Umklapp scattering in unconventional superconductors: Microwave conductivity shows that κ−(BEDT−TTF)2Cu[N(CN)2]Br is a dxy superconductor

Abstract

Microwave conductivity experiments can directly measure the quasiparticle scattering rate in the superconducting state. We show that this, combined with knowledge of the Fermi surface geometry, can distinguish between closely related superconducting order parameters, e.g., ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{xy}$ superconductivity. We benchmark this method on ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and, unsurprisingly, confirm that this is a ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ superconductor. We then apply our method to $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{Br}$, which we discover is a ${d}_{xy}$ superconductor.