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Jahn Teller orbital glass state

10/21/2014
Jahn Teller orbital glass state

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
temperature, Tc of 35 K upon compression. The effect of the localised t1u3 electronic configuration on the properties of the material is not well-understood. Here we studied the relationship between the intrinsic crystallographic C603- orientational disorder and the molecular Jahn–Teller (JT) effect dynamics in the Mott insulating state. The high-resolution 13C magic-angle-spinning (MAS) NMR spectrum at room temperature comprises three peaks in the intensity ratio 1 : 2 : 2 consistent with the presence of three crystallographically-inequivalent carbon sites in the fcc unit cell and revealing that the JT-effect dynamics are fast on the NMR time-scale of 10-5 s despite the presence of the frozen-in C603-merohedral disorder disclosed by the 133Cs MAS NMR fine splitting of the tetrahedral and octahedral 133Cs resonances. Cooling to sub-liquid-nitrogen temperatures leads to severe broadening of both the 13C and 133Cs MAS NMR multiplets, which provides the signature of an increased number of inequivalent 13C and 133Cs sites. This is attributed to the freezing out of the C603- JT dynamics and the development of a t1u electronic orbital glass state guided by the merohedral disorder of the fcc structure. The observation of the dynamic and static JT effect in the Mott insulating state of the metrically cubic but merohedrally disordered Cs3C60 fulleride in different temperature ranges reveals the intimate relation between charge localization, magnetic ground state, lifting of electronic degeneracy, and orientational disorder in these strongly-correlated systems.

 

Electronic phase diagram of fcc A3C60 fullerides.

(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
A. Potočnik, A. Y. Ganin, Y. Takabayashi, M. T. McDonald, I. Heinmaa, P. Jeglič, R. Stern, M. J. Rosseinsky, K. Prassides and D. Arčon

Chem. Sci., 2014, 5, 3008.

 

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