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Interplay between magnetism and superconductivity near the pressure induced Mott transition in the Cs3C60 compound


Unconventional superconductivity (SC) is generally claimed for materials which display a phase diagram where SC is proximate to a magnetic phase. This is clearly the case for the High Tc cuprates for which the ground state of the undoped compound is an insulating antiferromagnet (AF). This state is called a Mott insulator as the electronic correlations (EC) (coulomb interactions) localise the electrons on the Cu sites. This Mott state is only driven into a metallic SC by chemical introduction of carriers in the CuO2 planes, so that the cuprates are doped Mott insulators. Here we shall consider a Mott insulator which becomes a metal upon application of an external pressure p which reduces the distance between molecular sites and then favours the delocalisation of the electrons. As will be summarised here, this so called Mott insulator to metal transition has been found to occur in the specific case of Cs3C60 [1][2]. This is a member of the A3C60 compounds family (A=alkali metal), in which the C60 balls (Fig.1) are pushed apart by the insertion of three A per unit cell which give their three electrons per C60 ball. All other A3C60 do display SC at ambient pressure and were considered as conventional SC [3][4]. As the Cs is the alkali with the largest ionic radius Cs3C60 is the most expanded compound and happens to be a Néel AF insulator at ambient p. It is only driven into a metal (SC with optimal Tc 38K) after the Mott transition under application of a pressure p> p0=5 kbar. This is emphasised in our recent work dedicated to a detailed study of the (p,T) phase diagram of Fig. 2 in which SC is quite proximate to the Mott insulating state, so that the importance of EC cannot be neglected in the SC state.

NMR techniques have allowed us to distinguish the two isomeric phases of Cs3C60 [2], which slightly differ in their insulating state magnetic properties. We have chosen here to perform a detailed study of the A15 phase of Fig.1, in which we did evidence that AF and SC phases coexist only in a narrow p range about p0 (Fig.2). This establishes the first order thermodynamic character in agreement with expectations for a Mott transition in a three dimensional (3D) material. A detailed analysis of the 133Cs NMR spectrum allowed us to preclude any cell symmetry change, which is a hallmark of a genuine Mott transition driven by the electronic density of states. We do evidence a slight variation of the lattice parameter at the transition due to lattice expansion, as occurs in e.g. V2O3.

Fig 1 : Unit cell structure of the A15-Cs3C60 phase. Here the body centred cell of C60 balls is completed by pairs of Cs atoms lying on the cube faces which are organised in chains in the three lattice directions.

At ambient p in the insulating state, 13C NMR shift data allowed us to evidence that the electronic paramagnetic susceptibility follows a Curie-Weiss law associated with a local spins S=½, instead of a high spin S=3/2 predicted by Hund’s rules for ionic C603+. This violation of Hund’s rules due to a modification of the C60 spherical shape (Jahn -Teller Distortion) has been predicted for charged C60n+ ions[5][6].

At fixed p>p0, in the metallic state one can probe the evolution of the SC properties when the ECs are progressively increased toward p0. For instance, the variation with T of (T1T)-1, where T1 is the 133Cs or 13C NMR spin-lattice relaxation yields for T< Tc a measurement of the SC gap 2Δ (see Fig. 3). We had recently shown that 2Δ increases with decreasing p towards p0, while Tc decreases on the SC dome, so that upon approaching the Mott transition 2Δ/kBTc increases regularly with respect to the BCS value observed in conventional SC.

Fig. 2 (Left) : The (p,T) phase diagram of A15-Cs3C60 displays at low T a narrow first order transition at p0 between a Néel AF and a SC state. At higher pressures the thermal expansion at fixed p induces a recovery of the insulating paramagnetic state through a transition which broadens with increasing p up to a critical point pc 7 kbar. The variety of data shown here represent specific points in the step like variation seen in Fig. 3 as well as on the 133Cs NMR shift (see References for details)).
Fig. 3 (Right) : The 133Cs nuclear spin lattice relaxation time T1 allows to evidence the SC gap below 30K for p>5 kbar (blue data, details in References) and the AF gap for p < 5 kbar (filled red symbols).The step increase between the metallic and the insulating paramagnetic behaviours locates the Mott transition given in Fig. 2.

One can also see that (T1T)-1 displays a second step like increase in Fig. 3 centred at a temperature T’ >Tc and joins then a behaviour quite identical to that found at ambient p in the Mott phase. This is therefore the signature of a high T recovery of the insulating behaviour. The Mott transition broadens and smears out progressively for increasing p , which has led us to establish the (p,T) phase diagram presented in Fig. 2 with a critical point at pc 7 kbar similar to that known for the liquid-vapour transition.

This ensemble of results bring clear evidence that the increasing ECs near the 3D Mott transition are not significantly detrimental to superconductivity. They rather suggest that repulsive electron interactions might even reinforce electron-phonon superconductivity, being then partly responsible for the large Tc values, as proposed by theoretical models taking the ECs as a key ingredient [7]. Furthermore the normal state properties do not exhibit the pseudogap phase which is quite characteristic of the cuprate phase diagram in which the insulator metal transition is controlled by hole doping.

[1] Y. Takabayashi et al, Science 323, 1585 (2009)
[2] Y. Ihara et al, PRL104, 256402 (2010) ; EPL 94, 37007 (2011)
[3] O. Gunnarsson, RMP 69, 575 (1997).
[4] C. H. Pennington et al, RMP 68, 855 (1996).
[5] M. Capone et al, Phys. Rev. B 62 ,7619 (2000).
[6] V. Brouet et al, Phys. Rev. B 66, 155123(2002)
[7] M. Capone et al, RMP 81, 943 (2009).

References

Mott Transition in the A15 Phase of Cs3C60 : Absence of a Pseudogap and Charge Order
H. Alloul, P. Wzietek, T. Mito, D. Pontiroli, M. Aramini, M. Riccò, J.P. Itie, and E. Elkaim
Physical Review Letters 118, 237601 (2017). doi:10.1103/PhysRevLett.118.237601
P. Wzietek, T. Mito, H. Alloul, D. Pontiroli, M. Aramini and M. Riccò, PRL 112, 066401 (2014).

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Henri Alloul