Terahertz Spin-Orbital Excitations in the paramagnetic state of   multiferroic Sr$_2$FeSi$_2$O$_7$
  We studied the novel multiferroic material Sr$_2$FeSi$_2$O$_7$, and found 3 absorption modes above the magnetic ordering transition temperature using time-domain terahertz spectroscopy. These absorption modes can be explained as the optical transitions between the spin-orbit coupling and crystal field split 3d$^6$ Fe$^{2+}$ ground state term in this material. Consideration of the compressed tetrahedral environment of the Fe$^{2+}$ site is crucial to understand the excitations. We point out, however, discrepancies between the single-site atomic picture and the experimental results. 
Thuc T. Mai, C. Svoboda, M.T. Warren, T.-H. Jang, J. Brangham, Y.H.   Jeong, S-W. Cheong, and R. Vald\'es Aguilar
cond-mat.str-el