1. parent compound metallic reflects the lifted degeneracy change in selection rules Magnetic Mott–Jahn–Teller ground states in expanded trivalent fullerides revealed by vibrational spectroscopy Gyöngyi Klupp, Péter Matus, Katalin Kamarás Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, Budapest, Hungary Alexey Y. Ganin, Alec McLennan, Matthew J. Rosseinsky Department of Chemistry, University of Liverpool, Liverpool, United Kingdom Yasuhiro Takabayashi, Martin T. McDonald, Kosmas Prassides Department of Chemistry, University of Durham, Durham, United Kingdom Jahn–Teller distortion in C 60 3- Introduction IR spectra Cs 3 C 60 Expanded fulleride : Cs 3 C 60 A15 fcc (merohedral disorder) At ambient pressure both polymorphs exhibit Mott insulating ground state whereas C 60 3- ions are in low spin state (S = 1/2) Slight decrease of the lattice constant can induce a strongly correlated superconducting ground state Y. Ihara, H. Alloul, P. Wzietek, D. Pontiroli, M. Mazzani, M. Riccò EPL 94 37007 (2011) band structure 0.5 eV LUMO : t 1u threefold degeneracy t 1u C 60 C 60 3- level splitting Distortion is very small ~0.04 Å BUT! Vibrational spectroscopy can detect even small molecular deformations that lower the symmetry KBr pellet pressed in glove box Liquid He cooled continuous flow cryostat Bruker IFS 66v/S FTIR spectrometer Resolution: 0.25 cm -1 / 1 cm -1 A15 rich sample: 71% A15 Cs 3 C 60 + 14% fcc Cs 3 C 60 + 15% bco Cs 4 C 60 fcc rich sample: 86% fcc Cs 3 C 60 + 3% A15 Cs 3 C 60 + 7% bco Cs 4 C 60 + 4% CsC 60 Funds: Hungarian National Research Fund (OTKA) No. 75813, Engineering and Physical Sciences Research Council EP/G037132 and EP/G037949 Splitting of T 1u modes Activation of previously silent modes symmetry reduction of C 60 3- Experimental Possible causes of the observed splitting: multiphase nature of the sample — excluded crystal field — excluded Jahn–Teller effect — candidate Solid-state conformers T-dependence of the observed spectra gradual transformation of two temperature dependent solid-state conformers to a single one typical and unique for dynamic Jahn–Teller systems Conclusions Molecular distortions dominated by the dynamic Jahn–Teller effect have been revealed by vibrational spectroscopy As a cubic structure does not lift the degeneracy of the t 1u -based conduction band allowing metallicity to exist, our results prove the relevance of the dynamic Jahn–Teller effect overcoming Hund’s rule causing a loss of degeneracy, leading to a correlation-driven localized state, namely the magnetic Mott–Jahn–Teller insulator state Two inequivalent distortions due to the crystal field Y. Takabayashi, A.Y. Ganin, P. Jeglič, D. Arčon, T. Takano, Y. Iwasa, Y. Ohishi, M. Takata, N. Takeshita, K. Prassides, M.J. Rosseinsky Science 323, 1585 (2009) A.Y. Ganin, Y. Takabayashi, P. Jeglič, D. Arčon, A. Potočnik, P.J. Baker, Y. Ohishi, M.T. McDonald, M.D. Tzirakis, A. McLennan, G.R. Darling, M. Takata, M.J. Rosseinsky, K. Prassides Nature 466, 221 (2010) A.Y. Ganin, Y. Takabayashi, Y.Z. Khimyak, S. Margadonna, A. Tamai, M.J. Rosseinsky, K. Prassides Nat. Mater 7, 367 (2008) C.C. Chancey, M.C.M. O’Brien: The Jahn-Teller Effect in C 60 and Other Icosahedral Complexes. Princeton University Press, 1997 A. Auerbach, N. Manini, E. Tosatti: Phys. Rev. B 49, 12998 (1994) M. Capone, M. Fabrizio, C. Castellani, E. Tosatti Rev. Mod. Phys. 81, 943 (2009) References Peculiar spectra compare to the well known metallic and the present localized state