Abstract
The reflectance of the insulating quasi-two-dimensional quantum magnet SrCu2(BO3)2 has been examined over a wide temperature and frequency range for light polarized parallel (a axis) and perpendicular (c axis) to the copper- and boron-oxygen sheets. The spectra have been measured for temperatures below the structural phase transition Ts=395 K for both polarizations; above Ts a limited study of the in-plane properties was undertaken in the far-infrared region only. Several new modes appear in the reflectance just below Ts along the a and c axes, while others are visible only for T⪡Ts. Below Ts, the intensity of some of the new modes displays little or no temperature dependence, while the intensity of some vibrations increases dramatically with decreasing temperature. Ab initio calculations have been performed for the room-temperature phase using density-functional theory, and the frequencies and atomic characters of the infrared-active phonons at the zone center were obtained using the direct method. The agreement between the calculated and experimentally observed frequencies is quite good, and assignments of the modes are discussed. The vibrational features that are observed only at low temperature appear to be magnetic in origin.