arXiv subjects

D. A. Kozlov

Publications and source records attributed to D. A. Kozlov.

31 records · Page 2Linked to original sources

arXiv2018

High frequency impact ionization and nonlinearity of photocurrent induced by intense terahertz radiation in HgTe-based quantum well structures

We report on a strong nonlinear behavior of the photogalvanics and photoconductivity under excitation of HgTe quantum wells (QWs) by intense terahertz (THz) radiation. The increasing radiation intensity causes an inversion of the sign of the photocurrent and transition to its superlinear dependence on the intensity. The photoconductivity also shows a superlinear raise with the intensity. We show that the observed photoresponse nonlinearities are caused by the band-to-band \emph{light} impact ionization under conditions of a photon energy less than the forbidden gap. The signature of this kind of impact ionization is that the angular radiation frequency $ω=2πf$ is much higher than the reciprocal momentum relaxation time. Thus, the impact ionization takes place solely because of collisions in the presence of a high-frequency electric field. The effect has been measured on narrow HgTe/CdTe QWs of 5.7\,nm width; the nonlinearity is detected for linearly and circularly polarized THz radiation with different frequencies ranging from $f=0.6$ to 1.07\,THz and intensities up to hundreds of kW/cm$^2$. We demonstrate that the probability of the impact ionization is proportional to the exponential function, $\exp(-E_0^2/E^2)$, of the radiation electric field amplitude $E$ and the characteristic field parameter $E_0$. The effect is observable in a wide temperature range from 4.2 to 90\,K, with the characteristic field increasing with rising temperature.

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arXiv2018

Sign-alternating photoconductivity and magnetoresistance oscillations induced by terahertz radiation in HgTe quantum wells

We report on the observation of terahertz radiation induced photoconductivity and of terahertz analog of the microwave-induced resistance oscillations (MIRO) in HgTe-based quantum well (QW) structures of different width. The MIRO-like effect has been detected in QWs of 20 nm thickness with inverted band structure and a rather low mobility of about 3 $\times$ 10$^5$ cm$^2$/V s. In a number of other structures with QW widths ranging from 5 to 20 nm and lower mobility we observed an unconventional non-oscillatory photoconductivity signal which changes its sign upon magnetic field increase. This effect was observed in structures characterized by both normal and inverted band ordering, as well as in QWs with critical thickness and linear dispersion. In samples having Hall bar and Corbino geometries an increase of the magnetic field resulted in a single and double change of the sign of the photoresponse, respectively. We show that within the bolometric mechanism of the photoresponse these unusual features imply a non-monotonic behavior of the transport scattering rate, which should decrease (increase) with temperature for magnetic fields below (above) the certain value. This behavior is found to be consistent with the results of dark transport measurements of magnetoresistivity at different sample temperatures. Our experiments demonstrate that photoconductivity is a very sensitive probe of the temperature variations of the transport characteristics, even those that are hardly detectable using standard transport measurements.

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arXiv2017

Circular and linear magnetic quantum ratchet effects in dual-grating-gate CdTe-based nanostructures

We report on the observation and systematic study of polarization sensitive magnetic quantum ratchet effects induced by alternating electric fields in the terahertz frequency range. The effects are detected in (Cd,Mn)Te-based quantum well (QW) structures with inter-digitated dual-grating-gate (DGG) lateral superlattices. A dc electric current excited by cw terahertz laser radiation shows 1/B-periodic oscillations with an amplitude much larger than the photocurrent at zero magnetic field. Variation of gate voltages applied to individual grating gates of the DGG enables us to change the degree and the sign of the lateral asymmetry in a controllable way. The data reveal that the photocurrent reflects the degree of lateral asymmetry induced by different gate potentials. We show that the magnetic ratchet photocurrent includes the Seebeck thermoratchet effect as well as the effects of "linear" and "circular" ratchets, which are sensitive to the corresponding polarization of the driving electromagnetic force. Theoretical analysis performed in the framework of semiclassical approach and taking into account Landau quantization describes the experimental results well.

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arXiv2017

Robust helical edge transport at $ν=0$ quantum Hall state

Among the most interesting predictions in two-dimensional materials with a Dirac cone is the existence of the zeroth Landau level (LL), equally filled by electrons and holes with opposite chirality. The gapless edge states with helical spin structure emerge from Zeeman splitting at the LL filling factor $ν=0$ gapped quantum Hall state. We present observations of a giant nonlocal four-terminal transport in zero-gap HgTe quantum wells at the $ν=0$ quantum Hall state. Our experiment clearly demonstrates the existence of the robust helical edge state in a system with single valley Dirac cone materials.

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arXiv2017

Magneto-resistance oscillations induced by high-intensity terahertz radiation

We report on observation of pronounced terahertz radiation-induced magneto-resistivity oscillations in AlGaAs/GaAs two-dimensional electron systems, the THz analog of the microwave induced resistivity oscillations (MIRO). Applying high power radiation of a pulsed molecular laser we demonstrate that MIRO, so far observed at low power only, are not destroyed even at very high intensities. Experiments with radiation intensity ranging over five orders of magnitude from $0.1$ W/cm$^2$ to $10^4$ W/cm$^2$ reveal high-power saturation of the MIRO amplitude, which is well described by an empirical fit function $I/(1 + I/I_s)^β$ with $β\sim 1$. The saturation intensity Is is of the order of tens of W/cm$^2$ and increases by six times by increasing the radiation frequency from $0.6$ to $1.1$ THz. The results are discussed in terms of microscopic mechanisms of MIRO and compared to nonlinear effects observed earlier at significantly lower excitation frequencies.

