arXiv subjects

K. Dybko

Publications and source records attributed to K. Dybko.

16 recordsLinked to original sources

arXiv2026

Tunable chiral anomaly in electron magnetotransport in the Weyl semimetallic Pb$_{1-x}$Sn$_x$Te:Cr alloy

We study magnetotransport properties of semiconductor substitutional alloy Pb$_{1-x}$Sn$_x$Te, known to exhibit Sn-content dependent properties of topological crystalline insulators with a semimetallic zero-gap state at a specific band inversion point. We experimentally verify the theoretically predicted role of chemical disorder in this multivalley electron system, which leads to sequential band inversions in various valleys and places the Fermi level close to the pairs of Weyl nodes, as identified in the density functional theory calculations. Doping with mixed-valence Cr resonant impurities enables exploitation of the unique properties of dopant resonant states, which provide an effective means of tuning carrier concentration. The combination of these two effects results in the pinning of the Fermi level in the vicinity of the nodal touching points across a wide range of composition. To address the above issues, we grow Bridgman bulk crystals of Pb$_{1-x}$Sn$_x$Te heavily doped with chromium and covering the full range of tin (0 $\leq x \leq$ 1), i.e. spanning both the topological crystalline insulator and trivial electronic regimes. We observe the emergence of the three dimensional (3D) Weyl semimetal phase over a range of Sn compositions, namely for $0.25 < x < 0.45$. We provide magnetotransport evidence for this and verify the relationship between the magnitude of the experimentally determined Berry curvature and the electrical properties of these materials. Quantum transport regime observed in magnetoresistance is also independently confirmed by thermal conductivity measurements.

cond-mat.mtrl-sci↗

arXiv2025

Cr resonant impurity for studies of band inversion and band offsets in IV-VI semiconductors

Understanding the electronic structure of transition-metal dopants in IV-VI semiconductors is critical for tuning their band structure. We analyze properties of Cr dopant in $Pb_{1-x}Sn_xTe$ and PbSe by magnetic and transport measurements, which are interpreted based on density functional calculations. We demonstrate that the pinning of the Fermi energy to the chromium resonant level occurs for both n-type and p-type $Pb_{1-x}Sn_xTe$ in the whole composition range. This enables us to determine the valence band and conduction band offsets at the PbTe/SnTe/PbSe heterointerfaces, which is important for designing high-prformance 2D transistors. Furthermore, the magnetic measurements reveal the presence of Cr ions in three charge states, $Cr^{3+}$, $Cr^{2+}$, and $Cr^{1+}$. The last one corresponds to the Cr dopants incorporated at the interstitial, and not the substitutional, sites. The measured concentrations of the interstitial and substitutional Cr are comparable.

cond-mat.mtrl-sci↗

arXiv2024

PbTe/SnTe heterostructures -- candidate platform for studying spin-triplet superconductivity

This paper explores the potential for spin-triplet superconductivity in molecular beam epitaxy grown PbTe/SnTe semiconductor heterostructures. We present convincing evidence for spin-triplet pairing by soft point-contact spectroscopy experiments, using both spin-polarized and unpolarized electrons and additionally, by detailed analysis of the upper critical field, as inferred from the four probe resistance measurements. The experimental data are described in terms of the Anderson-Brinkman-Morel model of p-wave electron pairing. Our results confirm the predictions on strain-induced topological superconductivity by E.Tang and L. Fu (Nature Physics, 10, 964, 2014).

cond-mat.supr-con↗

arXiv2022

Thermoelectric PbTe-CdTe bulk nanocomposite

The preparation method of thermoelectric PbTe-CdTe semiconductor nanocomposite in the form of a bulk material doped with Bi, I or Na, intended for production the mid-temperature thermoelectric energy generators is presented. The method takes advantage of the extremely low mutual solubility of both semiconductors, resulting from their different crystal structure, and is based on a specifically designed Bridgman growth procedure. It is shown that the formation of zinc-blende crystalline CdTe grains in the rock-salt matrix of thermoelectric PbTe can be forced during the synthesis of a composite by introducing Cd in the form of CdTe compound and choosing the growth temperature above the melting point of PbTe but below the melting point of CdTe. X-ray diffraction and SEM-EDX spectroscopy analyzes as well as basic electric and thermoelectric characterization of the nanocomposite samples containing 2, 5 and 10 at. \% of Cd showed that using proposed growth procedure, it is possible to obtain both n-type (Bi- or I-doped) and p-type (Na-doped) material with carrier concentration of 1{\div}5 x 10\^{19} cm\^{-3} and uniformly distributed CdTe grains with a diameter of the order of 100 nm.

