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These nanocrystal structures are emerging as materials for low-cost, flexible, thin-film electronics. Still, the mechanism of electron and exciton transport of these structures is not understood. The goal of this workshop is to bring together leading experts in this field. Participation in the event is by invitation only. We welcome participation of graduate students and post docs. Please contact us with any questions by email at (CETNA2017 @ physics.umn.edu). Alexander Efros (Navy Research Laboratory) Uwe Kortshagen (University of Minnesota) Boris Shklovskii (FTPI, University of Minnesota)ZapperZ's recent post about new work on the pseudogap in high temperature superconductors has made me think about how to try to explain something like this to scientifically literate nonspecialists. Here's an attempt, starting from almost a high school chemistry angle. Chemists (and spectroscopists) like energy level diagrams. You know - like this one - where a horizontal line at a certain height indicates the existence of a particular (electronic) energy level for a system at some energy.
The higher up the line, the higher the energy. In extended solid state systems, there are usually many, many levels. That means that an energy level diagram would have zillions of horizontal lines. These tend to group into bands, regions of energy with many energy levels, separated by gaps, regions of energy with no levels. So, what happens in systems where the electron-electron interaction does matter a lot? In that case, you should think of the energy levels as rearranging and redistributing themselves depending on how many electrons are in the system. This all has to happen self-consistently. One particularly famous example of what can happen is the Mott insulating state. (Strictly speaking, I'm going to describe a version of this related to the Hubbard model.) Suppose there are N real-space sites, and N electrons to place in there. In the noninteracting case, the highest occupied level would not be near a gap - it would be in the middle of a band. Because the electrons can shuffle around in space without any particular cost to doubly occupying a site, the system would be a metal.
However, suppose it costs an energy U to park two electrons on any site. The lowest energy state of the whole system would be each of the N sites occupied by one electron, with an energy gap of U separating that ground state from the first excited state. So, in the presence of strong interactions, at exactly "half-filling", you can end up with a gap. Even without this lattice site picture, in the presence of disorder, it's possible to see signs of the formation of a gap near the highest occupied level (for experts, in the weak disorder limit, this is the Altshuler-Aronov reduction in the density of states; in the strong disorder limit, it's the Efros-Shklovskii Coulomb gap). Whenever I read a super-enthusiastic news story about how devices based on new material XYZ are the greatest thing ever and are going to be an eventual replacement for silicon-based electronics, I immediately think that the latter clause is likely not true. People have gotten very spoiled by silicon (and to a lesser degree, III-V compound semiconductors like GaAs), and no wonder: it's at the heart of modern technology, and it seems like we are always coaxing new tricks out of it.
Of course, that's because there have been millions of person-years worth of research on Si. Any new material system (be it graphene, metal oxide heterostructures, or whatever) starts out behind the eight ball by comparison. This paper on the arxiv this evening is an example of why this business is hard. It's about Bi2Se3, one of the materials classified as "topological insulators". These materials are meant to be bulk insulators (well, at low enough temperature; this one is actually a fairly small band gap semiconductor), with special "topologically protected" surface states. One problem is, very often the material ends up doped via defects, making the bulk relatively conductive. Another problem, as studied in this paper, is that exposure to air, even for a very brief time, dopes the material further, and creates a surface oxide layer that seems to hurt the surface states. This sort of problem crops up with many materials. It's truly impressive that we've learned how to deal with these issues in Si (where oxygen is not a dopant, but does lead to a surface oxide layer very quickly).
This kind of work is very important and absolutely needs to be done well.... Texas governor Rick Perry has proposed (as a deliberately provocative target) that the state's (public) universities should be set up so that a student can get a bachelor's degree for $10,000 total (including the cost of books). Hey, I'm all for moon shot-type challenges, but there is something to be said for thinking hard about what you're suggesting. This plan (which would set costs per student cheaper than nearly all community colleges, by the way) is not well thought-out at all, which is completely unsurprising. To do this, the handwave argument is that professors should maximize online content for distance learning, and papers could be graded by graduate students or (apparently very cheaply hired) instructors. Even then, it's not clear that you could pull this off. Let me put it this way: I can argue that the world would benefit greatly from a solar electric car that costs $1,000, but that doesn't mean that one you'd want to own can actually be produced in an economically sustainable way at that price.
This is classic Perry, though. Like Amy Chua, I'm choosing to be deliberately provocative in what I write below, though unlike her I don't have a book to sell. I recently heard a talk where a well reputed science educator (not naming names) argued that those of us teaching undergraduates need to adapt to the learning habits of "millennials". That is, these are a group of people who have literally grown up with google (a thought that makes me feel very old, since I went to grad school w/ Sergei Brin) - they are used to having knowledge (in the form of facts) at their fingertips in a fraction of a second. They are used to nearly continuous social networking, instantaneous communication, and constant multitasking (or, as a more stodgy person might put it, complete distraction, attention deficit behavior, and a chronic inability to concentrate). This academic argued that we need to make science education mimic real research if we want to produce researchers and get students jazzed about science.
Moreover, this academic argued that making students listen to lectures and do problem sets was (a) ineffective, since that's not how they were geared to learn, and (b) somewhere between useless and abusive, being slavishly ruled by a culture of "covering material" without actually educating. Somehow we should be more in tune with how Millennials learn, and appeal to that, rather than being stodgy fogies who force dull, repetitious "exercises at the end of the chapter" work. (The US, that is.) More people need to read this. This past week I hosted Seth Putterman for a physics colloquium here at Rice, and one of the things he talked about is some of his group's work related to triboelectricity, or the generation of charge separation by friction/rubbing. When you think about it, it's quite amazing that we have no first-principles explanation of a phenomenon we're all shown literally as children (rub a balloon on your hair and it builds up enough "static" charge that it will stick to a plaster wall, unless you live in a very humid place like Houston).