arXiv subjects

A. V. Nikolaev

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

At least 19 recordsLinked to original sources

arXiv2026

Spin relaxation in $X$-wave magnets with $X=p, d, f, g, i$

Spin relaxation results in the spin decoherence and a finite spin lifetime, which are detrimental to spintronic devices. To achieve a long spin lifetime desirable for spintronic devices, elucidating the spin relaxation mechanism and factors influencing the spin lifetime is of vital importance. Here, we investigate the spin relaxation in $X$-wave magnets ($X=p, d, f, g, i$) with Rashba spin-orbit coupling within the framework of D'yakonov-Perel' mechanism. We calculate the general matrix of the spin relaxation time for an arbitrary Néel vector direction of the $X$-wave magnet. As an illustration, we study the spin relaxation for the Néel vector along the $[001]$ direction. It is found that the reciprocal spin-relaxation-time matrices are anisotropic and diagonal for the $d$-, $f$-, $g$- and $i$-wave magnets. For the $p$-wave magnet, we derive the analytical expressions for the temporal evolution of spins. Moreover, the spin relaxation rate is proportional to the momentum relaxation time, Rashba and altermagnetic spin-split strengths for all $X$-wave magnets. Our results shine more light on the fundamental understanding of the spin relaxation mechanism in $X$-wave magnets.

cond-mat.other↗

arXiv2026

Exact solution for a periodically driven magnetic multilayer system

Periodic driving serves as an effective method for controlling the properties of physical systems. Called "Floquet engineering," it is a broad field of theoretical and experimental activity. Whereas original Floquet theory was proposed to a system of ordinary differential equations, the quantum systems with time-dependent potential require using partial differential equations. Among different methods of analysis of such systems, time series is a most common one. Though general scheme was developed in a number of works, its application to specific problems often faces significant difficulties. In particular, the class of the problems describing magnetic multilayers with time-dependent potential (e.g., rotating magnetization of some of the layers) leads to significant complication of the problem due to two-component wave function and matching conditions at the interfaces. Taking as an example a two-layer system containing magnetic layer with rotating magnetization, we construct a class of solution containing arbitrary but finite number of the terms. The structure of the solution is analyzed. In particular, we show that boundary conditions, which seem a natural generalization of that for a stationary problem, cannot be imposed in the case of rotating magnetizations.

math-ph↗

arXiv2026

Alternative treatment of relativistic effects in linear augmented plane wave (LAPW) method: application to Ac, Th, ThO2 and UO2

We examine the influence of the relativistic effects within the linear augmented plane wave method (LAPW) for solids and propose a few alternative ways to accurately take them into account: (1) we introduce new radial dependencies for LAPW (Bloch-type) basis functions, based on two actual radial solutions of the Dirac equation for j=l-1/2 and j=l+1/2 states. The proposed radial 6p functions receive more weight from the Dirac p-1/2 solution and, due to this, can on average correctly describe completely filled $6p$ bands even without the additional p-1/2 local atomic function, as is done in the LAPW+p-1/2 method; (2) the canonical LAPW matrix elements for the spherically symmetric component of the potential, assuming non-relativistic radial wave functions, should be corrected; (3) we argue that for a realistic spin-orbit (SO) energy splitting of the semicore 6p-states the spin-orbit interaction constant zeta(p) should be calculated with the 6p-3/2 radial component, because the value of zeta(p) obtained with the canonical mixing of the 6p-1/2 and 6p-3/2 components overestimates the SO splitting. Different ways of taking into account relativistic effects can change the equilibrium lattice constant up to 0.15 A and the elastic modulus up to 26 GPa. We find that in the full treatment of the spin-orbit coupling UO2 has a small gap of forbidden states (0.2-0.4 eV) at the Fermi level, which persists for all k-vectors and, therefore, UO2 should be classified as a semimetal. We also discuss the peculiarities of the electron band structure of actinium, which result in an overestimation of its lattice constant.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic control of nonlinear transport induced by the quantum metric

The quantum geometry plays a crucial role in the nonlinear transport of quantum materials. Here, we use the Boltzmann transport formalism to study the magnetic control of nonlinear transport induced by the quantum metric in two-dimensional systems with different types of spin-orbit coupling (SOC). It is shown that the nonlinear conductivity is strongly dependent on the direction of a field and reveals significant spatial anisotropy. Moreover, the field-direction dependent relations are distinct for different SOCs. In addition, it is demonstrated that the contributions from the quantum metric and Drude mechanism are distinguishable due to their opposite signs or distinct anisotropy relations. We further derive the analytical formulas for the anisotropic nonlinear conductivity, in exact agreement with numerical results. Our work shines more light on the interplay between the nonlinear transport and quantum geometry.

