Markus Hoinkis
Phase Transitions in Low-Dimensional Transition Metal Compounds
Supervisor: Prof. Dr. Ralph Claessen [Experimental physics II]
Date of oral examination: 01/26/2007
133 pages, english , OPUS (Online-Publikations-Server) der Universitätsbibliothek Augsburg
The materials in the scope of this dissertation belong to the domain where Peierls and Mott physics meet – in other words both electron-phonon coupling and electronic correlations play an essential role in these systems. With 1T-TaSe2 a layered transition metal compound was investigated that can be regarded as a paradigm quasi-two-dimensional charge density wave (CDW) system. The appeal of this material certainly lies in the occurrence of a surface Mott metal-insulator transition, which is driven by CDW-induced changes of the electronic bandwidth and can thus be controlled simply by varying the temperature. In this thesis, a detailed examination of the electronic structure in the presence of the CDW is presented. The results of DFT calculations make it possible to identify a separated conduction band with a strongly reduced width compared to the undistorted state, which explains the rather unusual appearance of Mott physics in a 5d transition metal compound. This observation corroborates the Mott-Hubbard scenario with star-of-David clusters as relevant sites of the corresponding Hubbard picture. On the experimental side, clear evidence for the Mott transition at the surface of 1T-TaSe2 is given in form of angle-resolved photoemission data. This represents one of the few examples where one is able to observe the evolution of the spectral function while going through the transition by tuning the crucial ration U/W within the same single crystal.

Compared to the charge-Peierls transition, its counterpart involving the spin degree of freedom – i.e., the spin-Peierls transition - must be considered a much rarer phenomenon. The compounds TiOCl and TiOBr studied in the course of this dissertation are, together with CuGeO3, the only known inorganic materials to exhibit this instability. In this thesis x-ray diffraction experiments provide direct evidence for the spin-Peierls nature of TiOCl , thus leading to a coherent spin-Peierls picture of the oxyhalides with two successive phase transitions. The existence of an extended fluctuation regime above the intermediate phase is confirmed by measurements of the heat capacity, which show a strongly delayed release of entropy and thus hint towards the major role played by fluctuations. The orbital sector as possible origin of these fluctuations can be ruled out on the basis of photoemission experiments utilizing selection rules for polarized radiation.

In a comprehensive study focussing on the normal state electronic structure, both the momentum-integrated and momentum resolved spectral function was determined experimentally by means of photoelectron spectroscopy. Complemented by various DFT and model calculations, these measurements show that the electronic structure of TiOCl and TiOBr cannot be understood in a one-particle picture. The best fit to the data is accomplished by a 1D-Hubbard-model calculation. However, a comparison of the two compounds, which reveals the much greater importance of interchain coupling in the bromide, leads to the conclusion that multi-band effects and/or the magnetic interchain interaction on the triangular lattice have to be taken into account in order to arrive at a realistic theoretic modelling of these systems.

It is exactly this triangular lattice with its geometric frustration of magnetic interactions that fuels the expectations that another fascinating phenomenon – the RVB state – is not far from being realized in the titanium oxyhalides. In this regard the results of pressure-dependent optical transmittance and reflectance measurements appear particularly intriguing: The observed closure of the correlation gap at about 12 GPa is clear evidence for an insulator-metal transition. Assuming that changes of the crystal-structure symmetry are not involved, this would be the first observation of a bandwidth-controlled Mott transition induced by external pressure. The interesting point concerning this experimental discovery is that according to RVB theory the proximity to a metallic state gives reason to anticipate promising chances for finding a novel, RVB-type superconductivity realized in suitably doped oxyhalides.