Friday, March 25, 2011

Mind The Pseudogap

If there is a sequel to "Beware of the Pseudogap", this would be it.

In the earlier paper, they reported observing two distinctive gaps for the cuprate superconductors - one that is associated with the superconducting gap, while the other is of a different nature and thus, competes with superconductivity. Finding the true nature of the pseudogap is extremely important, since it can be determined, once and for all, if this gap that exists above Tc, the critical temperature, has anything to do with superconducting mechanism at all - is it a precursor to superconductivity, or is it competing with it. This paper said yes to both.

Now comes another interesting report[1]. This time they studied the optimally-doped cuprate superconducting compound Bi2201 using three different techniques: ARPES, polar Kerr effect, and time-resolved reflectivity. What they discovered is quite interesting. They claim that the onset of T* (the pseudogap temperature) is a phase transition into a non-superconducting broken-symmetry state.

But then, what about the question on whether there are two distinctive gaps in the superconducting phase, as claimed in the earlier paper? This is what they have to say:

Below Tc, the nodal arc is gapped with a dwave–like structure suggestive of a dominantly superconducting origin (38). In contrast, in the antinodal region, rather than one order being dominant, or the two gaps of both orders adding in quadrature, the spectral function develops a complex structure with two energy scales below EF of mixed origin, a larger one being primarily associated with the pseudogap order and a smaller one with the superconducting order.

Fascinating! Essentially, they observe the same thing, i.e. two different gaps, albeit it in the antinodal region of the Brillouin zone. So there is some consistencies here with this respect.

I certainly don't doubt that more studies on this will be forthcoming. But at least now we can start to consider separating the two different origins of the gaps this amazingly-complex material.


[1] R.-H. He et al., Science v.331, p.1579 (2011).

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