On the Nature of Scaling Relations in Cuprate HTSCs

Abstract

The nature of the normal state and the mechanism of superconductivity in HTSC cuprates remain under discussion until now. However, a number of experimental facts may be considered firmly established: 1) There is a d-wave superconducting energy gap on the 2D Fermi surface (FS) of optimally doped HTSC cuprates, with zeros in node directions [1]. 2) The transition to normal state is accompanied by the closing of the gap across parts of the FS and the formation of Fermi arcs centered at the nodes. Meanwhile, the gap in antinode directions virtually does not change across T c ("pseudogap") and closes at some temperature Т*well above T c [2,3]. 3) As the doping level is reduced, the pseudogap increases and deviates from the simple d-wave behavior [4]. In deeply underdoped samples, Cooper pairs form islands in k-space around the nodal regions [5]. 4) Above Т с , underdoped samples exhibit giant Nernst effect and anomalous diamagnetism [6,7]. Here we suggest a simple physical model of HTSC cuprates that provides explanation of the features listed above. According to this model, the unusual properties of these compounds result from their unique electronic structure favourable for the formation of diatomic negative-U centers (NUCs) and realization of an unusual mechanism of electron−electron interaction.The suggested model is based on the mechanism of the formation of NUCs in HTSCs that we proposed previously [8]. Its essence can be understood from Fig. 1a, which shows the electronic spectrum of a CuO 2 plane of an undoped HTSC. It is known that the electronic structure of the insulating phase of HTSCs in the vicinity of the Fermi energy E F is described well by the model of an insulator with a charge-transfer gap [9]. In this scheme, the energy of the lowest-lying excitation Δ сt (~2 eV) is related to the transfer of an electron from an oxygen ion to a neighboring copper ion, 3d 9 L→3d 10 L -(here, 3d 9 L denotes the state with a hole on the 3d-shall of Cu ion and a completely occupied 2p-shell of the neighboring ligand (oxygen), and 3d 10 Lis the state with a completely occupied 3d-shell of Cu and a hole on the ligand ion). Note that the Cu3d 10 L state is considerably higher in energy than Cu3d 10 L -.