TIOCL, TIOBR - ARE THESE RVB D1, S=1/2 MATERIALS - THE RESULTS OF SCANDIUM SUBSTITUTION SET IN THE CONTEXT OF OTHER S=1/2 SYSTEMS OF CURRENT INTEREST FOR HIGH-TEMPERATURE SUPERCONDUCTIVITY AND THE METAL-INSULATOR-TRANSITION

The title materials display anomalous electrical, magnetic and optical properties. They have the characteristics of Mott insulators showing high electrical resistance and ligand-field optical spectra, but at the same time they adopt a magnetic ground state which is neither of Curie-Weiss type nor indeed of some standard antiferromagnetically ordered condition. The observed state is one of lowish, isotropic, and almost temperature-independent paramagnetism. One solution in such circumstances would be some systematically distorted crystal structure in which the cation spins become paired up into chemical bonds. However, IR work shows that the TiOX structure is not distorted, at least not in a manner that breaks symmetry and/or increases the cell size. We have studied the magnetic properties of Sc-doped TiOX to cast light on the problem, seeking possible indications of the existence of the resonant valence bond or RVB state-the state currently suggested by some to be responsible for high-temperature superconductivity in the mixed-valence cuprates. It is necessary to distinguish between RVB and some extreme case of magnetic frustration in these S = 1/2 2D oxyhalide systems. In this paper we have tried to put the detailed behaviour of the oxyhalide materials in context with other S = 1/2 systems. In many the S = 1/2 quantum fluctuation problem is compounded by low dimensionality, by geometric frustration of antiferromagnetic order, and by the approach to the metal-insulator transition. The diversity of low-temperature behaviour is sinking, but it is clear that RVB Must constitute a rarely encountered ground state rather than the norm. Our tentative conclusion is that TiOX presents an RVB ground state which Sc doping renders glassy and static.