arXiv subjects

A. Yu. Kuntsevich

Publications and source records attributed to A. Yu. Kuntsevich.

At least 37 records · Page 2Linked to original sources

arXiv2020

Simple mechanism that breaks the Hall effect linearity at low temperatures

Hall resistance $R_{\textrm{xy}}$ is commonly suggested to be linear-in-magnetic-field $B$, provided the field is small. We argue here that at low temperatures this linearity is broken due to weak localization/antilocalization phenomena in inhomogeneous systems, while in a uniform medium these effects do not affect the linear field dependence of $R_{\textrm{xy}}(B)$. We calculate the Hall resistance for different two-component media using a mean-field approach and show that this non-linearity is experimentally observable.

cond-mat.mes-hall↗

arXiv2019

Strain-driven nematicity of the odd-parity superconductivity in Sr$_x$Bi$_2$Se$_3$

We present a novel experimental evidence for the odd-parity nematic superconductivity in high-quality single crystals of doped topological insulator Sr$_x$Bi$_2$Se$_3$. The X-ray diffraction shows that the grown single crystals are either weakly stretched or compressed uniaxially in the basal plane along one of the crystal axis. We show that in the superconducting state, the upper critical magnetic field $H_{c2}$ has a two-fold rotational symmetry and depends on the sign of the strain: in the stretched samples, the maximum of $H_{c2}$ is achieved when the in-plane magnetic field is transverse to the strain axis, while in the compressed samples this maximum is observed when the field is along the strain direction. This result is naturally explained within a framework of the odd-parity nematic superconductivity coupled to the strain. Magnetoresistance in the normal state is independent of the current direction and also has a two-fold rotational symmetry that demonstrates the nematicity of the electronic system in the normal state.

cond-mat.supr-con↗

arXiv2019

Density of states measurements for heavy subband of holes in HgTe quantum wells

Valence band in narrow HgTe quantum wells contains well-conductive Dirac-like light holes at the $Γ$ point and poorly conductive heavy hole subband located in the local valleys. Here we propose and employ two methods to measure the density of states for these heavy holes. The first method uses a gate-recharging technique to measure thermodynamical entropy per particle. As the Fermi level is tuned with gate voltage from light to heavy subband, the entropy increases dramatically, and the value of this increase gives an estimate for the density of states. The second method determines the density of states for heavy holes indirectly from the gate voltage dependence of the period of the Shubnikov-de Haas oscillations for light holes. The results obtained by both methods are in the reasonable agreement with each other. Our approaches can be applied to measure large effective carrier masses in other two-dimensional gated systems.

cond-mat.mes-hall↗

arXiv2019

Effect of Sr doping on structure, morphology and transport properties of Bi$_2$Se$_3$ epitaxial thin films

We report molecular beam epitaxy growth of Sr-doped Bi$_2$Se$_3$ films on (111) BaF$_2$ substrate, aimed to realize unusual superconducting properties inherent to Sr$_x$Bi$_2$Se$_3$ single crystals. Despite wide range of the compositions, we do not achieve superconductivity. To explore the reason for that we study structural, morphological and electronic properties of the films and compare them to the corresponding properties of the single crystals. The dependence of the c-lattice constant in the films on Sr content appears to be more than an order of magnitude stronger than in the crystals. Correspondingly, all other properties also differ substantially, indicating that Sr atoms get different positions in lattices. We argue that these structural discrepancies come from essential differences in growth conditions. Our research calls for more detailed structural studies and novel growth approaches for design of superconducting Sr$_x$Bi$_2$Se$_3$ thin films.

cond-mat.mtrl-sci↗

arXiv2018

Probing spin susceptibility of a correlated two-dimensional electron system by transport and magnetization measurements

