Nonlinear optical spectroscopy of transition metal oxides and their interfaces

Transition metal oxides are an important class of materials exhibiting unusual electronic properties associated with strong electron-electron correlations. In other words, these materials cannot be understood with single-particle models, even qualitatively. The most well known property of some of these materials is high-temperature superconductivity, which is still one of the major unexplained puzzles of condensed matter physics. Other such materials have unusual magnetic properties, such as the "colossal magneto-resistance", or electric properties, such as ferroelectricity. Striking properties are also observed at the surfaces of these materials or at the interface between two of them.

As an example, few years ago the interface between LaAlO₃ (LAO) and SrTiO₃ (STO), two normal band insulators, has been surprisingly found to be conductive! It has been later established that a bidimensional gas of electrons form at the interface and allows electrical conduction. This phenomenon is very promising for novel electronic applications and is currently the subject of very intense studies around the world.

The approach we pursue in our lab for studying the physics of transition metal oxides is based on using nonlinear optical spectroscopies. These techniques have found many applications in other fields of the condensed matter physics and material science, but their application to transition metal oxides has been very limited so far. In particular, we are presently mainly focusing on the technique of second harmonic generation (SHG), which is particularly useful for investigating surfaces and interfaces. We have recently obtained very promising results on the LAO/STO interface [see e.g., Phys. Rev. B 80, 075110 (2009) and figure on the side] and are actively working on the extension of these results.

Other nonlinear optical (or non-standard) techniques that we have recently applied to the study of transition metal oxides include the coherent antiStokes Raman spectroscopy (CARS), which we have applied to studying the high-temperature superconductor YBCO see, e.g., Opt. Express 16, 9054-9059 (2008)], and the time-resolved luminescence, which we have applied to studying the recombination dynamics of electrons and holes in STO [see, e.g., J. Appl. Phys. 106, 103515 (2009) and references therein].

This activity is presently the main focus of our lab. This project is carried out in collaboration with the HTS group in Napoli (Fabio Miletto, Umberto Scotti di Uccio et al.), within the framework of Coherentia CNR center. There are thesis subjects available on this project both for dottorato di ricerca (PhD) and for laurea magistrale (MSc). Contact Lorenzo Marrucci for getting more details about them.

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Last updated: 23-feb-2011