Valence-band photoemission spectroscopy of the giant magnetoresistive spinel compound ${\mathrm{Fe}}_{0.5}{\mathrm{Cu}}_{0.5}{\mathrm{Cr}}_2{\mathrm{S}}_4$

Electronic structures of the giant magnetoresistive ${\mathrm{Fe}}_{0.5}{\mathrm{Cu}}_{0.5}{\mathrm{Cr}}_2{\mathrm{S}}_4$ (FCCS) spinel compound have been investigated using photoemission spectroscopy (PES). Resonant PES measurements around the Cu, Fe, Cr $3\overrightarrow{p}3d$ absorption edges exhibit negligible resonant interference behavior for the Cu $3d$ valence electrons, indicating the monovalent valence state of the Cu ion in FCCS. The top of the valence band is found to be predominantly of the Cr $3d$ and the nearly filled Cu $3d$ electron character, whereas the Fe $3d$ electron character is distributed over the whole valence band. The measured valence-band PES spectra of FCCS yield better agreement with the $\mathrm{LSDA}+U$ calculation than with the local spin-density approximation (LSDA) calculation, suggesting the importance of the large Coulomb interactions U between d electrons. On the other hand, the low spectral intensity near $E_F$ in the measured valence-band spectrum suggests an extra localization in FCCS, not explained by the large U alone.