arXiv subjects

S. S. Krishtopenko

Publications and source records attributed to S. S. Krishtopenko.

At least 19 recordsLinked to original sources

arXiv2026

Electrically driven Rabi dynamics of magnetic-field-induced corner states in a two-dimensional topological insulator

We study coherent electric manipulation of magnetic-field-induced localized states at a double kink of a helical edge in a HgTe/CdHgTe quantum well. An in-plane magnetic field opens a gap in the one-dimensional edge spectrum, while changes in the edge orientation generate localized in-gap states at the kinks. We show that, for suitable geometry and magnetic-field direction, two such states form an effective lithographically defined two-level subsystem. Using an edge-state model that includes both the localized levels and the continuum states outside the magnetic-field-induced gap, we calculate the electric-dipole matrix elements and solve the time-dependent problem under resonant driving. The resulting dynamics exhibits Rabi oscillations with linear frequencies of $20$--$40$~GHz for realistic parameters. We find that the continuum states provide a leakage channel whose strength is strongly controlled by the driving amplitude: reducing the electric field suppresses leakage below the percent level while preserving GHz-scale coherent oscillations. These results establish a route from magnetic-field-induced corner-state physics to electrically driven two-level dynamics in a realistic two-dimensional topological-insulator edge geometry.

cond-mat.mes-hall↗

arXiv2025

Many-particle hybridization of optical transitions from zero-mode Landau levels in HgTe quantum wells

We present far-infrared magnetospectroscopy measurements of a HgTe quantum well in the inverted band structure regime over the temperature range of 2 to 60 K. The particularly low electron concentration enables us to probe the temperature evolution of all four possible optical transitions originating from zero-mode Landau levels, which are split off from the edges of the electron-like and hole-like bands. By analyzing their resonance energies, we reveal an unambiguous breakdown of the single-particle picture indicating that the explanation of the anticrossing of zero-mode Landau levels in terms of bulk and interface inversion asymmetries is insufficient. Instead, the observed behavior of the optical transitions is well explained by their hybridization driven by electron-electron interaction. We emphasize that our proposed many-particle mechanism is intrinsic to HgTe quantum wells of any crystallographic orientation, including (110) and (111) wells, where bulk and interface inversion asymmetries do not induce the anticrossing of zero-mode Landau levels.

cond-mat.mes-hall↗

arXiv2025

Quasiparticle picture of topological phase transitions induced by interactions

We present a general recipe to describe topological phase transitions in condensed matter systems with interactions. We show that topological invariants in the presence of interactions can be efficiently calculated by means of a non-Hermitian quasiparticle Hamiltonian introduced on the basis of the Green's function. As an example analytically illustrating the application of the quasiparticle concept, we consider a topological phase transition induced by the short-range electrostatic disorder in a two dimensional system described by the Bernevig-Hughes-Zhang model. The latter allows us to explicitly demonstrate the change in the $\mathbb{Z}_2$ topological invariant and explain the quantized values of the longitudinal conductance in a certain range of the Fermi energy and the disorder strength found previously in numerical calculations.

cond-mat.mtrl-sci↗

arXiv2025

Nonlinear Terahertz Electroluminescence from Dirac Landau Polaritons

We report Dirac Landau polaritons observed by terahertz (THz) magnetoreflectivity spectroscopy, demonstrating strong coupling between cyclotron transitions of two-dimensional (2D) Dirac fermions in HgTe quantum wells and optical cavity modes. Under pulsed electrical injection we observe efficient nonlinear electroluminescence, with a strongly out-of-equilibrium polariton distribution dominated by emission from the upper polariton branches. Model analysis of the bias-dependent emission intensity and spectral narrowing indicates a polariton occupancy per mode approaching unity, with a possible contribution from stimulated polariton emission in the spectral region of the upper anticrossing. These results open prospects toward Dirac Landau polariton condensates and low-threshold, tunable THz polariton lasers based on cyclotron emission.

cond-mat.mes-hall↗

arXiv2024

Spin-dependent photovoltage in graphene/MoS2-based field-effect transistors

It has recently been shown that Terahertz sensors can effectively detect the spin resonances of Dirac fermions in graphene. The associated photovoltaic measurement technique allows for the investigation of the intrinsic spin-orbit coupling in graphene as well as its topological properties from microwave to Terahertz frequencies. In this work, using graphene/MoS2-based Field-Effect Transistors, we observed a magnetic resonance photovoltage signal in the Gigahertz range that is independent of the gate bias. The dispersion of the associated spin-flip transitions remains intriguingly unaffected by the MoS2 layer. In parallel, the spin-related signal consistently appears as a drop in photovoltage, regardless of the signal's polarity or origin, whether it arises from plasma wave rectification or thermoelectric effects. This behavior is interpreted as a decrease in the system's spin polarization due to spin-dependent recombination or scattering of photocarriers. Understanding the various photovoltaic signals in highly sensitive Gigahertz/Terahertz sensors paves the way for exploring spin-dependent mechanisms in two-dimensional quantum materials, influenced by proximity effects such as spin-orbit coupling, topology, and magnetism.

