Alexander Yu. Sipatov

The dependences of the electrical conductivity, the Hall coefficient, and the Seebeck coefficient on the layer thickness d (d = 18−600 nm) of p-type topological insulator Bi₂Te₃ thin films grown by thermal evaporation in vacuum on glass substrates were obtained at room temperature. In the thickness range of d = 18–100 nm, sustained oscillations with a substantial amplitude were revealed. The observed oscillations are well approximated by a harmonic function with a period Δd = (9.5 ± 0.5) nm. At d > 100 nm, the transport coefficients practically do not change as d is increased. The oscillations of the kinetic properties are attributed to the quantum size effects due to the hole confinement in the Bi₂Te₃ quantum wells. The results of the theoretical calculations of Δd within the framework of a model of an infinitely deep potential well are in good agreement with the experimental results. It is suggested that the substantial amplitude of the oscillations and their sustained character as a function of d are connected with the topologically protected gapless surface states of Bi₂Te₃ and are inherent to topological insulators.

This is a comprehensive study of the effect of strong magnetic fields on superconductivity in PbTe/PbS heterostructures with semiconducting layers of different thicknesses. Metallic conductivity and superconductivity (critical temperature T_c ≤ 6.5 K) in PbTe/PbS heterostructures are caused by inversion of bands along a continuous network of misfit dislocations that develops at the interfaces between semiconductor layers of sufficient thickness (d > 80 nm). With decreasing d the continuity of the superconducting interface is disrupted, T_c decreases, and the metallic conductivity changes to a semiconducting type. Disruption of the continuity of the superconducting interface is found to be a necessary condition for observing a magnetic-field induced superconductor-insulator transition (SIT) and has a significant influence on its features: a fan-like set of resistance curves R(T); intersection of the R(B) curves for fields perpendicular, as well as parallel, to the interface; and, negative magnetoresistance. A scaling analysis based on Fisher's theoretical model is carried out for these samples. No evidence of a SIT was observed in heterostructures with a perfect interface. It appears that the SIT effect is related to percolation phenomena characteristic of granular superconductors.

It is established that the room-temperature dependences of transport properties on the total thickness of PbSe layers d in PbSe/EuS superlattices exhibit an oscillatory behavior. It is shown that the oscillation period Δd practically coincides with the period of the thickness oscillations observed earlier in single PbSe/EuS quantum well. The non-monotonic character of these dependences is attributed to quantum size effects. The theoretically estimated and experimentally determined Δd values are in good agreement.

Single crystalline Bi nanowires were grown by extrusion from Bi/Co thin films. The films were obtained by thermal evaporation in high vacuum. The average diameter, length and density of obtained nanowires were 100 nm, 30 μm and 6.5×10⁵ cm⁻², respectively. The non-catalyzed self-organized process of whisker formation on the surface of immiscible composite thin film was exploited for nanowire growth. It was shown that the whiskers had formed during and after a thin film deposition. The value of residual stresses in a whole thin film coating as well as in its bismuth component was measured using X-ray diffraction technique. It was revealed that local compressive stresses, that had induced the whisker growth, had been formed by a segregation of Bi layers into Bi globules. A simple model of the whisker formation to minimize free energy in the Bi/Co system was proposed taking into account interfacial and elastic deformation energies. The obtained results can be utilized for growing of nanowires of other low-melting-point metals and semiconductors from immiscible composite thin films.

The magnetism of ferromagnetic metal/ferromagnetic semiconductor PbS/EuS/Fe/PbS thin films has been studied. The presence of antiferromagnetic alignment of neighboring ferromagnetic layers in direct contact with each other was observed. The films also showed an exchange bias effect which appeared due to the formation (likely during film deposition) of antiferromagnetic FeS at the Fe/PbS interface. Antiferromagnetic interfacial exchange coupling between Fe and FeS was also observed. It was shown that the exchange anisotropy could be controlled by the remnant state of Fe magnetization during the cooling process, that in turn could be tuned by an external cooling field. The influence of exchange anisotropy and exchange coupling effects on the field and temperature dependencies of the film magnetization was examined.