Jahn Teller orbital glass state10/21/2014
The most expanded fcc-structured alkali fulleride, Cs3C60, is a Mott insulator at ambient pressure because of the weak overlap between the frontier t1u molecular orbitals of the C603- anions. It has a severely disordered antiferromagnetic ground state that becomes a superconductor with a high critical
(a) Electronic phase diagram of fcc A3C60 fullerides shown as a function of volume occupied per C603- ion. The red solid line shows the superconducting transition temperature, Tc and the green solid line the antiferromagnetic ordering temperature, TN. The dotted line delineates the temperature evolution of the volume occupied per C603- ion in fcc Cs3C60 at ambient pressure4 and marks the path along which the present temperature-dependent MAS NMR measurements were undertaken. The line is color-coded to indicate how the correlation time for the Jahn–Teller deformational dynamics compare with the time-scale of the MAS NMR experiments (10-5 s) – at high temperatures, this time-scale is significantly shorter (red color) but as the temperature is lowered it becomes longer (blue color). Open rectangles mark two representative regions of the phase diagram, where the MAS NMR results reveal different dynamics of the Jahn–Teller effect as depicted in (b) and (c).
(b) At high temperatures in the Mott–Jahn–Teller-insulating phase, the rate of interconversion between different Jahn–Teller conformers is very fast on the NMR time-scale (orbital-liquid state) thus restoring the high symmetry of the C603- site and allowing for the observation of three distinct 13C MAS NMR peaks assigned to the C1, C2 and C3 crystallographically inequivalent carbon sites.
(c) As the rate of the Jahn–Teller dynamics slows down at low temperatures, the m-3 molecular symmetry is locally broken leading to the multiplication of the number of inequivalent carbon sites with different local molecular distortions and different charge densities, which are marked schematically by the red, blue and green regions of the C603- units. As a result, a very broad featureless 13C MAS NMR spectrum is measured at 56 K. In this regime, the merohedral disorder removes the degeneracy in the energy of the Jahn–Teller distortions and freezes-in the randomness of the JT conformations leading to a Jahn–Teller orbital glass state.
This joint study between Institute Jozef Stefan, University of Durham, University of Liverpool and National Institute of Chemical Physics and Biophysics, Tallinn was published in
Jahn–Teller orbital glass state in the expanded fcc Cs3C60 fulleride