Far-infrared study of the two-dimensional dimer spin system SrCu2(BO3)(2)

Using far-infrared spectroscopy in magnetic fields up to 12 T we have studi ed a two-dimensional dimer spin gap system SrCu2(BO3)(2) We found several i nfrared-active modes in the dimerized state (below 10 K) in the frequency r ange from 3 to 100 cm(-1). The measured splitting from the ground state to the excited triplet Ms = 0 sublevel is Delta(1) = 24.2 cm(-1) and the other two triplet state sublevels in zero magnetic field are 1.4 cm(-1) below an d above the M-S = O sublevel. Another multiplet is at Delta(2) = 37.6 cm(-1 ) from the ground state. A strong electric dipole active transition polariz ed in the (ab) plane is activated in the dimer spin system below 15 K at 52 cm(-1).