Ordering Phenomena in Transition Metal Oxides

Titanium oxyhalides: A playground for electron correlation physics   

The titanium-based oxyhalides (TiOX with X=Cl,Br) can be viewed as prototypical materials for the study of electronic correlation effects. They are characterized by an intimate coupling between charge, spin, and lattice degrees of freedom. The strong on-site Coulomb interaction drives these materials into a Mott-insulating state [1]. Due to the nearly triangular coordination of the Ti³⁺ ions in the quasi-2D crystal structure magnetic frustration between the local s=1/2 spins is expected and has led to speculations on the possible occurrence of a resonating valence bond (RVB) state. However, it is found instead that an unusual spin Peierls instability occurs which results from strong spin-lattice coupling and indicates a more 1D character of the material [2].   
In my contribution I will focus on our recent attempts to drive these prototypical Mott-insulators into a metallic state by electron doping through alkaline intercalation. Photoemission data show the evolution of new states within the Mott gap which follow the spectral weight distribution expected for a bandfilling-controlled Mott transition and carry the signatures of strong (spin-)polaronic coupling.

[1] M. Hoinkis, M. Sing, J. Schäfer, M. Klemm, S. Horn, H. Benthien, E. Jeckelmann, T. Saha-Dasgupta, L. Pisani, R. Valentí, and R. Claessen, Phys. Rev. B 72,  125127 (2005).

[2] M. Hoinkis, M. Sing, S. Glawion, L. Pisani, R. Valentí, S. van Smaalen, M. Klemm, S. Horn, and R. Claessen, Phys. Rev. B 75, 245124 (2007).