arXiv subjects

S. V. Syzranov

Publications and source records attributed to S. V. Syzranov.

At least 19 recordsLinked to original sources

arXiv2022

Effect of vacancy defects on geometrically frustrated magnets

Quenched disorder may prevent the formation of the widely sought quantum-spin-liquid states (QSLs) or mask their signatures by inducing a spin-glass state, which is why considerable experimental efforts are directed at purifying materials that may host QSLs. However, in geometrically frustrated (GF) magnets, the largest class of materials in which QSLs are sought, the glass-transition temperature $T_g$ grows with decreasing the density of vacancy defects, accompanied by a simultaneous growth of the magnetic susceptibility. In this paper, we develop a phenomenological theory of glass transitions and magnetic susceptibility in 3D geometrically frustrated (GF) magnetic materials. We consider a model of a GF magnet in which the glass transition occurs in the absence of vacancies, e.g., due to other types of quenched disorder. We show that disorder that creates weak local perturbations, e.g. weak random strain, leads to the growth of the transition temperature $T_g$. By contrast, vacancies reduce $T_g$ for small vacancy concentrations. Another consequence of the presence of vacancies is the creation of quasispins, effective magnetic moments localised near the vacancies, that contribute to the magnetic susceptibility of the system together with the bulk spins. We show that increasing the vacancy density leads to an increase of the total magnetic susceptibility.

cond-mat.mtrl-sci↗

arXiv2022

The Geometrically Frustrated Spin Glass (Fe1-pGap)2TiO5

The unusual anisotropy of the spin glass transition in the pseudobrookite system Fe$_2$TiO$_5$ has been interpreted as arising from an induced, van der Waals-like, interaction among magnetic clusters. Here we present susceptibility ($χ$) and specific heat data (C) for Fe2TiO5 diluted with non-magnetic Ga, (Fe$_{1-p}$Ga$_p$)$_2$TiO$_5$, for disorder parameter p = 0, 0.11, and 0.42, and elastic neutron scattering data for p = 0.20. A uniform suppression of T{_g} is observed upon increasing p, along with a value of $χ(T_g)$ that increases as T$_g$ decreases, i.e. $dχ(T_g)/dT_g< 0$. We also observe C(T) $\propto$ T$^2$ in the low temperature limit. The observed behavior places (Fe$_{1-p}$Ga$_p$)$_2$TiO$_5$ in the category of a strongly geometrically frustrated spin glass.

cond-mat.dis-nn↗

arXiv2022

Eminuscent phase in frustrated magnets: a challenge to quantum spin liquids

A geometrically frustrated (GF) magnet consists of localised magnetic moments, spins, whose orientation cannot be arranged to simultaneously minimise their interaction energies. Such materials may host novel fascinating phases of matter, such as fluid-like states called quantum spin liquids. GF magnets have, like all solid-state systems, randomly located impurities whose magnetic moments may ``freeze'' at low temperatures, making the system enter a spin-glass state. We analyse the available data for spin-glass transitions in GF materials and find a surprising trend: the glass-transition temperature grows with decreasing impurity concentration and reaches a finite value in the impurity-free limit at a previously unidentified, ``hidden'', energy scale. We propose a scenario in which the interplay of interactions and entropy leads to a crossover in the permeability of the medium that assists glass freezing at low temperatures. This low-temperature, ``eminuscent'', phase may obscure or even destroy the widely-sought spin-liquid states in rather clean systems.

cond-mat.str-el↗

arXiv2020

Fluctuation-Induced Interactions and the Spin Glass Transition in $Fe_2TiO_5$

We investigate the spin-glass transition in the strongly frustrated well-known compound $Fe_2TiO_5$. A remarkable feature of this transition, widely discussed in the literature, is its anisotropic properties: the transition manifests itself in the magnetic susceptibly only along one axis, despite $Fe^{3+}$ $d^5$ spins having no orbital component. We demonstrate, using neutron scattering, that below the transition temperature $T_g = 55 K$, $Fe_2TiO_5$ develops nanoscale surfboard shaped antiferromagnetic regions in which the $Fe^{3+}$ spins are aligned perpendicular to the axis which exhibits freezing. We show that the glass transition may result from the freezing of transverse fluctuations of the magnetization of these regions and we develop a mean-field replica theory of such a transition, revealing a type of magnetic van der Waals effect.

