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 Pb_1−xSn_xSe. It is shown that highly conductive surface electron states are present in Pb_1−xSn_xSe both in the inverse and direct electron energy spectrum.

Terahertz photoconductivity in heterostructures based on n-type Hg_1−xCd_xTe 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.

A review of recent results of experimental investigations devoted to studying unusual properties of impurity states in doped narrow-gap lead telluride-based semiconductors is presented. These results are analyzed in the framework of existing theoretical concepts.

We report on the observation of the giant negative magnetoresistance effect in PbTe doped simultaneously with Yb and Mn. In some impurity concentration range, the sample resistance at the liquid helium temperature drops by about three orders of magnitude when the magnetic field increases up to 5–6 T, with subsequent rise by about one order of magnitude as the field increases to 17.5 T. The effect is observed up to T=35 K. The possible origin of the effect is discussed.

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.

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 (Cd_1−xZn_x)₃As₂. We performed a study on a range of (Cd_1−xZn_x)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.