Electronic Structure Evolution across the Peierls Metal-Insulator Transition in 
a Correlated Ferromagnet

Bhobe, P.; Kumar, A.; Taguchi, M.; Eguchi, R.; Matsunami, M.; Takata, Y.; Nandy, A.; Mahadevan, P.; Sarma, D.; Neroni, A.; Sasioglu, E.; Lezaic, M.; Oura, M.; Senba, Y.; Ohashi, H.; Ishizaka, K.; Okawa, M.; Shin, S.; Tamasaku, K.; Kohmura, Y.; Yabashi, M.; Ishikawa, T.; Hasegawa, K.; Isobe, M.; Ueda, Y.; Chainani, A.

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Electronic Structure Evolution across the Peierls Metal-Insulator Transition in a Correlated Ferromagnet

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Kumar, A.
Department Solid State Spectroscopy (Bernhard Keimer), Max Planck Institute for Solid State Research, Max Planck Society;
Department Nanoscale Science (Klaus Kern), Max Planck Institute for Solid State Research, Max Planck Society;

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Isobe, M.
Scientific Facility Crystal Growth (Masahiko Isobe), Max Planck Institute for Solid State Research, Max Planck Society;
Department Quantum Materials (Hidenori Takagi), Max Planck Institute for Solid State Research, Max Planck Society;

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Citation

Bhobe, P., Kumar, A., Taguchi, M., Eguchi, R., Matsunami, M., Takata, Y., et al. (2015). Electronic Structure Evolution across the Peierls Metal-Insulator Transition in a Correlated Ferromagnet. Physical Review X, 5(4): 041004.

Cite as: https://hdl.handle.net/21.11116/0000-000E-CD0A-1

Abstract

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.