cond-mat.mes-hall↗

arXiv2025

Anisotropic nonlinear transport in two-dimensional ferroelectrics

The longitudinal nonlinear response plays a crucial role in the nonreciprocal charge transport and may provide a simple electrical means to probe the spin-orbit coupling, magnetic order and polarization states, etc. Here, we report on a study on the polarization and magnetic field control of longitudinal nonlinear transport in two-dimensional (2D) ferroelectrics with in-plane polarization. Based on the Boltzmann transport theory, we first study that using a general Hamiltonian model and show that the nonlinear conductivity can be significantly tuned by the polarization and magnetic field. In addition, the nonlinear conductivity reveals a strong spatial anisotropy. We further derive the analytical formulas for the anisotropic nonlinear conductivity in exact accordance with numerical results. Then, we exemplify those phenomena in the 2D ferroelectric SnTe monolayer in the presence of an external magnetic field based on the density functional theory calculations. It is also revealed that the polarity of nonlinear conductivity is locked to the direction of the polarization, thus pointing to the possibility of the nonlinear detection of polarization states. Our work uncovers intriguing features of the longitudinal nonlinear transport in 2D ferroelectrics and provides guidelines for designing the polarization control of rectifying devices.

cond-mat.mes-hall↗

arXiv2024

Accounting all contributions for the Van Vleck paramagnetism and the Langevin diamagnetism from first principles: application to diamond

A general method for calculating magnetic susceptibility ($χ$) in dielectrics within a single choice of magnetic gauge for the whole crystal is presented. On the basis of the method, accounting for all contributions to the Van Vleck paramagnetism and Langevin (Larmore) diamagnetism, a full-scale ab initio calculation of $χ$ in diamond is performed. Unfamiliar contributions to $χ$ includes a Van Vleck contribution from the interstitial region and an offset contribution from the muffin-tin (MT) sphere, appearing due to the change of the MT-sphere magnetic moment when the sphere is displaced from the origin. Although the Langevin diamagnetism explicitly depends on the choice of the origin, its sum with the Van Vleck term remains invariant, which is demonstrated on the basis of the gauge invariance of the magnetic vector potential. The derived expressions have been applied to ab initio calculations of magnetic susceptibility of the crystalline diamond within the linear augmented plane wave method (LAPW). With the diamond unit cell having the inversion symmetry, the magnetic (Van Vleck) calculations require the irreducible part of the Brillouin zone accounting for half of the whole zone, i.e. 24 times larger than that in the absence of magnetic field. Investigating possible anisotropy of $χ$, we calculate it for 74 different directions of H (belonging to Lebedev surface grid points), and demonstrate that the actual value of $χ$ remain isotropic. The obtained volume magnetic susceptibility in diamond lies in the range $16.27-16.72 (with the Langevin contribution -39.22-39.94 and the Van Vleck contribution -22.94-23.22), in units 10^{-7}, which compares well with the experimental data and other calculations.

cond-mat.mtrl-sci↗

arXiv2023

Energy barriers of Be and B in passing through the C60 fullerene cage

We have studied the potential barriers for the penetration of atomic beryllium or boron inside the C60 fullerene by performing ab initio density functional theory (DFT) calculations with three variants for the exchange and correlation: B3LYP (hybrid functional), PW91 and PBE. Four principal trajectories to the inner part of C60 for the penetrating atom have been considered: through the center of six-member-carbon ring (hexagon), five-member-carbon ring (pentagon), and also through the center of the double C-C bond (D-bond) and the center of the single C-C bond (S-bond). Averaging over the three DFT variants yields the following barriers for beryllium penetrating inside a deformable fullerene: 3.2 eV (hexagon), 4.8 eV (S-bond), 5.3 eV (D-bond), 5.9~eV (pentagon). These barriers correspond to the slow and adiabatic penetration of Be, in contrast to the fast (non-adiabatic) penetration through the rigid cage of C60 resulting in 5.6 eV (hexagon), 16.3 eV (pentagon), 81.8 eV (S-bond) and 93.4 eV (D-bond). The potential barriers for the boron penetrating inside deformable/rigid C60 are: 3.7/105.4 eV (D-bond), 4.0/86.8 eV (S-bond), 4.7/7.8 eV (hexagon), 6.8/14.0 eV (pentagon). The potential barriers for Be and B escaping from the inner part of C$_{60}$ are higher by the value of 0.84 eV for Be and 0.81 eV for B. The considerable reduction of the potential barriers for the deformable fullerene is ascribed to the formation of the Be-C and B-C bonds. We discuss the difference between Be and B, compare three variants of DFT, and analyze the role of the dispersion interaction.

