Origin of Pseudogap in High-Tc Superconductors

TL;DR

The paper explains the pseudogap phenomenon in high-Tc cuprates using the paired cluster model, attributing it to singlet coupled magnetic clusters affecting electronic states and resulting in non-BCS gap properties.

Contribution

It demonstrates that the paired cluster model can account for the pseudogap's origin and properties, providing a non-phononic explanation for high-Tc superconductivity.

Findings

Pseudogap appears at T_CF >= T >= T_c with d-wave symmetry.

The energy gap does not vanish at T_c and remains almost T-independent.

The model explains the absence of NMR coherence peak and other gap-related properties.

Abstract

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.