Tuesday, January 27, 2009

Is the Pseudogap Competing With The Superconducting Gap?

The issue on the nature of the pseudogap in the spectra of high-Tc superconductors is one of the most puzzling aspect of this family of material. Briefly, the pseudogap is the gap seen in the single-particle spectrum ABOVE Tc, i.e. before it condenses into the superconducting state where the superconducting gap develops. The pseudogap occurs above Tc without any kind of condensation, and it is more pronounced in the underdoped regime. Not only that, as one go further into the underdoped regime, the upper temperature that the pseudogap exists also increases, meaning that you start seeing this gap in the spectrum at an even higher temperature as the doping decreases.

Since it was first discovered, the main question that has been circulating is whether this pseudogap is simply a precursor to the superconducting gap? Is this the signature of electron pairing, forming Cooper pairs, but without the long-range coherence needed to form the superconducting fluid? This is the pre-formed pair scenario. The other school of thought is that the pseudogap is simply a pairing that competes with superconductivity. The charge carriers that are forming the pairing are taken out of the "pool" of carriers that later on will form the Cooper pair and condenses into the supercurrent below Tc.

This question has continued till today, with evidence being presented for one camp or the other. One of the latest by H.B. Yang et al. using ARPES[1]. When they reconstructed the particle-hole symmetry, they arrive at the conclusion that the result supports the preformed pair scenario. However, another report J.H. Ma et al.[2] using STM and ARPES results reported two distinct gaps that behave differently from each other, supporting the idea that the pseudogap competes with superconductivity. This work even had a press release. So is this a done deal?

Not quite yet! Interestingly enough, just a few days ago, a theoretical preprint out of the Univerisity of Chicago discussed this issue[3]. They discussed how, within their preformed concept, that one can have different temperature dependence for the preformed gap that forms at the nodal and antinodal directions of the Cu-O plane of the cuprates. So such observation does not rule out the preformed pair scenario.

In other words, there is still no smoking gun to pick one over the other as far as the origin of the pseudogap. So the story continues.

Zz.

[1] H.B. Yang et al. Nature v.456, p.77 (2008).
[2] J.H. Ma et al. Phys. Rev. Lett. v.101, p.207002 (2008).
[3] C.C. Chien et al. http://arxiv.org/abs/0901.3151

3 comments:

Heumpje said...

Hi Zz,
I guess you want to change "over" in the first alinea into "under"...

Cheers

ZapperZ said...

Thanks for the correction. I don't know why I was going for the "overdoped" regime.

Zz.

Heumpje said...

Because the underdoped regime is a mess. :)

Perhaps this paper interests readers as well. It is the most recent collection of experimental data regarding the two gaps. The authors come to the conclusion that the pseudogap is sets the pairing scale, while the sc gap measures the condensation energy.