cond-mat.mes-hall↗

arXiv2022

Reply to Comment on "Multiple locations of boron atoms in the exohedral and endohedral C60 fullerene" by J. Xu and G.-L. Hou

In three out of five cases considered in our work, DFT calculations presented by Xu and Hou in their Comment give the same ground state confirmations. On the other hand, depending on the choice of the exchange-correlation functional, the geometry optimization within DFT results in different ground state confirmations for B@C60 and B60, Table I of the Comment. Therefore, the energy balance between nearest confirmations in these molecular complexes is subtle, and various methods can give different ground state structures. Consequently, the results of our method - the Hartree-Fock (HF) approach with the second order Møller-Plesset perturbation theory (MP2) - should be compared with the DFT results on equal ground, we cannot agree that the DFT method used in the Comment is superior to HF-MP2. In the Reply, we also present additional HF calculations with the 6-31G* basis set (used in the Comment for the geometry optimization) to show that the polarization functions do not change the ground state confirmations obtained by us earlier at the HF/6-31G level.

cond-mat.mtrl-sci↗

arXiv2022

Possible quadrupole-order-driven commensurate-incommensurate phase transition in B20 CoGe

The B20-type cobalt germanide CoGe was investigated by measuring the specific heat, resistivity, and $^{59}$Co nuclear magnetic resonance (NMR). We observed a phase transition at $T_Q=13.7$ K, evidenced by a very narrow peak of the specific heat and sharp changes of the nuclear spin-spin ($T_2^{-1}$) and spin-lattice ($T_1^{-1}$) relaxation rates. The fact that the entropy release is extremely small and the Knight shift is almost independent of temperature down to low temperatures as anticipated in a paramagnetic metal indicates that the $T_Q$ transition is of non-magnetic origin. In addition, we detected a crossover scale $T_0\sim30$ K below which the resistivity and the NMR linewidth increase, and $T_1^{-1}$ is progressively distributed in space, that is, a static and dynamical spatial inhomogeneity develops. While the order parameter for the $T_Q$ transition remains an open question, a group-theoretical analysis suggests that the finite electric quadrupole density arising from the low local site symmetry at cobalt sites could drive the crystal symmetry lowering from the P2$_1$3 symmetry that is commensurate to the R3 symmetry with an incommensurate wavevector, which fairly well accounts for the $T_Q$ transition. The quadrupole-order-driven commensurate-incommensurate phase transition may be another remarkable phenomenon arising from the structural chirality inherent in the noncentrosymmetric B20 family.

cond-mat.str-el↗

arXiv2021

Saddle point anomaly of Landau levels in graphenelike structures

Studying the tight binding model in an applied rational magnetic field (H) we show that in graphene there are very unusual Landau levels situated in the immediate vicinity of the saddle point (M-point) energy epsilon_M. Landau levels around $ε_M$ are broadened into minibands (even in relatively weak magnetic fields ~40-53 T) with the maximal width reaching 0.4-0.5 of the energy separation between two neighboring Landau levels though at all other energies the width of Landau levels is practically zero. In terms of the semiclassical approach a broad Landau level or magnetic miniband at epsilon_M is a manifestation of the so called self-intersecting orbit signifying an abrupt transition from the semiclassical trajectories enclosing the $Γ$ point to the trajectories enclosing the K point in the momentum space. Remarkably, the saddle point virtually does not affect the diamagnetic response of graphene, which is caused mostly by electron states in the vicinity of the Fermi energy ε_F. Experimentally, the effect of the broading of Landau levels can possibly be observed in twisted graphene where two saddle point singularities can be brought close to the Fermi energy.

