Thermal Hall effect in cuprate superconductors

We report measurements of the thermal Hall conductivity κ_xy in cuprate superconductors, in magnetic fields large enough to supress superconductivity.
In a previous study on underdoped YBCO at a doping p = 0.11, we observed a negative κ_xy at low temperature [1], consistent with the electron-like Fermi surface characteristic of the charge-density-wave phase at that doping [2]. We found that the Wiedemann-Franz law, κ_xy/T = L₀ σ_xy, is satisfied in the T = 0 limit [1].
Here we turn to the overdoped regime, with a study of Nd-LSCO at p = 0.24, a doping immediately above the critical doping where the pseudogap phase ends, at p* = 0.23 [3]. We observe a positive κ_xy at low temperature, consistent with the large hole-like Fermi surface characteristic of the overdoped regime [3,4]. Despite the non-Fermi-liquid nature of the transport, with its linear T dependence down to T → 0 [3,5], we find that the Wiedemann-Franz law is again satisfied in the T = 0 limit.

[1] G. Grissonnanche et al., Phys. Rev. B 93, 064513 (2016).
[2] O. Cyr-Choiniere et al., Phys. Rev. X 7, 031042 (2017).
[3] C. Collignon et al., Phys. Rev. B 95, 224517 (2017).
[4] S. Badoux et al., Nature 531, 210 (2016).
[5] R. Daou et al., Nat. Phys. 5, 31 (2009).