No abstract

The Seebeck coefficient S of the cuprate superconductor La 2−x Sr x CuO 4 (LSCO) was measured in magnetic fields large enough to access the normal state at low temperatures, for a range of Sr concentrations from x ¼ 0.07 to x ¼ 0.15. For x ¼ 0.11, 0.12, 0.125, and 0.13, S=T decreases upon cooling to become negative at low temperatures. The same behavior is observed in the Hall coefficient R H ðTÞ. In analogy with other hole-doped cuprates at similar hole concentrations p, the negative S and R H show that the Fermi surface of LSCO undergoes a reconstruction caused by the onset of charge-density-wave modulations. Such modulations have indeed been detected in LSCO by x-ray diffraction in precisely the same doping range. Our data show that in LSCO this Fermi-surface reconstruction is confined to 0.085 < p < 0.15. We argue that in the field-induced normal state of LSCO, charge-density-wave order ends at a critical doping p CDW ¼ 0.15 AE 0.005, well below the pseudogap critical doping p ⋆ ≃ 0.19.

The Seebeck coefficient S of the cuprate YBa 2 Cu 3 O y is measured in magnetic fields large enough to suppress superconductivity, at hole dopings p ¼ 0.11 and p ¼ 0.12, for heat currents along the a and b directions of the orthorhombic crystal structure. For both directions, S=T decreases and becomes negative at low temperature, a signature that the Fermi surface undergoes a reconstruction due to broken translational symmetry. Above a clear threshold field, a strong new feature appears in S b , for conduction along the b axis only. We attribute this feature to the onset of 3D-coherent unidirectional charge-densitywave modulations seen by x-ray diffraction, also along the b axis only. Because these modulations have a sharp onset temperature well below the temperature where S=T starts to drop towards negative values, we infer that they are not the cause of Fermi-surface reconstruction. Instead, the reconstruction must be caused by the quasi-2D bidirectional modulations that develop at significantly higher temperature. The unidirectional order only confers an additional anisotropy to the already reconstructed Fermi surface, also manifest as an in-plane anisotropy of the resistivity.