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We study the possibility of pressure-induced transitions from a normal semiconductor to a topological insulator (TI) in bismuth tellurohalides using density functional theory and tight-binding method. In BiTeI this transition is realized through the formation of an intermediate phase, a Weyl semimetal, that leads to modification of surface state dispersions. In the topologically trivial phase, the surface states exhibit a Bychkov-Rashba type dispersion. The Weyl semimetal phase exists in a narrow pressure interval of 0.2 GPa. After the Weyl semimetal-TI transition occurs, the surface electronic structure is characterized by gapless states with linear dispersion. The peculiarities of the surface states modification under pressure depend on the band-bending effect. We have also calculated the frequencies of Raman active modes for BiTeI in the proposed high-pressure crystal phases in order to compare them with available experimental data. Unlike BiTeI, in BiTeBr and BiTeCl the topological phase transition does not occur. In BiTeBr, the crystal structure changes with pressure but the phase remains a trivial one. However, the transition appears to be possible if the low-pressure crystal structure is retained. In BiTeCl under pressure, the topological phase does not appear up to 18 GPa due to a relatively large band gap width in this compound.

Strong topological insulators (TIs) support topological surfaces states on any crystal surface. In contrast, a weak, time-reversal-symmetry-driven TI with at least one non-zero v 1 , v 2 , v 3 ℤ 2 index should host spin-locked topological surface states on the surfaces that are not parallel to the crystal plane with Miller indices ( v 1   v 2   v 3 ). On the other hand, mirror symmetry can protect an even number of topological states on the surfaces that are perpendicular to a mirror plane. Various symmetries in a bulk material with a band inversion can independently preordain distinct crystal planes for realization of topological states. Here we demonstrate the first instance of coexistence of both phenomena in the weak 3D TI Bi 2 TeI which ( v 1   v 2   v 3 ) surface hosts a gapless spin-split surface state protected by the crystal mirror-symmetry. The observed topological state has an even number of crossing points in the directions of the 2D Brillouin zone due to a non-TRIM bulk-band inversion. Our findings shed light on hitherto uncharted features of the electronic structure of weak topological insulators and open up new vistas for applications of these materials in spintronics.

The results of a theoretical study of the Fe 3 SiTe 2 compound, belonging to a promising class of the van der Waals systems with high Curie temperature and gigantic magnetoresistance, are presented. It is found out that Fe 3 SiTe 2 represents a ferromagnetic metal characterized by high spin-polarization values at the Fermi level. It is shown that the charge transport occurs due to a contribution from the spin-down electrons and is formed prevailingly in the plane of five-layer blocks.

The results of a theoretical study of the Fe.sub.3SiTe.sub.2 compound, belonging to a promising class of the van der Waals systems with high Curie temperature and gigantic magnetoresistance, are presented. It is found out that Fe.sub.3SiTe.sub.2 represents a ferromagnetic metal characterized by high spin-polarization values at the Fermi level. It is shown that the charge transport occurs due to a contribution from the spin-down electrons and is formed prevailingly in the plane of five-layer blocks.

The scattered-wave method is used to calculate electron spectra of central Ni8Ti, Ti8Ni, and Ti8Al nanoclusters in nanoparticles of (Ni26Ti64) Al1 and (Ti26Ni64) Al1 alloys in the B2 structure. The single Al atom is shown to have a better chance for accommodation on the titanium sublattice because of high binding energy.

We apply both analytical and ab-initio methods to explore heterostructures composed of a threedimensional topological insulator (3D TI) and an ultrathin normal insulator (NI) overlayer as a proof ground for the principles of the topological phase engineering. Using the continual model of a semi-infinite 3D TI we study the surface potential (SP) effect caused by an attached ultrathin layer of 3D NI on the formation of topological bound states at the interface. The results reveal that spatial profile and spectrum of these near-surface states strongly depend on both the sign and strength of the SP. Using ab-initio band structure calculations to take materials specificity into account, we investigate the NI/TI heterostructures formed by a single tetradymite-type quintuple or septuple layer block and the 3D TI substrate. The analytical continuum theory results relate the near-surface state evolution with the SP variation and are in good qualitative agreement with those obtained from density-functional theory (DFT) calculations. We predict also the appearance of the quasi-topological bound state on the 3D NI surface caused by a local band gap inversion induced by an overlayer.

The electronic structure and the energy of the ground state of nanoclusters of a number of 3d transition metals and their compounds have been studied in terms of the method of scattered waves. It is shown that a substantial difference in the electronic structure of cluster fragments and bulky materials leads to significantly different dependences of the differences of the energies of crystalline modifications on the average electron concentration.[PUBLICATION ABSTRACT]

The unoccupied states in topological insulators Bi_2Se_3, PbSb_2Te_4, and Pb_2Bi_2Te_2S_3 are studied by the density functional theory methods. It is shown that a surface state with linear dispersion emerges in the inverted conduction band energy gap at the center of the surface Brillouin zone on the (0001) surface of these insulators. The alternative expression of Z_2 invariant allowed us to show that a necessary condition for the existence of the second Gamma Dirac cone is the presence of local gaps at the time reversal invariant momentum points of the bulk spectrum and change of parity in one of these points.

On the basis of relativistic ab-initio calculations we show that the driving mechanism of simultaneous emergence of parabolic and M-shaped 2D electron gas (2DEG) bands at the surface of layered topological insulators as well as Rashba-splitting of the former states is an expansion of van der Waals (vdW) spacings caused by intercalation of metal atoms or residual gases. The expansion of vdW spacings and emergence of the 2DEG states localized in the (sub)surface region are also accompanied by a relocation of the topological surface state to the lower quintuple layers, that can explain the absence of interband scattering found experimentally.

The unoccupied part of the band structure of topological insulators Bi\\(_2\\)Te\\(_{x}\\)Se\\(_{3-x}\\) (\\(x=0,2,3\\)) is studied by angle-resolved two-photon photoemission and density functional theory. For all surfaces linearly-dispersing surface states are found at the center of the surface Brillouin zone at energies around 1.3 eV above the Fermi level. Theoretical analysis shows that this feature appears in a spin-orbit-interaction induced and inverted local energy gap. This inversion is insensitive to variation of electronic and structural parameters in Bi\\(_2\\)Se\\(_3\\) and Bi\\(_2\\)Te\\(_2\\)Se. In Bi\\(_2\\)Te\\(_3\\) small structural variations can change the character of the local energy gap depending on which an unoccupied Dirac state does or does not exist. Circular dichroism measurements confirm the expected spin texture. From these findings we assign the observed state to an unoccupied topological surface state.