Effect of controlled disorder on quasiparticle thermal transport in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$

Low temperature thermal conductivity, $\kappa$, of optimally-doped Bi2212 was studied before and after the introduction of point defects by electron irradiation. The amplitude of the linear component of $\kappa$ remains unchanged, confirming the universal nature of heat transport by zero-energy quasiparticles. The induced decrease in the absolute value of $\kappa$ at finite temperatures allows us to resolve a nonuniversal term in $\kappa$ due to conduction by finite-energy quasiparticles. The magnitude of this term provides an estimate of the quasiparticle lifetime at subkelvin temperatures.