RIXS study of strain-tuned coupled valence and spin-state transition in epitaxial $(\mathrm{Pr}_{0.85}\mathrm{Y}_{0.15})_{0.7}\mathrm{Ca}_{0.3}\mathrm{CoO}_{3}$ films

Abstract

Strain-tuned $(\mathrm{Pr}_{0.85}\mathrm{Y}_{0.15})_{0.7}\mathrm{Ca}_{0.3}\mathrm{CoO}_{3}$ thin films provide a model system for controlling coupled valence, spin-state, structural and metal-insulator transitions (VSSTs) in perovskite cobaltite materials. Here, we use Co $L_{2,3}$ and Pr $M_{4,5}$ X-ray Absorption Spectroscopy (XAS), along with Co $L_{3}$ edge Resonant Inelastic X-ray Scattering (RIXS), to probe the local electronic structure across the strain-controlled VSST in epitaxial (Pr,Y,Ca)CoO$_{3}$ thin films. In compressively strained films on YAlO$_{3}$ (YAO) substrates, cooling through the transition produces Pr$^{4+}$-like spectral weight, suppresses Co$^{4+}$-like character, and favors predominantly low-spin Co$^{3+}$. The changes at the Pr and Co edges are quantitatively consistent with a reduction of approximately 0.15 holes per Co in the Co--O network. By contrast, tensile-strained thin films on La$_{0.18}$Sr$_{0.82}$Al$_{0.59}$Ta$_{0.41}$O$_{3}$ (LSAT) substrates have nearly temperature-independent spectra. Multiplet calculations reproduce the principal XAS lineshapes and local $dd$ excitations at high energy loss. Additionally, low-energy RIXS intensity recovered above the transition is consistent with a combination of Co$^{4+}$-like local excitations and a broader particle--hole continuum associated with the metallic phase. These results provide element-resolved evidence for charge compensation between the Pr and Co--O sublattices and support cooperative coupling among charge-transfer, spin-state, and lattice structure at the strain-controlled VSST.