cond-mat.mtrl-sci↗

arXiv2022

Electronic properties of TaAs2 topological semimetal investigated by transport and ARPES

We have performed electron transport and ARPES measurements on single crystals of transition metal dipnictide TaAs2 cleaved along the ($\overline{2}$ 0 1) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov-de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between the magnetic field and the [$\overline{2}$ 0 1] direction. The results indicate the elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved ($\overline{2}$ 0 1) surface and it was found that bulk states pockets at the constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level is increased, which is due to a small n-doping of the samples. This shifts the Fermi level closer to the Dirac point, allowing to investigate the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.

cond-mat.other↗

arXiv2020

Controlled electron transmission by lead chalcogenide barrier potential

Transmission of electrons across a rectangular barrier of IV-VI semiconductor compounds is considered. Conduction electrons arrive at the barrier and are reflected or transmitted through it depending on the relative values of the barrier potential $V_b$ and the electron energy $E$. The theory, in close analogy to the Dirac four component spinors, accounts for the boundary conditions on both sides of the barrier. The calculated transmission coefficient $T_C$ is an oscillatory function of the barrier voltage varying between zero (for full electron reflection) and unity (for full electron transmission). Character of electron wave functions outside and inside the barrier is studied. There exists a total current conservation, i. e. the sum of transmitted and reflected currents is equal to the incoming current. The transmission $T_C$ is studied for various barrier widths and incoming electron energies. Finally, the transmission coefficient $T_C$ is studied as a function of $V_b$ for decreasing energy gaps $E_g$ of different Pb$_{1-x}$Sn$_x$Se compounds in the range of 150 meV $\geq E_g \geq$ 2 meV. It is indicated that for very small gap values the behaviour of $T_C$ closely resembles that of the chiral electron tunneling by a barrier in monolayer graphene. For $E_g$ =0 (Pb$_{0.81}$Sn$_{0.19}$Se) the coefficient $T_C$ reaches the value of 1 independently of $V_b$.

cond-mat.mes-hall↗

arXiv2020

Conductance spectra of (Nb, Pb, In)/NbP -- superconductor/Weyl semimetal junctions

The possibility of inducing superconductivity in type-I Weyl semimetal through coupling its surface to a superconductor was investigated. A single crystal of NbP, grown by chemical vapor transport method, was carefully characterized by XRD, EDX, SEM, ARPES techniques and by electron transport measurements. The mobility spectrum of the carriers was determined. For the studies of interface transmission, the (001) surface of the crystal was covered by several hundred nm thick metallic layers of either Pb, or Nb, or In. DC current-voltage characteristics and AC differential conductance through the interfaces as a function of the DC bias were investigated. When the metals become superconducting, all three types of junctions show conductance increase, pointing out the Andreev reflection as a prevalent contribution to the subgap conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is satisfactorily described by modified Blonder-Tinkham-Klapwijk model. The absolute value of the conductance is much smaller than that for the bulk crystal, indicating that the transmission occurs through only a small part of the contact area. An opposite situation occurs in In-NbP junction, where the conductance at the peak reaches the bulk value indicating that almost whole contact area is transmitting and, additionally, a superconducting proximity phase is formed in the material. We interpret this as a result of indium diffusion into NbP, where the metal atoms penetrate the surface barrier and form very transparent superconductor-Weyl semimetal contact inside. However, further diffusion occurring already at room temperature leads to degradation of the effect, so it is observed only in the pristine structures. Despite of this, our observation directly demonstrates possibility of inducing superconductivity in a type-I Weyl semimetal.

