Long-sought Zhang-Rice singlet visualized directly in cuprate superconductor

Researchers at Tsinghua University and the Chinese Academy of Sciences have directly visualized a localized Zhang-Rice singlet for the first time in the unconventional cuprate superconductor, hole-doped Ca₂CuO₂Cl₂ (CCOC). This groundbreaking work, published in Nature Physics, sheds light on the processes responsible for high-temperature superconductivity in cuprate materials.

The study was motivated by the central question of how superconductivity emerges when holes are introduced into an antiferromagnetic Mott insulator. Most previous research focused on either the undoped parent compound or on samples that had already become superconducting. However, the crucial intermediate stage—the formation of new electronic states and superconducting pairs from isolated holes—remained unexplored.

By preparing a series of CCOC samples with varying doping levels, the researchers performed large-area, high-resolution atomic-scale imaging to map the spatial distribution of electronic states at different energies. This allowed them to directly observe the formation of a Zhang-Rice singlet associated with an individual doped hole. This is the first time such a singlet has been directly visualized in real space.

As the hole concentration increased, the researchers found that isolated Zhang-Rice singlets spontaneously merged to form new electronic entities called "molecules." These molecular states exhibited distinctive electronic structures, including stripe-like molecular orbitals and a size of approximately four lattice constants. The merging of these molecular states eventually led to the high-temperature superconducting state observed in the material.

The direct visualization of Zhang-Rice singlets provides strong experimental support for this long-standing theoretical concept. The researchers believe that this work establishes a crucial link between the microscopic physics of individual doped holes and macroscopic superconductivity. By observing the formation of these singlets and their subsequent merging into electronic molecules, the study sheds light on the transition from undoped states to superconductivity.

These findings could open new possibilities for understanding high-temperature superconductivity and potentially guiding the design of new materials with advantageous properties. The researchers plan to further investigate the spatial configurations and interaction conditions under which doped holes first develop a superconducting gap.

Additionally, they aim to develop a unified picture of electronic molecules to explain various experimental observations in cuprate materials, such as transport measurements and angle-resolved photoemission spectroscopy.