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Singular Band Behavior of High-Temperature Superconductors

We revise mean-field approximation of the slave-boson technique and show that through the consistent treatment of ground state energy and particle-propagators the Luttinger sum rule is preserved. The approximation results with the effectively free electron system where the effects of strong Coulomb interaction are manifested through the renormalization of the band-parameters. Applying this approximation on the extended Emery model for the CuO2 of the high-Tc superconductors, electron band structure is calculated, characterized by three parameters: the site-energy difference \Delta_{pf} &#61472 ; ; ; between Cu and O, the Cu-O overlap t and the O-O overlap t&#8217 ; ; ; Performing analytically slave boson renormalisation due to large U on the Cu site, it is shown in the regime \Delta_{pf} >>t, t', that the model with small t' maps onto the model with t'=0 with modified values of parameters where Brinkman-Rice metal-insulator transition takes place. This mapping though fails in the region of large bare values of \Delta_{pf}, where the renormalized \Delta_{pf}saturates at 4|t'|, showing that the copper band never crosses the oxygen band. The effective band parameters are then used to fit Fermi surfaces and energy bands of LSCO, Bi2212 and Y123, observed in the ARPES experiments. It is shown that the evolution of the Fermi surface with doping, measured in LSCO, can be explained within the above renormalisation scheme, if the underdoped regime is associated with strong renormalisation of t and saturation of \Delta_{pf}. Doping then increases strongly the value of t with respect to t', while the value of \Delta_{pf} stays practically unaffected as suggested by slave boson calculation.

high-Tc superconductors; slave boson approximation; electronic band-structure; ARPES measurements

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