Electronic properties of strongly correlated complex oxide nano-structures

with Jacques Chakhalian

Understanding the physics of strongly correlated electrons is the central challenge of modern condensed matter physics. Complex oxide systems are the most common class of materials found on Earth. Earliest examples known to humanity include Fe₂O₃ - ironstone and Fe₃O₄ - magnetite. Magnetite can be found in the brain cells of a tuna fish and is believed to be responsible for tuna’s ability to navigate the ocean in a way which is similar to the Giant Magneto-Resistance effect (GMR). On the other hand magnetite is also known as a half-metallic compound and is actively researched in the new field called spintronics. Recently another class of complex oxides called manganites (e.g. La_2/3Ca_1/3MnO₃) has generated a lot of attention. The manganites possess a rather peculiar feature called colossal magneto-resistance and show dramatic changes in resis-tivity with applied external magnetic field. This feature alone made manganites a prime candidate for nano-magnetic switches. As a final example, the most exiting recent discovery in condensed matter physics is connected to the group of oxides called: high-temperature super-conducting cuprates (e.g. La_1-xSr_xCuO₄, YBa₂Cu₃O₇ or Bi₂Sr₂SaCu₂O₈; see Figure 1).

Figure 1. Crystal structure of the most studies high-temperature superconductor YBa2Cu3O7 with the transition temperature as high as TSC=90K.

From the fundamental physics point of view, there exist another extremely interesting class of oxides known as Mott-Hubbard insulators. Typical examples of those include anti-ferromagnetic (AFM) oxides such as NiO, CoO, MnO etc. In spite of their obvious chemical simplicity Mott insulators are thrilling because of our complete inability to predict their electronic and magnetic properties from quantum theory. For instance, after decades of advances in theory multiplied by the power of modern computers we still cannot fully explain why NiO is an insulator and not a metal as predicted by the band theory. Among Mott insulators there exist another sub-group of oxides (e.g. Va₂O₃ or Ti₂O₃) showing a metal-to-insulator (MI) transition, which occurs below certain characteristic temperature T_M-I. Again the presence of the MI transition is one of the un-explained puzzles of modern physics. A common feature which unites the oxides with those rather "bizarre" properties is the presence of de-localized d-electrons of transition metals (e.g. Cu, Ni, Fe, Ru etc.) strongly coupled to oxygen p-orbitals. The enormous diversity and excep-tional physical properties arise mostly from a delicate balance between de-localized electrons and effects of Coulomb interaction, which tends to localize electrons.

Figure 2. Results of theoretical simulations of the copper 2p x-ray absorption spectra of Cs2KCuF6 (bottom) and La2Li1/2Cu1/2O4 (top), in comparison with the experimental spectra.

Our group is actively involved in fabrication and advanced characterization of nano-structures built of complex oxides similar to described above. The nano-scale adds an extra degree of com-plexity in to already extremely rich physics. In order to investigate complex oxide nano-structures we use synchrotron soft X-ray radiation (400 eV-3000 eV) and polarized neutron scattering. All experiments have been performed at the world-class facilities such as Advance Photon Source (Argonne National Lab, USA), the European Synchrotron Radiation Facility (Grenoble, France) and ISIS (Rutherford Appleton Laboratory, UK). By using soft X-ray radia-tion we are able to probe electronic structure of oxides directly. Moreover by using left and right circularly popularized light combined with polarized neutron reflectivity we were able to investi-gate their magnetic properties on nano-scale (down to a sub-monolayer!). In order to quantify the obtained experimental results we need to compare them with the most powerful computa-tional scheme developed for this purpose - a cluster multiplet model (see Fig. 2). The main goal of the project is to learn how to predict electronic properties of selected nano-structured oxides and to extract physical parameters relevant to strong correlations.

We are inviting qualified students to participate in this exciting new project. The required skill-set includes: good working knowledge of basic quantum mechanics and atomic physics, basics of inorganic chemistry, good computational skills and ability to work independently under time constraint.