Electronic structure evolution across the Peierls metal-insulator transition in a correlated ferromagnet

Transition metal compounds often undergo spin-charge-orbital ordering due to strong electron-electron correlations. In contrast, low-dimensional materials can exhibit a Peierls transition arising from low-energy electron-phonon-coupling-induced structural instabilities. We study the electronic structure of the tunnel framework compound K₂Cr₈O₁₆, which exhibits a temperature-dependent (T-dependent) paramagnetic-to-ferromagnetic-metal transition at T_C=180 K and transforms into a ferromagnetic insulator below T_MI=95 K. We observe clear T-dependent dynamic valence (charge) fluctuations from above TC to TMI, which effectively get pinned to an average nominal valence of Cr^+3.75 (Cr⁴⁺∶Cr³⁺ states in a 3∶1 ratio) in the ferromagnetic-insulating phase. High-resolution laser photoemission shows a T-dependent BCS-type energy gap, with 2G(0)~3.5(k_BT_MI)~35 meV. First-principles band-structure calculations, using the experimentally estimated on-site Coulomb energy of U~4 eV, establish the necessity of strong correlations and finite structural distortions for driving the metal-insulator transition. In spite of the strong correlations, the nonintegral occupancy (2.25 d−electrons/Cr) and the half-metallic ferromagnetism in the t_2g up-spin band favor a low-energy Peierls metal-insulator transition