Saturday, March 21, 2009

The APS March Meeting 2009

The largest yearly meeting of physicist is held every march by the American Physical Society --- about 6000 of us show up. Sometimes during these conferences it is fun to play “spot the physicist” – trying to see if you can figure out which people on the street are the physicists here for the conference, and which people are the natives of the city. Unfortunately, this game is usually way too easy. Just look for the person whose lunch money you stole in 5th grade.

This year the meeting was in Pittsburgh. The city really has improved since the last time we had the March Meeting there over a decade ago. I think that was the only time I have seen prostitutes trying to get business at a physics conference. They seemed a bit sad, bored, and annoyed not to be getting any work. But physicists are not really the type to hire prostitutes.

This year there were no hookers, so that is an improvement. There were a couple of decent restaurants and they were pretty cheap too. The city is still run-down in places. I stayed in a motel just outside of the city which was kind of dump, but no bedbugs or roaches and the bed was comfortable, so it was fine.

Most years at the March meeting there are one or two new exciting topics that generate a whole lot of excitement. A few years back, the big thing was Magnesium Boride. Here is my report of the big things from this year.


Graphene: Carbon is an amazing molecule. The same single atom can form several different types of materials: Diamond, Graphite, Buckyballs, Nanotubes, Graphene. Graphene is a single layer of graphite – carbon atoms forming a single planar hexagonal lattice. A few years back (ca, 2004) it was discovered that graphene is really easy to make. You take a chunk of graphite (pencil lead) and touch it to a piece of Scotch tape. Then you put the tape down on a nice clean surface and some microscopic layers of the graphite come off on the surface. If you look carefully at the surface with a microscope you discover that some of the pieces that were transferred to the surface are just a single atomic layer thick – this is graphene.

Since the discovery of graphene there has been an absolute explosion of research on the topic. For the experimentalist it is a new material with some amazing properties – and it is simple to make (everyone knows how to use Scotch tape). For the theorist, the electrons in graphene actually appear as if they are in the relativistic limit, unlike electrons in most metals. This means there is 50 years of condensed matter theory to be re-considered with this interesting new hook.

The most impressive talk I saw on graphene was a demonstration of an impressive new growth technique for the material. Of course everyone loves the Scotch tape method, but it has its limitations -- you typically don’t make pieces of graphene bigger than a few microns. This is a bit of a limitation. This new method is epitaxial -- (epi meaning above, or surface and taxis meaning motion). Many conventional, and industrially important semiconductors, like galium arsenide or indium phosphide are grown by epitaxis. In this method atomic layers are deposited one at a time on the top of the surface and the atoms move around on the layer to find their proper positions. The group from Georgia is growing epitaxial graphene on the surface of Silicon Carbide – about 100 layers of it. This may sound like it totally defeats the purpose. You would think that if you have 100 atomic layers, you no longer have graphene, you have graphite again. But it turns out that each atomic layer that is grown, for some unknown reason, is actually slightly misaligned from the last, so there is no chemical bonding between layers and you can consider each layer as being an independent graphene. While this sounds rather unlikely, the experimental evidence that it is true (at least the evidence I saw) looked very compelling. This looks to me like the first real step in taking graphene from the lab and making it a material that can really be used in the semiconductor industry (maybe).


Pnictides: There is a new class of material, known as the “pnictides”. The word “pnictide” comes from the word “pnictogen” which means any element from same column as nitrogen in the periodic table. Last year, it was discovered that some of these complicated compounds, containing iron, arsenic, and a smattering of other elements, superconduct at extremely high temeperature (the record being something like a balmy 50K (around -360 Farenheit). This means at temperatures below this, they can carry electricity with absolutely no resistance or loss. If you start current flowing around a large ring of this material, it could keep going for the age of the universe.

At any rate, in the last year, there have been a heap of experiments trying to sort out what is going on in these materials. Hopes have been high that understanding these materials might tell us something about other mysterious superconductors. Apparently the chemistry of these materials (complicated though they are) is relatively forgiving, in that it is easy to grow pristine samples so many of the experiments are relatively free of dirt and other problems. Nonetheless, there is still a huge amount of disagreement between the result of experiments from different groups that needs to be worked out. Within a few years, things should start to converge.

In addition to the literally hundreds of talks on graphene and pnictides, there were plenty of talks on quantum Hall and topological phases of matter to keep me happy. There is a nice resurgence of these topics. A few years back, there were hardly any talks on quantum Hall and most people thought the field was dead. To a large extent, we have Microsoft to thank for the resurgence.

I gave this talk and this talk. Both went reasonably well. I also had to chair two sessions, which were unfortunately back to back, which made for a rather exhausting day.

2 comments:

Marianne said...

Don't you mean look for the person who also got his lunch money stolen in the 5th grade?

Steve said...

I'll have you know I only had my lunch money stolen once --- ok, it was by a girl, but still.