A. V. Galeeva

Photoconductivity of the narrow-gap Pb1 − xSnxTe(In) semiconductors in the terahertz spectral range

The spectral dependence of photoconductivity in the doped narrow-gap semiconductor Pb0.75Sn0.25Te(In) under the action of terahertz laser radiation pulses has been studied at temperatures 4.2–30 K. It is shown that the photoconductivity spectrum of the semiconductor spreads at least up to the wavelength of 500 μm. This value is more than twice higher than the red cutoff wavelength of 220 μm in uniaxially stressed Ge(Ga) which is known as the most long-wavelength photodetector among sensitive photon detectors of radiation. Mechanisms responsible for photosensitivity of PbSnTe(In) in the terahertz spectral range are discussed.

Electron energy relaxation under terahertz excitation in (Cd1−xZnx)3As2 Dirac semimetals

We demonstrate that measurements of the photo-electromagnetic effect using terahertz laser radiation provide an argument for the existence of highly conductive surface electron states with a spin texture in Dirac semimetals (Cd1− xZnx)3As2. We performed a study on a range of (Cd1− xZnx)3As2 mixed crystals undergoing a transition from the Dirac semimetal phase with an inverse electron energy spectrum to trivial a semiconductor with a direct spectrum in the crystal bulk by varying the composition x. We show that for the Dirac semimetal phase, the photo-electromagnetic effect amplitude is defined by the number of incident radiation quanta, whereas for the trivial semiconductor phase, it depends on the laser pulse power, irrespective of wavelength. We assume that such behavior is attributed to a strong damping of the interelectron interaction in the Dirac semimetal phase compared to the trivial semiconductor, which may be due to the formation of surface electron states with a spin texture in Dirac semimetals.

Manifestation of topological surface electron states in the photoelectromagnetic effect induced by terahertz laser radiation

We demonstrate that measurements of the photoelectromagnetic effect using terahertz laser radiation may provide a unique opportunity to discriminate between the topological surface states and other highly conductive surface electron states. We performed a case study of mixed (BiIn x )2Se3 crystals undergoing a topological phase transformation due to the transition from the inverse to the direct electron energy spectrum in the crystal bulk at variation of the composition x. We show that for the topological insulator phase, the photoelectromagnetic effect amplitude is defined by the number of incident radiation quanta, whereas for the trivial insulator phase, it depends on the power in a laser pulse irrespective of its wavelength. We assume that such behavior is attributed to a strong damping of the electron–electron interaction in the topological insulator phase compared to the trivial insulator.

Magnetic-field-induced terahertz photogeneration in PbTe(Ga)

It has been shown that a magnetic field in PbTe(Ga) single crystals induces the appearance of a positive photoresponse in the terahertz spectral range. Nonequilibrium charge carriers are generated from states located against the background of the continuum of the conduction band near the quasi-Fermi level. The density of states on the quasi-Fermi level is a critical parameter responsible for the process of generation.

Non-equilibrium electron transport induced by terahertz radiation in the topological and trivial phases of Hg1−xCdxTe

Terahertz photoconductivity in heterostructures based on n-type Hg1− xCdxTe epitaxial films both in the topological phase (x < 0.16, inverted band structure, zero band gap) and the trivial state (x > 0.16, normal band structure) has been studied. We show that both the positive photoresponse in films with x < 0.16 and the negative photoconductivity in samples with x > 0.16 have no low-energy threshold. The observed non-threshold positive photoconductivity is discussed in terms of a qualitative model that takes into account a 3D potential well and 2D topological Dirac states coexisting in a smooth topological heterojunction.

Terahertz Photoconductivity in Hg 1− x Cd x Te near the transition from the direct to inverted spectrum

For the Hg1−xCdxTe-based structures, it is shown that the transition from the direct to invertеd spectrum is accompanied by the sign change for the signals related to the terahertz photoconductivity and to the magnetophotogalvanic effect. Within the range of chemical compositions corresponding to the inverted spectrum, the photoconductivity kinetics exhibits specific features, which can result from the surface topological states.

Intermetallic solid solution Fe{sub 1-x}Co{sub x}Ga{sub 3}: Synthesis, structure, NQR study and electronic band structure calculations

Unlimited solid solution Fe1� xCoxGa3 was prepared from Ga flux. Its crystal structure was refined for Fe0.5Co0.5Ga3 (P42/mnm, a¼6.2436(9), c ¼ 6.4654(13), Z¼ 4) and showed no ordering of the metal atoms. A combination of the electronic band structure calculations within the density functional theory (DFT) approach and 69,71 Ga nuclear quadrupole resonance (NQR) spectroscopy clearly shows that the Fe–Fe and Co–Co dumbbells are preferred to the Fe–Co dumbbells in the crystals structure. The band structure features a band gap of about 0.4 eV, with the Fermi level crossing peaks of a substantial density of electronic states above the gap for x40. The solid solution is metallic for x40.025. The study of the nuclear spin–lattice relaxation shows that the rate of the relaxation, 1/T1, is very sensitive to the Co concentration and correlates well with the square of the density of states at the Fermi level, N 2 (EF).

New class of highly sensitive terahertz detectors

The properties of doped semiconductors based on lead telluride, which can be used to design highly sensitive terahertz photodetectors, are described. It is shown that these properties allow one to develop photodetectors with much better parameters than those of the best world analogs.