= 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. https://doi.org/10.1103/PhysRevB.48.4219[Hirschfeld and Goldenfeld (1993), abstract: crossover of low-temperature penetration depth from linear to quadratic behavior under strong scattering]{ref}
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