Infrared Magneto-Spectroscopy of Rare-Earth-Based Triangular-Lattice Antiferromagnet KCeS2

Triangular-lattice antiferromagnets (TLAFMs) have long been recognized as promising candidates for exploring magnetic frustration. While extensive research has been conducted on TLAFMs with 3d transition metals, recent studies increasingly focus on rare-earth-based TLAFMs. Several delafossite compounds of the form ARX2 (where R being a rare-earth ion) show strong potential for hosting a quantum spin liquid (QSL) ground state. In contrast, the iso-structural Ce-based compound KCeS2 exhibits antiferromagnetic ordering below T_N = 0.38 K, as confirmed by specific heat and neutron diffraction measurements. In this study, we perform infrared magneto-spectroscopy on powdered KCeS2 samples under magnetic fields up to 17.5 T. Our results indicate that both the crystal electric field (CEF) and Zeeman splittings in the J=5/2 and J=7/2 states can be effectively modeled through our analytical calculations, which suggest large g-anisotropy. Additionally, we find that phonons play a significant role in this compound, as their interactions with CEF modes substantially influence the spectral characteristics. By examining KCeS2 under magnetic fields, our ultimate objective is to understand the factors that enable some ARX2 compounds to exhibit QSL behavior while others display magnetic order at low temperatures.