Hidden magnetic texture in the pseudogap phase of high-Tc YBa2Cu3O6.6

The pseudogap state in high-T-c superconducting copper oxides in one of the most mysterious quantum states of matter whose understanding remains a challenge for condensed matter physicists. The authors report a hidden magnetic texture deep in the pseudogap state highlighting another piece of the puzzle. Despite decades of intense research, the enigmatic pseudo-gap (PG) phase of superconducting cuprates remains unsolved. In the last 15 years, various symmetry breaking states were discovered in the PG phase, including an intra-unit cell (IUC) magnetism, which preserves the lattice translational (LT) symmetry but breaks the time-reversal and parity symmetries, and an additional incipient charge density wave breaking the LT symmetry. However, none of these states can (alone) account for the partial gapping of the Fermi surface. Here we report a hidden LT-breaking magnetism using polarized neutron diffraction. Our measurements reveal magnetic correlations, in two different underdoped YBa2Cu3O6.6 single crystals that set in at the PG onset temperature with (i) a planar propagation wave vector (pi, 0) equivalent to (0, pi), yielding a doubling or quadrupling of the magnetic unit cell and (ii) magnetic moments mainly pointing perpendicular to the CuO2 layers. The LT-breaking magnetism is at short-range suggesting the formation of clusters of 5-6 unit cells. Together with the previously reported IUC magnetism, it yields a hidden magnetic texture of the CuO2 unit cells hosting loop currents, forming large supercells that may be helpful for elucidating the PG puzzle.