Abstract
The superconducting properties of the graphite intercalation compound CaC6, available as high-quality c-axis-oriented polycrystals, were investigated through muon-spin spectroscopy measurements. An unconventional TF-μSR procedure, applicable to superconductors characterized by close-lying critical fields, was successfully adopted for measuring the muon-spin relaxation rates. Field-dependent measurements provide a value λab(0)=62(4) nm for the in-plane magnetic field penetration depth, κ=1.4(2) for the GL parameter, and hint at the presence of a slight gap anisotropy in the ab plane. The dependence of relaxation on temperature confirms the s-wave character of CaC6 superconductivity, with an average zero-temperature superconducting gap of 2.1(1) meV.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Despite many years of study, graphite and graphite intercalation compounds (GICs) still have many surprises in store. Among these, the unexpected appearance of superconductivity below 12 K in CaC6 implies a series of questions concerning the role of the donor, the pairing mechanism, the exact value of the gap and its possible anisotropy. To investigate the superconducting properties of CaC6, we make use of the muon-spin rotation (μSR) technique, in which implanted spin-polarized muons probe the field modulation induced by the superconducting vortices. CaC6, however, presents an additional challenge: due to the close-lying lower and upper critical fields (Hc1 and Hc2), its superconducting region is narrow and hence difficult to access through standard measurements.
Main results. By employing the special strategy depicted in the figure, we were able to measure the muon-spin relaxation rate σ as a function of temperature and applied field in the whole superconducting range. From these measurements we obtain information concerning the penetration depth, the symmetry of the superconducting gap and, consequently, the pairing mechanism.
Wider implications. The unconventional transverse-field μSR procedure used here could also be applied to many other superconductors characterized by close-lying critical fields.
Figure. (a) For a superconductor with critical fields Hc1 
Hc2, a standard fixed-field scan is sufficient for measuring σ(T). (b) However, for Hc1 
Hc2 a 'zig-zag' path is needed. This consists of piece-wise sequences of field-cooling (horizontal dashed line), followed by a measuring scan on heating, at the same field (full line, displaced for clarity), and by a field change (vertical dashed line).