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

Abstract

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 $100{\mathrm{cm}}^{\ensuremath{-}1}.$ The measured splitting from the ground state to the excited triplet ${M}_{S}=0$ sublevel is ${\ensuremath{\Delta}}_{1}=24.2{\mathrm{cm}}^{\ensuremath{-}1}$ and the other two triplet state sublevels in zero magnetic field are $1.4{\mathrm{cm}}^{\ensuremath{-}1}$ below and above the ${M}_{S}=0$ sublevel. Another multiplet is at ${\ensuremath{\Delta}}_{2}=37.6{\mathrm{cm}}^{\ensuremath{-}1}$ from the ground state. A strong electric dipole active transition polarized in the $(\mathrm{ab})$ plane is activated in the dimer spin system below 15 K at $52{\mathrm{cm}}^{\ensuremath{-}1}.$