Ab initio investigation of layered and low-symmetry oxides and interfaces: multiferroic junctions, TCO’s, and a ferroelectric metal.

This essay is intended to be a review of the entire resear h work developed during my PhD s hool, highly based on published papers and enri hed by ba kground introdu tions of the debated topi s. My resear h fo used on several distin t topi s sharing the use of ab-initio density fun tional theory al ulations for studying di erent types of materials aimed to te h- nologi al appli ations. First, it deals with the study of the tunneling magneto-resistan e and interfa e magneto- ele tri e e ts of the SrRuO3/PbTiO3/SrRuO3 multi-ferroi tunnel jun tion (MFTJ) with asymmetri interfa es, by using standard DFT approa hes. A work about V doping of PbTiO3, a proper multiferroi ompound, is also presented. Se ond, an on-going work about a layered perovskite Bi5Ti5O17 is presented. We predi t it as the rst ferroele tri metal: it has a non-zero density of states at the Fermi level, metal- like ondu tivity along with spontaneous polarization in zero eld and a depolarizing eld in a nite system. Third, it reports the investigation of the orrelated material CaCu2Ti4O12 by using dif- ferent density fun tional methods, where the pSIC method, developed by my resear h group, manages to give a orre t interpretation of the four opti al transitions assessed experimen- tally. Fouth, a study about the wide-gap semi ondu tor Ga2O3 is presented. By means of beyond-DFT approa hes su h as Hybrid fun tionals, the famous GW method and the pSIC method, we assess the anisotropy of opti al absorption in phase of Ga2O3, supporting the experimental spe tra. In addition, it presents a further DFT-GGA investigation of the doping of −Ga2O3 with In. We assess its ele troni and strutural properties, maximum In solubility, and the band alignment of interfa e between -Ga2O3 and (Ga1−xInx)2O3. Finally, it reports my ontribution to enhan e the development of the pSIC ode, providing it with two new tools, namely two routines for al ulating the diele tri fun tion and the X-ray absorption ross se tion, whi h aims to obtain a more dire t omparison between theoreti al band stru ture and experimental measure of opti al properties. The ode to al ulate the diele tri fun tion has been used in the gap hara terization of −Ga2O3. Some test appli ations are shown, where XAS routine, implemented to exploit pSIC method, allows us to a urately reprodu e experimental spetra of di erent types of materials.