Despite decades of intense researches, the enigmatic pseudo-gap phase of superconducting cuprates remains an unsolved mystery. In the last 15 years, condensed matter physicists discovered that this phase hosts symmetry breaking states as an intra-unit cell (or q=0) magnetism, interpreted in terms of loop current patterns , preserving lattice translation and breaking the time-reversal and parity symmetries, followed, upon cooling, by an additional incipient charge density wave  breaking the lattice translational symmetry. However, none of these states can (alone) account for the partial gapping of the Fermi surface.
Our recent polarized neutron diffraction measurements in YBa2Cu3O6+x single crystals with different hole doping levels [3-4] reveal a novel hidden bi-axial magnetism that may be crucial to elucidate the pseudogap puzzle. This short-range magnetism, with typical correlations over 5-6 unit cells, is carried by the CuO2 layers and settles in at the pseudogap onset temperature. Distinct from the q=0 magnetism, its planar propagation wave vector is (π,0)≡(0,π), yielding a (2x2) quadrupling of the magnetic unit cell (q=1/2 magnetism). It further displays a strong out-of-plane anisotropy of the associated magnetic moments, predominantly pointing perpendicular to the CuO2 planes, consistent with a loop current phase.
We discovered that the q=0 and q=1/2 magnetisms could be embedded within a single complex and highly spread-out magnetic texture. This phase could correspond to the smallest possible domain of loop current supercell breaking the lattice translational symmetry, recently proposed to account for the pseudogap opening . The existence of such a broad magnetic texture reveals an unexpected aspect of the pseudogap physics that may modify our understanding of that state of matter.
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 B. Keimer et al., Nature 518, 179 (2015).
 D. Bounoua et al., Comm. Phys 5, 268 (2022)
 D. Bounoua et al, arXiv:2302.01870 (2023), under review in Phys. Rev. B.
 C.M. Varma, Phys. Rev. B, 99, 224516 (2019).
Dr. Jitae Park
Dr. Theresia Heiden-Hecht