cond-mat.mes-hall↗

arXiv2021

Fano-type effect in hydrogen-terminated pure nanodiamond

Two novel properties, unique for semiconductors: a negative electron affinity [1-2], and a high p-type surface electrical conductivity [3-4], were discovered in diamond at the end of the last century. Both properties appear when the diamond surface is hydrogenated. A natural question arises: is the influence of the surface hydrogen on diamond limited only to the electrical properties? Here, we report the first observation of a transparency peak at 1328 cm-1 in IR absorption of hydrogen-terminated pure (undoped) nanodiamonds. This new optical property is ascribed to Fano-type destructive interference between zone-center phonons and free carriers (holes) appearing in the near-surface layer of hydrogenated nanodiamond. Our work opens the way to exploring the physics of electron-phonon coupling in undoped diamonds and promises the application of the H-terminated nanodiamonds as a new optical material with an induced transparency in IR optical range.

physics.optics↗

arXiv2020

Ab initio based description of the unusual temperature increase of the electric field gradient at Ti sites in rutile TiO2

Combining a precise ab initio electron band structure calculation of the TiO2 rutile structure with the temperature evolution of the Ti mean-square displacements, we reproduce a puzzling temperature increase of the electric field gradient at Ti sites in TiO2, observed experimentally. Our method employs a procedure of averaging two quadrupole electron density components (L = 2) inside a sphere vibrating with the Ti nucleus at its center, where the key factor introducing the temperature dependence is the square root of the Debye-Waller factor. Although the Debye-Waller factor always reduces the corresponding Fourier component, in TiO2 due to the interplay between terms of opposite signs, it results in a net increase of the whole sum with temperature, leading to the growth of the electric field gradient. Quantitatively, we find that the increase of electric field gradient is only half of the experimental value, which we ascribe to anharmonic effects or a strong oxygen position influence. In addition, our method reproduces the unusual temperature dependence of the asymmetry parameter eta, which first decreases with temperature, goes to zero and then increases.

cond-mat.mtrl-sci↗

arXiv2020

Peculiar chemical bonding between thorium and a carbon hexagon in carbon nanomaterials

We explore an unusual nature of chemical bonding of the thorium atom with a ring of six carbon atoms (hexagon) in novel carbon materials. Our ab initio calculations of Th-based metallofullerenes (Th@C60, Th@C20) and Th bound to benzene or coronene at the Hartree-Fock level with the second order perturbation (MP2) correction accounting for the van der Waals interactions, demonstrate that the optimal position of the thorium atom is where it faces the center of a hexagon and is located at a distance of 2.01-2.07 A from the center. For Th encapsulated in C60 it is found at 2.01 A, whereas the other local energy minima are shifted to larger energies (0.22 eV and higher). Inside C60 the highest local minimum at 1.17 eV is observed when Th faces the center of the five member carbon ring (pentagon). Based on our calculations for Th with benzene and coronene where the global minimum for Th corresponds to its position at 2.05 A (benzene) or 2.02 A (coronene) from the hexagon center, we conclude that a well pronounced minimum is likely to present in graphene and in a single wall carbon nanotube. The ground state of Th is singlet, other high spin states (triplet and quintet) lie higher in energy (> 1.62 eV). We discuss a potential use of the carbon nanomaterials with the 229Th isotope having the nuclear transition of the optical range, for metrological purposes.

cond-mat.mtrl-sci↗

arXiv2020

Towards an ab initio theory for the temperature dependence of electric-field gradient in solids: application to hexagonal lattices of Zn and Cd

Based on ab initio band structure calculations we formulate a general theoretical method for description of the temperature dependence of electric field gradient in solids. The method employs a procedure of averaging multipole electron density component (l \neq 0) inside a sphere vibrating with the nucleus at its center. As a result of averaging each Fourier component (K \neq 0) on the sphere is effectively reduced by the square root of the Debye-Waller factor [exp(-W)]. The electric field gradient related to a sum of K-components most frequently decreases with temperature (T), but under certain conditions because of the interplay between terms of opposite signs it can also increase with T. The method is applied to calculations of the temperature evolution of the electric field gradients of pristine zinc and cadmium crystallized in the hexagonal lattice. For calculations within our model of crucial importance is the temperature dependence of mean-square displacements which can be taken from experiment or obtained from the phonon modes in the harmonic approximation. For the case of Zn we have used data obtained from single crystal x-ray diffraction. In addition, for Zn and Cd we have calculated mean-square displacements with the density functional perturbation treatment of the Quantum Espresso package. With the experimental data for displacements in Zn our calculations reproduce the temperature dependence of the electric field gradient very accurately. Within the harmonic approximation of the Quantum Espresso package the decrease of electric field gradients in Zn and Cd with temperature is overestimated. Our calculations indicate that the anharmonic effects are of considerable importance in the temperature dependence of electric field gradients.

