Penetration depth studies of unconventional superconductors

The magnetic penetration depth lambda of various unconventional superconductors have been measured down to ∼0.1 K using a tunnel-diode based, self-inductive technique at 21 and 28 MHz. The samples measured are the heavy-fermion superconductors CeCoIn5 and PrOs4Sb12, the rare-earth borocarbides RENi2B2C (RE = Y, Lu, Er, Ho), the spin-triplet superconductor Sr2RuO4, and elemental Pb. From these studies we see how superconductivity is affected by the presence of another order, such as paramagnetism/antiferromagnetism (in ErNi2B2C) or spin-density-wave (in Pb). We also see how the superfluid response is modified by nonlocal corrections in CeCoIn5 and Sr2RuO4. Hints of multi-band superconductivity are seen in YNi2B2C, LuNi2B 2C and Sr2RuO4. Finally, we observe point-node behavior in PrOs4Sb12, whose superconductivity is suggested to be non-magnetic in origin---the first of its kind. Various theoretical models have been used to fit the data. Some of the work are still ongoing.