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arXiv2017

Magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with a lateral asymmetric superlattice

We report on the observation of magnetic quantum ratchet effect in (Cd,Mn)Te- and CdTe-based quantum well structures with an asymmetric lateral dual grating gate superlattice subjected to an external magnetic field applied normal to the quantum well plane. A dc electric current excited by cw terahertz laser radiation shows 1/B-oscillations with an amplitude much larger as compared to the photocurrent at zero magnetic field. We show that the photocurrent is caused by the combined action of a spatially periodic in-plane potential and the spatially modulated radiation due to the near field effects of light diffraction. Magnitude and direction of the photocurrent are determined by the degree of the lateral asymmetry controlled by the variation of voltages applied to the individual gates. The observed magneto-oscillations with enhanced photocurrent amplitude result from Landau quantization and, for (Cd,Mn)Te at low temperatures, from the exchange enhanced Zeeman splitting in diluted magnetic heterostructures. Theoretical analysis, considering the magnetic quantum ratchet effect in the framework of semiclassical approach, describes quite well the experimental results.

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arXiv2016

Photogalvanic probing of helical edge channels in 2D HgTe topological insulators

We report on the observation of a circular photogalvanic current excited by terahertz (THz) laser radiation in helical edge channels of HgTe-based 2D topological insulators (TIs). The direction of the photocurrent reverses by switching the radiation polarization from right-handed to left-handed one and, for fixed photon helicity, is opposite for the opposite edges. The photocurrent is detected in a wide range of gate voltages. With decreasing the Fermi level below the conduction band bottom, the current emerges, reaches a maximum, decreases, changes its sign close to the charge neutrality point (CNP), and again rises. Conductance measured over a 7 $μ$m distance at CNP approaches 2e2/h, the value characteristic for ballistic transport in 2D TIs. The data reveal that the photocurrent is caused by photoionization of helical edge electrons to the conduction band. We discuss the microscopic model of this phenomenon and compare calculations with the experimental data.

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arXiv2016

MIRO-like oscillations of magneto-resistivity in GaAs heterostructures induced by THz radiation

We report on the study of terahertz radiation induced MIRO-like oscillations of magneto-resistivity in GaAs heterostructures. Our experiments provide an answer on two most intriguing questions - effect of radiation helicity and the role of the edges - yielding crucial information for understanding of the MIRO origin. Moreover, we demonstrate that the range of materials exhibiting radiation-induced magneto-oscillations can be largely extended by using high-frequency radiation.

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arXiv2015

Probing quantum capacitance in a 3D topological insulator

We measure the quantum capacitance and probe thus directly the electronic density of states of the high mobility, Dirac type of two-dimensional electron system, which forms on the surface of strained HgTe. Here we show that observed magneto-capacitance oscillations probe, in contrast to magnetotransport, primarily the top surface. Capacitance measurements constitute thus a powerful tool to probe only one topological surface and to reconstruct its Landau level spectrum for different positions of the Fermi energy.

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arXiv2015

Cyclotron Resonance Assisted Photocurrents in Surface States of a 3D Topological Insulator Based on a Strained High Mobility HgTe Film

We report on the observation of cyclotron resonance induced photocurrents, excited by continuous wave terahertz radiation, in a 3D topological insulator (TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent formation is supported by complimentary measurements of magneto-transport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the effective masses and the mobility of 2D Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band.

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arXiv2014

Quantum Oscillations of Photocurrents in HgTe Quantum Wells with Dirac and Parabolic Dispersions

We report on the observation of magneto-oscillations of terahertz radiation induced photocurrent in HgTe/HgCdTe quantum wells (QWs) of different widths, which are characterized by a Dirac-like, inverted and normal parabolic band structure. The photocurrent data are accompanied by measurements of photoresistance (photoconductivity), radiation transmission, as well as magneto-transport. We develop a microscopic model of a cyclotron-resonance assisted photogalvanic effect, which describes main experimental findings. We demonstrate that the quantum oscillations of the photocurrent are caused by the crossing of Fermi level by Landau levels resulting in the oscillations of spin polarization and electron mobilities in spin subbands. Theory explains a photocurrent direction reversal with the variation of magnetic field observed in experiment. We describe the photoconductivity oscillations related with the thermal suppression of the Shubnikov-de Haas effect.

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arXiv2013

Transport properties of a 3D topological insulator based on a strained high mobility HgTe film

We investigated the magnetotransport properties of strained, 80nm thick HgTe layers featuring a high mobility of mu =4x10^5 cm^2/Vs. By means of a top gate the Fermi-energy is tuned from the valence band through the Dirac type surface states into the conduction band. Magnetotransport measurements allow to disentangle the different contributions of conduction band electrons, holes and Dirac electrons to the conductivity. The results are are in line with previous claims that strained HgTe is a topological insulator with a bulk gap of ~15meV and gapless surface states.

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arXiv2013

Giant spin-polarized current in a Dirac fermion system at cyclotron resonance

We report on the observation of the giant spin-polarized photocurrent in HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum emerges. Exciting QW of 6.6 nm width by terahertz (THz) radiation and sweeping magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative (-0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical gating. The photocurent data, accompanied by measurements of radiation transmission as well as Shubnikov-de Haas and quantum Hall effects, give an evidence that the enhancement of the photocurrent is caused by cyclotron resonance in a Dirac fermion system. The developed theory shows that the current is spin polarized and originates from the spin dependent scattering of charge carriers heated by the radiation.

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