We report temperature and density dependences of the spin susceptibility of strongly interacting electrons in Si inversion layers. We measured (i) the itinerant electron susceptibility $χ^*$ from the Shubnikov-de Haas oscillations in crossed magnetic fields and (ii) thermodynamic susceptibility $χ_{\rm T}$ sensitive to all the electrons in the layer. Both $χ^*$ and $χ_{\rm T}$ are strongly enhanced with lowering the electron density in the metallic phase. However, there is no sign of divergency of either quantity at the density of the metal-insulator transition $n_c$. Moreover, the value of $χ_{\rm T}$, which can be measured across the transition down to very low densities deep in the insulating phase, increases with density at $n<n_c$, as expected. In the absence of magnetic field, we found the temperature dependence of $χ^*$ to be consistent with Fermi-liquid-based predictions, and to be much weaker than the power-law, predicted by non-Fermi-liquid models. We attribute a much stronger temperature dependence of $χ_{\rm T}$ to localized spin droplets. In strong enough in-plane magnetic field, we found the temperature dependence of $χ^*$ to be stronger than that expected for the Fermi liquid interaction corrections.

cond-mat.str-el↗

arXiv2018

Structural distortion behind the nematic superconductivity in Sr$_x$Bi$_2$Se$_3$

An archetypical layered topological insulator Bi$_2$Se$_3$ becomes superconductive upon doping with Sr, Nb or Cu. Superconducting properties of these materials in the presence of in-plane magnetic field demonstrate spontaneous symmetry breaking: 180$^\circ$-rotation symmetry of superconductivity versus 120$^\circ$-rotation symmetry of the crystal. Such behavior brilliantly confirms nematic topological superconductivity. To what extent this nematicity is due to superconducting pairing in these materials, rather than due to crystal structure distortions? This question remained unanswered, because so far no visible deviations from the 3-fold crystal symmetry were resolved in these materials. To address this question we grow high quality single crystals of Sr$_x$Bi$_2$Se$_3$, perform detailed X-ray diffraction and magnetotransport studies and reveal that the observed superconducting nematicity direction correlates with the direction of small structural distortions in these samples( $\sim 0.02$\% elongation in one crystallographic direction). Additional anisotropy comes from orientation of the crystallite axes. 2-fold symmetry of magnetoresistance observed in the most uniform crystals well above critical temperature demonstrates that these structural distortions are nevertheless strong enough. Our data in combination with strong sample-to-sample variation of the superconductive anisotropy parameter are indicative for significance of the structural factor in the apparent nematic superconductivity in Sr$_x$Bi$_2$Se$_3$.

cond-mat.supr-con↗

arXiv2018

Quantum Interference Controls the Electron Spin Dynamics in n-GaAs

Manifestations of quantum interference effects in macroscopic objects are rare. Weak localization is one of the few examples of such effects showing up in the electron transport through solid state. Here we show that weak localization becomes prominent also in optical spectroscopy via detection of the electron spin dynamics. In particular, we find that weak localization controls the free electron spin relaxation in semiconductors at low temperatures and weak magnetic fields by slowing it down by almost a factor of two in $n$-doped GaAs in the metallic phase. The weak localization effect on the spin relaxation is suppressed by moderate magnetic fields of about 1 T, which destroy the interference of electron trajectories, and by increasing the temperature. The weak localization suppression causes an anomalous decrease of the longitudinal electron spin relaxation time $T_1$ with magnetic field, in stark contrast with well-known magnetic field induced increase in $T_1$. This is consistent with transport measurements which show the same variation of resistivity with magnetic field. Our discovery opens a vast playground to explore quantum magneto-transport effects optically in the spin dynamics.

cond-mat.mes-hall↗

arXiv2018

Simple mechanisms that impede the Berry phase identification from magneto-oscillations

The phase of quantum magneto-oscillations is often associated with the Berry phase and is widely used to argue in favor of topological nontriviality of the system (Berry phase $2πn+π$). Nevertheless, the experimentally determined value may deviate from $2πn+π$ arbitrarily, therefore more care should be made analyzing the phase of magneto-oscillations to distinguish trivial systems from nontrivial. In this paper we suggest two simple mechanisms dramatically affecting the experimentally observed value of the phase in three-dimensional topological insulators: (i) magnetic field dependence of the chemical potential, and (ii) possible nonuniformity of the system. These mechanisms are not limited to topological insulators and can be extended to other topologically trivial and non-trivial systems.

