Far-infrared study of the two-dimensional dimer spin systemSrCu2(BO3)2

Using far-infrared spectroscopy in magnetic fields up to 12 T we have studied a two-dimensional dimer spin gap system ${\mathrm{SrCu}}_2({\mathrm{BO}}_3{)}_2.$ We found several infrared-active modes in the dimerized state (below 10 K) in the frequency range from 3 to $100{\mathrm{cm}}^{-1}.$ The measured splitting from the ground state to the excited triplet $M_S=0\mathrm{}$ sublevel is ${\Delta }_1=24.2\mathrm{}{\mathrm{cm}}^{-1}$ and the other two triplet state sublevels in zero magnetic field are $1.4{\mathrm{cm}}^{-1}$ below and above the $M_S=0\mathrm{}$ sublevel. Another multiplet is at ${\Delta }_2=37.6\mathrm{}{\mathrm{cm}}^{-1}$ from the ground state. A strong electric dipole active transition polarized in the $\mathrm{}\left(\mathrm{ab}\right)\mathrm{}$ plane is activated in the dimer spin system below 15 K at $52{\mathrm{cm}}^{-1}.$