7.1 A₃C₆₀

In conclusion we have observed the LF relaxation of Mu@C₆₀ in the A₃ superconductors. The temperature dependence of the relaxation rates exhibit Korringa behaviour above T_c and a small strongly field dependent coherence peak and strong activated behaviour in the superconducting state. The superconducting energy gap can be extracted from the low temperature behaviour, and the large uncertainty in its value is mainly due to uncertainty in the appropriate form of the temperature dependent superconducting DOS. For Rb₃ and K₃ and the range of models considered, the reduced gap $2\Delta/kT_c$ lies in the range 3-5. However, for consistency between the high and low field temperature dependences, a weakly temperature dependent broad DOS is favoured, and the reduced gap values lie between 3.2 and 4.0. The upper end of this range as well as the broad low temperature DOS peaks are consistent with the tunneling and optical results of Koller.[195] The small size of the peak can be due to one or several factors, such as the moderately strong electron-phonon coupling suggested, for example, by tunneling.[195,198] Following Pennington and Stenger[34] we conclude that, provided the results of Akis[125] carry over directly to the case of A₃C₆₀, which possesses a very broad and complicated phonon spectrum, the small size of the 1.5 T coherence peak implies $T_c/E_{\log} \approx 0.2$.A ratio as large as this is incompatible with the near weak-coupling value of the energy gap which gives[195] $T_c/E_{\log} < 0.1$. Thus strong-coupling alone will not consistently account for the both the small coherence peak and the gap. Together with the strong field dependence of the coherence peak, this suggests that an additional coherence peak suppression mechanism connected with the inhomogeneous vortex state must be present. A full explanation of such an effect would require extension of the theories discussed in section IV.D[144,147] to the regime of strong-coupling.

In transverse field we find broadening of the $\mu$SR precession line due to the inhomogeneous fields of the vortex state. There is no clear flux lattice lineshape, so we can only estimate the magnitude of $\lambda$, which for example in Rb₃ lies in the range 3000 - 7000 Å . This range is definitely inconsistent with the values of Chu and McHenry[90], and this discrepancy is likely due, not to sample dependence of $\lambda$, but rather to the systematic differences between the two techniques.

The weaker activated dependence in s-Na₂Cs is likely due to coexistence of the ns-Na₂Cs phase. Quench-rate dependent experiments may be able to clarify this and allow measurement of the properties of both phases. By comparing the coherence peak in Fm$\bar{3}$m materials with s-Na₂Cs, we find no evidence for an effect on the coherence peak due to the degree of molecular order. In the Fm$\bar{3}$m materials, quench rate dependence would also help to clarify the role of frozen orientational disorder in the value of $\lambda$ and in the residual low temperature T₁ relaxation.

Finally we note that there are interesting aspects of Mu@C₆₀ in A₃ such as its stability and very weak interaction with the conduction band that suggest further theoretical invesitagtion of the detailed properties of endohedral fullerene species.