Terahertz Spin-Orbital Excitations in the paramagnetic state of multiferroic Sr${}_2$FeSi${}_2$O${}_7$

Abstract

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.

5 pages, 3 figures, 1 table. Submitted. Small correction to figure 3 and corresponding discussion