arXiv subjects

A. V. Rozhkov

Publications and source records attributed to A. V. Rozhkov.

At least 37 records · Page 2Linked to original sources

arXiv2021

Unconventional hysteretic transition in a charge density wave

Hysteresis underlies a large number of phase transitions in solids, giving rise to exotic metastable states that are otherwise inaccessible. Here, we report an unconventional hysteretic transition in a quasi-2D material, EuTe4. By combining transport, photoemission, diffraction, and x-ray absorption measurements, we observed that the hysteresis loop has a temperature width of more than 400 K, setting a record among crystalline solids. The transition has an origin distinct from known mechanisms, lying entirely within the incommensurate charge-density-wave (CDW) phase of EuTe4 with no change in the CDW modulation periodicity. We interpret the hysteresis as an unusual switching of the relative CDW phases in different layers, a phenomenon unique to quasi-2D compounds that is not present in either purely 2D or strongly-coupled 3D systems. Our findings challenge the established theories on metastable states in density wave systems, pushing the boundary of understanding hysteretic transitions in a broken-symmetry state.

cond-mat.mtrl-sci↗

arXiv2020

Spin density wave and electron nematicity in magic-angle twisted bilayer graphene

We study theoretically many-body properties of magic-angle twisted bilayer graphene for different doping levels. Our investigation is focused on the emergence, stability, and manifestations of nematicity of the ordered low-temperature electronic state. It is known that, at vanishing interactions, the low-energy spectrum of the system studied consists of four almost-flat almost-degenerate bands. Electron-electron repulsion lifts this degeneracy. To account for such an interaction effect, a numerical mean-field theory is used. Assuming that the ground state has spin-density-wave-like order, we introduce a multicomponent order parameter describing spin magnetization. Our simulations show that the order parameter structure depends on the doping level. In particular, doping away from the charge neutrality point reduces the rotational symmetry of the ordered state, indicating the appearance of an electron nematic state. Manifestations of the nematicity can be observed in the spatial distribution of the spin magnetization within a moir{é} cell, as well as in the single-electron band structure. The nematicity is strongest at half-filling (two extra electron or holes per supercell). We argue that nematic symmetry breaking is a robust feature of the system ground state, stable against model parameters variations. Specifically, it is shown that, away from the charge neutrality point, it persists for all three parametrizations of the interlayer hopping amplitudes discussed in the paper. Obtained theoretical results are consistent with the available experimental data.

cond-mat.str-el↗

arXiv2020

Can bilayer graphene become a fractional metal?

It is known that electron interactions can cause a perfect spin polarization of the Fermi surface of a metal. In such a situation only half of the non-interacting Fermi surface is available, and thus this phase is commonly referred to as a 'half-metal'. Here we argue that, in multi-band electronic systems with nesting, further 'fractionalization' of the Fermi surface is possible. Taking the AA bilayer graphene as a convenient test case, we demonstrate that, under suitable conditions imposed on the electron interactions, doped AA bilayer graphene can host a 'quarter-metal' state. In such a state, only one quarter of the non-interacting Fermi surface (Fermi contour) reaches the Fermi energy. At higher doping level, other 'fractional' metals can emerge. We briefly analyze the transport properties of these proposed phases.

cond-mat.str-el↗

arXiv2020

Competition of spatially inhomogeneous states in antiferromagnetic Hubbard model

In this work we study zero-temperature phases of the anisotropic Hubbard model on a three-dimensional cubic lattice in a weak coupling regime. It is known that, at half-filling, the ground state of this model is antiferromagnetic (commensurate spin-density wave). For non-zero doping, various types of spatially inhomogeneous phases, such as phase-separated states and the state with domain walls ("soliton lattice"), can emerge. Using the mean-field theory, we evaluate the free energies of these phases to determine which of them could become the true ground state in the limit of small doping. Our study demonstrates that the free energies of all discussed states are very close to each other. Smallness of these energy differences suggests that, for a real material, numerous factors, unaccounted by the model, may arbitrary shift the relative stability of the competing phases. This further implies that purely theoretical prediction of the true ground state in a particular many-fermion system is unreliable.

cond-mat.str-el↗

arXiv2020

Amplitude dynamics of charge density wave in LaTe$_3$: theoretical description of pump-probe experiments

