Frustrated magnetism in superconducting fullerides

Muon explores fcc Cs3C60

Muon explores fcc Cs3C60
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Researchers from the universities of Liverpool and Durham have used ISIS muons as part of a study published in this week’s Nature, showing how structural control of superconductivity can be used to discern the way electronic correlations underpin the mechanism of high-temperature superconductivity.

Adding certain metal ions to solid carbon-60 molecules creates a superconducting material that can conduct electricity without resistance. Using caesium gives the highest superconducting transition temperature for this family. In this study a new variant of Cs3C60 was synthesized with a face-centred cubic (fcc) crystal structure that becomes superconducting under applied pressure.

By comparing the properties of superconducting C60 materials a universal trend for this family was found that identifies the proximity to a transition between metallic and insulating states as the key parameter controlling the superconducting behaviour. This indicates that the correlations between electrons are crucial to the superconducting mechanism. Other high-temperature superconducting materials, such as the copper oxide materials which have the highest superconducting transition temperatures, can only be made with one structure – a 2D square motif – whereas C60 systems can be made with different high-symmetry structures, allowing the effect of structural changes to be probed.

High-temperature superconductors are also known to be on the verge of magnetically ordered states, and fcc Cs3C60 is no exception. However, its face-centred cubic lattice acts to frustrate the magnetic interactions between neighbouring C60 molecules, suppressing the magnetic ordering temperature. In fcc Cs3C60 magnetic order only occurs below 2.2 K, compared to 46 K in the unfrustrated body-centred cubic analogue; such suppression by magnetic frustration leads to the intriguing situation where superconductivity emerges at 35 K with pressure out of a rapidly fluctuating magnetic state. ISIS muons elucidated this weak low-temperature magnetically ordered state and the transition to a higher temperature disordered state evident in other measurements.

These results demonstrate the universal way electronic correlations control the magnetic and superconducting properties of alkali metal doped carbon-60 compounds. This discovery will help to guide theoretical models of other families of high-temperature superconductors leading to an understanding of the underlying mechanism and the possibility of designing materials that are better suited to applications.

Alexey Y. Ganin (University of Liverpool), Yasuhiro Takabayashi (Durham University), Peter Jeglic (Institute Jozef Stefan), Denis Arcon (Institute Jozef Stefan & University of Ljubljana, Slovenia), Anton Potocnik (Institute Jozef Stefan), Peter J. Baker (ISIS),
Yasuo Ohishi (SPring-8), Martin T. McDonald, Manolis D. Tzirakis (Durham University), Alec McLennan, George R. Darling (University of Liverpool), Masaki Takata (SPring-8 & RIKEN),
Matthew J. Rosseinsky (University of Liverpool) & Kosmas Prassides (Durham University).

Research date: December 2009

Further Information

See A. Ganin, Y. Takabayashi, et al. Nature 466, 221 (2010).

This study was funded by EPSRC. STFC provided access to the muon beamline MuSR at ISIS and x-ray beamlines at ESRF and Diamond.

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