cond-mat.mes-hall↗

arXiv2024

Multi-probe analysis to separate edge currents from bulk currents in quantum spin Hall insulators and to analyze their temperature dependence

We present a multi-probe transport analysis that effectively separates bulk and edge currents in large Hall bar devices with standard geometries. Applied to transport measurements on all possible four-probe configurations of six-probe Hall bar devices made of inverted three-layer InAs/GaInSb quantum wells (QWs), our analysis not only reveals the presence of dissipative edge currents in the topological gap, but also allows the temperature dependence of bulk and edge conductivity to be evaluated separately. The temperature dependence of the edge conductivity for Hall bar channels from 10 $\mu$m to 70~$\mu$m in the range of 1.5 K to 45 K is consistent with the theoretical expectation of weakly interacting helical edge electrons with backscattering due to localized magnetic moments of charge impurities. We argue that these charge impurities are naturally associated with intrinsic Ga-antisite defects, which act as double acceptors in InAs/Ga(In)Sb-based QWs.

cond-mat.mes-hall↗

arXiv2024

Quantum Hall effect and zero plateau in bulk HgTe

The quantum Hall effect, which exhibits a number of unusual properties, is studied in a gated 1000-nm-thick HgTe film, nominally a three-dimensional system. A weak zero plateau of Hall resistance, accompanied by a relatively small value of Rxx of the order of h/e^2, is found around the point of charge neutrality. It is shown that the zero plateau is formed by the counter-propagating chiral electron-hole edge channels, the scattering between which is suppressed. So, phenomenologically, the quantum spin Hall effect is reproduced, but with preserved ballisticity on macroscopic scales (larger than 1mm). It is shown that the formation of the QHE occurs in a two-dimensional (2D) accumulation layer near the gate, while the bulk carriers play the role of an electron reservoir. Due to the exchange of carriers between the reservoir and the 2D layer, an anomalous scaling of the QHE is observed not with respect to the CNP, but with respect to the first electron plateau.

cond-mat.mes-hall↗

arXiv2024

Magnetophotogalvanic Effects Driven by Terahertz Radiation in CdHgTe Crystals with Kane Fermions

We report on the observation and comprehensive study of the terahertz radiation induced magneto-photogalvanic effect (MPGE) in bulk CdHgTe crystals hosting Kane fermions. The MPGE has been detected in Cd$_{x}$Hg$_{1-x}$Te films with Cd contents $x = 0.15$ and $0.22$ subjected to an in-plane magnetic field. At liquid helium temperature we observed multiple resonances in MPGE current upon variation of magnetic field. In the $x = 0.22$ with noninverted band structure, the resonances are caused by cyclotron resonance (CR) and photoionization of an impurity level. In the $x = 0.15$ films with an inverted band structure, they originate from the CR and interband optical transitions. Band structure calculated by the Kane model perfectly describes positions of all resonances. In particularly, the resonant MPGE caused by interband transitions excited by THz radiation is caused by the gapless energy spectrum of Kane fermions realized in materials with certain Cd contents and temperature range. In addition to the resonant MPGE current we detected a nonresonant one due to indirect optical transitions (Drude-like). This contribution has a nonmonotonic magnetic field dependence increasing linearly at low magnetic field $B$, approaching a maximum at moderate field and decreasing at high $B$. While the nonresonant MPGE decreases drastically with increasing temperature, it is well measurable up to room temperature. The developed theory demonstrates that the MPGE current arises due to cubic in momentum spin-dependent terms in the scattering probability. The asymmetry caused by these effects results in a pure spin current which is converted into an electric current due to the Zeeman effect.

