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

TL;DR

This study investigates terahertz absorption modes in the multiferroic Sr$_2$FeSi$_2$O$_7$, revealing spin-orbit and crystal field excitations in the paramagnetic state, with implications for understanding its electronic structure.

Contribution

It provides the first detailed analysis of terahertz spin-orbital excitations in Sr$_2$FeSi$_2$O$_7$ above magnetic ordering, highlighting the importance of local environment effects.

Findings

Identified three absorption modes above magnetic transition temperature.

Attributed modes to optical transitions involving spin-orbit and crystal field effects.

Noted discrepancies between atomic models and experimental data.

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.