Origin of Pseudogap in High-Tc Superconductors
  We have proposed earlier a paired cluster (PC) model for high T_c superconductivity mechanism. We show here that this model is able to explain the pseudogap origin and other gap related properties of cuprates. As per PC model, singlet coupled magnetic cluster pairs are present in the cuprates, in the material's otherwise paramagnetic state below a temperature T_CF, where an ionic spin of cluster 1 forms a singlet pair with a corresponding ionic spin of cluster 2. The conducting electrons (CEs) for T>=T_c and both the CEs and the Cooper pairs (CPs) for T<T_c interact with the singlet coupled ion pairs which enhances the CE, CP energy, causing a redistribution of the filled electronic density of states (DOS). Due to this a pseudogap appears in the electronic DOS at the Fermi surface, for T_CF>=T>=T_c, with d-wave symmetry which superimposes over the BCS superconducting state (SS) energy gap for T<T_c resulting in (i) a mixed anisotropic s-, d- wave symmetry for the observed below T_c energy gap, (ii) nondisappearance of the gap at T_c on heating and almost T independence of the gap width, (iii) presence of states in the gap and (iv) several other gap behaviour related properties, like the absence of NMR spin relaxation rate coherence peak, which give impression of a non-BCS, nonphononic cuprate superconductivity. 
J. K. Srivastava (Tata Institute of Fundamental Research, Mumbai,   India)