cond-mat.dis-nn↗

arXiv2020

Magnetotransport and internodal tunnelling in Weyl semimetals

Internodal dynamics of quasiparticles in Weyl semimetals manifest themselves in hydrodynamic, transport and thermodynamic phenomena and are essential for potential valleytronic applications of these systems. In an external magnetic field, coherent quasiparticle tunnelling between the nodes modifies the quasiparticle dispersion and, in particular, opens gaps in the dispersion of quasiparticles at the zeroth Landau level. We study magnetotransport in a Weyl semimetal taking into account mechanisms of quasiparticle scattering both affected by such gaps and independent of them. We compute the longitudal resistivity of a disordered Weyl semimetal with two nodes in a strong magnetic field microscopically and demonstrate that in a broad range of magnetic fields it has a strong angular dependence $ρ(η)\propto C_1+C_2 \cos^2η$, where $η$ is the angle between the field and the separation between the nodes in momentum space. The first term is determined by the coherent internodal tunnelling and is important only at angles $η$ close to $π/2$. This contribution depends exponentially on the magnetic field, $\propto \exp\left(-B_0/B\right)$. The second term is weakly dependent on the magnetic field for realistic concentrations of the impurities in a broad interval of fields.

cond-mat.mes-hall↗

arXiv2019

Duality between disordered nodal semimetals and systems with power-law hopping

Nodal semimetals (e.g. Dirac, Weyl and nodal-line semimetals, graphene, etc.) and systems of pinned particles with power-law interactions (trapped ultracold ions, nitrogen defects in diamonds, spins in solids, etc.) are presently at the centre of attention of large communities of researchers working in condensed-matter and atomic, molecular and optical physics. Although seemingly unrelated, both classes of systems are abundant with novel fundamental thermodynamic and transport phenomena. In this paper, we demonstrate that low-energy field theories of quasiparticles in semimetals may be mapped exactly onto those of pinned particles with excitations which exhibit power-law hopping. The duality between the two classes of systems, which we establish, allows one to describe the transport and thermodynamics of each class of systems using the results established for the other class. In particular, using the duality mapping, we establish the existence of a novel class of disorder-driven transitions in systems with the power-law hopping $\propto1/r^γ$ of excitations with $d/2<γ<d$, different from the conventional Anderson-localisation transition. Non-Anderson disorder-driven transitions have been studied broadly for nodal semimetals, but have been unknown, to our knowledge, for systems with long-range hopping (interactions) with $γ<d$.

cond-mat.mes-hall↗

arXiv2019

Interaction-induced transition in the quantum chaotic dynamics of a disordered metal

We demonstrate that a weakly disordered metal with short-range interactions exhibits a transition in the quantum chaotic dynamics when changing the temperature or the interaction strength. For weak interactions, the system displays exponential growth of the out-of-time-ordered correlator (OTOC) of the current operator. The Lyapunov exponent of this growth is temperature-independent in the limit of vanishing interaction. With increasing the temperature or the interaction strength, the system undergoes a transition to a non-chaotic behaviour, for which the exponential growth of the OTOC is absent. We conjecture that the transition manifests itself in the quasiparticle energy-level statistics and also discuss ways of its explicit observation in cold-atom setups.

cond-mat.mes-hall↗

arXiv2018

Adiabatic dechiralisation and thermodynamics of Weyl semimetals

We study thermodynamic manifestations of the chiral anomaly in disordered Weyl semimetals. We focus, in particular, on the effect which we call 'adiabatic dechiralization,' the phenomenon in which a change in temperature and/or an absorption or release of heat results from applying parallel electric and magnetic fields that change the imbalance of quasiparticles with different chiralities (at different Weyl nodes). This effect is similar to that of adiabatic demagnetization, which is commonly used as a method of low-temperature refrigeration. We describe this phenomenon quantitatively and discuss experimental conditions favorable for its observation. A related phenomenon, which we analyze and which is readily observable in experiments, is the dependency of the heat capacity of a Weyl semimetal on parallel electric and magnetic fields.

