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 T-C to T-MI, 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) similar to 3.5(k(B)T(MI)) similar to 35 meV. First-principles band-structure calculations, using the experimentally estimated on-site Coulomb energy of U similar to 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.