Abstract
The Seebeck coefficient $S$ of the cuprate YBa$_{2}$Cu$_{3}$O$_{y}$ was measured in magnetic fields large enough to suppress superconductivity, at hole dopings $p = 0.11$ and $p = 0.12$, 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 $S_{\rm b}$, for conduction along the $b$ axis only. We attribute this feature to the onset of 3D-coherent unidirectional charge-density-wave 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.
Highlights
In the past decade, various transport measurements in high magnetic fields have revealed that the Fermi surface of hole-doped cuprate superconductors undergoes a reconstruction at low temperature in a doping interval centered at p ≃ 0.12 [1]
The key feature of this Fermisurface reconstruction (FSR) is the presence of a small electronlike pocket, detected by quantum oscillations [2,3,4,5], combined with sign changes in the temperature dependence of the Hall (RH) and Seebeck (S) coefficients, from positive at high temperature to negative at low temperature
The anomaly in Sb we observe in YBCO at p 1⁄4 0.11 is confined to a region of the H-T diagram (Fig. 4) that is essentially the same region where 3D unidirectional CDW order has been observed by x-ray diffraction (XRD) [26,27,28]
Summary
Various transport measurements in high magnetic fields have revealed that the Fermi surface of hole-doped cuprate superconductors undergoes a reconstruction at low temperature in a doping interval centered at p ≃ 0.12 [1]. The mechanism by which CDW order produces a small electron pocket in the Fermi surface of hole-doped cuprates remains a puzzle. This is because CDW order is thought to be unidirectional (or “stripelike”) in at least some cuprates, and a unidirectional CDW modulation does not, in general, produce a closed electron pocket [22], at least not at “nodal” locations in the Brillouin zone, away from the antinodal pseudogap [23]. Bidirectional CDW order (with in-plane modulations along both high-symmetry directions of the tetragonal or orthorhombic lattice) readily produces a closed electron pocket at nodal locations [24,25]
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