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.

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 (Bi -x 1 In x ) 2 Se 3 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.

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Microstructure, charge transport and photoelectric properties are studied for PbTe(In) films deposited on amorphous substrates kept at different temperatures Ts during deposition. All the films have a column‐like structure with the mean column diameter increasing from 60 nm at the lowest Ts = ‐120 °C up to 300 nm at the maximal Ts = 250 °C. The substrate temperature increase results in appearance of a preferred orientation of grains and in an inversion of the conductivity type from hole to electron. Electric transport and photoelectric properties of the films were studied in the temperature interval 4.2 – 300 K in DC and AC modes at frequencies varied from 20 Hz up to 1 MHz. The impedance spectra analysis was done in terms of the equivalent circuit model. Modification of charge transport mechanisms is correlated with the variation of microstructure of the films grown at different substrate temperatures. Films with disordered grains possess all features of non‐homogeneous systems, while the films with a preferred orientation of grains are analogous to PbTe(In) single crystals in their transport and photoelectric properties. Mechanisms of the persistent photoconductivity observed in the films are qualitatively distinguished. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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 Pb Sn Se. 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 Pb Sn Se.Index Terms-IV-VI semiconductor materials, photoelectromagnetic effect, surface conductivity, terahertz, topological insulators (TIs). and also with the P.N. Lebedev Physical Institute,

We report on the observation of a new effect-the appearance of a galvanic signal in the narrow-gap semiconductor Pb 1−x Sn x Te(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.

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