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 the 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 a series of 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. Application of pressure suppresses the low-T upturn in S/T, as it did the upturn in ρ and R_H [3]. For a heat current perpendicular to the CuO₂ planes, 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*.
[1] Badoux et al., Nature 531, 210 (2016)
[2] Collignon et al., Phys. Rev. B 95, 224517 (2017)
[3] Doiron-Leyraud et al., Nature Comm. 8, 2044 (2017).