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 similar or equal to 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 similar or equal to 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 p(CDW) similar or equal to 0.19 as the critical doping beyond which there is no CDW-induced Fermi surface reconstruction. The fact that p(CDW) 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.