Search for Q ~ 0 Order near a Forbidden Bragg Position in Bi_2.1Sr_1.9CaCu₂O_8+x with Resonant Soft X-ray Scattering

Identifying what broken symmetries are present in the cuprates has become a major area of research. Many authors have reported evidence for the so-called “Q ~ 0” order that involves broken inversion, mirror, chiral, or time-reversal symmetry that is uniform in space. Here we use resonant soft x-ray scattering to search for Q ~ 0 order in Bi_2.1Sr_1.9CaCu₂O_8+x by measuring the region of a forbidden Bragg peak, (0, 0, 3), which is normally extinguished by symmetry but may become allowed on resonance if valence band order is present. Using circularly polarized light, we found that this reflection becomes allowed on the Cu L₃ resonance for temperatures T_c < T < T^*, though remains absent in linear polarization and at other temperatures. This observation suggests the existence of spatially uniform valence band order near the pseudogap temperature.