Discovery of an electronic crystal in very underdoped RBCO*

The quantum electronic state in underdoped copper oxide high temperature superconductors tends to break the translational symmetry of the parent lattice. The resulting emergent, collective phases of matter compete with superconductivity and manifest in two main forms – charge/spin stripes and incommensurate charge-density-waves (CDWs) – but in all cases emerge out of a strongly correlated metallic state. 

In the present study, we used a combination of resonant X-ray scattering, electronic transport, and muon spin rotation measurements to study these electronic orders in the RBa₂Cu₃O₇ family (RBCO; R = rare earth), using rare earth substitution to reach the very underdoped regime. There, we discovered a multiorbital density wave originating from Cu and Pr orbitals, signaling the presence of a spatially modulated electronic state deep in the ‘Mott’ limit.

Our observations of an electronic crystal state in a magnetic insulating cuprate reveal that this electronic state is a new and potentially ubiquitous form of electronic symmetry breaking in the CuO_­2 planes. Ultimately, these findings strongly hint at a strong coupling origin of charge order in the RBCO family and underscore the prominent role of Coulomb-frustrated electronic phase separation across all cuprates.