Hidden magnetic texture in the pseudogap phase of high-Tc superconducting YBa2Cu3O6+x

by Dr Dalila Bounoua (LLB, CEA)

Monday 18 Dec 2023, 14:30 → 15:30 Europe/Berlin

PH HS 3 (Physics Department)

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 [1], preserving lattice translation and breaking the time-reversal and parity symmetries, followed, upon cooling, by an additional incipient charge density wave [2] 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 YBa₂Cu₃O_6+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 [5]. 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.

[1] P. Bourges, D. Bounoua, Y. Sidis, C.R. Phys 22, 1 (2021).
[2] B. Keimer et al., Nature 518, 179 (2015).
[3] D. Bounoua et al., Comm. Phys 5, 268 (2022)
[4] D. Bounoua et al, arXiv:2302.01870 (2023), under review in Phys. Rev. B.
[5] C.M. Varma, Phys. Rev. B, 99, 224516 (2019).