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RES - New Application Activity: Towards understanding strong electronic correlation in transition-metal oxides from first principles: a theoretical spectroscopy approach

Complex solids with correlated electrons represent one of the most challenging open problems in condensed-matter physics. They are materials where electrons occupy narrow orbitals, as in partially filled d or f shells. Because of the spatial confinement in those orbitals, electrons experience strong Coulomb repulsion and tend to localize, competing with the opposite tendency to spread over other lattice sites to minimize the kinetic energy.
In recent years, the study of strongly correlated materials has led to the introduction of a new paradigm of complexity in solid-state physics, in which extreme sensitivity to the control parameters and emergence show up as key factors [1]. In fact, in solids with strongly correlated electrons, small variations of external parameters, such as temperature, pressure or doping, can induce dramatic changes in the properties of the system. These are materials that exhibit phase transitions with resistivity changes of many orders of magnitude [2], huge volume variations [3], high Tc superconductivity [4], or colossal magnetoresistance [5]. Moreover, by assembling different strongly correlated electronic systems one can create new materials with properties that did not preexist in their constituents. This is a spectacular realization of the emerging concept of "more is different" [6] . For instance, recent investigations have shown that the interface between two insulatin g perovskite oxides can become a metal [7] or even a superconductor [8].
For these reasons, correlated solids display a broad range of very interesting properties, which make the possible technological applications from these materials very exciting and promising. Researchers are optimistic to find in a new "oxide electronics" better solutions to outperform semiconductor devices and obtain new material properties so far only dreamed [9]. On the other hand, the ability to devise new functionalities of new devices crucially depends on the microscopic understanding of the electronic excitations induced by external perturbations, as probed also by spectroscopy experiments. First-principles theories, based on either many-body perturbation theory (MBPT) or time-dependent density-functional theory (TDDFT) are now a well-established tool in the interpretation of electronic excitations [10]. In sp semiconductors or metals, for instance, reliable band structure calculations in the GW approximation of MBPT are routinely performed and the obtained results are usually in striking agreement with experiment [11]. The main goal of this project is to push further the frontiers of the traditional domains of these ab initio methods towards the broad class of strongly correlated materials, transition-metal oxides in particular. This implies the exploration of the limits of the presently working approximations, the study of new strategies to overcome new kinds of problems, and the application of the new developments to technologically relevant systems.
This research project is part of the activities of the San Sebastian Scientific Development Centre of the European Theoretical Spectroscopy Facility (see, funded by the EU FP7 with the project I3-ETSF (INFRA-2007-1.2.2, Grant Agreement N. 211956) and by the Spanish MICINN ACI Promociona (ACI2009-1036). Moreover it is supported also by a Juan de la Cierva Fellowship (Matteo Gatti).


Matteo Gatti (Angel Rubio)
Amilcare Iacomino
Pierluigi Cudazzo
Federico Iori

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