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Vadim M. Loktev

Publications and source records attributed to Vadim M. Loktev.

At least 19 recordsLinked to original sources
arXiv2025

Impurity effects in twisted carbon nanotubes

We consider electronic spectra of twisted carbon nanotubes and their perturbation by impurity atoms absorbed at different positions on nanotube surface within the framework of Anderson hybrid model. A special attention is given to the cases when 1D Weyl (massless Dirac) modes are present in the nanotube spectrum and their hybridization with localized impurity states produces, with growing impurity concentration c, onset of a mobility gap near the impurity level and then opening, at yet higher c, of some narrow range of delocalized states within this mobility gap. Such behaviors are compared with similar effects in the previously studied 2D graphene, carbon nanoribbons, and non-twisted carbon nanotubes. Some possible practical applications are discussed.

cond-mat.mes-hall↗
arXiv2022

Impurity effects on Dirac modes in graphene armchair nanoribbons

We consider finite ribbons of graphene with armchair orientation of their edges to study in detail impurity effects on specific Dirac-like modes. In the framework of Anderson hybrid model of impurity perturbation, a possibility for Mott localization and for opening of a mobility gap under local impurity perturbations is found and analyzed in function of this model parameters: the impurity energy level, its hybridization with the host Dirac modes, and the impurity concentration. Possible electronic phase states in such disordered system and subsequent phase transitions between them are discussed.

cond-mat.dis-nn↗
arXiv2020

Impurity resonance effects in graphene $vs$ impurity location, concentration and sublattice occupation

Unique electronic band structure of graphene with its semi-metallic features near the charge neutrality point is sensitive to impurity effects. Using the Lifshitz and Anderson impurity models, we study in detail the disorder induced spectral phenomena in the electronic band structure of graphene, namely, the formation of resonances, quasi-gaps, bound states, impurity sub-bands, and their overall impact on the electronic band restructuring and the associated Mott-like metal-insulator transitions. We perform systematic analytical and numerical study for realistic impurities, both substitutional and adsorbed, focusing on those effects that stem from the impurity adatoms locations (top, bridge, and hollow positions), concentration, host sublattice occupation, perturbation strengths, etc. Possible experimental and practical implications are discussed as well.

cond-mat.mes-hall↗
arXiv2015

Biased doped silicene as a source for advanced electronics

Restructuring of electronic spectrum in a buckled silicene monolayer under some applied voltage between its two sublattices and in presence of certain impurity atoms is considered. A special attention is given to formation of localized impurity levels within the band gap and the to their collectivization at finite impurity concentration. It is shown that a qualitative restructuring of quasiparticle spectrum within the initial band gap and then specific metal-insulator phase transitions are possible for such disordered system and can be effectively controlled by variation of the electric field bias at given impurity perturbation potential and concentration. Since these effects are expected at low impurity concentrations but at not too low temperatures, they can be promising for practical applications in nanoelectronics devices.

cond-mat.dis-nn↗
arXiv2013

Impurity effects on electronic transport in ferropnictide superconductors

Effects of impurities and disorder on transport properties by electronic quasiparticles in superconducting iron pnictides are theoretically considered. The most prominent new features compared to the case of pure material should appear at high enough impurity concentration when a specific narrow band of conducting quasiparticle states can develop within the superconducting gap, around the position of localized impurity level by a single impurity center. The predicted specific threshold effects in the frequency dependent optical conductivity and temperature dependent thermal conductivity and also in Seebeck and Peltier coefficients can have interesting potentialities for practical applications.

cond-mat.supr-con↗
arXiv2013

Magnetic Dynamics of a Multiferroic with an Antiferromagnetic Layer

Shape effects in magnetic particles are widely studied, because of the ability of the shape and the size to control the parameters of a sample during its production. Experiments with nano-sized samples show that the shape can affect also the properties of antiferromagnetic (AFM) materials. However, the theoretical interpretation of these effects is under discussion. We propose a model to study the shape-induced effects in AFM particles at the AFM resonance frequency. The Lagrange function method is used to calculate the spectrum of resonance oscillations of the AFM vector in a synthetic multiferroic (piezoelectric + antiferromagnet). The influence of the specimen shape on the AFM resonance frequency in the presence of an external magnetic field is studied. Conditions for a resonance under the action of an external force or for a parametric resonance to arise in the magnetic subsystem are considered.

