Discovery of charge order in a cuprate Mott insulator*

Cuprate superconductors exhibit multiple interacting electronic phases. Among them, charge/spin stripes and incommensurate charge density waves (CDWs) have been experimentally observed In underdoped cuprates with correlated metallic ground states. Here, we search for signatures of CDW order in very lightly hole-doped cuprates from the RBa₂Cu₃O_7 − δ family (RBCO; R = Y or rare earth) by using resonant X-ray scattering, electron transport, and muon spin rotation measurements to resolve its electronic and magnetic ground states. Specifically, we use Pr to substitute Y at the R-site to systematically underdope YBCO and access the insulating regime of the cuprate phase diagram without changing the oxygen stoichiometry of YBCO. Resonant X-ray scattering data on Pr-doped YBCO thin films reveal an in-plane CDW order with a peak onset temperature (T_CDW) above 300 K, similar to the T_CDW of the 3D charge order seen by Ruiz et al. in this system [1]. This in-plane CDW follows and extends the linear evolution of the wave vector versus hole concentration relationship present in oxygen-underdoped YBCO [2] all the way to the insulating and magnetically ordered Mott limit. Combined with the recent observation of the charge crystal phase on an insulating surface of Bi₂Sr₂CaCu₂O_8 + z [3], our results in RBCO suggest that this electronic symmetry breaking is universally present in very lightly doped CuO₂ planes. These findings bridge the gap between the Mott insulating and underdoped metallic states and underscore the prominent role that Coulomb-frustrated electronic phase separation plays among all cuprates.

[1] A. Ruiz et al. Nat. Comm. 13, article 6197 (2022)

[2] S. Blanco-Canosa et al. Phys. Rev. B 90, page 054513 (2014)

[3] H. Zhao et al. Nat. Mat. 18, page 103 (2019)