arXiv subjects

A. T. Burkov

Publications and source records attributed to A. T. Burkov.

7 recordsLinked to original sources

arXiv2025

Orbital magnetic susceptibility of multifold fermions

Topological semimetals are intensively studied in recent years. Besides the well known Weyl and Dirac semimetals, some materials possess nodes with linear crossing of multiple bands. Low energy excitations around these nodes are called multifold fermions and can be described by $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian with pseudospin greater than 1/2. In the present work we investigate the contribution of these states into orbital magnetic susceptibility $χ$. We have found that, similarly to Weyl semimetals, the dependence of susceptibility on chemical potential $μ$ shows an extremum when $μ$ is close to the band crossing energy. In the case of half-integer pseudospin, this extremum is a minimum and the susceptibility is negative (diamagnetic). While in the case of integer pseudospin, the susceptibility is large and positive (paramagnetic) due to the contribution of dispersionless band, corresponding to zero pseudospin projection. This leads also to nonmonotonic temperature dependence of $χ$. As an example, we considered the case of cobalt monosilicide, where the states near the $Γ$ point correspond to pseudospin 1 without spin-orbital interaction, and to a combination of Weyl node and pseudospin-3/2 states taking into account spin-orbit coupling.

cond-mat.mtrl-sci↗

arXiv2020

Low-temperature thermal conductivity of Co$_{1-x}$M$_x$Si (M=Fe, Ni) alloys

We study the low-temperature electrical and thermal conductivity of CoSi and Co$_{1-x}$M$_x$Si alloys (M = Fe, Ni; $x \leq$ 0.06). Measurements show that the low-temperature electrical conductivity of Co$_{1-x}$Fe$_{x}$Si alloys decreases at $x > $ 0.01 by an order of magnitude compared with that of pure CoSi. It was expected that both the lattice and electronic contributions to thermal conductivity would decrease in the alloys. However, our experimental results revealed that at temperatures below 20K the thermal conductivity of Fe- and Ni-containing alloys is several times larger than that of pure CoSi. We discuss possible mechanisms of the thermal conductivity enhancement. The most probable one is related to the dominant scattering of phonons by charge carriers. We propose a simple theoretical model that takes into account the complex semimetallic electronic structure of CoSi with nonequivalent valleys, and show that it explains well the increase of the lattice thermal conductivity with increasing disorder and the linear temperature dependence of the thermal conductivity in the Co$_{1-x}$Fe$_x$Si alloys below 20K.

cond-mat.mtrl-sci↗

arXiv2018

The effect of energy-dependent electron scattering on thermoelectric transport in novel topological semimetal CoSi

These compounds have long been known as promising thermoelectric materials. Recently it was revealed, that they also have unconventional electronic topology. This renewed interest to the investigation of their transport properties. In order to improve theoretical description of thermoelectric transport in these compounds, we take into account electron scattering beyond commonly used constant relaxation time approximation. Using first principle calculations, we investigate the scattering of charge carriers by phonons and point defects. The dependence of the scattering rate on the energy correlates with that for the total density of states. This implies that in this material not only the intraband, but also the interband scattering is important, especially for bands with low density of states. The Seebeck coefficient and the electrical resistivity of CoSi and of dilute solid solutions Co$_{1-x}$M$_x$Si (M=Fe or Ni, $x<0.1$) are calculated as a function of temperature and the alloy composition. We show that the account of strong energy dependence of relaxation time is important for the description of experimentally observed rapid increase of the resistivity and qualitative change of its temperature dependence with the substitution of cobalt for iron, as well as for the description of the magnitude of the Seebeck coefficient, its temperature and composition dependence.

