Anisotropy of the Seebeck Coefficient in the Cuprate Superconductor YBa2Cu3Oy: Fermi-Surface Reconstruction by Bidirectional Charge Order

Abstract

The Seebeck coefficient S of the cuprate YBa2Cu3Oy is 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 Sb , 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. The unidirectional order only confers an additional anisotropy to the already reconstructed Fermi surface, also manifest as an in-plane anisotropy of the resistivity.

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Plain Language Summary

Many superconductors operate at very low temperatures. Copper oxide materials known as cuprates, however, exhibit superconductivity at record high temperatures (around −120∘C ). The origin of this remarkable phenomenon is a puzzle, in part because other electronic behaviors coexist with superconductivity and compete with it in ways that are not well understood. These behaviors can be described by changes in an abstract entity known as the Fermi surface, a boundary defining the free electrons’ allowed momentums. In the cuprate YBa2Cu3Oy , it is thought that one of two types of charge-density waves (CDWs)—modulations in the density of charge carriers—reconstructs the Fermi surface, but it is not clear which one is responsible. We present experiments that identify the culprit.

The two types of CDWs in YBa2Cu3Oy are (i) two-dimensional bidirectional short-range modulations and (ii) three-dimensional unidirectional long-range ones. We measured the Seebeck coefficient—a parameter that indicates how much voltage is created when one side of a material is heated—for both the short and long axes of an YBa2Cu3Oy crystal and found a striking new feature that coincides with the onset of 3D CDWs. However, the signature of Fermi-surface reconstruction, a change in the Seebeck coefficient to negative values, appears at much higher temperatures along with growth of 2D waves seen by x-ray diffraction. This indicates that the 2D CDWs are responsible for the reconstruction.

We expect that our findings will settle the debate on the cause of the Fermi-surface reconstruction in YBa2Cu3Oy , as well as other cuprates, and will help clarify how the CDWs relate to superconductivity.