Colloquium on March 1, 2010

Hao Tjeng
Max Planck Institute for Chemical Physics of Solids, Dresden

Electron spectroscopy on 3d^1 transition-metal oxides: model studies for testing new theoretical approaches

One of the long standing topics in theoretical solid state physics concerns the single-particle spectral weight distribution in Mott-Hubbard systems in the vicinity of the metal-insulator transition. The class of $d^1$ perovskites has been recognized in the last decade as a materialization of this topic on which experimental tests can be carried out.

We have recently carried out a bulk-sensitive valence band photoemission study on high quality pure and doped LaTiO$_3$ and YTiO$_3$ single crystals. Our measurements on the pure systems reveal that these small gap $d^1$ Mott insulators have Ti $3d$ spectra which are much broader and with very different lineshapes than the occupied $3d$ bands as calculated by band theories. The mean-field inclusion of the Hubbard $U$ explains the band gap but produces even narrower bands, indicating the complete breakdown of standard mean-field theories in describing excitation spectra. Encouraging is that dynamical mean field theories (DMFT) is able to reproduce the spectra to a large extent. We associate the observed spectra with the propagation of a hole in a system with surprisingly well suppressed charge fluctuations thereby showing characteristics of a $t$-$J$ model. Upon doping we observed a remarkable rapid increase of a 'metallic' peak at the Fermi level, an observation which can be described well by DMFT.

We have also carried out photoemission experiments on Ti2O3 and VO2, and have observed features which indicate the presence of 'electronic' dimers. This is important in view of the discussion to what extent c-axis dimers play a role in the metal-insulator transitions in V2O3. It is important also to note that the electronic structure of these systems cannot be reproduced by single-site DMFT using realistic band structure parameters. We infer that it is crucial to consider the k-dependence of the self-energy, and in particular, the inter-site spin-spin correlations. An extension of the DMFT in terms of a well-chosen cluster is necessary.