Unconventional isotope effects in superconducting fulleridesM. Riccò1, F. Gianferrari1, D. Pontiroli1, M. Belli1, C. Bucci1 and T. Shiroka1, 2
1 CNISM and Dipartimento di Fisica, Università di Parma - Via G. P. Usberti 7/a, 43100 Parma, Italy
2 Laboratory for Muon-Spin Spectroscopy, Paul Scherrer Institut - CH-5232 Villigen PSI, Switzerland
received 14 September 2007; accepted in final form 4 January 2008; published March 2008
published online 31 January 2008
Although widely accepted, the phonon-mediated, BCS-like theory of superconducting A3C60 fullerides (A = K, Rb, Cs) cannot reproduce correctly all their parameters, even in its strong-coupling, Migdal-Eliashberg limit. The fundamental difficulty, ascribed to intrinsically close phonon and electron energy scales (respectively at 0.2 and 0.25 eV), has been overcome by dynamical mean-field theories (DMFT), which, unlike ME, consider electron-phonon and electron-electron interactions on an equal footing. The unconventional phenomena predicted in the new framework include, among others, isotope effects on spin susceptibility, totally absent from standard theories. We have tested these predictions, finding a significant dependence on the isotopic mass in both Tc and, more importantly, in the normal-state Pauli susceptibility . The comparative measurement of in two different K3C60 samples (85% 13C-enriched vs. natural abundance), both by SQUID magnetometry as well as by 13C NMR Knight shift, definitely confirms the presence of an isotope effect on susceptibility, although a quantitative agreement with theory is still missing.
71.20.Tx - Fullerenes and related materials; intercalation compounds.
74.25.Ha - Properties of type I and type II superconductors: Magnetic properties.
76.60.Cq - Chemical and Knight shifts.
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