cond-mat.mes-hall↗

arXiv2019

Two-valence band electron and heat transport in monocrystalline PbTe-CdTe solid solutions with high Cd content

High quality p-type PbTe-CdTe monocrystalline alloys containing up to 10 at.$\%$ of Cd are obtained by self-selecting vapor transport method. Mid infrared photoluminescence experiments are performed to follow the variation of the fundamental energy gap as a function of Cd content. The Hall mobility, thermoelectric power, thermal conductivity and thermoelectric figure of merit parameter $ZT$ are investigated experimentally and theoretically paying particular attention to the two-valence band structure of the material. It is shown that the heavy-hole band near the $Σ$ point of the Brillouin zone plays an important role and is responsible for the Pb$_{1-x}$Cd$_x$Te hole transport at higher Cd-content. Our data and their description can serve as the standard for Pb$_{1-x}$Cd$_x$Te single crystals with $x$ up to 0.1. It is shown, that monocrystalline Pb$_{1-x}$Cd$_x$Te samples with relatively low Cd content of about 1 at.\% and hole concentration of the order of 10$^{20}$ cm$^{-3}$ may exhibit $ZT \approx$ 1.4 at 600 K.

cond-mat.mtrl-sci↗

arXiv2019

Gating effects in antiferromagnetic CuMnAs

Antiferromagnets (AFs) attract much attention due to potential applications in spintronics. Both the electric current and the electric field are considered as tools suitable to control properties and the Néel vector direction of AFs. Among AFs, CuMnAs has been shown to exhibit specific properties that result in the existence of the current-induced spin-orbit torques commensurate with spin directions and topological Dirac quasiparticles. Here, we report on the observation of a reversible effect of an electric field on the resistivity of CuMnAs thin films, employing ionic liquid as a gate insulator. The data allow to determine the carrier type, concentration, and mobility independently of the Hall effect that may be affected by an anomalous component.

cond-mat.mtrl-sci↗

arXiv2019

Experimental search for the origin of low-energy modes in topological materials

Point-contact spectroscopy of several non-superconducting topological materials reveals a low temperature phase transition that is characterized by a Bardeen-Cooper-Schrieffer-type of criticality. We find such a behavior of differential conductance for topological surfaces of non-magnetic and magnetic Pb$_{1-y-x}$Sn$_y$Mn$_x$Te. We examine a possible contribution from superconducting nanoparticles, and show to what extent our data are consistent with Brzezicki's et al. theory [arXiv:1812.02168], assigning the observations to a collective state adjacent to atomic steps at topological surfaces.

cond-mat.supr-con↗

arXiv2018

Probing spatial extent of topological surface states by weak antilocalization experiments

Weak antilocalization measurements has become a standard tool for studying quantum coherent transport in topological materials. It is often used to extract information about number of conducting channels and dephasing length of topological surface states. We study thin films of prototypical topological crystalline insulator SnTe. To access microscopic characteristic of these states we employ a model developed by Tkachov and Hankiewicz, [Physical Review B 84, 035444]. Using this model the spatial decay of the topological states is obtained from measurements of quantum corrections to the conductivity in perpendicular and parallel configurations of the magnetic field. Within this model we find interaction between two topological boundaries which results in scaling of the spatial decay with the film thickness. We attribute this behavior to bulk reservoir which mediates interactions by scattering events without phase breaking of topological carriers.

cond-mat.mes-hall↗

arXiv2017

Experimental evidence for topological surface states wrapping around bulk SnTe crystal

We demonstrate that the metallic topological surface states wrap on all sides the 3D topological crystalline insulator SnTe. This is achieved by studying oscillatory quantum magneto-transport and magnetization at tilted magnetic fields which enables us to observe simultaneous contributions from neighbouring sample sides. Taking into account pinning of the Fermi energy by the SnTe reservoir we successfully describe theoretically the de Haas-van Alphen oscillations of magnetization. The determined π-Berry phase of surface states confirms their Dirac fermion character. We independently observe oscillatory contributions of magneto-transport and magnetization originating from the bulk SnTe reservoir of high hole density. It is concluded that the bulk and surface Landau states exist in parallel. Our main result that the bulk reservoir is surrounded on all sides by the topological surface states has an universal character.