cond-mat.mes-hall↗

arXiv2017

Conductivity of a Weyl semimetal with donor and acceptor impurities

We study transport in a Weyl semimetal with donor and acceptor impurities. At sufficiently high temperatures transport is dominated by electron-electron interactions, while the low-temperature resistivity comes from the scattering of quasiparticles on screened impurities. Using the diagrammatic technique, we calculate the conductivity $σ(T,ω,n_A,n_D)$ in the impurities-dominated regime as a function of temperature $T$, frequency $ω$, and the concentrations $n_A$ and $n_D$ of donors and acceptors and discuss the crossover behaviour between the regimes of low and high temperatures and impurity concentrations. In a sufficiently compensated material [$|n_A-n_D|\ll(n_A+n_D)$] with a small effective fine structure constant $α$, $σ(ω,T)\propto T^2/(T^{-2}-iω\cdot\text{const})$ in a wide interval of temperatures. For very low temperatures or in the case of an uncompensated material the transport is effectively metallic. We discuss experimental conditions necessary for realising each regime.

cond-mat.mes-hall↗

arXiv2017

High-Dimensional Disorder-Driven Phenomena in Weyl Semimetals, Semiconductors and Related Systems

It is commonly believed that a non-interacting disordered electronic system can undergo only the Anderson metal-insulator transition. It has been suggested, however, that a broad class of systems can display disorder-driven transitions distinct from Anderson localisation that have manifestations in the disorder-averaged density of states, conductivity and other observables. Such transitions have received particular attention in the context of recently discovered 3D Weyl and Dirac materials but have also been predicted in cold-atom systems with long-range interactions, quantum kicked rotors and all sufficiently high-dimensional systems. Moreover, such systems exhibit unconventional behaviour of Lifshitz tails, energy-level statistics and ballistic-transport properties. Here we review recent progress and the status of results on non-Anderson disorder-driven transitions and related phenomena.

cond-mat.mes-hall↗

arXiv2017

Out-of-time-order correlators in finite open systems

We study out-of-time order correlators (OTOCs) of the form $\langle\hat A(t)\hat B(0)\hat C(t)\hat D(0)\rangle$ for a quantum system weakly coupled to a dissipative environment. Such an open system may serve as a model of, e.g., a small region in a disordered interacting medium coupled to the rest of this medium considered as an environment. We demonstrate that for a system with discrete energy levels the OTOC saturates exponentially $\propto \sum a_i e^{-t/τ_i}+const$ to a constant value at $t\rightarrow\infty$, in contrast with quantum-chaotic systems which exhibit exponential growth of OTOCs. Focussing on the case of a two-level system, we calculate microscopically the decay times $τ_i$ and the value of the saturation constant. Because some OTOCs are immune to dephasing processes and some are not, such correlators may decay on two sets of parametrically different time scales related to inelastic transitions between the system levels and to pure dephasing processes, respectively. In the case of a classical environment, the evolution of the OTOC can be mapped onto the evolution of the density matrix of two systems coupled to the same dissipative environment.

cond-mat.mes-hall↗

arXiv2016

Electron transport in nodal-line semimetals

We study the electrical conductivity in a nodal-line semimetal with charged impurities. The screening of the Coulomb potential in this system is qualitatively different from what is found in conventional metals or semiconductors, with the screened potential $ϕ$ decaying as $ϕ\propto 1/r^2$ over a wide interval of distances $r$. This unusual screening gives rise to a rich variety of conduction regimes as a function of temperature, doping level and impurity concentration. In particular, nodal-line semimetals exhibit a diverging mobility $\propto 1/|μ|$ in the limit of vanishing chemical potential $μ$, a linearly-increasing dependence of the conductivity on temperature, $σ\propto T$, and a large weak-localization correction with a strongly anisotropic dependence on magnetic field.

cond-mat.mes-hall↗

arXiv2016

Multifractality at non-Anderson disorder-driven transitions in Weyl semimetals and other systems

Systems with the power-law quasiparticle dispersion $ε_{\bf k}\propto k^α$ exhibit non-Anderson disorder-driven transitions in dimensions $d>2α$, as exemplified by Weyl semimetals, 1D and 2D arrays of ultracold ions with long-range interactions, quantum kicked rotors and semiconductor models in high dimensions. We study the wavefunction structure in such systems and demonstrate that at these transitions they exhibit fractal behaviour with an infinite set of multifractal exponents. The multifractality persists even when the wavefunction localisation is forbidden by symmetry or topology and occurs as a result of elastic scattering between all momentum states in the band on length scales shorter than the mean free path. We calculate explicitly the multifractal spectra in semiconductors and Weyl semimetals using one-loop and two-loop renormalisation-group approaches slightly above the marginal dimension $d=2α$.

