a - b Anisotropy of the Intra-Unit-Cell Magnetic Order in YBa2Cu3O6.6

Within the complex phase diagram of the hole-doped cuprates, seizing the nature of the mysterious pseudogap phase is essential for unraveling the microscopic origin of high-temperature superconductivity. Below the pseudogap temperature T*, evidence for intra-unit-cell orders breaking the fourfold rotation symmetry have been provided by neutron diffraction and scanning tunneling spectroscopy. Using polarized neutron diffraction on a detwinned YBa2Cu3O6.6 sample, we here report a distinct a - b anisotropy of the intra-unit-cell magnetic structure factor below T*, highlighting that intra-unit-cell order in this material breaks the mirror symmetry of the CuO2 bilayers. This is likely to originate from a crisscrossed arrangement of loop currents within the CuO2 bilayer, resulting in a bilayer mean toroidal axis along the b direction.