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Iori F.,Nano bio Spectroscopy Group and Scientific Development Center | Gatti M.,Nano bio Spectroscopy Group and Scientific Development Center | Rubio A.,Nano bio Spectroscopy Group and Scientific Development Center | Rubio A.,Fritz Haber Institute
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We present a systematic study of the electronic structure of several prototypical correlated transition-metal oxides: VO 2, V 2O 3, Ti 2O 3, LaTiO 3, and YTiO 3. In all these materials, in the low-temperature insulating phases the local and semilocal density approximations (LDA and GGA, respectively) of density-functional theory yield a metallic Kohn-Sham band structure. Here we show that, without invoking strong-correlation effects, the role of nonlocal exchange is essential to cure the LDA/GGA delocalization error and provide a band-structure description of the electronic properties in qualitative agreement with the experimental photoemission results. To this end, we make use of hybrid functionals that mix a portion of nonlocal Fock exchange with the local LDA exchange-correlation potential. Finally, we discuss the advantages and the shortcomings of using hybrid functionals for correlated transition-metal oxides. © 2012 American Physical Society.


Helbig N.,Nano bio Spectroscopy Group and Scientific Development Center | Tokatly I.V.,Nano bio Spectroscopy Group and Scientific Development Center | Tokatly I.V.,Ikerbasque | Rubio A.,Nano bio Spectroscopy Group and Scientific Development Center | Rubio A.,Fritz Haber Institute
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

We investigate the suitability of natural orbitals as a basis for describing many-body excitations. We analyze to which extent the natural orbitals describe both bound as well as ionized excited states and show that depending on the specifics of the excited state the ground-state natural orbitals may yield a good approximation. We show that the success of reduced density-matrix functional theory in describing molecular dissociation lies in the flexibility provided by fractional occupation numbers while the role of the natural orbitals is minor. © 2010 The American Physical Society.


Cudazzo P.,Nano bio Spectroscopy Group and Scientific Development Center | Attaccalite C.,CNRS Neel Institute | Tokatly I.V.,Nano bio Spectroscopy Group and Scientific Development Center | Tokatly I.V.,Ikerbasque | And 2 more authors.
Physical Review Letters | Year: 2010

Using first principles many-body theory methods (GW+Bethe-Salpeter equation) we demonstrate that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Strong electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out of plane and in plane, respectively. The presence of such bound excitons opens the path towards an excitonic Bose-Einstein condensate in graphane that can be observed experimentally. © 2010 The American Physical Society.


Palummo M.,European Theoretical Spectroscopy Facility | Palummo M.,Massachusetts Institute of Technology | Giorgi G.,Tokyo University of Science | Giorgi G.,University of Tokyo | And 4 more authors.
Journal of Physical Chemistry C | Year: 2012

Because of rapid progress in the synthesis methods, TiO 2-based nanomaterials are nowadays the object of strong research interest for their promising performances in photocatalysis and photovoltaics applications. In particular, large quantities of two-dimensional (2D) nanosheets (NSs) can be produced and used for further assembling of new nanostructured materials with different morphologies and functionalities. Investigating the microscopic nature of their opto-electronic properties is a fundamental prerequisite for rationalizing experimental data and improving devices performances. By means of first-principles excited-state simulations, we reveal here the excitonic nature of radiative transitions (in the vis-UV spectral range) of TiO 2-based NSs. Furthermore these calculations on top of finite temperature molecular dynamics (MD) simulations explain the large Stokes shifts experimentally observed and confirm how the nature of optical transitions in these two-dimensional oxide materials is due to strongly bound excitons. Our study shows that the inclusion of many-body effects plays a fundamental role for a correct interpretation of the experimental data regarding photoexcited processes in low-dimensional titanium dioxide materials. © 2012 American Chemical Society.


Kramberger C.,University of Tokyo | Roth F.,Leibniz Institute for Solid State and Materials Research | Schuster R.,Leibniz Institute for Solid State and Materials Research | Kraus R.,Leibniz Institute for Solid State and Materials Research | And 7 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Ab initio calculations of the loss function of potassium-intercalated and electron-loaded bundles of single-walled carbon nanotubes yield a channeled-charge-carrier plasmon without perpendicular dispersion. Experimentally, we probe the momentum-dependent loss function of thin bundles consisting of only a few potassium-intercalated single-walled carbon nanotubes by angle-resolved electron-energy-loss spectroscopy and confirm this intrinsic channeling. The charge-carrier-plasmon energy is via in situ intercalation and is tunable in the near-visible infrared-energy range from 0.85 to 1.15 eV. © 2012 American Physical Society.

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