MatQ:

Thermopoweracross the phase diagram of the cuprate La1.6−xNd0.4SrxCuO4 : signatures of the pseudogap andcharge-density-wave phases

Abstract :
The Seebeck coefficient(thermopower) S of the cupratesuperconductor La1.6−xNd0.4SrxCuO4 was measured across its doping phasediagram (from p= 0.12 to p=0.25), at various temperatures down to T ≈ 2K, in the normal state accessed by suppressing superconductivity with amagnetic field up to H= 37.5T. The magnitude ofS/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 dopingfor the onset ofthe pseudogap phase. This confirms that the pseudogap phase causes a largereduction of the carrierdensity n, consistent with a dropfrom n= 1 +p above p* ton=p below p*, as previously inferred from measurements of the Hallcoefficient, resistivity and thermal conductivity. When the doping is reduced below p= 0.19,a qualitative change is observed wherebyS/T decreases as T→0,eventuallyto reach negative values at T=0. In prior work on other cuprates, negative values of S/T atT→0 were shown to result froma reconstruction of the Fermi surface caused by charge-density-wave (CDW) order. We thereforeidentify pCDW ≈ 0.19as the critical doping beyond whichthere is no CDW-induced Fermi surface reconstruction. The fact that pCDW is well separated from p* revealsthat there is a doping range below p* where the transport signatures of thepseudogap phase are unaffectedby CDW correlations, as previously found in YBa2Cu3Oy and La2−xSrxCuO4.