N. N. Stepanov

Electrical conductivity and band structure of thin polycrystalline EuS films

A study of the electrical resistance of thin polycrystalline EuS films (0.4–0.8 μm thick) in the temperature range 120–480 K has provided the basis for a model of the band structure of this substance. It has been shown that the main impurity levels in thin polycrystalline EuS films are those related with localized states near the conduction band bottom, as well as the Ei donor levels of Eu ions outside regular lattice sites. The “tail” of the localized states extends in energy up to at least −0.45 eV.

Thermoelectric properties of TmTe under pressure up to 20 GPa

The effect of quasi-hydrostatic pressure up to 20 GPa on thermopower of single-crystal samples of thulium monotelluride and monosulfide was studied. The pressure dependences of the thermopower of thulium monotelluride were found to have features in pressure regions of 2 and 6 GPa, which can be associated with phase transitions to the cation mixed-valence state and tetragonal structure, respectively. The upper pressure boundary of stability of the thulium ion mixed-valence state in monotelluride was evaluated by comparing the pressure dependences of the thermopower of TmTe and TmS.

Investigation of the temperature dependences of the electrical resistance of SmS single crystals at different pressures

The pressure-induced shift of impurity levels under hydrostatic pressure at T = 300 K (−9.6 meV/MPa) has been obtained from measurements of the temperature dependences of the electrical resistance of SmS single crystals at different pressures. The obtained value confirms the validity of the existing model of the semiconductor-metal phase transition in samarium monosulfide.

Semiconductor-metal phase transition in LaBi under high pressure

The effect of pressure up to 22 GPa on the electrical resistance and thermopower of lanthanum monobismuthide at room temperature has been studied. A semiconductor-metal phase transition in the pressure range of 4–6 GPa has been revealed from the change in the sign of thermopower and the thermal dependence of the electrical resistance of LaBi. The observed inflections in the pressure dependences of the thermopower and electrical resistance of LaBi samples in the pressure range of 8–11 GPa can be ascribed to the structural phase transition from the B1 phase to the PT and B2 phases.

Investigation of impurity levels in thin polycrystalline SmS films

Data obtained in the study of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films (thickness ∼0.5–0.8 μm) performed in the temperature region 4.2–440 K have been used to correct the band structure model of this material. It has been shown that the main impurity levels in thin polycrystalline SmS films are levels corresponding to localized states close to the conduction band bottom, as well as the impurity donor levels Ei which belong to Sm ions filling vacancies in the S sublattice. The tail of localized states has been found to extend up to the energy of impurity donor levels.

Study of the electrical properties of thin SmS films at high pressures

The pressure-induced shift of impurity levels under hydrostatic compression (−1.9 × 10−2 meV/MPa) at T = 300 K has been derived from measurements of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films on glass substrates at different pressures. The difference between the pressure-induced shifts of impurity levels in thin films and single crystals has been attributed to the effect of elastic properties of the substrate material. It has been shown that the semiconductor-metal phase transition in SmS films does not occur at pressures of up to 1000 MPa, because the impurity levels triggering the mechanism of phase transition at such pressures are not in the conduction band.

Mechanism of stabilization of the Sm1 − xGdxS metallic modification at the pressure-induced semiconductor-metal phase transition

A comparative analysis of the structural features and electrical parameters of Sm0.85Gd0.15S single crystals in the semiconducting phase and the metallic phase stable at atmospheric pressure suggests that, in materials with the Sm1 − xGdxS composition, the stabilization of the metallic phase formed under a high hydrostatic pressure occurs as a result of the increase in the volume fraction of defects in the material. The latter is reflected in a decrease in the size of coherent X-ray scattering regions. The persistence of the metallic SmS phase formed in the near-surface layer after the polishing of the sample surface allows a similar explanation.

Study of electrical resistivity of semiconductor SmS in the absence of a metallic phase on the surface

The temperature dependences of the electrical resistivity of samarium monosulfide single-crystal samples subjected to chemical treatment to remove a metallic phase from their surfaces have been measured in the range of 1.5–400 K at atmospheric pressure and at a pressure of 0.3 GPa. The temperature dependences of the activation energy of conduction electrons at these pressures and the piezoresistance coefficient of uniform compression have been calculated. It has been shown that the known model of the structure of the impurity-level spectrum in SmS remains partially valid at temperatures higher than 15 K. At lower temperatures, the existence of shallow donor centers in SmS and the hopping conduction over them should be taken into account.

Hopping conduction in EuS polycrystals

The dependences of the electrical conductivity of europium monosulfide (EuS) polycrystals on temperature in the range 150–400 K and pressures to 620 MPa have been studied. It is shown that the electrotransfer is mainly performed due to a hopping mechanism.

Thermopower of lanthanum monochalcogenides subjected to uniform compression up to 22 GPa

It is shown that lanthanum monochalcogenides (LaS, LaSe, LaTe), in which metal ions are trivalent at least up to 22 GPa, may be used as reference materials in finding the stability domains of a variable-valence state in rare-earth elements in different compounds studied under pressure. The thermopower of these materials throughout the pressure interval studied varies between 1 and 4 μV/K.