Signatures of phonon splitting in the infrared spectra of a quantum magnet SrCu$_2$(BO$_3$)$_2$

Infrared spectroscopy studies of SrCu$_2$(BO$_3$)$_2$ have been performed along both the in-plane and c-axis crystallographic directions. The reflectance will be reported 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). In the in-plane spectra we observe a new feature developing at 443 cm$^{-1}$ (55 meV) below about 20 K. Detail temperature, magnetic field and polarization dependence of this feature will be reported. All the results point toward close relation of 443 cm$^{-1}$ mode with the development of singlet ground state in SrCu$_2$(BO$_3$)$_2$.