Model of Preformed Hole-Pairs in Cuprate Superconductors

Abstract

A model of preformed hole-pairs in cuprate superconductors has been proposed based on some experimental results i.e., 1) electron paramagnetic resonance spectra of quenched superconductors which show very frequently the fragment (CuO) 4 broken off from the CuO 2 layer in the structure, 2) 41 meV peak observed in neutron diffraction and nuclear magnetic resonance spectra of superconductors, 3) Heisenberg exchange interaction leading to ferromagnetism observed in CuO which is an essential ingredient of all superconductors and some generally accepted conclusions i.e., a) that the order parameter in superconductors has 2 2 x y d  symmetry and b) coherence length is of the order of 15 -20 Angstrom. Heisenberg exchange interaction between two (CuO 4 ) plaquettes each containing a lattice hole binds the two holes which are the charge carriers in the cuprate superconductors. It is not very clear whether the hole-pair is in the triplet or singlet state, but the triplet state is supported by the experimental observation of ferromagnetism in the parent material CuO. The proposed hole-pair singlet is different from Zhang-Rice singlet.

Hole-Pair Formation in Cuprate Superconductors despite Antiferromagnetic Fluctuations

“…[4] because the detailed treatment given in [4] cannot be reproduced here. However, some salient points of the paper [4] are given here which may be of help to readers. As prepared, cuprate superconductors are electron paramagnetic resonance (EPR) silent because of antiferromagnetic (AFM) coupling of Cu 2+ ions in the all-important CuO 2 plane.…”

“…For better understanding of this paper, it should be read in conjunction with the paper in Ref. [4] because the detailed treatment given in [4] cannot be reproduced here. However, some salient points of the paper [4] are given here which may be of help to readers.…”

“…As prepared, cuprate superconductors are electron paramagnetic resonance (EPR) silent because of antiferromagnetic (AFM) coupling of Cu 2+ ions in the all-important CuO 2 plane. When cuprate superconductors are deoxygenated, they show EPR spectra of Cu-monomer, dimmer, tetramer and octamer, but more frequently (CuO) 4 tetramer. (CuO) 4 tetramer is the unit cell of two-dimensional (2-D) CuO 2 plane and by understanding its properties, one can understand the properties of CuO 2 plane which is the seat of superconductivity.…”

“…When cuprate superconductors are deoxygenated, they show EPR spectra of Cu-monomer, dimmer, tetramer and octamer, but more frequently (CuO) 4 tetramer. (CuO) 4 tetramer is the unit cell of two-dimensional (2-D) CuO 2 plane and by understanding its properties, one can understand the properties of CuO 2 plane which is the seat of superconductivity. Spectra of (CuO) 4 and other fragments [6]- [13] given above are observed also in constituents of superconductors as BaCuO 2 , SrCuO 2 , CaCuO 2 , Bi 2 CuO 4 , CuO etc.…”

“…(CuO) 4 tetramer is the unit cell of two-dimensional (2-D) CuO 2 plane and by understanding its properties, one can understand the properties of CuO 2 plane which is the seat of superconductivity. Spectra of (CuO) 4 and other fragments [6]- [13] given above are observed also in constituents of superconductors as BaCuO 2 , SrCuO 2 , CaCuO 2 , Bi 2 CuO 4 , CuO etc. On deoxygenation, (CuO) 4 unit gets magnetically isolated from the bulk by breaking 8 Cu-O bonds situated at its 4 corners.…”