ИСТИНА

The discovery of high-temperature superconductivity up to 100 K in a monolayer FeSe on SrTiO3 has caused a variety of disputes on how superconductivity evolves in such materials from bulk to film, because bulk FeSe exhibits a TC not higher than 10 K. Moreover, for multilayer FeSe charge carrier doping convert non-superconducting films with various thicknesses into superconductors with Tc up to 48 K. In order to predict changes of electronic structure leading to TC increase we need to carefully describe electronic structure of low-dimensional iron chalcogenides and understand mechanisms of charge-transfer in such systems. In this work we have investigated changes of basic superconducting properties and electronic structures during the transition from bulk material to two-dimensional one for iron chalcogenides FeSe and FeTe by combination of experimental and computational techniques. Computational band structures and density of states (of FeSe, FeTe, for 1, 2, 3, 4, 5, 10, 15 layers and bulk material) were performed by ab initio calculations using projector augmented plane-wave (PAW) method with PBE exchange correlation functional. Bulk single crystals of tetragonal iron selenide and telluride for experimental studies were grown in evacuated quartz ampoules using flux technique (in melts of halides of alkali metals and aluminum) at a steady-state temperature gradient along the ampoule. The growth technique was developed in our laboratory and allows us to obtain high-quality single crystals avoiding such problems as crystal defects and heterogeneity associated with changing of thermodynamic parameters during the process of flux technique synthesis with non-constant temperature gradient. The microstructure and chemical composition of the samples were studied by SEM/EDX, HRTEM, X-ray diffraction. We have found that all produced crystals are tetragonal, samples have satisfactorily homogeneous composition and correct tetragonal shape of the crystals. Low-dimensional samples such as monolayers and multilayers will be obtained using mechanical exfoliation of these single crystals. Our results present systematization of electronic structure data for bulk and low-dimensional iron chalcogenides. For now we are in process of computational data analyzing and initial characterization and measuring temperature dependences of resistivity and other superconducting parameters for different samples. Acknowledgements The reported study was funded by RFBR according to the research project No.16-32-00435 мол_a.