cond-mat.mes-hall↗
arXiv2012

Peculiarities of the stochastic motion in antiferromagnetic nanoparticles

Antiferromagnetic (AFM) materials are widely used in spintronic devices as passive elements (for stabilization of ferromangetic layers) and as active elements (for information coding). In both cases switching between the different AFM states depends in a great extent from the environmental noise. In the present paper we derive the stochastic Langevin equations for an AFM vector and corresponding Fokker-Planck equation for distribution function in the phase space of generalised coordinate and momentum. Thermal noise is modeled by a random delta-correlated magnetic field that interacts with the dynamic magnetisation of AFM particle. We analyse in details a particular case of the collinear compensated AFM in the presence of spin-polarised current. The energy distribution function for normal modes in the vicinity of two equilibrium states (static and stationary) in sub- and super-critical regimes is found. It is shown that the noise-induced dynamics of AFM vector has pecuilarities compared to that of magnetisation vector in ferromagnets.

cond-mat.mes-hall↗
arXiv2012

Symmetry and the macroscopic dynamics of antiferromagnetic materials in the presence of spin-polarized current

Antiferromagnetic (AFM) materials with zero or vanishingly small macroscopic magnetization are nowadays the constituent elements of spintronic devices. However, possibility to use them as active elements that show nontrivial controllable magnetic dynamics is still discussible. In the present paper we extend the theory [A.F.Andreev, V.I.Marchenko, Sov. Phys. --- Uspekhi, 23 (1980), 21] of macroscopic dynamics in AFMs for the cases typical for spin-valve devices. In particular, we consider the solid-like magnetic dynamics of AFMs with strong exchange coupling in the presence of spin-polarized current and give an expression for the current-induced Rayleigh dissipation function in terms of the rotation vector for different types %generalized potential of AFMs. Basing on the analysis of linearized equations of motion we predict the current-induced reorientation and AFM resonance, and found the values of critical currents in terms of AFMR frequencies and damping constants. We show the possibility of current-induced spin-diode effect and second-harmonic generation in AF layer. The proposed approach is generalized for the description of current-related phenomena in inhomogeneous AFMs.

cond-mat.mtrl-sci↗
arXiv2012

Magnetoelastic Coupling and Possibility of Spintronic Electromagnetomechanical Effects

Nanoelectromangetomechanical systems (NEMMS) open up a new path for the development of high speed autonomous nanoresonators and signal generators that could be used as actuators, for information processing, as elements of quantum computers etc. Those NEMMS that include ferromagnetic layers could be controlled by the electric current due to effects related with spin transfer. In the present paper we discuss another situation when the current-controlled behaviour of nanorod that includes an antiferro- (instead of one of ferro-) magnetic layer. We argue that in this case ac spin-polarized current can also induce resonant coupled magneto-mechanical oscillations and produce an oscillating magnetization of antiferromagnetic (AFM) layer. These effects are caused by \emph{i}) spin-transfer torque exerted to AFM at the interface with nonmagnetic spacer and by \emph{ii}) the effective magnetic field produced by the spin-polarized free electrons due to $sd$-exchange.The described nanorod with an AFM layer can find an application in magnetometry and as a current-controlled high-frequency mechanical oscillator.

cond-mat.mes-hall↗
arXiv2011

Adatom interaction effects in surface diffusion

Motivated by recent research of Nikitin et al. (J.Phys.D vol. 49, 055301(2009)), we examine the effects of interatomic interactions on adatom surface diffusion. By using a mean-field approach in the random walk problem, we derive a nonlinear diffusion equation and analyze its solutions. The results of our analysis are in good agreement with direct numerical simulations of the corresponding discrete model. It is shown that by analyzing a time dependence of adatom concentration profiles one can estimate the type and strength of interatomic interactions.

cond-mat.stat-mech↗
arXiv2011

Theory of a Narrow roton Absorption Line in the Spectrum of a Disk-Shaped SHF Resonator

