Valence-band photoemission study of $R_3{\mathrm{S}}_4$ $(R=\mathrm{La},$ Ce)

Electronic structures of ${\mathrm{R}}_3{\mathrm{S}}_4$ (R=La, Ce) has been investigated by using photoemission spectroscopy (PES) and electronic structure calculations performed in the LSDA and the $\mathrm{LSDA}+U$ method. The valence-band spectrum of ${\mathrm{La}}_3{\mathrm{S}}_4$ reveals that the electronic states near $E_{\mathrm{F}}$ have mainly La $5d$ character, in agreement with the LSDA calculation. The Ce $4f$ PES spectrum exhibits three prominent features at about $-0.5\hspace{0.5em}\mathrm{eV},$ $-2.5\hspace{0.5em}\mathrm{eV},$ and $-5.5\hspace{0.5em}\mathrm{eV}.$ We interpret the Ce $4f$ spectrum as the essentially trivalent Ce $4f$ spectrum, and identify the peaks at $-0.5\hspace{0.5em}\mathrm{eV}$ and $-2.5\hspace{0.5em}\mathrm{eV}$ as the correlation-split $4f$ states, and the $-5.5\hspace{0.5em}\mathrm{eV}$ peak as the hybridized $4f/5d$ states with S p electrons. The CIS and CFS yield spectra show that ${\mathrm{Ce}}_3{\mathrm{S}}_4$ has formally trivalent ${\mathrm{Ce}}^{3+}$ ions. Nearly half-metallic nature is obtained for ferromagnetic ${\mathrm{Ce}}_3{\mathrm{S}}_4,$ and the $\mathrm{LSDA}+U$ calculation provides a reasonably good description of the electronic structure of ${\mathrm{Ce}}_3{\mathrm{S}}_4.$ Superconductivity in ${\mathrm{La}}_3{\mathrm{S}}_4$ and the absence of the tetragonal distortion in ${\mathrm{Ce}}_3{\mathrm{S}}_4$ have been discussed based on their electronic structures.