cond-mat.mes-hall↗

arXiv2023

Disorder-induced phase transitions in double HgTe quantum wells

By using the self-consistent Born approximation, we investigate topological phase transitions in double HgTe quantum wells (QWs) induced by the short-range impurities. Following the evolution of the density-of-states and the spectral function, we demonstrate multiple closings and openings of the band-gap with the increase of the disorder strength due to the mutual inversions between the first and second electron-like and hole-like subbands. We show that starting from a band insulator in the clean limit, under the influence of disorder, the double HgTe QW undergoes a transition, first into a semimetal state similar to "bilayer graphene", and then into a high-order topological insulator state with a double band inversion. We find out that all disorder-induced transitions can be fully characterized by introducing a non-Hermitian quasiparticle Hamiltonian encoding the band structure renormalization and quasiparticle decay.

cond-mat.mes-hall↗

arXiv2023

Gate tunable terahertz cyclotron emission from two-dimensional Dirac fermions

Two-dimensional Dirac fermions in HgTe quantum wells close to the topological phase transition can generate significant cyclotron emission that is magnetic field tunable in the Terahertz (THz) frequency range. Due to their relativistic-like dynamics, their cyclotron mass is strongly dependent on their electron concentration in the quantum well, providing a second tunability lever and paving the way for a gate-tunable, permanent-magnet Landau laser. In this work, we demonstrate the proof-of-concept of such a back-gate tunable THz cyclotron emitter at fixed magnetic field. The emission frequency detected at 1.5 Tesla is centered on 2.2 THz and can already be electrically tuned over 250 GHz. With an optimized gate and a realistic permanent magnet of 1.0 Tesla, we estimate that the cyclotron emission could be continuously and rapidly tunable by the gate bias between 1 and 3 THz, that is to say on the less covered part of the THz gap.

cond-mat.mes-hall↗

arXiv2023

Terahertz cyclotron emission from two-dimensional Dirac fermions

Since the emergence of graphene, we have seen several proposals for the realization of Landau lasers tunable over the terahertz frequency range. The hope was that the non-equidistance of the Landau levels from Dirac fermions would suppress the harmful non-radiative Auger recombination. Unfortunately, even with this non-equidistance an unfavorable non-radiative process persists in Landau-quantized graphene, and so far no cyclotron emission from Dirac fermions has been reported. One way to eliminate this last non-radiative process is to sufficiently modify the dispersion of the Landau levels by opening a small gap in the linear band structure. A proven example of such gapped graphene-like materials are HgTe quantum wells close to the topological phase transition. In this work, we experimentally demonstrate Landau emission from Dirac fermions in such HgTe quantum wells, where the emission is tunable by both the magnetic field and the carrier concentration. Consequently, these results represent an advance in the realization of terahertz Landau lasers tunable by magnetic field and gate-voltage.

cond-mat.mes-hall↗

arXiv2022

Temperature Dependent Zero-Field Splittings in Graphene

Graphene is a quantum spin Hall insulator with a 45 $μ$eV wide non-trivial topological gap induced by the intrinsic spin-orbit coupling. Even though this zero-field spin splitting is weak, it makes graphene an attractive candidate for applications in quantum technologies, given the resulting long spin relaxation time. On the other side, the staggered sub-lattice potential, resulting from the coupling of graphene with its boron nitride substrate, compensates intrinsic spin-orbit coupling and decreases the non-trivial topological gap, which may lead to the phase transition into trivial band insulator state. In this work, we present extensive experimental studies of the zero-field splittings in monolayer and bilayer graphene in a temperature range 2K-12K by means of sub-Terahertz photoconductivity-based electron spin resonance technique. Surprisingly, we observe a decrease of the spin splittings with increasing temperature. We discuss the origin of this phenomenon by considering possible physical mechanisms likely to induce a temperature dependence of the spin-orbit coupling. These include the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electron-phonon interactions, and the presence of a magnetic order at low temperature. Our experimental observation expands knowledge about the non-trivial topological gap in graphene.

cond-mat.mtrl-sci↗

arXiv2022

Large inverted band-gap in strained three-layer InAs/GaInSb quantum wells

Quantum spin Hall insulators (QSHIs) based on HgTe and three-layer InAs/GaSb quantum wells (QWs) have comparable bulk band-gaps of about 10--18~meV. The former however features a band-gap vanishing with temperature, while the gap in InAs/GaSb QSHIs is rather temperature-independent.Here, we report on the realization of large inverted band-gap in strained three-layer InAs/GaInSb QWs. By temperature-dependent magnetotransport measurements of gated Hall bar devices, we extract a gap as high as 45 meV. Combining local and non-local measurements, we attribute the edge conductivity observed at temperatures up to 40 K to the topological edge channels with equilibration lengths of a few micrometers. Our findings pave the way toward manipulating edge transport at high temperatures in QW heterostructures.

cond-mat.mes-hall↗

arXiv2021

HgTe quantum wells for QHE metrology under soft cryomagnetic conditions: permanent magnets and liquid ${^4He}$ temperatures