cond-mat.str-el↗

arXiv2016

Low temperature Hall effect in bismuth chalcogenides thin films

Bismuth chalcogenides are the most studied 3D topological insulators. As a rule, at low temperatures thin films of these materials demonstrate positive magnetoresistance due to weak antilocalization. Weak antilocalization should lead to resistivity decrease at low temperatures; in experiments, however, resistivity grows as temperature decreases. From transport measurements for several thin films (with various carrier density, thickness, and carrier mobility), and by using purely phenomenological approach, with no microscopic theory, we show that the low temperature growth of the resistivity is accompanied by growth of the Hall coefficient, in agreement with diffusive electron-electron interaction correction mechanism. Our data reasonably explain the low-temperature resistivity upturn.

cond-mat.mes-hall↗

arXiv2016

Novel Energy Scale in the Interacting 2D Electron System Evidenced from Transport and Thermodynamic Measurements

We study how the non-Fermi-liquid two-phase state reveals itself in transport properties of high-mobility Si-MOSFETs. We have found features in zero-field transport, magnetotransport, and thermodynamic spin magnetization in a 2D correlated electron system that may be directly related with the two-phase state. The features manifest above a density dependent temperature $T^*$ that represents a novel high-energy scale, apart from the Fermi energy. More specifically, in magnetoconductivity, we found a sharp onset of the novel regime $δσ(B,T) \propto (B/T)^2$ above a density-dependent temperature $T_{\rm kink}(n)$, a high-energy behavior that "mimics" the low-temperature diffusive interaction regime. The zero-field resistivity temperature dependence exhibits an inflection point $T_{\rm infl}(n)$. In thermodynamic magnetization, the weak-field spin susceptibility per electron, $\partial χ/\partial n$ changes sign at $T_{dM/dn}(n)$. All three notable temperatures, $T_{\rm kink}$, $T_{\rm infl}$, and $T_{d M/ d n}$, behave critically $\propto (n-n_c)$, are close to each other, and are intrinsic to high-mobility samples solely, we therefore associate them with an energy scale $T^*$ caused by interactions in the 2DE system.

cond-mat.str-el↗

arXiv2015

Temperature dependent nonlinear Hall effect in macroscopic Si-MOS antidot array

By measuring magnetoresistance and Hall effect in classically moderate perpendicular magnetic field in Si-MOSFET-type macroscopic antidot array we found a novel effect: nonlinear with field, temperature- and density-dependent Hall resistivity. We discuss qualitative explanation of the phenomenon and suggest that it might originate from strong temperature dependence of the resistivity and mobility in the shells of the antidots.

cond-mat.mes-hall↗

arXiv2015

Strongly correlated two-dimensional plasma explored from entropy measurements

Charged plasma and Fermi liquid are two distinct states of electronic matter intrinsic to dilute two-dimensional electron systems at elevated and low temperatures, respectively. Probing their thermodynamics represents challenge because of lacking an adequate technique. Here we report thermodynamic method to measure the entropy per electron in gated structures. Our technique appears to be three orders of magnitude superior in sensitivity to the ac calorimetry, allowing entropy measurements with only $10^8$ electrons. This enables us to investigate the correlated plasma regime, previously inaccessible experimentally in two-dimensional electron systems in semiconductors. In experiments with clean two-dimensional electron system in Si-based structures we traced entropy evolution from the plasma to Fermi-liquid regime by varying electron density. We reveal that the correlated plasma regime can be mapped onto the ordinary non-degenerate Fermi gas with an interaction-enhanced temperature dependent effective mass. Our method opens up new horizons in studies of low-dimensional electron systems.

cond-mat.mes-hall↗

arXiv2015

Novel Energy Scale in the Interacting 2D Electron System Evidenced from Transport and Thermodynamic Measurements