We formulate a dynamical model to describe a photo-induced charge density wave (CDW) quench transition and apply it to recent multi-probe experiments on LaTe$_3$ [A. Zong et al., Nat. Phys. 15, 27 (2019)]. Our approach relies on coupled time-dependent Ginzburg-Landau equations tracking two order parameters that represent the modulations of the electronic density and the ionic positions. We aim at describing the amplitude of the order parameters under the assumption that they are homogeneous in space. This description is supplemented by a three-temperature model, which treats separately the electronic temperature, temperature of the lattice phonons with stronger couplings to the electronic subsystem, and temperature of all other phonons. The broad scope of available data for LaTe$_3$ and similar materials as well as the synergy between different time-resolved spectroscopies allow us to extract model parameters. The resulting calculations are in good agreement with ultra-fast electron diffraction experiments, reproducing qualitative and quantitative features of the CDW amplitude evolution during the initial few picoseconds after photoexcitation.

cond-mat.mes-hall↗

arXiv2020

Dynamical spin susceptibility of spin-valley half-metal

A few years ago we predicted theoretically that in systems with nesting of the Fermi surface the spin-valley half-metal has lower energy than the spin density wave state. In this paper we suggest a possible way to distinguish these phases experimentally. We calculate dynamical spin susceptibility tensor for both states in the framework of the Kubo formalism. Discussed phases have different numbers of the bands: four bands in the spin-valley half-metal and only two bands in the spin density wave. Therefore, their susceptibilities, as functions of frequency, have different number of peaks. Besides, the spin-valley half-metal does not have rotational symmetry, thus, in general the off-diagonal components of susceptibility tensor are non-zero. The spin density wave obeys robust rotational symmetry and off-diagonal components of the susceptibility tensor are zero. These characteristic features can be observed in experiments with inelastic neutron scattering.

cond-mat.str-el↗

arXiv2019

Many-body effects in twisted bilayer graphene at low twist angles

We study the zero-temperature many-body properties of twisted bilayer graphene with a twist angle equal to the so-called `first magic angle'. The system low-energy single-electron spectrum consists of four (eight, if spin label is accounted) weakly-dispersing partially degenerate bands, each band accommodating one electron per Moir{é} cell per spin projection. This weak dispersion makes electrons particularly susceptible to the effects of interactions. Introducing several excitonic order parameters with spin-density-wave-like structure, we demonstrate that (i)~the band degeneracy is partially lifted by the interaction, and (ii)~the details of the low-energy spectrum becomes doping-dependent. For example, at or near the undoped state, interactions separate the eight bands into two quartets (one quartet is almost filled, the other is almost empty), while for two electrons per Moiré cell, the quartets are pulled apart, and doublets emerge. When the doping is equal to one or three electrons per cell, the doublets split into singlets. Hole doping produces similar effects. As a result, electronic properties (e.g., the density of states at the Fermi energy) demonstrate oscillating dependence on the doping concentration. This allows us to reproduce qualitatively the behavior of the conductance observed recently in experiments [Cao et al., Nature {\bf 556}, 80 (2018)]. Near half-filling, the electronic spectrum loses hexagonal symmetry indicating the appearance of a many-body nematic state.

cond-mat.str-el↗

arXiv2019

Disorder correction to the Néel temperature of ruthenium-doped BaFe$_2$As$_2$: Theoretical analysis

We analyze theoretically nuclear magnetic resonance data for the spin-density wave phase in the ruthenium-doped BaFe$_2$As$_2$. Since inhomogeneous distribution of Ru~atoms introduces disorder into the system, experimentally observable random spatial variations of the spin-density wave order parameter emerge. Using perturbation theory for the Landau functional, we estimate the disorder-induced correction to the Néel temperature for this material. Calculated correction is significantly smaller than the Néel temperature itself for all experimentally relevant doping levels. This implies that the Néel temperature is quite insensitive to the disorder created by the dopants.

