 Disorder qualification of nodal power laws

The nodal quasiparticle power laws of a clean d-wave superconductor assume that the excitation energy exceeds any disorder-induced low-energy scale. Scattering from impurities changes the quasiparticle spectrum and can introduce a finite low-energy density of states. A measured power law can consequently cross over as temperature falls, even if the underlying pairing symmetry stays the same.

For example, strong impurity scattering can replace a clean linear-in-temperature London penetration depth increment by a quadratic law at sufficiently low temperature. A power-law exponent alone therefore does not determine the pairing symmetry without a stated scattering regime. Comparing several observables, sample purities and temperature intervals helps distinguish a robust gap feature from a disorder crossover. ^[ref]