Infrared reflectivity measurements of a superconducting energy scale in Rb3c60

AN understanding of the superconductivity and other electronic properties of the alkali-metal-doped fullerenes1 will require measurements of their solid-state physical properties. In the normal state one would like to know the electron effective mass, the Fermi energy and conduction bandwidth, and the mean free path for electron transport; the superconducting state, meanwhile, can be characterized at the most basic level by length and energy scales such as the coherence length, penetration depth and superconducting energy gap. Here we describe the use of far-infrared reflectivity measurements to probe the low-energy response of Rb3C60. We derive a value for the characteristic energy scale associated with reflectivity, which allows us to estimate a value for the gap energy 2δ of 3–5 kTC, in reasonable agreement with tunnelling measurements2. Combining this result with data obtained in previous studies, we estimate other length and energy scales, such as the bandwidth and penetration depth.