Direct observation of charge order in underdoped and optimally doped Bi2(Sr, La)2CuO6+δ by resonant inelastic x-ray scattering

Charge order in underdoped and optimally doped high-Tc superconductors Bi2Sr2-xLaxCuO6+δ (Bi2201) is investigated by Cu L3 edge resonant inelastic x-ray scattering. We have directly observed charge density modulation in the optimally doped Bi2201 at momentum transfer Q≃0.23 reciprocal lattice units, with smaller intensity and correlation length with respect to the underdoped sample. This demonstrates the short-range charge order in Bi2201 persists up to optimal doping, as in other hole-doped cuprates. We explored the nodal (diagonal) direction and found no charge order peak, confirming that charge density modulates only along the Cu-O bond directions. We measured the out-of-plane dependence of charge order, finding a flat response and no maxima at half integer L values. This suggests there is no out-of-plane phase correlation in single layer Bi2201, at variance from YBa2Cu3O6+x and La2-x(Ba,Sr)xCuO4. Combining our results with data from the literature we assess that charge order in Bi2201 exists in a large doping range across the phase diagram, i.e., 0.07p0.16, demonstrating thereby that it is intimately entangled with the antiferromagnetic background, the pseudogap, and superconductivity.