Pustovit Y. Electronic structure of FeSe1-xTex in vicinity to Fermi level and its temperature induced changes

PhD thesis is devoted to the investigation of characteristic features of electronic structure of the FeSe and FeSe-based superconductors and to experimental investigation of temperature-induced changes of the electronic structure of FeSeTe crystals. In the case of FeSe, experimentally obtained band structure shows a downward shift of the zones in the corner of the Brillouine zone and an upward shift in the center. These experimentally observed shifts happen in the directions that are opposite to those suggested by the calculation of the FeSe electronic band structure. By using a quadratic approximation of experimentally obtained and calculated bands renormalization factors have been obtained for different bands of various FeSe-based compounds. It has been shown that renormalization factor values lie in a much narrower interval (1-9), than it was evaluated in previously published papers (2-20). Using two dimensional hopping model and taking into account hopping between three nearest neighbors, the bands in Г-M cut for various FeSe-based superconductors have been approximated. Comparison between the approximation coefficients of the experimentally obtained and calculated bands suggests that in the frame of the hopping model such shifts can be explained by blocking of the hopping between nearest neighbors. The reason for this dramatic decrease of hopping probability between nearest neighbors can be appearance of some kind of ordering that have not been taken into account in the calculations. ARPES spectra of Z point of FeSeTe have been obtained in a wide temperature range (20-250K) using synchrotron radiation with horizontal and vertical polarization to investigate temperature-induced changes of the band structure. MDC and EDC analysis together with digital image processing methods have been used for processing experimentally obtained spectra. It has been established that best precision of band position determination is achieved by curvature method. Processing of ARPES data obtained by the radiation with vertical polarization has given possibility to determine dyz band with 1 meV resolution. From EDC analysis the direction of dxy band movement has been determined and maximal (2.1 meV per 10K) and minimal (0.8meV per 10K) rate of temperature changes have been evaluated. Analogous analysis of ARPES spectra obtained by radiation with horizontal polarization confirms dyz band position determined from horizontal data and enabled determination of the direction and rate of dxz band evolution. It has been shown that temperature induced changes of dyz band position in Z point of FeSeTe in wide temperature range could have been explained by strong temperature dependence of quasiparticle self-energy, however taking into account changes of band structure in the Brillouin zone corner such model can’t be used, because these changes break electron-hole parity. Also changes in the number of carriers violates Luttinger theorem, which is implausible. Based on these considerations conclusions are made that temperature induced changes of band structure is the result of redistribution of electrons between different phases, coexisting in the sample, or the result of delocalization of electrons.