Angular dependence of the penetration depth in unconventional superconductors

We examine the Meissner state nonlinear electrodynamic effects on the field and angular dependence of the low-temperature penetration depth δ of superconductors in several kinds of unconventional pairing states, with nodes or deep minima (“quasinodes”) in the energy gap. Our calculations are prompted by the fact that, for typical unconventional superconducting material parameters, the predicted size of these effects for δ exceeds the available experimental precision for this quantity by a much larger factor than for others. We obtain expressions for the nonlinear component of the penetration depth Δδ for different two- and three-dimensional nodal or quasinodal structures. Each case has a characteristic signature as to its dependence on the size and orientation of the applied magnetic field. This shows that Δδ measurements can be used to elucidate the nodal or quasinodal structure of the energy gap. For nodal lines we find that Δδ is linear in the applied field, while the dependence is quadratic for point nodes. For layered materials with YBa₂Cu₃O_7−δ type anisotropy, our results for the angular dependence of Δδ differ greatly from those for tetragonal materials and are in agreement with experiment. For the two- and three-dimensional quasinodal cases, Δδ is no longer proportional to a power of the field and the field and angular dependences are not separable, with a suppression of the overall signal as the node is filled in.