We calculate the probability of the birth of a circular phonon (c-phonon) in He II by a c-photon of the resonator. It is shown that this probability has sharp maxima at frequencies, where the effective group velocity of the c-phonon is equal to zero; the density of states of c-phonons strongly grows at such frequencies. For He II, these frequencies correspond to a roton and a maxon. From the probability of the c-roton birth, we calculate the roto line width which is found to approximately agree with the experimental one. We conclude that the roton line observed in the super-high-frequency (SHF) absorption spectrum of helium is related to the birth of c-rotons. A possible interpretation of the Stark effect observed for the roton line is also proposed.

cond-mat.other↗
arXiv2010

Metal-insulator transition in hydrogenated graphene as manifestation of quasiparticle spectrum rearrangement of anomalous type

We demonstrate that the spectrum rearrangement can be considered as a precursor of the metal-insulator transition observed in graphene dosed with hydrogen atoms. The Anderson-type transition is attributed to the coincidence between the Fermi level and the mobility edge, which appearance is induced by the spectrum rearrangement. Available experimental data are thoroughly compared to the theoretical results for the Lifshitz impurity model.

cond-mat.dis-nn↗
arXiv2010

Electrical conductivity in graphene with point defects

The electrical conductivity of graphene containing point defects is studied within the binary alloy model in its dependence on the Fermi level position at the zero temperature. It is found that the minimal conductivity value does not have a universal character and corresponds to the impurity resonance energy rather than to the Dirac point position in the spectrum. The substantial asymmetry of the resulting dependence of the conductivity on the gate voltage magnitude is attributed as well to this same shift of the conductivity minimum to the resonance state energy.

cond-mat.dis-nn↗
arXiv2010

On the theory of magnetization in multiferroics: competition between ferro- and antiferromagnetic domains

Many technological applications of multiferroics are based on their ability to reconstruct the domain structure (DS) under the action of small external fields. In the present paper we analyze the different scenarios of the DS behavior in a multiferroic that shows simultaneously ferro- and antiferromagnetic ordering on the different systems of magnetic ions. We consider the way to control a composition of the DS and macroscopic properties of the sample by an appropriate field treatment. We found out that sensitivity of the DS to the external magnetic field and the magnetic susceptibility in a low-field region are determined mainly by the destressing effects (that have magnetoelastic origin). In a particular case of Sr$_{2}$Cu$_{3}$O$_{4}$Cl$_{2}$ crystal we anticipate the peculiarities of the elastic and magnetoelastic properties at $T\approx 100$ K.

cond-mat.mes-hall↗
arXiv2009

Spin transfer and current-induced switching in antiferromagnets

We present theoretical description of the precessional switching processes induced by simultaneous application of spin-polarized current and external magnetic field to antiferromagnetic component of the "pinned" layer. We found stability ranges of different static and dynamic regimes. We showed the possibility of steady current-induced precession of antiferromagnetic vector with frequency that linearly depends on the bias current. Furthermore, we found an optimal duration of current pulse required for switching between different orientations of antiferromagnetic vector and current and field dependence of switching time. Our results reveal the difference between dynamics of ferro- and antiferromagnets subjected to spin transfer torques.

cond-mat.other↗
arXiv2007

Shape-induced phenomena in the finite size antiferromagnets

It is of common knowledge that the direction of easy axis in the finite-size ferromagnetic sample is controlled by its shape. In the present paper we show that a similar phenomenon should be observed in the compensated antiferromagnets with strong magnetoelastic coupling. Destressing energy which originates from the long-range magnetoelastic forces is analogous to demagnetization energy in ferromagnetic materials and is responsible for the formation of equilibrium domain structure and anisotropy of macroscopic magnetic properties. In particular, crystal shape may be a source of additional uniaxial magnetic anisotropy which removes degeneracy of antiferromagnetic vector or artificial 4th order anisotropy in the case of a square cross-section sample. In a special case of antiferromagnetic nanopillars shape-induced anisotropy can be substantially enhanced due to lattice mismatch with the substrate. These effects can be detected by the magnetic rotational torque and antiferromagnetic resonance measurements.

cond-mat.stat-mech↗