Zhang-Rice singlets fall apart

A quarter of century has already passed since the discovery of cuprate superconductors. The theoretical understanding has been a central problem in condensed matter physics. Perhaps the most frequently utilized model to model their behaviors is based on the so-called Zhang-Rice singlets. Such singlet is spin less and consists of an O hole and a Cu hole. Although, this picture has been prevailing in literature over so many years, loopholes gradually show up in both experimental and theoretical studies (please see previous blog entries). Here I mention another study that reveals this fallacy. It was published last year. [PRL 103, 087402 (2009)]

X-ray absorption spectra on the overdoped high-temperature superconductors Tl2Ba2CuO6þ and La2 xSrxCuO4 reveal a striking departure in the electronic structure from that of the underdopedregime. The upper Hubbard band, identified with strong correlation effects, is not observed on the oxygenK edge, while the lowest-energy prepeak gains less intensity than expected above p 0:21. This suggests a breakdown of the Zhang-Rice singlet approximation and a loss of correlation effects or a significant shift in the most fundamental parameters of the system, rendering single-band Hubbard models inapplicable. Such fundamental changes suggest that the overdoped regime may offer a distinct route to understanding in the cuprates.