Density of states in high-Tc superconductors

Abstract

The density of staleS and the low temperature specific heat of higb-Tc superconductors are calculated in a functional i n.formali$m using the slave boson technique. The manybody calculation in a saddle point approximation shows that the Iowener&Y sector is dominated by 3 single band. The calculated values of density of states are in cood apeemcot witb experimental results. 1.lntroductioaHigh temperature superronductivity in ceramic oxides raises serious questions regarding the mech~ anism of normal and superconducting state properties of these systems. Ever since the discovery [1), various theoretical models have been suggested and different mechanisms ranging from conventional weak coupling BCS-like to novel strong coupling formulations have been proposed (2-4). But still there is no general consensus on the relevant parameter regime and the mechanism of superconductivity in these systems.It is believed that the essential physics of nonnal and superconducting states reside in the Cu-O planar subsystem which is common to the copper oxide superconductors.Many theoretical models begin with a two-dimensional extended Hubbard model or Anderson lattice model with a strong on-site repulsive interaction [5-7}. The introduction of nearest neighbour repulsion in the extended Hubbard model [8] emphasizes the relevance of charge degrees of freedom. In the l-J models and RVB theories the low energy sector of the problem is dominated by the spin degrees of freedom [9,10].In this paper we analyze the extended model without nearest neighbour repulsion, but with a direct oxygen-oxygen hopping between the Po orbitals which brings to the scene, in addition to the spin degrees of freedom, the charge degrees of freedom.These models have been previously studied usin&a< mean field scheme [11,12]. We studied this stroaIlt ~ correlated problem using a slave boSOD (13) lem. At half filling, the materials are antifef'l'ODtit netically spin ordered insulators. The proximity. being a magnetic insulator makes strong corrdatiol relevant to the doped case.. Here we consider a model, in which the on-site .. pulsion imposes a double occupancy constraint., the copper 3d;cl_y2 orbital. We study tbis using a JaJp N expansion technique which is nonperturbative it the CQupling constants. The study is restricted to the mean field theory (N = 00 ). The partition f\mct\on and the free energy are derived in section 2. The expressions for the specific heat and the density of states have been derived in section 3. The numerical computations are presented in section 4. The co.clusions concerning the re normalized energy bands and the comparison with experiments are also discussed in section 4. The modelThe model we consider in this paper has three essential bands. As a consequence of the oxygen ligand enviYonment the copper 3d degeneracy is lifted and 0921-4534/92/$05.00