Zhang-Rice singlet directly imaged in cuprate superconductor

Scientists at Tsinghua University and the Chinese Academy of Sciences have reported the first direct visualization of a Zhang-Rice singlet, a localized quantum state that forms when a hole is introduced into a copper-oxide plane of a cuprate superconductor. The study, published in Nature Physics, used hole-doped Ca₂CuO₂Cl₂ (CCOC), a material chosen for its simple crystal structure and exceptionally clean surface, which allows high-resolution atomic-scale imaging. By partially substituting calcium with sodium, the team prepared a series of samples spanning doping levels from the extremely lightly doped regime to the onset of superconductivity.

For each doping level, the researchers performed large-area, high-resolution imaging to map the spatial distribution of electronic states at different energies. This approach allowed them to continuously follow the evolution of the electronic structure as holes were added, rather than observing only the final superconducting state. The measurements revealed the real-space electronic structure of an individual Zhang-Rice singlet associated with a single doped hole, providing direct experimental support for a theoretical concept that has been central to cuprate physics for decades.

As the hole concentration increased, the initially isolated Zhang-Rice singlets did not simply accumulate. They spontaneously merged to form new physical entities the team dubbed "electronic molecules." These structures appear as plaquettes approximately four lattice constants in size, with stripe-like molecular orbitals. With further doping, the electronic molecules gradually connect with one another, eventually evolving into the high-temperature superconducting state.

Senior author Yayu Wang said the work establishes a missing experimental link between the microscopic physics of individual doped holes and macroscopic superconductivity. The direct visualization of Zhang-Rice singlets and their evolution into electronic molecules could improve theoretical models of high-temperature superconductors and inform the design of new materials. The researchers plan to further investigate the spatial configurations and interaction conditions under which doped holes first develop a superconducting gap, and to test whether the electronic-molecule framework can unify diverse experimental observations in cuprates.

The study was conducted using scanning tunneling microscopy and spectroscopy techniques on the CCOC samples. The paper, "Visualization of the Zhang–Rice singlet, electronic molecules and Cooper pair formation in a cuprate superconductor," appears in Nature Physics (2026) with DOI 10.1038/s41567-026-03375-4.