Hole-Pair Formation in Cuprate Superconductors despite Antiferromagnetic Fluctuations

Abstract

We have earlier proposed models of preformed hole pairs based on the results of our electron paramagnetic resonance experiments. A hole doped in a cuprate superconductor causes ferromagnetic alignment of the spins of the holes of 4 Cu 2+ ions of the plaquette (CuO) 4 in which it enters. Spin alignments undergo oscillations from vertically upward to vertically downward of the CuO 2 plane. Vertical projections of spins go on changing when they pass through different plaquettes going to zero when they pass through the CuO 2 plane. Ferromagnetic alignments of spins produce magnetic fields on the plane proportional to their vertical projections. When two holes travelling in CuO 2 plane come across each other at a certain distance between them, they are attracted towards each other by Heisenberg exchange interaction and their path is decided by the magnetic field produced due to spin alignments. Their path is similar to 3 dx 2 - y 2 atomic orbital. Y-123 has been chosen as an example. Due to plethora of evidence of antiferromagnetic fluctuations in cuprates, hole-pair formation has been tried in Y-123 assuming antiferromagnetic fluctuations in it. It has been found that hole-pair formation in spite of AFM fluctuations can be explained on the same lines as done earlier. Hole-pair formation was tried in Tl-2201 to test whether the same rules apply in cuprates with very high coherence lengths. Coherence length in Tl-2201 = 52 &Aring , whereas in Y-123 = 15 20 &Aring in CuO 2 plane. It has been reported that in Tl-2201 the CuO 2 plane is very flat and smooth. From this it was concluded that high coherence length is the result of the smoothness of the plane. Further it was concluded that the smoothness of the CuO 2 plane depends upon the nature of the near neighbors of the CuO 2 plane. Near neighbors of Y-123 and Tl-2201 have been compared.