The CuO2 plane is supposed to underlie the basic physics of cuprate superconductors. Such a plane is made of three square Braviss lattices, two of which are occupied by O ions and one by Cu ions. In a work published last year, both the O- (Zhang-Rice orbital) and Cu-sub-lattice contribute one orbital per site. The inter-electron correlations are negligible on O ions but essential on Cu ions. The correlation is signified by a Hubbard repulsion on Cu sites. The low energy sector thus consists of one orbital from O and one from the lower-band of Cu, per site. What results is a theory that might be able to explain a heap of perplexing phenomena discovered recently. I would say, in many aspects this model resembles the single-band Hubbard model. The difference, which is critical in producing the p-n asymmetry, is that, this model is two-band.
In the latest Nature Physics, a perspective comes up highlighting some aspects of the single-band model. Particularly, the authors focus on two things: (1) the strange metal phase with linear T-dependence resistivity and (2) the spectral weight transfer that suggests electron correlations. By analyzing relevant experiments, they advocate composite, rather than fractionalized, excitations, which they call doublons. These doublons constitute the freedom degrees from the upper Hubbard band. Again, such model ignores the p-n asymmetry, which is, I think, a key to the understanding of cuprate SC physics. [nature physics | VOL 6 | DECEMBER 2010 | www.nature.com/naturephysics]
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