title: Nanoscale orbital excitations and the infrared spectrum of a molecular Mott insulator: A15-Cs3C60.
abstract: The quantum physics of ions and electrons behind low-energy spectra of strongly correlated molecular conductors, superconductors and Mott insulators is poorly known, yet fascinating especially in orbitally degenerate cases. The fulleride insulator Cs3C60(A15), one such system, exhibits infrared (IR) spectra with low temperature peak features and splittings suggestive of static Jahn-Teller distortions with a breakdown of orbital symmetry in the molecular site. That is puzzling, since there is no detectable static distortion, and because the features and splittings disappear upon modest heating, which they should not. Taking advantage of the Mott-induced collapse of electronic wavefunctions from lattice-extended to nanoscale localized inside a caged molecular site, we show that the unbroken spin and orbital symmetry of the ion multiplets explains the IR spectrum without adjustable parameters. This demonstrates the importance of a fully quantum treatment of nuclear positions and orbital momenta in the Mott insulator sites, dynamically but not statically distorted. The observed demise of these features with temperature is explained by the thermal population of a multiplet term whose nuclear positions are essentially undistorted, but whose energy is very low-lying. That term is in fact a scaled-down orbital excitation analogous to that of other Mott insulators, with the same spin 1/2 as the ground state, but with a larger orbital momentum of two instead of one.
authors.affiliation: Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA.
authors.forename: S S
authors.initials: SS
authors.lastname: Naghavi
authors.affiliation: International School for Advanced Studies (SISSA), and CNR-IOM Democritos National Simulation Center, Via Bonomea 265, I-34136 Trieste, Italy. tosatti@sissa.it.
authors.forename: M
authors.initials: M
authors.lastname: Fabrizio
authors.affiliation: International School for Advanced Studies (SISSA), and CNR-IOM Democritos National Simulation Center, Via Bonomea 265, I-34136 Trieste, Italy. tosatti@sissa.it and Institut für Theoretische Physik, Goethe-Universität, 60438 Frankfurt am Main, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Qin
authors.affiliation: International School for Advanced Studies (SISSA), and CNR-IOM Democritos National Simulation Center, Via Bonomea 265, I-34136 Trieste, Italy. tosatti@sissa.it and International Centre for Theoretical Physics (ICTP), Strada Costiera 11, I-34151 Trieste, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Tosatti