# a-b anisotropy of the intra-unit-cell magnetic order in $\rm YBa_2Cu_3O_{6.6}$

## TL;DR

This study reveals an a-b anisotropy in intra-unit-cell magnetic order in YBa2Cu3O6.6 below the pseudogap temperature, indicating mirror symmetry breaking likely due to a specific loop current arrangement.

## Contribution

It provides the first polarized neutron diffraction evidence of a-b anisotropy in intra-unit-cell magnetic order in detwinned YBa2Cu3O6.6, suggesting a new symmetry-breaking mechanism.

## Key findings

- Intra-unit-cell magnetic order breaks mirror symmetry of CuO2 bilayers.
- An a-b anisotropy appears below the pseudogap temperature T*.
- The magnetic structure likely involves a crisscrossed loop current pattern.

## Abstract

Within the complex phase diagram of the hole-doped cuprates, seizing the nature of the mysterious pseudo-gap phase is essential to unravel the microscopic origin of high-temperature superconductivity. Below the pseudo-gap temperature $\rm T^{\star}$, evidences for intra-unit-cell orders breaking the 4-fold rotation symmetry have been provided by neutron diffraction and scanning tunneling spectroscopy. Using polarized neutron diffraction on a detwinned $\rm YBa_2Cu_3O_{6.6}$ sample, we here report a distinct a-b anisotropy of the intra-unit-cell magnetic structure factor below $\rm T^{\star}$, highlighting that intra-unit-cell order in this material breaks the mirror symmetry of the CuO$_2$ bilayers. This is likely to originate from a crisscrossed arrangement of loop currents within the $\rm CuO_2$ bilayer, resulting in a bilayer mean toroidal axis along the $\rm {\bf b}$ direction.