The role of dimers in the metal-insulator transitions in VO${_2}$ and Ti${_2}$O${_3}$

The nature of the metal-insulator-transition (MIT) in VO$_2$ has been subject of debate for a long time. Recently the scenario of an orbital assisted MIT has been proposed on the basis of the dramatic change of the orbital occupation as observed by soft-X-ray absorption spectroscopy measurements.\footnote{M.W. Haverkort {\it et al.}, PRL {\bf 95}, 196404 (2005).} We present the results of our bulk sensitive photoemission study on the MIT in VO${_2}$ using high quality single crystals. We observe a huge transfer of spectral weight across the MIT, supporting recent LDA+DMFT cluster calculations.\footnote{S. Biermann {\it et al.}, PRL {\bf 94}, 026404 (2005).} To investigate the possible role of dimers herein, we also studied the insulating phase of Ti${_2}$O${_3}$. Here we clearly observe a double peak structure which can be taken as a characteristic for the presence of dimers.