cond-mat.mes-hall↗

arXiv2016

Critical exponents at the unconventional disorder-driven transition in a Weyl semimetal

Disordered non-interacting systems in sufficiently high dimensions have been predicted to display a non-Anderson disorder-driven transition that manifests itself in the critical behaviour of the density of states and other physical observables. Recently the critical properties of this transition have been extensively studied for the specific case of Weyl semimetals by means of numerical and renormalisation-group approaches. Despite this, the values of the critical exponents at such a transition in a Weyl semimetal are currently under debate. We present an independent calculation of the critical exponents using a two-loop renormalisation-group approach for Weyl fermions in $2-\varepsilon$ dimensions and resolve controversies currently existing in the literature.

cond-mat.mes-hall↗

arXiv2015

Disorder-driven transition in a chain with power-law hopping

We study a 1D system with a power-law quasiparticle dispersion $\propto |k|^α\sign k$ in the presence of a short-range-correlated random potential and demonstrate that for $α<1/2$ it exhibits a disorder-driven quantum phase transition with the critical properties similar to those of the localisation transition near the edge of the band of a semiconductor in high dimensions, studied in Refs. 1 and 2. Despite the absence of localisation in the considered 1D system, the disorder-driven transition manifests itself, for example, in a critical form of the disorder-averaged density of states. We confirm the existence of the transition by numerical simulations and find the critical exponents and the critical disorder strength as a function of $α$. The proposed system thus presents a convenient platform for numerical studies of the recently predicted unconventional high-dimensional localisation effects and has the potential for experimental realisations in chains of ultracold atoms in optical traps.

cond-mat.mes-hall↗

arXiv2015

Critical transport in weakly disordered semiconductors and semimetals

Motivated by Weyl semimetals and weakly doped semiconductors, we study transport in a weakly disordered semiconductor with a power-law quasiparticle dispersion $ξ_{\bf k}\propto k^α$. We show, that in $2α$ dimensions short-correlated disorder experiences logarithmic renormalisation from all energies in the band. We study the case of a general dimension $d$ using a renormalisation group, controlled by an $\varepsilon=2α-d$-expansion. Above the critical dimensions, conduction exhibits a localisation-delocalisation phase transition or a sharp crossover (depending on the symmetries of the Hamiltonian) as a function of disorder strength. We utilise this analysis to compute the low-temperature conductivity in Weyl semimetals and weakly doped semiconductors near and below the critical disorder point.

cond-mat.mes-hall↗

arXiv2015

Unconventional localisation transition in high dimensions

We study non-interacting systems with a power-law quasiparticle dispersion $ξ_{\bf k}\propto k^α$ and a random short-range-correlated potential. We show that, unlike the case of lower dimensions, for $d>2α$ there exists a critical disorder strength (set by the band width), at which the system exhibits a disorder-driven quantum phase transition at the bottom of the band, that lies in a universality class distinct from the Anderson transition. In contrast to the conventional wisdom, it manifests itself in, e.g., the disorder-averaged density of states. For systems in symmetry classes that permit localisation, the striking signature of this transition is a non-analytic behaviour of the mobility edge, that is pinned to the bottom of the band for subcritical disorder and grows for disorder exceeding a critical strength. Focussing on the density of states, we calculate the critical behaviour (exponents and scaling functions) at this novel transition, using a renormalisation group, controlled by an $\varepsilon=2α-d$ expansion. We also apply our analysis to Dirac materials, e.g., Weyl semimetal, where this transition takes place in physically interesting three dimensions.

cond-mat.mes-hall↗

arXiv2013

Strongly anisotropic Dirac quasiparticles in irradiated graphene

We study quasiparticle dynamics in graphene exposed to a linearly-polarized electromagnetic wave of very large intensity. Low-energy transport in such system can be described by an effective time-independent Hamiltonian, characterized by multiple Dirac points in the first Brillouin zone. Around each Dirac point the spectrum is anisotropic: the velocity along the polarization of the radiation significantly exceeds the velocity in the perpendicular direction. Moreover, in some of the points the transverse velocity oscillates as a function of the radiation intensity. We find that the conductance of a graphene p-n junction in the regime of strong irradiation depends on the polarization as $G(θ)\propto|\sinθ|^{3/2}$, where $θ$ is the angle between the polarization and the p-n interface, and oscillates as a function of the radiation intensity.

cond-mat.mes-hall↗