By analyzing the in-plane field magnetoconductivity, zero field transport, and thermodynamic spin magnetization in 2D correlated electron system in high mobility Si-MOS samples, we have revealed a novel high energy scale $T^*$, beyond the Fermi energy. In magnetoconductivity, we found a sharp onset of the novel regime $δσ(B,T) \propto (B/T)^2$ above a density dependent temperature $T_{\rm kink}(n)$, the high-energy behavior that "mimics" the low-temperature diffusive interaction regime. The zero field resistivity temperature dependence exhibits an inflection point $T_{\rm infl}$. In thermodynamic magnetization, the weak field spin susceptibility per electron, $\partial χ/\partial n$ changes sign at $T_{dM/d n}$. All three notable temperatures, $T_{\rm kink}$, $T_{\rm infl}$, and $T_{d M/ d n}$, behave critically $\propto (n-n_c)$, are close to each other, and are intrinsic to high mobility samples only; we therefore associate them with a novel energy scale $T^*$ caused by interactions in the 2DE system.

cond-mat.str-el↗

arXiv2015

Temperature derivative of the chemical potential and its magnetooscillations in two-dimensional system

We report first thermodynamic measurements of the temperature derivative of chemical potential (dμ/dT) in two-dimensional (2D) electron systems. In order to test the technique we have chosen Schottky gated GaAs/AlGaAs heterojunctions and detected experimentally in this 2D system quantum magnetooscillations of dμ/dT. We also present a Lifshits-Kosevitch type theory for the dμ/dT magnetooscillations in 2D systems and compare the theory with experimental data. The magnetic field dependence of the dμ/dT value appears to be sensitive to the density of states shape of Landau levels. The data in low magnetic field domain demonstrate brilliant agreement with theory for non-interacting Fermi gas with Lorentzian Landau level shape.

cond-mat.str-el↗

arXiv2013

Electron-electron interaction correction and magnetoresistance in tilted fields in Si-based 2D systems

We study diffusive electron-electron interaction correction to conductivity by analyzing simultaneously $ρ_{xx}$ and $ρ_{xy}$ for disordered 2D electron systems in Si in tilted magnetic field. Tilting the field is shown to be a straightforward tool to disentangle spin and orbital effects. In particular, by changing the tilt angle we prove experimentally that in the field range $gμ_BB>k_BT$ the correction depends on modulus of magnetic field rather than on its direction, which is expected for a system with isotropic $g$-factor. In the high-field limit the correction behaves as $\ln (B)$, as expected theoretically (Lee, Ramakrishnan, Phys. Rev. B{\bf 26}, 4009 (1982)). Our data prove that the diffusive electron-electron interaction correction to conductivity is not solely responsible for the huge and temperature dependent magnetoresistance in parallel field, typically observed in Si-MOSFETs.

cond-mat.mes-hall↗

arXiv2012

Spin-Droplet State of an Interacting 2D Electron System

We report thermodynamic magnetization measurements of two-dimensional electrons in several high mobility Si metal-oxide-semiconductor field-effect transistors. We provide evidence for an easily polarizable electron state in a wide density range from insulating to deep into the metallic phase. The temperature and magnetic field dependence of the magnetization is consistent with the formation of large-spin droplets in the insulating phase. These droplets melt in the metallic phase with increasing density and temperature, although they survive up to large densities.

cond-mat.str-el↗

arXiv2011

Thermodynamic magnetization of two-dimensional electron gas measured over wide range of densities

We report measurements of dm/dn in Si MOSFET, where m is the magnetization of the two-dimensional electron gas and n is its density. We extended the density range of measurements from well in the metallic to deep in the insulating region. The paper discusses in detail the conditions under which this extension is justified, as well as the corrections one should make to extract dm/dn properly. At low temperatures, dm/dn was found to be strongly nonlinear already in weak magnetic fields, on a scale much smaller than the characteristic scales, expected for interacting two-dimensional electron gas. Surprisingly, this nonlinear behavior exists both in the dielectric, and in the metallic region. These observations, we believe, provide evidence for strong coupling of the itinerant and localized electrons in Si-MOSFET.

cond-mat.str-el↗

arXiv2009

Thermodynamic magnetization of a strongly interacting two-dimensional system

We report thermodynamic magnetization measurements of a 2-dimensional electron gas for several high mobility Si-MOSFETs. The low-temperature magnetization is shown to be strongly sub-linear function of the magnetic field. The susceptibility determined from the zero-field slope diverges as 1/T^α, with α=2.2-2.6 even at high electron densities, in apparent contradiction with the Fermi-liquid picture.

cond-mat.str-el↗