Infrared optical properties of the spin-1/2 quantum magnet TiOCl

We report results on the electrodynamic response of TiOCl, a low-dimensional spin-1/2 quantum magnet that shows a spin gap formation for T<T-c1 = 67 K. The Fano-like shape of a few selected, infrared active phonons suggests an interaction between lattice vibrations and a continuum of low-frequency (spin) excitations. The temperature dependence of the phonon mode parameters extends over a broad temperature range well above T-c1, indicating the presence of an extended fluctuation regime. In the temperature interval between 200 K and T-c1, there is a progressive dimensionality crossover (from two to one), as well as a spectral weight shift from low towards high frequencies. This allows us to identify a characteristic energy scale of about 430 K, ascribed to a pseudo-spin-gap.