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.
Highlights
The electron-correlation-driven metal-to-insulator transition (MIT) remains a central problem in condensedmatter physics
We report a combination of hard x-ray, soft x-ray and laser photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), as well as generalized gradient approximation (GGA) þ U and parametrized Hartee-Fock ðHFÞ þ U calculations for K2Cr8O16
U can be estimated by the equation U 1⁄4 E2p − 1⁄2hν − BEAuger − 2ε3d, where E2p corresponds to the BE of the Cr 2p main peak, hν is the incident photon energy, and BEAuger is the BE of the corresponding Auger peak
Summary
The electron-correlation-driven metal-to-insulator transition (MIT) remains a central problem in condensedmatter physics. A recent firstprinciples study on transition-metal compounds (including titanates, manganites, and ferrites) showed that if the system is strongly hybridized, a formal change of the oxidation state is compensated by a lowering of the ligand energy states In such a situation, doping leads to ligand-character holes and a very small change in delectron count [14]. It is noted that a Mott-Peierls transition is expected to show spectral changes in the 3d density of states (DOS) over large energy scales of the order of the on-site Coulomb energy U, as is well known from the case of VO2 [18,19,20] We experimentally answer this question in the present work. The results indicate that strong correlations set up the large-energy-scale electronic structure, while the structural distortion causes a weak-coupling BCS-like transition within about 35 meV of EF in K2Cr8O16
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
