Hybrid exchange-correlation functional for accurate prediction of the electronic anD structural properties of ferroelectric oxides. / D. I. Bilc; R. Orlando; R. Shaltaf; G. M. Rignanese; J. Íñiguez; Ph. Ghosez. -In: PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS. -ISSN 1098-ISSN -0121. -77:16(2008, pp. 165107-1-165107-13. Original Citation:Hybrid exchange-correlation functional for accurate prediction of the electronic anD structural properties of ferroelectric oxides. Published version:DOI:10.1103/PhysRevB.77.165107 Terms of use:Open Access (Article begins on next page) Anyone can freely access the full text of works made available as "Open Access". Works made available under a Creative Commons license can be used according to the terms and conditions of said license. Use of all other works requires consent of the right holder (author or publisher) if not exempted from copyright protection by the applicable law. Using a linear combination of atomic orbitals approach, we report a systematic comparison of various density functional theory ͑DFT͒ and hybrid exchange-correlation functionals for the prediction of the electronic and structural properties of prototypical ferroelectric oxides. It is found that none of the available functionals is able to provide, at the same time, accurate electronic and structural properties of the cubic and tetragonal phases of BaTiO 3 and PbTiO 3 . Some, although not all, usual DFT functionals predict the structure with acceptable accuracy, but always underestimate the electronic band gaps. Conversely, common hybrid functionals yield an improved description of the band gaps, but overestimate the volume and atomic distortions associated with ferroelectricity, giving rise to an unacceptably large c / a ratio for the tetragonal phases of both compounds. This supertetragonality is found to be induced mainly by the exchange energy corresponding to the generalized gradient approximation ͑GGA͒ and, to a lesser extent, by the exact exchange term of the hybrid functional. We thus propose an alternative functional that mixes exact exchange with the recently proposed GGA of Wu and Cohen ͓Phys. Rev. B 73, 235116 ͑2006͔͒ which, for solids, improves over the treatment of exchange of the most usual GGA's. The new functional renders an accurate description of both the structural and electronic properties of typical ferroelectric oxides. Availability: This is the author's manuscript

We describe the intrinsic mechanism of 2-dimensional electron confinement at the n-type SrTiO3/LaAlO3 interface as a function of the sheet carrier density n(s) via advanced first-principles calculations. Electrons localize spontaneously in Ti 3d(xy) levels within a thin (≲2  nm) interface-adjacent SrTiO3 region for n(s) lower than a threshold value n(c)∼10(14)  cm(-2). For n(s)>n(c) a portion of charge flows into Ti 3d(xz)-d(yz) levels extending farther from the interface. This intrinsic confinement can be attributed to the interface-induced symmetry breaking and localized nature of Ti 3d t(2g) states. The sheet carrier density directly controls the binding energy and the spatial extension of the conductive region. A direct, quantitative relation of these quantities with n(s) is provided.

Thermoelectrics are promising for addressing energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but less by maximizing the power factor. The latter imposes apparently conflicting requirements on the band structure: a narrow energy distribution and a low effective mass. Quantum confinement in nanostructures and the introduction of resonant states were suggested as possible solutions to this paradox, but with limited success. Here, we propose an original approach to fulfill both requirements in bulk semiconductors. It exploits the highly directional character of some orbitals to engineer the band structure and produce a type of low-dimensional transport similar to that targeted in nanostructures, while retaining isotropic properties. Using first-principle calculations, the theoretical concept is demonstrated in Fe2YZ Heusler compounds, yielding power factors 4 to 5 times larger than in classical thermoelectrics at room temperature. Our findings are totally generic and rationalize the search of alternative compounds with similar behavior. Beyond thermoelectricity, these might be relevant also in the context of electronic, superconducting, or photovoltaic applications.

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Scanning tunneling spectroscopy images of Bi 2 Se 3 doped with excess Bi reveal electronic defect states with a striking shape resembling clover leaves. With a simple tight-binding model, we show that the geometry of the defect states in Bi 2 Se 3 can be directly related to the position of the originating impurities. Only the Bi defects at the Se sites five atomic layers below the surface are experimentally observed. We show that this effect can be explained by the interplay of defect and surface electronic structure.