Optical response of the superconducting state of K3C60 and Rb3C60

Do organic and other exotic superconductors fail universal scaling relations?

Universal scaling relations are of tremendous importance in science, as they reveal fundamental laws of nature. Several such scaling relations have recently been proposed for superconductors; however, they are not really universal in the sense that some important families of superconductors appear to fail the scaling relations, or obey the scaling with different scaling pre-factors. In particular, a large group of materials called organic (or molecular) superconductors are a notable example. Here, we show that such apparent violations are largely due to the fact that the required experimental parameters were collected on different samples, with different experimental techniques. When experimental data is taken on the same sample, using a single experimental technique, organic superconductors, as well as all other studied superconductors, do in fact follow universal scaling relations.

The mean free path for electron conduction in metallic fullerenes

The electrical resistivity, p, of a metal is usually interpreted in terms of the mean free path (the average distance, l, an electron travels before it is scattered). As the temperature is raised, the resistivity increases and the apparent mean free path is correspondingly reduced. In this semi-classical picture, the mean free path cannot be much shorter than the distance, d, between two atoms. This has been confirmed for many systems and was considered to be a universal behaviour. Recently, some apparent exceptions were found, including alkali-doped fullerenes and high-temperature superconductors. However, there remains the possibility that these systems are in exotic states, with only a small fraction of the conduction electrons contributing to the conductivity; the mean free path would then have to be correspondingly larger to explain the observed resistivity. Here we report a model calculation of electron conduction in alkali-doped fullerenes, in which the electrons are scattered by intramolecular vibrations. The resistivity at large temperatures implies l << d, demonstrating that there is no fundamental principle requiring l > or = d. At high temperatures, the semi-classical picture breaks down, and the electrons cannot be described as quasiparticles.