Low-energy quasi-circular electron correlations with charge order wavelength in Bi₂Sr₂CaCu₂O_8+δ

Charge order correlations are ubiquitous to cuprate superconductors. However, very little is still known about the dynamic behavior of these correlations and what influence they may have to the transport properties of the cuprates, such as the strange metal phase. We report high-resolution resonant inelastic x-ray scattering (RIXS) experiments that reveal the presence of dynamic electron correlations over the q_x-q_y scattering plane in underdoped Bi₂Sr₂CaCu₂O_8+δ with T_c=54 K. Here we use the softening of the RIXS-measured bond stretching phonon line as a marker for the presence of charge-order-related dynamic electron correlations. The experiments show that these dynamic correlations exist at energies below approximately 70 meV and are centered around a quasi-circular manifold in the q_x-q_y scattering plane with radius equal to the magnitude of the charge order wave vector. We also demonstrate how this phonon-tracking procedure provides the necessary experimental precision to rule out fluctuations of short-range directional order as the origin of the observed correlations. The existence of low energy quasi-circular correlations demonstrated by these experiments further promotes the concept where dynamic charge order correlations mediate the isotropic scattering responsible for the strange metal behavior of the cuprates.