HgTe quantum wells with a thickness of ${\sim}$7 nm may have a graphene-like band structure and have been recently proposed to be potential candidates for quantum Hall effect (QHE) resistance standards under the condition of operation in the fields above certain critical field $B_c$, above which the topological phase (with parasitic edge conduction) disappears. We present experimental studies of the magnetoresistance of different of HgTe quantum wells as a function temperature and magnetic field, determining the critical magnetic field $B_c$. We demonstrate that for QWs of specific width $B_c$ becomes low enough to grant observation of remarkably wide QHE plateaus at the filling factor ${v=-1}$ (holes) in relaxed cryomagnetic conditions: while using commercial 0.82 T Neodymium permanent magnets and temperature of a few Kelvin provided by ${^4He}$ liquid system only. Band structure calculations allow us to explain qualitatively observed phenomena due to the interplay between light holes and heavy holes energy sub-bands (side maxima of the valence band). Our work clearly shows that the peculiar band structure properties of HgTe QWs with massless Dirac fermions make them an ideal platform for developing metrological devices with relaxed cryomagnetic conditions.

cond-mat.mes-hall↗

arXiv2020

Many-particle effects in optical transitions from zero-mode Landau levels in HgTe quantum wells

We report on the far-infrared magnetospectroscopy of HgTe quantum wells with inverted band ordering at different electron concentrations. We particularly focus on optical transitions from zero-mode Landau levels, which split from the edges of electron-like and hole-like bands. We observe a pronounced dependence of the transition energies on the electron concentration varied by persistent photoconductivity effect. This is striking evidence that in addition to the already well-documented crystalline and interface asymmetries, electron-electron interactions also have a significant impact on the usual behavior of the optical transitions from zero mode Landau levels.

cond-mat.mes-hall↗

arXiv2019

Massless Dirac fermions in III-V semiconductor quantum wells

We report on the clear evidence of massless Dirac fermions in two-dimensional system based on III-V semiconductors. Using a gated Hall bar made on a three-layer InAs/GaSb/InAs quantum well, we restore the Landau levels fan chart by magnetotransport and unequivocally demonstrate a gapless state in our sample. Measurements of cyclotron resonance at different electron concentrations directly indicate a linear band crossing at the $Γ$ point of Brillouin zone. Analysis of experimental data within analytical Dirac-like Hamiltonian allows us not only determing velocity $v_F=1.8\cdot10^5$ m/s of massless Dirac fermions but also demonstrating significant non-linear dispersion at high energies.

cond-mat.mes-hall↗

arXiv2018

Unconventional Reentrant Quantum Hall Effect in a HgTe/CdHgTe Double Quantum Well

We report on observation of an unconventional structure of the quantum Hall effect (QHE) in a $ p$-type HgTe/Cd$_x$Hg$_{1-x}$Te double quantum well (DQW) consisting of two HgTe layers of critical width. The observed QHE is a reentrant function of magnetic field between two $i=2$ states (plateaus at $ρ_{xy}=h/ie^2$) separated by an intermediate $i=1$ state, which looks like some anomalous peak on the extra-long $i=2$ plateau when weakly expressed. The anomalous peak apparently separates two different regimes: a traditional QHE at relatively weak fields for a small density of mobile holes $p_s$ and a high-field QH structure with a $2-1$ plateau--plateau transition corresponding to much larger $p_s$. We show that only a part of holes, residing in an additional light hole subband in the DQW, participate in QHE at weak fields while the rest of holes is excluded into the reservoir formed in the lateral maximum of the valence subband. All the holes come into play at high fields due to a peculiar behavior of the zero-mode levels.

cond-mat.mes-hall↗

arXiv2018

Spin splitting of surface states in HgTe quantum wells

We report on beating appearance in Shubnikov-de Haas oscillations in conduction band of 18-22nm HgTe quantum wells under applied top-gate voltage. Analysis of the beatings reveals two electron concentrations at the Fermi level arising due to Rashba-like spin splitting of the first conduction subband H1. The difference dN_s in two concentrations as a function of the gate voltage is qualitatively explained by a proposed toy electrostatic model involving the surface states localized at quantum well interfaces. Experimental values of dN_s are also in a good quantitative agreement with self-consistent calculations of Poisson and Schrodinger equations with eight-band kp Hamiltonian. Our results clearly demonstrate that the large spin splitting of the first conduction subband is caused by surface nature of $H1$ states hybridized with the heavy-hole band.

cond-mat.mes-hall↗