This week in cond-mat

Here are a couple of recent papers that seem likely to generate quite a bit of interest.

arxiv:0903.5359 - Voggu et al., A New Method of Obtaining High Enrichment of Metallic Single-Walled Carbon Nanotubes
One of the major challenges in using nanotubes for various electronics purposes is the large number of tube types. Carbon nanotubes can be metallic or semiconducting depending on just how their graphene-like mesh is wrapped into a cylinder, and most common nanotube growth methods produce a whole mixture of different tube types. Sometimes it would be very nice to produce only a single tube type. Well, the authors here seem to have found an approach that gives them a high yield (90%) of metallic nanotubes, using a particular catalyst chemistry in a carbon arc furnace. Sounds promising, though scale-up to industrial levels (that is, kg quantities of tubes) is likely to be pretty challenging. These approaches also can be devilishly tricky to reproduce - three nominally identical setups can grow different compositions of material because of sensitivity to tiny variations in conditions.

arxiv:0903.5260 - Hicks et al., Evidence for Nodal Superconductivity in LaFePO from Scanning SQUID Susceptometry
A big outstanding question in the new iron-based superconductors is, what is the pairing symmetry of the superconducting wavefunction? In ordinary low-T_c superconductors, the electrons pair up in "s-wave" pairing. That is, each Cooper pair consists of a spin singlet with zero orbital angular momentum. In contrast, the high-T_c cuprates have d-wave pairing - again a spin singlet, but each Cooper pair has two units of orbital angular momentum. The big significance of this is that the pair wavefunction then must have nodes where it changes sign (just like d orbitals in atoms have nodes as a function of "longitude" when going around the atomic center). The presence of nodes means that there are certain directions in the material where the superconducting gap is zero, and therefore it costs very little energy to make electron- (or hole-)like excitations. These manifest themselves in the temperature dependence of various quantities like magnetic penetration depth. Well, the authors here have used a very powerful (but challenging) technique to measure the penetration depth locally as a function of temperature in LaFePO, and they argue that they see evidence of nodes. This is exciting, because some people argue that the iron pnictides should have s-wave pairing (though a funny kind, where electron and hole pockets in the band structure each have superconducting order with opposite signs of order parameter). It'll be neat to see how this all shakes out....