Browsing by Author "Himmetoglu, B."
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Item Determination of the Mott-Hubbard gap in GdTiO3(American Physical Society, 2015-08-06) Bjaalie, L.; Verma, A.; Himmetoglu, B.; Janotti, Anderson; Raghavan, S.; Protasenko, V.; Steenbergen, E. H.; Jena, D.; Stemmer, S.; Van de Walle, Chris G.; L. Bjaalie, A. Verma, B. Himmetoglu, A. Janotti, S. Raghavan, V. Protasenko, E. H. Steenbergen, D. Jena, S. Stemmer, and C. G. Van de Walle; Janotti, AndersonThe band gaps of rare-earth titanates are commonly reported to be 0.2–0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report experimental and theoretical results on GdTiO3 (GTO) indicating that the gap is significantly larger. Photoluminescence (PL) measurements show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. First-principles calculations, based either on density-functional theory (DFT) with a hybrid functional or on DFT+U, consistently show that the gap is close to 2 eV. We also propose an interpretation of the previously reported optical absorption spectra. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials.Item Structural and electronic properties of SrZrO3 and Sr(Ti,Zr)O3 alloys(American Physical Society, 2015-08-11) Weston, L.; Janotti, Anderson; Cui, X. Y.; Himmetoglu, B.; Stampfl, C.; Van de Walle, Chris G.; L. Weston, A. Janotti, X. Y. Cui, B. Himmetoglu, C. Stampfl, and C. G. Van de Walle; Janotti, AndersonUsing hybrid density functional calculations, we study the electronic and structural properties of SrZrO3 and ordered Sr(Ti,Zr)O3 alloys. Calculations were performed for the ground-state orthorhombic (Pnma) and high-temperature cubic (Pm3m) phases of SrZrO3. The variation of the lattice parameters and band gaps with Ti addition was studied using ordered SrTix Zr1−x O3 structures with x = 0, 0.25, 0.5, 0.75, and 1. As Ti is added to SrZrO3, the lattice parameter is reduced and closely follows Vegard’s law. On the other hand, the band gap shows a large bowing and is highly sensitive to the Ti distribution. For x = 0.5, we find that arranging the Ti and Zr atoms into a 1 × 1 SrZrO3/SrTiO3 superlattice along the [001] direction leads to interesting properties, including a highly dispersive single band at the conduction-band minimum (CBM), which is absent in both parent compounds, and a band gap close to that of pure SrTiO3. These features are explained by the splitting of the lowest three conduction-band states due to the reduced symmetry of the superlattice, lowering the band originating from the in-plane Ti 3dxy orbitals. The lifting of the t2g orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. Our results demonstrate how short-period SrZrO3/SrTiO3 superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk SrTiO3.