Infrared studies of a quantum magnet SrCu$_2$(BO$_3$)$_2$

We will report results of our infrared studies of SrCu$_2$(BO$_3 $)$_2$, a two-dimensional spin system with a disordered ground state even at very low temperatures, and a spin gap of about 24 cm$^{-1}$ (3 meV). This material has recently attracted attention because of a possibility that doping may lead to a superconductivity mediated by antiferromagnetic fluctuations, possibly similar to high-T$_c$ cuprates. Using polarized light we have probed both crystallographic directions over a broad range of frequencies (from about 30 cm$^{-1}$ to 20,000 cm$^{-1} $) and temperatures (from 4.2 K to 300 K). The results reveal significant differences between the ab-plane and c-axis directions. We will discuss these findings in relation with the resonance effects observed in inelastic light scattering experiments from collective magnetic excitations.