Seamus Davis on “Atomic Scale Visualization of ‘Pseudogap’ Electronic Matter in Cuprates”

Kavli Institute on Theoretical Physics at Santa Barbara, KITP, has a great program of posting videos and slides of their numerous talks online. (You can also subscribe to their numerous RSS feeds and get them as podcasts)

This week I listened to an excellent talk by Seamus Davis of Cornell on Atomic Scale Visualization of ‘Pseudogap’ Electronic Matter in Cuprates .

This talk was a part of an on-going symposium on strongly correlated systems and cold atoms. There was also another talk by Davis a few days before, providing a nice introduction to STS and d-wave superconductivity.

The second talk covered some older results, which were published in Science earlier this year – Y. Kohzaka et al., Science 315, 1380 (2007), as well as a few other famous papers – such as J. Lee et al., Nature 442, 546 (2006) and K. McElroy et al., Science 309, 1048 (2005).

A very basic bottom-line summary of this work is that by using STM in spectroscopy mode – measuring tunneling current as a function of applied voltage for a specific site, one can probe electronic properties – for example superconducting or pseudo-gap with essentially atomic resolution. The information is similar to ARPES, but spatially resolved, rather than momentum resolved. The most fascinating discovery is that in BSCCO, which is the favorite compound for ARPES/STS and other surface-sensitive probes, the gap is not uniformly distributed, but rather shows some nanoscopic “patches”, as shown on the figure above. By using STM one could go several steps further and correlate dopants with the patchy gap distribution – for example gap is increased where dopant oxygen atoms are located. More recently they correlated gap with periodic “supermodulations” – distortions of the lattice. The implication here is that by changing the length of Cu-O bond, one could conceivably increase the superconducting gap (and therefore Tc) to higher temperatures.

The most intriguing (unpublished) results, in my opinion, are the measurements of d²I/dV² spectra (combined with O¹⁶ and O¹⁸ isotope effect measurements) that indicate that the bosonic “glue” mediating the superconductivity may indeed be some sort of localized electron-lattice interaction – though not necessarily, and in fact probably not likely phonons – instead it is something very local in nature.Ali Yazdani’s group at Princeton (formerly at UIUC), as well as numerous disciples of Davis’ group – for example Jenny Hoffman at Harvard and Kyle McElroy at Colorado, are also working on application of STS to a variety of these problems, so it’s likely we will see a lot more results like the ones described here soon.

You can dig for more information at Davis group website at Cornell.