Infrared optical properties of the spin-1/2 quantum magnet TiOCl
Low-dimensional quantum spin systems, based on complex transition metal oxides, recently attracted a lot of attention, particularly as a fascinating playground to study spin-charge separation, spin-gap states and quantum disorder. Proposals, that the exotic properties of low-dimensional spin -1/2 quantum magnets might also play a major role in shaping the mechanism for high temperature superconductivity, led to a vigorous experimental activity on materials involving Cu2+ ions with a 3d9 configuration (S = 1/2). Other examples of S = 1/2 are notably Ti3+ and V4+ systems in d1 configuration (i.e. one single d-electron occupies one of the t2g orbitals). In this respect, the layered TiOX (X = Cl and Br) compounds are most promising and are candidates for exotic electronic configurations as in the resonating-valence-bond (RVB) model and for superconductivity based on dimmer fluctuations. We have provided a thorough analysis of the optical properties of TiOCl, emphasizing particularly the temperature dependence of the phonon spectrum (Fig. 3.7). The temperature dependence of all relevant parameters, including the phonon linewidth and the spectral weight, occurs over a broad temperature interval. The pronounced narrowing of the IR modes with decreasing temperature (inset of Fig. 3.7) coincides with the suppression of low frequency spin fluctuations, also recognized in the so-called spin-gap phase of the NMR spectra. The behaviour of the IR spectral weight with temperature establishes the presence of a characteristic energy scale associated with the opening of a spin-gap. Although similar findings are frequently observed in 1D systems the present case is different due to the unusually high energy scale involved. Summing up, TiOCl is a member of a class of correlated electron systems where a pseudo-gap state is related to large phonon anomalies.