Henri Alloul (Orsay): NMR studies of the pressure induced Mott transition to superconductivity in the two phases of Cs3C60

Henri started by asking the audience whether they are familiar with the fact that fullerenes are superconducting and also strongly correlated systems [majority knew, by the way]

He also pointed out the major players in the field in a past: P.W Anderson and B. Batlogg

Highlight of the field: new results on the superconductivity induced by pressure, Takabayashi et
al, Science 2009, A. Ganin et al., Nature Materials, Aprl 2008

Outline of the talk:
- Introduction: A3C60 and their superconductivity
- electronic corrleations and Jahn-Teller Distotions
- Expanded magnetic moments
- Conclusion


He introduced the crystal structure (cubic and fcc - bipartite) of fullerides. Electronic structure from LDA: from molecular levels to the Bloch states [transfer integrals are weak W~ 0.5 eV] t_{1u} bands are filled by introducing dopants (alkali ions)

Exp. fact: Tc depends linearly on the lattice constant, a. It was originally interpreted as a sign that BCS formula Tc~ \hbar \omega_D exp[-1/(V N_0)] works well. Thus it was concluded at a time that it is a phonon-mediated superconductor [in 1991!]

1995: the story is not so simple - different behavior of the slopes Tc vs a [lattice constant] for Na2AC60 and A3C60 - signs of the correlations.

Furthermore, for A_nC_60 it was found that they have different (from metallic to insulating) behaviors. He gave two examples: A_4 C_60 (bct structure) and N2C60 (cubic). Both show small spin gap from NMR and large charge gap from optics - Mott insulators?!

The reason: large Coulomb interaction U, two electrons on a ball costs an energy U plus there is a Jahn-Teller effect [deformation of the molecule]. Remarkable result is that for n=2 and n=4
there is a larger energy gain per electron [these are results from the molecular structure calculations by Tosatti]. Especially there is additional U_{eff} which arises due to Jahn-Teller distortion and adds to the usual U for even n (in A_nC_60) and U-U_{eff} for odd n. It gives Mott insultor for n=2 and n=4.

Nevidomskii: why it is not simply a band inslutor: Answer: the reason is that n=2 and n=4 have a different crystal structure, it cannot be explained on the level of band insulator.

The rest of the talk was about odd value of n in A_nC_60

Special case: CsC_60 (A_1C_60): - Mott insulator. The reasoning: from a high_T_c cubic phase phase - to a polymer phase at 200K - and finally dimer phase at 77 K (all from NMR). Experimental justification comes from NMR which sees 3 different nuclei sites with different electronic surrounding. Studying the intensity of NMR you realize the proportional compositions of the phases. By doing that you find 12% of sites in a spin singlet state.

Then he moved to the A_nC_60 series with n=3: here the most intriguing perspective is a search for the Mott insulator (originally studied in 90's K_3C_60 is not a Mott insulator). The idea is to take a larger alkali ionic radius (going from Li to Cs). Success by chemists: Cs_3C_60 has been recently prepared. Here you do find the AF Mott state and overall the phase diagram as a function of pressure resembles many of those which are typical for Mott insulators: AF at small pressure, AF+SC at intermediate pressure (doping) and SC at larger doping with a dome like structure. The slight complication is that there is also a structural transition in these compounds: A15 structure for Cs_3 C_60 - you see a single Cs site with non-cubic local symmetry (MIT and SC part of the phase diagram); fcc (only Mott part) structure of Cs3C60 you find two Cs sites and the ratio 1:2. Difference allows for selective NMR experiments.

In the next few slides Henri has analyzed the magnetic dynamics and crystal structure by means of NMR in Cs3C60 in the fcc phase. Special emphasis was put on the enhanced magnetic fluctuations in the paramegnatuc phase. Upon pressure you find the transitions from fcc phase to A15 phase as well the transition from AF to SC phase. Important remark: Mott insulator to metal transition is not directly related to the crystal structure transition. (here the argument is that you do have MIT also in the A15 phase)

Superconductivity: definitely singlet superconductivity most likely s-wave (there is a Hebel-Slichter peak). But more measurements have to be taken.



Summary:
- fullerides are correlated
- original due to disorder, icosaedral symmetry of the soccer ball
- supercondctors near MIT
- static charge segregation in Cs1C60
- importance of the Jahn-Teller effect: different between odd and even n
- excellent possibility to study the multiorbital Mott transitions.


Questions: 1) blogger: is Jahn-Teller splitting is comparable to the bandwidth, Answer: there is no clear indication, calulations are done for the molecule.
2) Vojta: what is the role of possible spin frustration in the MIT Answer: not really known
Remark: coexistence region between SC and AFM is possibly an inhomogeniety effect
3) Nevidomskii: fcc phase can be a spin glass - any results for zero field. Answer: similar to the previous ones: the field just started, Remark from someone in the audience: there are data from muSR which indicates phase transition or something similar at 2K, origin is unclear.