Thermopower across the phase diagram of the cuprate La1.6-xNd0.4SrxCuO4: Signatures of the pseudogap and charge density wave phases

The Seebeck coefficient (thermopower) S of the cuprate superconductor La1.6-xNd0.4SrxCuO4 was measured across its doping phase diagram (from p=0.12 to p=0.25), at various temperatures down to T≃2 K, in the normal state accessed by suppressing superconductivity with a magnetic field up to H=37.5 T. The magnitude of S/T in the T=0 limit is found to suddenly increase, by a factor ≃5, when the doping is reduced below p=0.23, the critical doping for the onset of the pseudogap phase. This confirms that the pseudogap phase causes a large reduction of the carrier density n, consistent with a drop from n=1+p above p to n=p below p, as previously inferred from measurements of the Hall coefficient, resistivity, and thermal conductivity. When the doping is reduced below p=0.19, a qualitative change is observed whereby S/T decreases as T→0, eventually to reach negative values at T=0. In prior work on other cuprates, negative values of S/T at T→0 were shown to result from a reconstruction of the Fermi surface caused by charge density wave (CDW) order. We therefore identify pCDW≃0.19 as the critical doping beyond which there is no CDW-induced Fermi surface reconstruction. The fact that pCDW is well separated from p reveals that there is a doping range below p where the transport signatures of the pseudogap phase are unaffected by CDW correlations, as previously found in YBa2Cu3Oy and La2-xSrxCuO4.