A rather interesting paper that appeared in Nature this week. The authors studied the trilayer Bi2223 by observing Tc, the critical temperature, as they increase the pressure on the material[1]. [Side note: trilayer Bi2223 is called that because it has 3 copper-oxide planes per unit cell.] The interesting thing is that they first noticed an increase in Tc with increasing pressure, but then it started to drop before increasing again, to a maximum of around 136 K at 34 GPa.
The explanation given here is that Tc is first determined by two temperature scales: the critical temperature of pairing, i.e. when the Cooper pairs start to form, and the critical temperature for phase coherence, when all the pairs become "stiff" from phase fluctuation. This last part is when one gets the long-range phase coherence in a superconductor.
But it gets more interesting than that. It appears that with increasing pressure, one gets more hole doping onto the two outer copper-oxide planes. These two outer planes start to become more doped with holes and thus, might actually have a higher Tc, while the center copper-oxide plane remains underdoped. This is what the authors called "... The natural appearance of an inhomogeneous charge distribution among the inner and outer CuO2 planes...."
It is certainly an interesting proposal for a possible increase in Tc for these material.
Zz.
[1] X.-J. Chen et al., Nature v.466, p.950 (2010).
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