Seebeck Coefficient in a Cuprate Superconductor: Particle-Hole Asymmetry in the Strange Metal Phase and Fermi Surface Transformation in the Pseudogap Phase

Abstract

We report measurements of the Seebeck effect in both the ab plane (S_{a}) and along the c axis (S_{c}) of the cuprate superconductor La_{1.6-x}Nd_{0.4}Sr_{x}CuO_{4} (Nd-LSCO), performed in magnetic fields large enough to suppress superconductivity down to low temperature. We use the Seebeck coefficient as a probe of the particle-hole asymmetry of the electronic structure across the pseudogap critical doping p^{⋆}=0.23. Outside the pseudogap phase, at p=0.24>p^{⋆}, we observe a positive and essentially isotropic Seebeck coefficient as T→0. That S>0 at p=0.24 is at odds with expectations given the electronic band structure of Nd-LSCO above p^{⋆} and its known electronlike Fermi surface. We can reconcile this observation by invoking an energy-dependent scattering rate with a particle-hole asymmetry, possibly rooted in the non-Fermi-liquid nature of cuprates just above p^{⋆}. Inside the pseudogap phase, for p<p^{⋆}, S_{a} is seen to rise at low temperature as previously reported, consistent with the drop in carrier density n from n≃1+p to n≃p across p^{⋆} as inferred from other transport properties. In stark contrast, S_{c} at low temperature becomes negative below p^{⋆}, a novel signature of the pseudogap phase. The sudden drop in S_{c} reveals a change in the electronic structure of Nd-LSCO upon crossing p^{⋆}. We can exclude a profound change of the scattering across p^{⋆} and conclude that the change in the out-of-plane Seebeck coefficient originates from a transformation of the Fermi surface.