cond-mat.str-el↗

arXiv2018

Spin-valley half-metal in systems with Fermi surface nesting

Half-metals have fully spin polarized charge carriers at the Fermi surface. Such polarization usually occurs due to strong electron--electron correlations. Recently [Phys. Rev. Lett. {\bf{119}}, 107601 (2017)], we have demonstrated theoretically that adding (or removing) electrons to systems with Fermi surface nesting also stabilizes the half-metallic states even in the weak-coupling regime. In the absence of doping, the ground state of the system is a spin or charge density wave, formed by four nested bands. Each of these bands is characterized by charge (electron/hole) and spin (up/down) labels. Only two of these bands accumulate charge carriers introduced by doping, forming a half-metallic two-valley Fermi surface. Analysis demonstrates that two types of such half-metallicity can be stabilized. The first type corresponds to the full spin polarization of the electrons and holes at the Fermi surface. The second type, with antiparallel spins in electron-like and hole-like valleys, is referred to as a "spin-valley half-metal" and corresponds to the complete polarization with respect to the spin-valley operator. We analyze spin and spin-valley currents and possible superconductivity in these systems. We show that spin or spin-valley currents can flow in both half-metallic phases.

cond-mat.str-el↗

arXiv2018

Scattering on a rectangular potential barrier in nodal-line Weyl semimetals

We investigate single-particle ballistic scattering on a rectangular barrier in the nodal-line Weyl semimetals. Since the system under study has a crystallographic anisotropy, the scattering properties are dependent on mutual orientation of the crystalline axis and the barrier. To account for the anisotropy, we examine two different barrier orientations. It is demonstrated that, for certain angles of incidence, the incoming particle passes through the barrier with probability of unity. This is a manifestation of the Klein tunneling, a familiar phenomenon in the context of graphene and semimetals with Weyl points. However, the Klein tunneling in the Weyl-ring systems is observed when the angle of incidence differs from 90$^\circ$, unlike the cases of graphene and Weyl-point semimetals. The reflectionless transmission also occurs for the so-called `magic angles'. The values of `the magic angles' are determined by geometrical resonances between the barrier width and the de Broglie length of the scattered particle. In addition, we show that under certain conditions the wave function of the transmitted and reflected particles may be a superposition of two plane waves with unequal momenta. Such a feature is a consequence of the non-trivial structure of the iso-energy surfaces of the nodal-line semimetals.

cond-mat.mes-hall↗

arXiv2018

Spin-density wave state in simple hexagonal graphite

Simple hexagonal graphite, also known as AA graphite, is a metastable configuration of graphite. Using tight-binding approximation it is easy to demonstrate that AA graphite is a metal with well-defined Fermi surface. The Fermi surface consists of two sheets, each shaped like a rugby ball. One sheet corresponds to electron states, another corresponds to hole states. The Fermi surface demonstrates good nesting: a suitable translation in the reciprocal space superposes one sheet onto another. In the presence of the electron-electron repulsion a nested Fermi surface is unstable with respect to spin-density wave ordering. This instability is studied using the mean-field theory at zero temperature, and the spin-density wave order parameter is evaluated.

cond-mat.str-el↗

arXiv2017

Externally controlled band gap in twisted bilayer graphene

We theoretically study the effects of electron-electron interaction in twisted bilayer graphene in applied transverse dc electric field. When the twist angle is not very small, the electronic spectrum of the bilayer consists of four Dirac cones inherited from each graphene layer. Applied bias voltage leads to the appearance of two hole-like and two electron-like Fermi surface sheets with perfect nesting among electron and hole components. Such a band structure is unstable with respect to exciton band gap opening due to the screened Coulomb interaction. The exciton order parameter is accompanied by the spin-density-wave order. The value of the gap depends on the twist angle. More importantly, it can be controlled by applied bias voltage which opens new directions in manufacturing of different nanoscale devices.

cond-mat.str-el↗

arXiv2017

Spin-valley half-metal as a prospective material for spin-valley-tronics

Half-metallicity (full spin polarization of the Fermi surface) usually occurs in strongly correlated electron systems. We demonstrate that doping a spin-density wave insulator in the weak-coupling regime may also stabilize half-metallic states. The undoped spin-density wave is formed by four nested bands [i.e., each band is characterized by charge (electron/hole) and spin (up/down) labels]. Of these four bands only two accumulate the doped carriers, forming a half-metallic two-valley Fermi surface. Depending on parameters, the spin polarizations of the electron-like and hole-like valleys may be (i) parallel or (ii) antiparallel. The Fermi surface of (i) is fully spin-polarized (similar to usual half-metals). Case (ii), referred to as "a spin-valley half-metal", corresponds to complete polarization with respect to the spin-valley operator. The properties of these states are discussed.

