Ideal charge density wave order in the high-field state of superconducting YBCO

TL;DR

This study reveals a highly coherent, unidirectional charge density wave in high magnetic fields in clean YBCO crystals, significantly larger and more ordered than previously observed, indicating a potential ideal ground state for cuprates.

Contribution

It demonstrates the existence of a long-range, unidirectional CDW in high-field, clean YBCO, distinct from the zero-field CDW, with the largest correlation volume observed in cuprates.

Findings

Unidirectional CDW appears at a critical field proportional to Hc2.

The high-field CDW has a correlation volume 2-3 orders larger than at zero field.

The observed CDW is likely representative of the ideal, disorder-free cuprate ground state.

Abstract

The existence of charge density wave (CDW) correlations in cuprate superconductors has now been established. However, the nature of the ground state order has remained uncertain because disorder and the presence of superconductivity typically limit the CDW correlation lengths to a dozen unit cells or less. Here we explore the CDW correlations in YBa2Cu3Ox (YBCO) ortho-II and ortho-VIII crystals, which belong to the cleanest available cuprate family, at magnetic fields in excess of the resistive upper critical field (Hc2) where the superconductivity is heavily suppressed. We find an incommensurate, unidirectional CDW with a well-defined onset at a critical field strength that is proportional to Hc2. It is related to but distinct from the short-range bidirectional CDW that exists at zero magnetic field. The unidirectional CDW possesses a long inplane correlation length as well as significant correlations between neighboring CuO2 planes, yielding a correlation volume that is at least 2 - 3 orders of magnitude larger than that of the zero-field CDW. This is by far the largest CDW correlation volume observed in any cuprate crystal and so is presumably representative of the high-field ground-state of an "ideal" disorder-free cuprate.