cond-mat.mes-hall↗

arXiv2017

Band structure and unconventional electronic topology of CoSi

Crystalline semimetals with certain space group symmetries may possess unusual electronic structure topology, distinct from the conventional Weyl and Dirac semimetals. Characteristic property of these materials is the existence of band-touching points with multiple (higher than two-fold) degeneracy and nonzero topological charge. CoSi is a representative of this group of materials exhibiting the so-called "new fermions". We report on an ab initio calculation of the electronic structure of CoSi using density functional methods, taking into account the spin-orbit interactions. We demonstrate the existence of band-touching nodes with four- and six-fold degeneracy, located at the $Γ$ and $R$ points in the first Brillouin zone and near the Fermi energy. We show that these band-touching points carry topological charges of $\pm 4$ and describe the resulting Fermi arc surface states, connecting the projections of these nodes onto the surface Brillouin zone. We also discuss the influence of many body $G_0W_0$ corrections on the electronic band structure and the topological properties of CoSi.

cond-mat.mes-hall↗

arXiv2016

Energy filtering enhancement of thermoelectric performance of nanocrystalline Cr-Si composites

We report on thermoelectric properties of nanocrystalline Cr$_{\rm 1-x}$Si$_{\rm x}$ composite films. As-deposited amorphous films were transformed into a nanocrystalline state with average grain size of 10--20~nm by annealing during in-situ thermopower and electrical resistivity measurements. The partially crystallized films, i.e. the films consisting of crystalline grains dispersed in the amorphous matrix, are a new type of the heterogeneous material where the nanocrystalline phase plays the role of scattering centers giving rise to a large contribution to the thermopower. We show that the thermopower enhancement is related to the energy dependent scattering (energy filtering) of the charge carriers on the nanograin interfaces.

cond-mat.mtrl-sci↗

arXiv2008

Effect of static and dynamic disorder on electronic transport of $RCo_2$ compounds: a study of $Ho(Al_xCo_{1-x})_2$ alloys

We present experimental results on thermoelectric power ({\em S}) and electrical resistivity ($ρ$) of pseudobinary alloys Ho(Al$_x$Co$_{1-x}$)$_2$ ($0 \leq x \leq 0.1 $), in the temperature range 4.2 K to 300 K. The work focuses on the effects of static (induced by alloying) and dynamic (induced by temperature) disorder on the magnetic state and electronic transport in a metallic system with itinerant metamagnetic instability. Spatial fluctuations of the local magnetic susceptibility in the alloys lead to a development of a partially ordered magnetic ground state of the itinerant 3d electron system. This results in a strong increase of the residual resistivity and a suppression of the temperature-dependent resistivity. Thermopower exhibits a complex temperature variation in both the magnetically ordered and in the paramagnetic state. This complex temperature variation is referred to the electronic density of states features in vicinity of Fermi energy and to the interplay of magnetic and impurity scattering. Our results indicate that the magnetic enhancement of the Co 3d-band in RCo$_{\rm 2}$--based alloys upon a substitution of Co by non-magnetic elements is mainly related to a progressive localization of the Co -- 3d electrons caused by disorder. We show that the magnitude of the resistivity jump at the Curie temperature for RCo$_{\rm 2}$ compounds exhibiting a first order phase transition is a non-monotonic function of the Curie temperature due to a saturation of the 3d--band spin fluctuation magnitude at high temperatures.

cond-mat.str-el↗

arXiv2004

Anomalous magnetotransport in (Y$_{1-x}$Gd$_{x}$)Co$_{2}$ alloys: interplay of disorder and itinerant metamagnetism

New mechanism of magnetoresistivity in itinerant metamagnets with a structural disorder is introduced basing on analysis of experimental results on magnetoresistivity, susceptibility, and magnetization of structurally disordered alloys (Y$_{1-x}$Gd$_{x}$)Co$_{2}$. In this series, YCo$_{2}$ is an enhanced Pauli paramagnet, whereas GdCo$_{2}$ is a ferrimagnet (T$_{\rm c}$=400 K) with Gd sublattice coupled antiferromagnetically to the itinerant Co-3d electrons. The alloys are paramagnetic for $x < 0.12$. Large positive magnetoresistivity has been observed in the alloys with magnetic ground state at temperatures T$<$T$_{\rm c}$. We show that this unusual feature is linked to a combination of structural disorder and metamagnetic instability of itinerant Co-3d electrons. This new mechanism of the magnetoresistivity is common for a broad class of materials featuring a static magnetic disorder and itinerant metamagnetism.

cond-mat.str-el↗