D. A. Kozlov

Two-dimensional electron-hole system in a HgTe-based quantum well

A two-dimensional electron-hole system consisting of light high-mobility electrons with a density of Ns = (4–7) × 1010 cm−2 and a mobility of μn = (4–6) × 105 cm2/V s and heavier low-mobility holes with a density of Ps = (0.7–1.6) × 1011 cm−2 and a mobility of μp = (3–7) × 104 cm2/V s has been discovered in a quantum well based on mercury telluride with the (013) surface orientation. The system exhibits a number of specific magnetotransport properties in both the classical magnetotransport (positive magnetoresistance and alternating Hall effect) and the quantum Hall effect regime. These properties are associated with the coexistence of two-dimensional electrons and holes.

Cyclotron resonance of Dirac ferions in HgTe quantum wells

Cyclotron resonance of single-valley two-dimensional Dirac fermions in HgTe-based quantum wells has been experimentally investigated. The thickness of the wells is close to the critical value corresponding to the transition from the direct energy spectrum to the inverted spectrum. Under terahertz laser irradiation, transitions between the ground and first Landau levels, as well as between the first and second Landau levels, have been observed. Low magnetic fields corresponding to the cyclotron resonance, as well as the strong dependence of the position of the resonance on the electron density, indicate the Dirac character of the spectrum in these quantum wells. It has been shown that disorder plays an important role in the formation of the spectrum of two-dimensional Dirac fermions.

Cyclotron resonance in a two-dimensional semimetal based on a HgTe quantum well

Microwave cyclotron resonance of electrons and holes at the metal-to-semimetal transition in HgTe quantum wells with an inversed band structure has been investigated. The resonance has been studied by measuring microwave photoresistance in the frequency range of 35–170 GHz. The effective cyclotron masses of electrons and holes have been determined. A shift of the cyclotron resonance of the two-dimensional electrons at the metal-to-semimetal transition possibly caused by plasma effects in the two-dimensional semimetal has been discovered.

Two-dimensional electron systems in HgTe quantum wells

A brief review is given on the results of research on the properties of two-dimensional (2D) electron systems in quantum wells based on CdHgTe∕HgTe∕CdHgTe double heterojunctions with surface orientations of (100) and (013). The main features of the energy spectrum are described. The results of experiments yielding information about the parameters of the energy spectrum are presented. From cyclotron resonance measurements the effective mass of the 2D electrons in HgTe quantum wells with inverted band structure is obtained as a function of the density of such electrons, Ns, in the range 2.2×1011cm−2⩽Ns⩽9.6×1011cm−2. This density dependence indicates an appreciable nonparabolicity of the spectrum: the mass increases with increasing Ns in that range from a value (0.026±0.005)m0 to (0.0335±0.005)m0. The giant spin splitting observed in asymmetric HgTe quantum wells is discussed, and the results of experimental research on the transition from a quantum Hall liquid to an insulator and the plateau-plateau transit...

Weak localization of Dirac fermions in HgTe quantum wells

Weak localization in a system of gapless two-dimensional Dirac fermions in HgTe quantum wells with thickness d = 6.6 nm, which corresponds to the transition from a normal to an inverted spectrum, has been investigated experimentally. A negative logarithmic correction to the conductivity of the system has been observed both at the Dirac point and in the vicinity of this point. The anomalous magnetoresistance of two-dimensional Dirac fermions is positive. This indicates that weak localization in the system of two-dimensional Dirac fermions occurs owing to localization and interaction effects in the presence of rapid spin relaxation.

Two-dimensional semimetal in HgTe-based quantum wells

The first results are reported from a study of a new two-dimensional electron system, a two-dimensional semimetal, that is observed in wide quantum wells based on mercury telluride, which have an inverted band spectrum. Magnetotransport experiments confirm the existence of a semimetal state in quantum wells with (013) and (112) orientations and thicknesses of 18–21nm. These experiments show that the band overlap Δ=3–5meV. A comparison of the experimentally determined Δ with a theoretical calculation of the energy spectrum reveals the fundamental role of strain effects in the formation of the semimetal state. Scattering processes in the two-dimensional semimetal are studied and it is found that the jump in the electron mobility during electronic metal-two-dimensional semimetal transitions is caused by shielding of electron scattering on impurities by holes. The substantial, anomalous rise in the resistivity of the two-dimensional semimetal with increasing temperature is caused by electron-hole scattering. ...

Terahertz radiation-induced magnetoresistance oscillations of a high-density and high-mobility two-dimensional electron gas

The terahertz response of a high-density and high-mobility two-dimensional electron gas in 13-nm GaAs quantum wells at frequencies of 0.7 and 1.63 THz has been investigated. Terahertz radiation-induced magnetoresistance oscillations have been discovered. The oscillation maxima coincide with the harmonics of cyclotron resonance. It has been shown that a large number of harmonics (up to the ninth) appear under irradiation at a frequency of 0.7 THz. In this case, the effect is the analogue of microwave-induced oscillations. At a higher frequency, the oscillation amplitude decreases drastically with an increase in the harmonic number. This indicates a transition to the regime of ordinary cyclotron harmonics.

Quantum hall effect in a system of gapless Dirac fermions in HgTe quantum wells

The specific features of the quantum Hall effect (QHE) in a system of gapless Dirac fermions in HgTe quantum wells have been studied. It has been established that the behavior of the QHE is asymmetric with respect to the Dirac point: at the lowest temperature (0.2 K), quantization of hole fermions occurs in weak magnetic fields down to 0.15 T, whereas quantization of the electron part of the Dirac cone occurs only in fields of about 0.5 T. Such an asymmetry is caused by the effect of sideband maxima in the valence band forming the heavy-hole valleys, which play the role of a reservoir and screen out the fluctuation potential. The analysis of the behavior of the dissipative component of the conductivity near the Dirac point has led to a conclusion on the existence of the zeroth Landau level at this point.

Microwave response of a ballistic quantum dot

The microwave response (photovoltage and photoconductance) of a lateral ballistic quantum dot made of a high-mobility two-dimensional electron gas in an AlGaAs/GaAs heterojunction has been studied experimentally in the frequency range of 110–170 GHz. It has been found that the asymmetry of the photovoltage with respect to the sign of the magnetic field has mesoscopic character and depends on both the magnetic field and the microwave power. This indicates the violation of the Onsager reciprocity relations regarding the electron-electron interactions in the mesoscopic photovoltaic effect. A strong increase in the conductance of the quantum dot induced by the microwave radiation and unrelated to heating, as well as the microwave-induced magneto-oscillations, has been discovered.

Terahertz electron transport in a two-dimensional topological insulator in a HgTe quantum well

The terahertz response of a two-dimensional topological insulator in a HgTe quantum well to radiation with wavelengths of 118 and 184 μm is investigated. It is found that the photoconductivity is rather high (up to a few percent of dark conductivity) and is manifested in both the local and nonlocal responses of the system. This fact proves that the observed photoconductivity is caused by changes in the transport via edge current-carrying states. The sign and nonresonant character of the photoconductivity indicate that it is caused by the heating of electrons in the system. The analysis of experimental results makes it possible to suggest that this heating originates from the Drude absorption of terahertz radiation by metallic “droplets” appearing owing to fluctuations in the impurity potential and the gap and located in direct proximity to edge states.