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 CuO2 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 dx2–y2 symmetr and b) coherence length is of the order of 15 - 20 Angstrom. Heisenberg exchange interaction between two (CuO4) 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.
Highlights
Various mechanisms have been proposed for high temperature superconductivity (HTS), but as yet no consensus has been reached
The basic idea of this model has been derived from some experimental results as 1) electron paramagnetic resonance (EPR) spectra of quenched superconductors which very frequently show (CuO)4 fragments broken off from the active CuO2 layer in the cuprates, 2) 41 meV peak observed in neutron diffraction and nuclear magnetic resonance (NMR) spectra of superconductors and 3) Heisenberg exchange interaction leading to ferromagnetism (FM) in Cu-monomers. The frequent occurrence of (CuO) in nanoforms, sometimes even up to room temperature (RT) and some generally accepted conclusions as a) that the order-parameter of cuprates has dx2 y2 symmetry and b) that the coherence length is of the order of 15 - 20 Angstrom (A)
We have shown that when the wandering hole enters a (CuO)4 plaquette it experiences a magnetic field generated by the spin magnetic moments of the holes associated with the Cu2+ ions and their circulation around the boundary of (CuO)4 plaquette
Summary
Various mechanisms have been proposed for high temperature superconductivity (HTS), but as yet no consensus has been reached. To cross a plaquette because ten (CuO) units are crossed in 10–13sec which is the time period of spin oscillation Another important thing that will happen to (CuO) entity is that all the four Cu2+ holes have to circulate around the outer boundary of (CuO)4 [see Figure 2(a)]. Circulation of Cu2+ holes around the boundary of (CuO) plaquette is a necessity if coupling of nuclear spins of four copper ions has to be materialized whose signature is provided by EPR spectra. We have shown that when the wandering hole enters a (CuO) plaquette it experiences a magnetic field generated by the spin magnetic moments of the holes associated with the Cu2+ ions and their circulation around the boundary of (CuO) plaquette. We try to show how the two wandering holes form hole-pairs by virtue of Heisenberg exchange interaction
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