# Anisotropy of the Seebeck Coefficient in the Cuprate Superconductor YBa$_{2}$Cu$_{3}$O$_{y}$: Fermi-Surface Reconstruction by Bidirectional Charge Order

## TL;DR

This study investigates the anisotropic Seebeck coefficient in YBa₂Cu₃O_y under high magnetic fields, revealing Fermi-surface reconstruction linked to bidirectional charge order and unidirectional charge-density waves.

## Contribution

It provides evidence that Fermi-surface reconstruction in cuprates is caused by bidirectional charge modulations, distinct from the unidirectional charge-density waves.

## Key findings

- Seebeck coefficient becomes negative at low temperature, indicating Fermi-surface reconstruction.
- A new feature in S_b appears with charge-density-wave onset, but is not the cause of Fermi-surface reconstruction.
- Fermi-surface reconstruction is attributed to higher-temperature bidirectional modulations.

## 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.