cond-mat.str-el↗

arXiv2017

Magnetic field effects in electron systems with imperfect nesting

We analyze the effects of an applied magnetic field on the phase diagram of a weakly-correlated electron system with imperfect nesting. The Hamiltonian under study describes two bands: electron and hole ones. Both bands have spherical Fermi surfaces, whose radii are slightly mismatched due to doping. These types of models are often used in the analysis of magnetic states in chromium and its alloys, superconducting iron pnictides, AA-type bilayer graphene, borides, etc. At zero magnetic field, the uniform ground state of the system turns out to be unstable against electronic phase separation. The applied magnetic field affects the phase diagram in several ways. In particular, the Zeeman term stabilizes new antiferromagnetic phases. It also significantly shifts the boundaries of inhomogeneous (phase-separated) states. At sufficiently high fields, the Landau quantization gives rise to oscillations of the order parameters and of the Néel temperature as a function of the magnetic field.

cond-mat.str-el↗

arXiv2017

Single-electron gap in the spectrum of twisted bilayer graphene

We investigate the gap in the single-electron spectrum of twisted bilayer graphene. In a perfect infinite lattice of a twisted bilayer, the gap varies exponentially in response to weak changes of the twist angle. Such a large sensitivity makes theoretical predictions of the gap nearly impossible, since experimentally the twist angle is always known with finite accuracy. To address this issue, we numerically study finite clusters of twisted bilayer graphene. For finite systems, changing the twist angle causes a gradual crossover between gapless and gapped regimes. The crossover occurs when the finite-size quantization energy becomes comparable to the matrix elements responsible for the generation of the gap. We further argue that disorder scattering can induce similar crossover, in which the mean-free path plays the same role as the system size for the finite clusters. It is demonstrated that, to observe the gap experimentally, it is necessary to have a sample of suitable purity, and to possess the ability to tune the twist angle accurately.

cond-mat.mes-hall↗

arXiv2016

Electronic properties of graphene-based bilayer systems

This article reviews the theoretical and experimental work related to the electronic properties of bilayer graphene systems. Three types of bilayer stackings are discussed: the AA, AB, and twisted bilayer graphene. This review covers single-electron properties, effects of static electric and magnetic fields, bilayer-based mesoscopic systems, spin-orbit coupling, dc transport and optical response, as well as spontaneous symmetry violation and other interaction effects. The selection of the material aims to introduce the reader to the most commonly studied topics of theoretical and experimental research in bilayer graphene.

cond-mat.mes-hall↗

arXiv2016

Majorana fermion from Landau quantization in 2D topological superconductors

We study the generation of Majorana fermions in a two-dimensional topological superconductor placed in a transverse magnetic field B. A topological insulator/superconductor heterostructure and a two-dimensional p-wave superconductor are discussed. It is demonstrated that in these systems a single vortex creates two Majorana fermions, one hosted at the vortex core. The wave function of the second Majorana state is localized in the superconductor volume along a circle of radius $r^* \propto B^{-1}$ centered at the vortex core. In the case of many vortices, the sensitivity of $r^*$ to the magnetic field B may be used to control the coupling between the Majorana fermions. The latter property could be an asset for quantum computations.

cond-mat.mes-hall↗

arXiv2016

Heat Transport in Spin Chains with Weak Spin-Phonon Coupling

The heat transport in a system of $S=1/2$ large-$J$ Heisenberg spin chains, describing closely Sr$_2$CuO$_3$ and SrCuO$_2$ cuprates, is studied theoretically at $T\ll J$ by considering interactions of the bosonized spin excitations with optical phonons and defects. Treating rigorously the multi-boson processes, we derive a microscopic spin-phonon scattering rate that adheres to an intuitive picture of phonons acting as thermally populated defects for the fast spin excitations. The mean-free path of the latter exhibits a distinctive $T$-dependence reflecting a critical nature of spin chains and gives a close description of experiments. By the naturalness criterion of realistically small spin-phonon interaction, our approach stands out from previous considerations that require large coupling constants to explain the data and thus imply a spin-Peierls transition, absent in real materials.

cond-mat.str-el↗