mat.mes-hall↗

arXiv2023

BCS-like disorder-driven instabilities and ultraviolet effects in nodal-line semimetals

We study the effects of quenched disorder on electrons in a 3D nodal-line semimetal. Disorder leads to significant renormalisations of the quasiparticle properties due to ultraviolet processes, i.e., processes of scattering in a large band of momenta, of the width exceeding the inverse mean free path. As a result, observables such as the density of states and conductivity exhibit singular behaviour in a broad range of disorder strengths, excluding a small vicinity of the singular point. We find that, for example, the density of quasiparticle states diverges as a function of the disorder strength $g$ as $ρ(g,E)\propto |g_c(E)-g|^{-2}|E|$ for $g$ smaller than the critical value $g_c(E)$ and crosses over to a constant for $g$ very close to $g_c(E)$, where $E$ is the quasiparticle energy. For certain disorder symmetries, a 3D disordered nodal-line semimetal can be mapped to a 2D metal with attractive interactions. The described disorder-driven instabilities in such a nodal-line semimetal are mapped to Cooper and exciton-condensation instabilities in a 2D metal. For other disorder symmetries, the respective instabilities are similar but not exactly dual. We discuss experimental conditions favourable for the observation of the described effects.

cond-mat.mes-hall↗

arXiv2023

Quasispins of vacancy defects in Ising chains with nearest- and next-to-nearest-neighbour interactions

Motivated by frustrated magnets and quasi-one-dimensional magnetic materials, we study the magnetic properties of 1D Ising chains with nearest-neighbour (NN) and weaker next-to-nearest neighbour (NNN) interactions in the presence of vacancy defects. The effect of a vacancy on the magnetic susceptibility of a spin chain is two-fold: it reduces the length of the chain by an effective ``vacancy size'' and may also act as a free spin, a ``quasispin'', with a Curie-type $χ_\text{quasi}=\langle S^2\rangle/T$ contribution to the susceptibility. In chains with antiferromagnetic short-range order, the susceptibility of vacancy-free chains is exponentially suppressed at low temperatures, and quasispins dominate the effect of impurities on the chains' magnetic properties. For chains with antiferromagnetic NN interactions, the quasispin matches the value $\langle S^2\rangle=1$ of the Ising spins in the chain for ferromagnetic NNN interactions and vanishes for antiferromagnetic NNN interactions. For chains with ferromagnetic short-range order, quasispin effects are insignificant due to exponentially large low-temperature susceptibilities, and the dominant effect of a vacancy is effectively changing the length of the chain.

cond-mat.mtrl-sci↗

arXiv2023

Interactions-disorder duality and critical phenomena in nodal semimetals, dilute gases and other systems

We investigate classes of interacting systems that allow for a mapping to disordered noninteracting systems. As we show, such a mapping is possible for interacting systems with a suppressed density of states at the chemical potential, leading to suppressed screening, and systems near BCS-type instabilities. The mapping can also be applied qualitatively to other classes of systems that are not exactly dual to each other. The established duality suggests a new approach to analytical and numerical studies of many-body and disorder-driven phenomena in a variety of systems and allows to predict, e.g., new phase transitions dual to the previously known ones. Using the established duality, we predict new disorder-driven transitions in nodal-line semimetals and systems with long-range hopping dual to, respectively, the BCS and BEC-vacuum transitions in interacting systems and new interaction-driven transitions dual to previously known non-Anderson disorder-driven transitions. The established principle can also be used to classify and describe phase transitions in dissipative systems described by non-Hermitian Hamiltonians.

cond-mat.mes-hall↗

arXiv2022

Weyl hydrodynamics in a strong magnetic field

We study the hydrodynamic transport of electrons in a Weyl semimetal in a strong magnetic field. Impurity scattering in a Weyl semimetal with two Weyl nodes is strongly anisotropic as a function of the direction of the field and is significantly suppressed if the field is perpendicular to the separation between the nodes in momentum space. This allows for convenient access to the hydrodynamic regime of transport, in which electron scattering is dominated by interactions rather than by impurities. In a strong magnetic field, electrons move predominantly parallel to the direction of the field, and the flow of the electron liquid in a Weyl-semimetal junction resembles the Poiseuille flow of a liquid in a pipe. We compute the viscosity of the Weyl liquid microscopically and find that it weakly depends on the magnetic field and has the temperature dependence $η(T)\propto T^2$. The hydrodynamic flow of the Weyl liquid can be generated by a temperature gradient. The hydrodynamic regime in a Weyl-semimetal junction can be probed via the thermal conductance $G_q(B,T)\propto B^2 T$ of the junction.

cond-mat.mes-hall↗

arXiv2020

Non-Anderson critical scaling of the Thouless conductance in 1D

We propose and investigate numerically a one-dimensional model which exhibits a non-Anderson disorder-driven transition. Such transitions have recently been attracting a great deal of attention in the context of Weyl semimetals, one-dimensional systems with long-range hopping and high-dimensional semiconductors. Our model hosts quasiparticles with the dispersion $\pm |k|^α\mathrm{sign} k$ with $α<1/2$ near two points (nodes) in momentum space and includes short-range-correlated random potential which allows for scattering between the nodes and near each node. In contrast with the previously studied models in dimensions $d<3$, the model considered here exhibits a critical scaling of the Thouless conductance which allows for {an accurate} determination of the critical properties of the non-Anderson transition, with a precision significantly exceeding the results obtained from the critical scaling of the density of states, usually simulated at such transitions. We find that in the limit of the vanishing parameter $\varepsilon=2α-1$ the correlation-length exponent $ν=2/(3|\varepsilon|)$ at the transition is inconsistent with the prediction $ν_{RG}=1/|\varepsilon|$ of the perturbative renormalisation-group analysis. Our results allow for a numerical verification of the convergence of $\varepsilon$-expansions for non-Anderson disorder-driven transitions and, in general, interacting field theories near critical dimensions.

cond-mat.mes-hall↗

arXiv2018

Dynamo Effect and Turbulence in Hydrodynamic Weyl Metals

The dynamo effect is a class of macroscopic phenomena responsible for generation and maintaining magnetic fields in astrophysical bodies. It hinges on hydrodynamic three-dimensional motion of conducting gases and plasmas that achieve high hydrodynamic and/or magnetic Reynolds numbers due to large length scales involved. The existing laboratory experiments modeling dynamos are challenging and involve large apparatuses containing conducting fluids subject to fast helical flows. Here we propose that electronic solid-state materials -- in particular, hydrodynamic metals -- may serve as an alternative platform to observe some aspects of the dynamo effect. Motivated by recent experimental developments, this paper focuses on hydrodynamic Weyl semimetals, where the dominant scattering mechanism is due to interactions. We derive Navier-Stokes equations along with equations of magneto-hydrodynamics that describe transport of Weyl electron-hole plasma appropriate in this regime. We estimate the hydrodynamic and magnetic Reynolds numbers for this system. The latter is a key figure of merit of the dynamo mechanism. We show that it can be relatively large to enable observation of the dynamo-induced magnetic field bootstrap in experiment. Finally, we generalize the simplest dynamo instability model -- Ponomarenko dynamo -- to the case of a hydrodynamic Weyl semimetal and show that the chiral anomaly term reduces the threshold magnetic Reynolds number for the dynamo instability.

cond-mat.str-el↗