Monday, July 20, 2009

Physics Report from Dresden

The TOPO09 conference in Dresden last week (I dare you to find me in the group photo on that page... like where's waldo) was full of extremely good talks. I missed the first day of the conference, so I can’t comment on that. But in the remainder of the conference, there were two topics that stood out as themes: topological insulators and 5/2 physics.

Topological Insulators:

As everyone knows there are metals and there are insulators. In metals, the electrons can move around and conduct electricity, whereas in insulators, they can’t. When you have an insulator, the electrons can be non-mobile for any one of a number of reasons . But at some level their lack of mobility always comes down to the same type of story: The electrons fill up all of the low energy orbitals (like orbitals in an atom) – and the next lowest energy orbital is at some higher energy (i.e., there is an energy gap). So in order for an electron to move around, it needs to first jump up to a higher orbital, which at low temperatures, it cannot do (room temperature can be “low” compared to the relevant energy spacing).

Now consider the simplest possible model of an insulator: A simple crystal, with no disorder, and non-interacting electrons, such that all the low energy orbitals (bands) are filled and there is a gap to the next lowest state for the electrons (this is known as a band insulator). Since the physics of insulators was first described about 80 years ago, it was assumed that all such insulators are more or less the same. Well, we have recently discovered that they are not all the same. Some fundamentally new physics can occur when the constituent chemical elements of the insulator are down near the bottom of the periodic table.

What happens near the bottom of the periodic table is that relativity becomes important. (HUH?). Yes, that’s right – Einstein. Electrons in heavier elements are moving “faster” than electrons in the lighter elements. When they move fast enough you have to think about relativistic effects, and one of the first relativistic effect is that the spin of the electron and the orbital motion of the electron become coupled (so-called spin-orbit coupling --- in fact, you can argue that the existence of electron spin is relativistic in the first place). If the spin-orbit coupling is strong enough, the insulator can develop some completely new properties. However, unless you know what to look for, you won’t notice that it has changed, because it is still an insulator. I suppose this is why it took 80 years for us to figure out that all insulators are not the same.

Among the cool properties these new “topological” insulators is that they have conducting surface states that cannot be eliminated with any amount of disorder or structuring of the surface. This is extremely unusual and leads to all sorts of new possibilities.

At the Topo09 conference last week, Charlie Kane, a prof at UPenn, gave a super talk about these new topological insulators and all the cool stuff you can do with them. Kane is perhaps the person most credited with figuring out that this whole new class of materials exists. However, one of the other people who is highly credited for developing this new field is Rahul Roy – a lowly graduate student at the time he started making important contributions to the young field. I've managed to recruit Rahul to come to Oxford for a postdoc next year. I hope to work with him a lot on this new and exciting field.

5/2 physics:

I’ve blogged about this before. here and here, so I won’t belabor the point. But in brief: there are an increasing number of people discussing whether the new experiments on the “5/2 quantum Hall interferometer” is really showing evidence of a new type of particle – the nonabelion. I won’t say that I know what to make of the data – I certainly don’t know. The one thing I do think, however, is that the “orthodox” interpretation – what people want to see – is probably not what is going on. There are just too many problems with the story. In fact, the more I look at the data, the more nothing seems to fit. Add to this issue that the data is pretty ratty to begin with, and I think the evidence for the orthodox theory starts to look vanishingly small. I don't think the data is just noise though -- so something interesting is happening. But I think it will take a lot of headscratching to figure out what it is though -- and probably a whole lot more data.


Ilya said...

You're actually not so difficult to locate (or so I think!): start in the middle of the bottom row, with the fellow wearing the "ARMALL" (those are the only letters I can make out) t-shirt. Trace the line from his chest in the 2-o'clock direction, and you'll arrive at a german-looking fellow with a conspicuous name-tag on a gray shirt. Waldo is hiding just above this gentleman's left (from the gentleman's perspective) shoulder.

Has the "hopping-on-a-frustrated-kagome-lattice" story received any more attention? Not that it was super promising, but still :-)

Steve said...

You found waldo!

FYI: The ARMALL guy is Jason Alicea, currently a postdoc at Caltech (with a faculty position lined up at Irvine). You can contact him if you want to find out what his shirt actually says. The "german-looking guy" is an American, Greg Fiete.

I've chatted with Piers about the kagome paper, but we haven't made any real progress. He was going to visit Oxford this summer, but unfortunately, the only time he could make it was when I would be away.

Doug Natelson said...

Hey Steve - Have you seen Bob's latest on 5/2? The as-yet unpublished stuff?

By the way, for various reasons I'm polling theorists: what phase transitions in real systems are well described by mean field theory (order param goes like (1-T/Tc)^0.5)? The only ones I can think of are superconductivity and smectic-a to smectic-c. Any thoughts?

Steve said...

Hi Doug,

Yeah, Bob's published/unpublished work has been shown at about four conferences I've been to in the last month (yes, i am traveling too much). It is exactly this work that I'm referring too. Something interesting is going on, but it almost certainly is not the orthodox picture.

As for mean field: What you are asking is for what REAL physical system is the upper critical dimension 3 or smaller. If you google "upper critical dimension 3" you will find many additional cases listed. (Wetting, Roughening, tricrtical systems, ferroelectrics with dipolar uniaxial forces...). However, before stating any of these as fact, I would want to do my homework to make sure that google is not just giving me garbage.

Doug Natelson said...

Hi Steve - thanks for the useful search terms. I'll tell you sometime soon why I'm interested in this....