cond-mat.mtrl-sci↗

arXiv2018

Kinetic Scalar Curvature Extended $f(R)$ Gravity

In this work we study a modified version of vacuum $f(R)$ gravity with a kinetic term which consists of the first derivatives of the Ricci scalar. We develop the general formalism of this kinetic Ricci modified $f(R)$ gravity and we emphasize on cosmological applications for a spatially flat cosmological background. By using the formalism of this theory, we investigate how it is possible to realize various cosmological scenarios. Also we demonstrate that this theoretical framework can be treated as a reconstruction method, in the context of which it is possible to realize various exotic cosmologies for ordinary Einstein-Hilbert action. Finally, we derive the scalar-tensor counterpart theory of this kinetic Ricci modified $f(R)$ gravity, and we show the mathematical equivalence of the two theories.

gr-qc↗

arXiv2018

Landau diamagnetism of the free electron gas as a Fermi surface effect

The diamagnetic response of the free electron gas called the Landau diamagnetism is a complex and elusive effect requiring laborious computations. Here based on the semi-classical treatment of the problem I present a clear picture of the Landau diamagnetism at zero temperature, which offers a simple derivation of this effect and leads to important consequences: 1) the diamagnetic response is due to electron states in a very narrow Fermi surface region in the k-space, 2) small Fermi energy oscillations in an applied magnetic field are caused by redistribution (inflow or outflow) of electrons from the equatorial region of the Fermi surface. The consideration is based on a structure called magnetic tube whose electron states surround a certain Landau level in k-space. A completely filled magnetic tube does not change its energy in an applied magnetic field as if it complied with the Bohr -- van Leeuwen theorem. The intersection of tubes with the Fermi surface leads to the appearance of partially occupied tubes in the region of intersection. The reconstruction of electron states in a magnetic field in this very small narrow region gives rise to the Landau diamagnetic response. In addition to the Landau diamagnetism this approach fully describes the oscillatory de Haas - van Alphen contribution to the magnetic susceptibility from the equatorial region of the Fermi sphere.

cond-mat.mes-hall↗

arXiv2018

Magnetic, electronic and transport properties of high-pressure-synthesized chiralmagnets Mn$_{1-x}$Rh$_x$Ge (B20)

We report on structural, magnetic and transport properties of a new set of the high-pressuresynthesized compounds Mn$_{1-x}$Rh$x$Ge ($0 \leq x \leq 1$) with the chiral magnetic ordering. The magnetic and transport properties depend substantially on the concentration of rhodium (x) and the pressure. The saturation magnetic moment corresponds to a known high-spin value for pristine MnGe (x = 0) and decreases almost linearly with increasing concentration $x$. In addition, XMCD spectra taken at 10 K and 2 T indicate magnetic polarization of the Rh 4d electron states and Ge $4p$ states, which decreases with $x$, too. In rhodium rich compounds ($x \geq 0.5$) the temperature of the magnetic ordering increases significantly with pressure, whereas in manganese rich compounds ($x < 0.5$) the temperature decreases. Three different tendencies are also found for several structural and transport properties. In the intermediate range ($0.3 \leq x \leq 0.7$) samples are semiconducting in the paramagnetic phase, but become metallic in the magnetically ordered state. We carried out ab initio density-functional calculations of Mn$_{1-x}$Rh$_x$Ge at various concentrations $x$ and traced the evolution of electronic and magnetic properties. The calculation results are in good agreement with the measured magnetic moments and qualitatively explain the observed trends in transport properties.

cond-mat.str-el↗

arXiv2017

Exact solutions for scalar field cosmology in f(R) gravity

We look for exact solutions in scalar field cosmology. To achieve this we use $f(R)$ modified gravity with a scalar field and do not specify the the form of the $f(R)$ function. In particular, we study Friedmann universe assuming that acceleration of the scalar curvature is negligible. We first present solutions for special cases and then the general solution. Using initial conditions which represent the universe at the present epoch, we evaluated the constants of integration. This allows for the comparison of the scale factor in the new solutions with that of the $ΛCDM$ solution, thereby affecting the age of the universe in $f(R)$ gravity.

gr-qc↗