Vladimir Chernichkin

Detection of highly conductive surface electron states in topological crystalline insulators Pb1−xSnxSe using laser terahertz radiation

We suggest a method for detection of highly conductive surface electron states including topological ones. The method is based on measurements of the photoelectromagnetic effect using terahertz laser pulses. In contrast to conventional transport measurements, the method is not sensitive to the bulk conductivity. The method is demonstrated on an example of topological crystalline insulators Pb1−xSnxSe. It is shown that highly conductive surface electron states are present in Pb1−xSnxSe both in the inverse and direct electron energy spectrum.

Observation of local electron states linked to the quasi-Fermi level

We report on the observation of semiconductor local electron states linked to the quasi-Fermi level and, consequently, not characterized by the defined position in the energy spectrum which is familiar for shallow and deep impurities. This type of local electron states have been found in the doped narrow-gap semiconductor Pb1−xSnxTe(In). The binding energy of these states is less than 10 meV providing photoresponse at the wavelengths exceeding 100 μm.

Probing of local electron states in Pb 1−x Sn x Te(In) narrow-gap semiconductors using laser terahertz radiation

Local electron states in indium-doped lead telluride-based solid solutions exhibit a number of features which separate them from the diversity of impurity states in semiconductors. These features are most pronounced in terahertz photoconductivity. The results of the corresponding experiments performed during the last years and supported by the Russian Foundation for Basic Research are reviewed.

Monopolar photoelectromagnetic effect in Pb1−xSnxTe(In) under terahertz laser radiation

We report on the observation of a new effect?the appearance of a galvanic signal in the narrow-gap semiconductor Pb1-xSnxTe(In) in the magnetic field under the action of strong 100 ns-long terahertz laser pulses. The signal changes its sign and kinetics as the temperature increases from 4.2 K to 25 K. This semiconductor possesses inversion symmetry of the crystalline lattice making impossible the observation of the magnetophotogalvanic effect which looks similar in its experimental manifestation. On the other hand, the laser quantum energy is much less than the bandgap of the semiconductor making the effect considerably different from the conventional Kikoin?Noskov photoelectromagnetic effect. Possible mechanisms responsible for the appearance of the effect are discussed.

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.

PbTe(In) films with variable microstructure forphotodetection in IR and terahertz range

The work deals with studies of the grain size and surface state effect on photoelectric and transport properties of PbTe(In) films in the temperature range from 4.2 K up to 200 K under irradiation of a blackbody source and terahertz laser pulses. The PbTe(In) films were deposited on insulating substrates kept at the temperatures TS equal to -120 (see manuscript) 250C. AFM, SEM, Auger spectroscopy and X-ray diffraction were used to study the film microstructure. Increase of the TS value led to mean grain size growth from 60 up to 300 nm. All films had a column-like structure with the columns nearly perpendicular to the substrate plane. It is shown that microstructure of the films strongly affects the photoconductivity character in the terahertz region of the spectrum. Positive persistent photoresponse is observed at low temperatures in the polycrystalline films. For these films transport and photoelectric properties are determined by the grain volume and impurity state specifics. Nanocrystalline films have all features of non-homogeneous systems with band modulation. For these films only negative photoconductivity is observed in the whole temperature range. Possible mechanisms of the photoresponse formation are discussed.

Photoconductivity of PbTe:In films with variable microstructure

It is shown that the microstructure and features of formation of surface states in nanocrystalline and polycrystalline PbTe:In films most significantly affect the character of photoconductivity in the spectral range of 1–2.5 THz. We present the results of a study and comparative analysis of the character of conductivity of PbTe:In films in the temperature range from 4.2 to 300 K in a static mode and in variable electric fields with a frequency of up to 1 MHz with illumination with white light and under the effect of high-power terahertz laser pulses with a wavelength of up to 280 μm.

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.

Discrimination of Conductive Surface Electron States by Laser Terahertz Radiation in PbSe—A Base for Pb

We report on the influence of oxidation on features of surface electron states in PbSe films-materials belonging to the family of topological crystalline insulators Pb1-xSnxSe. These states are detected through observation of the photoelectromagnetic effect induced by terahertz laser pulses. It is demonstrated that highly conductive surface electron states in PbSe are inherent to the semiconductor itself and are not related to the material oxidation. This allows excluding surface states induced by oxidation as a reason for high surface conductivity in topological crystalline insulators based on Pb1-xSnxSe.

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A new type of semiconductor local states is revealed in lead-tin telluride solid solutions doped with indium. The energy position of these states is not linked to any specific location in the semiconductor energy spectrum, but follows the quasiFermi level position, which may be tuned by photoexcitation. The binding energy of these states is less than 10 meV providing appearance of photoresponse at wavelengths exceeding 100 μm. This conclusion is based on the results of experiments that use illumination of the samples by 100 ns – long pulses of an optically pumped NH3 terahertz laser with the wavelengths of 90, 148 and 280 μm corresponding to the quanta energies of 13.8, 8.4 and 4.4 meV, respectively.