Saturday, February 10, 2007

A scientific direction that I think is promising

I haven't written too much about my own research on this blog, mostly because I figure that people who really care about it can read my group homepage or my papers. However, there is one area out there that I think has real promise, and I'd like to get other folks thinking about it, at least in general terms.

Electronic transport measurements in nanoscale systems can be considered a kind of spectroscopy. In particular, when a chunk of conducting material is sufficiently small and relatively weakly coupled to leads (call them a "source" and a "drain", after transistor terminology), conduction can be dominated by one or a few specific quantum states of that material. There has been great work done by many groups over the past 15 years or so, looking at these individual electronic states in a bunch of systems, including metal nanoparticles, patches of doped semiconductor, and semiconductor nanowires and nanocrystals. As neat as these systems are, they're all comparatively simple from the electron-electron interaction point of view. With a few exceptions (like Kondo-based physics), you can pretty much work in a single-particle picture. That is, adding one more electron to these systems doesn't drastically change the spectrum of electronic states - the spectrum itself is mostly unchanged except for the population of the states, one of which has increased by 1.

Many interesting materials exist where strong electronic correlations are more important. For example, the high-Tc superconductors in their normal state are often "bad metals" that are not well described by a picture of weakly interacting electrons. There are similar phases in the heavy fermion compounds. Even magnetite (Fe3O4), a comparatively simple compound, has strong correlation effects: it's not really a metal or a semiconductor; it has a room temperature resistivity in the milliOhm-cm range (say 1000 times higher than Cu or Au), and that resistivity increases with decreasing temperature, but not in a simple way as in a semiconductor.

I think it would be very revealing for transport spectroscopy experiments to be performed on nanostructures made from these strongly correlated materials. This won't be easy for many practical reasons (e.g., stoichiometry can be tough to control in nanomaterials; noone knows how to make many of these systems in nanostructured forms yet), but I'm convinced that there is much to learn in such experiments.

5 comments:

Dan M said...

Gee and I was expecting a blog entry about the latest news out of Princeton concerning the ESP lab there...

BhpG said...

I bet he knew you'd say that before you posted...

:)

Sujit said...

Are there any references you can provide on work of this nature that may have already been done? You mention fabrication challenges - do you know of any work on this?

Is studying nanoscale-width strips of thin films relevant to what you are talking about? For example, I think I remember coming across work by a group at the University of Toronto who did STS on YBCO thin film strips of width ~50nm. I can't remember the reference off the top of my head but will try to dig it up.

Sujit said...

I don't know if anyone else is doing work of a similar nature, but the website for the UofT group I mentioned is here. The paper I was talking about is this one. There's a Proceedings of SPIE paper that summarizes some of the stuff available here.

Basically, they have done STS on YBCO thin-film strips and transport measurements on doped YBCO 'microwires'. What are your thoughts on this?

(Also, I came across another paper - "Nanoscale high-temperature superconductivity" - coauthored by them and Mohanty at BU, that mentions this work - available here.)

Doug Natelson said...

Hi Sujit - Relatively little work of this nature has been done, or at least published. The recent paper on field-effect modulation of Tc in a few-unit-cell-thick high-Tc layer is the best example that comes to mind. In my own group we've got some tantalizing unpublished results in one nanomaterial, but it's early days yet.

Regarding John Wei's nice work at Toronto: I don't think it really qualifies, in that the actual superconducting strips, while 50 nm thick, are 1 millimeter wide. That doesn't really fit my description, though it does raise the issue of what is the effective dimensionality for strongly correlated systems. If the correlation physics (e.g. strong on-site repulsions on copper ions, as in the copper oxide superconductors) is very local, does that mean that a few unit cells is enough to get essentially bulk material?

Raj's work is actually a good example of the fab challenges. I'm assuming there's a good reason why they haven't published anything else from those structures in the last three years.