Friday, January 27, 2012

Low Tc but strong Cooper pair binding

The plot below is an important one which I have been meaning to blog about for a while. It shows the strength of the diamagnetic signal [increasing from blue to purple to red]
from an underdoped cuprate superconductor as a function of magnetic field H and temperature T.
The figure is taken from a paper from Ong's group, discussed in an earlier post.
The superconducting transition temperature Tc is 12 K.  However, a magnetic field of about 45 tesla is required to destroy the diamagnetic signal which is associated with Cooper pairing. Furthermore, this "upper critical field" H_c2 is weakly temperature dependent. This large field scale reflects the large binding energy of the Cooper pairs. This can be seen by converting H_c2 to a coherence length (~30 A) and then an energy gap (~20 meV ~ 200 K) via a Pippard type formula [see this earlier Science paper by Ong et al.].  The energy gap is comparable to the pseudogap seen in ARPES and much smaller than k_B Tc.

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