TITLE:
Hole-Pair Formation in Cuprate Superconductors despite Antiferromagnetic Fluctuations
AUTHORS:
Rana Janardon Singh, Shakeel Khan
KEYWORDS:
Hole-Pair Formation in Cuprate Superconductors, Buckling Angle in CuO2 Plane, Coherence Length in a-b-Plane ζab
JOURNAL NAME:
World Journal of Condensed Matter Physics,
Vol.4 No.3,
August
29,
2014
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 Cu2+ ions of the plaquette
(CuO)4 in which it enters. Spin alignments undergo oscillations from
vertically upward to vertically downward of the CuO2 plane. Vertical
projections of spins go on changing when they pass through different plaquettes
going to zero when they pass through the CuO2 plane. Ferromagnetic
alignments of spins produce magnetic fields on the plane proportional to their
vertical projections. When two holes travelling in CuO2 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 3dx2 - y2 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 Å, whereas in Y-123 = 15 20 Å in CuO2 plane. It has been reported that in Tl-2201 the CuO2 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 CuO2 plane depends upon the nature of the near neighbors
of the CuO2 plane. Near neighbors of Y-123 and Tl-2201 have been
compared.