| TiOCl and the related isostructural oxyhalide TiOBr are layered spin-1/2 quantum magnets which display unusual magnetic properties. Because the Ti3+ ions form topologically a triangular-like lattice, it was speculated early in the nineties that these materials were magnetically highly frustrated and could display resonating valence bond (RVB)-type superconductivity, if driven into a metallic state. However, in case of TiOCl the Bonner-Fisher-type behavior of the susceptibility above Tc2=95K and the occurrence of a dimerized spin-Peierls phase below Tc1=67K indicate that the magnetic coupling is quasi-one-dimensional. This is supported by LDA+U calculations and angle-resolved photoelectron spectroscopy (ARPES) which find a pronounced electronic anisotropy with small but finite interchain coupling. The latter has been argued to be responsible for the non-canonical spin-Peierls behavior with an incommensurate precursor phase between Tc1 and Tc2, which may bring magnetic frustrations (and hence RVB physics) back into play. While TiOBr exhibits the same phase diagram, its one-dimensional character is less pronounced which makes it an even better candidate for bond dimer fluctuations. We will present our recent spectroscopic results on these oxyhalides and discuss first attempts to induce an insulator-metal transition by controlling band filling via alkali metal doping. |
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