cond-mat.mtrl-sci↗

arXiv2015

Nernst-Ettingshausen effect at the trivial-nontrivial band ordering in topological crystalline insulator Pb1-xSnxSe

The transverse Nernst Ettingshausen (N-E) effect and electron mobility in Pb$_{1-x}$Sn$_x$Se alloys are studied experimentally and theoretically as functions of temperature and chemical composition in the vicinity of vanishing energy gap $E_g$. The study is motivated by the recent discovery that, by lowering the temperature, one can change the band ordering from trivial to nontrivial one in which the topological crystalline insulator states appear at the surface. Our work presents several new aspects. It is shown experimentally and theoretically that the bulk N-E effect has a maximum when the energy gap $E_g$ of the mixed crystal goes through zero value. This result contradicts the claim made in the literature that the N-E effect changes sign when the gap vanishes. We successfully describe $dc$ transport effects in the situation of extreme band's nonparabolicity which, to the best of our knowledge, has never been tried before. A situation is reached in which both two-dimensional bands (topological surface states) and three-dimensional bands are linear in electron \textbf{k} vector. Various scattering modes and their contribution to transport phenomena in Pb$_{1-x}$Sn$_x$Se are analyzed. As the energy gap goes through zero, some transport integrals have a singular (nonphysical) behaviour and we demonstrate how to deal with this problem by introducing damping.

cond-mat.mtrl-sci↗

arXiv2013

Topological crystalline insulator states in Pb(1-x)Sn(x)Se

Topological insulators are a novel class of quantum materials in which time-reversal symmetry, relativistic (spin-orbit) effects and an inverted band structure result in electronic metallic states on the surfaces of bulk crystals. These helical states exhibit a Dirac-like energy dispersion across the bulk bandgap, and they are topologically protected. Recent theoretical proposals have suggested the existence of topological crystalline insulators, a novel class of topological insulators in which crystalline symmetry replaces the role of time-reversal symmetry in topological protection [1,2]. In this study, we show that the narrow-gap semiconductor Pb(1-x)Sn(x)Se is a topological crystalline insulator for x=0.23. Temperature-dependent magnetotransport measurements and angle-resolved photoelectron spectroscopy demonstrate that the material undergoes a temperature-driven topological phase transition from a trivial insulator to a topological crystalline insulator. These experimental findings add a new class to the family of topological insulators. We expect these results to be the beginning of both a considerable body of additional research on topological crystalline insulators as well as detailed studies of topological phase transitions.

cond-mat.mtrl-sci↗

arXiv2004

Thermoelectric studies of electronic properties of ferromagnetic GaMnAs layers

Thermoelectric power, electrical conductivity, and high field Hall effect were studied over a broad temperature range in ferromagnetic Ga(1-x)Mn(x)As epitaxial layers (0.015 70 K) the thermoelectric power in GaMnAs linearly increases with increasing temperature. That indicates the presence of a degenerate hole gas with the Fermi energy EF=220+-25 meV, nearly independent of Mn content (for 0.02<x<0.05). At lower temperatures GaMnAs layers with metallic-type conductivity show an additional contribution to the thermoelectric power with the maximum close to the Curie temperature. The layers exhibiting insulating electrical properties show 1/T-type increase of thermoelectric power at low temperatures.

cond-mat.mtrl-sci↗

arXiv2002

Dynamical scaling of the quantum Hall plateau transition

Using different experimental techniques we examine the dynamical scaling of the quantum Hall plateau transition in a frequency range f = 0.1-55 GHz. We present a scheme that allows for a simultaneous scaling analysis of these experiments and all other data in literature. We observe a universal scaling function with an exponent kappa = 0.5 +/- 0.1, yielding a dynamical exponent z = 0.9 +/- 0.2.

cond-mat.mes-hall↗