We present a study of thermal transport to access the electronic mean free path of d-wave quasiparticles in one of the most widely studied cuprate superconductors, Bi₂Sr₂CaCu2O_8+δ (Bi-2212). We have measured the thermal conductivity κ_xx and the thermal Hall conductivity κ_xy in underdoped, optimally-doped and overdoped samples of Bi-2212. In the overdoped and optimally-doped samples, we observe a clear enhancement in both κ_xx and κ_xy upon cooling below the critical temperature T_c, due to a suppression of the inelastic electron-electron scattering as electrons condense into pairs. The magnitude of the enhancement in κ_xy is controlled by the strength of the elastic impurity scattering. We use a prior model applied to the cuprate YBa₂Cu₃O_y (YBCO) [1] to estimate the mean free path from our κ_xy data, and find that the mean free path in Bi-2212 is 10 times shorter than in YBCO.

For the underdoped sample, the absence of any enhancement in either κ_xx or κ_xy points to a high degree of disorder in that sample.

We conclude that the thermal Hall technique is a good way to compare the mean free path of d-wave quasiparticles in various cuprate materials.

 

[1] Y. Zhang et al., Physical Review Letters 86, 890 (2001).