Fermi surface transformation across the pseudogap critical point in the cuprate Nd-LSCO from thermoelectric measurements

In cuprate superconductors, the nature of the pseudogap phase and its interplay with superconductivity are still unclear. Its onset at a doping p* is characterized by a drop in carrier density n from n=1+p above p* to n=p below p* [1]. In Nd_0.4La_1.6-xSr_xCuO₄ (Nd-LSCO), this shows up as a low-temperature upturn in the resistivity ρ and Hall coefficient R_H at dopings below p*=0.23 [2]. Here we present thermoelectric measurements in Nd-LSCO across p*, in magnetic fields large enough to suppress superconductivity. For a heat current in the CuO₂ planes, the Seebeck coefficient S shows a large increase at low temperature below p*, confirming the loss of carrier density. Alon the c-axis, we find that S is isotropic for p > p*, i.e. S_c/T and S_a/T are equal (and positive) in the T=0 limit. In sharp contrast, S_c becomes negative at low temperature when p < p*, revealing a profound change in the Fermi surface topology across p*. Using a Boltzmann transport model, which is shown to calculate accurately resistivity with a k-dependent scattering rate [3], we find that one needs an energy dependent scattering to get the correct sign and temperature dependence of S_a and S_c.
[1] Badoux et al, Nature 531, 210 (2016)
[2] Collignon et al, PRB 95, 224517 (2017)
[3] Fang et al, arXiv:2004.01725