Tuesday, March 04, 2014

March Meeting, Day 1

Observations from the first day of the APS March Meeting:
  • There has been a lot of progress and excitement in looking at layered materials "beyond graphene".  It's interesting to see a resurgence of interest in transition metal (Ti, but more frequently W and Mo) dichalcogenides (S, Se, Te), a topic of great activity in bulk materials growth in the 1970s and early 80s.  There are clearly a lot of bright people working on ways to grow these materials layer-by-layer, with the long-term idea of making structures somewhat like semiconductor heterostructures (e.g., GaAs/AlGaAs), but with the richer palette provided by these materials (exhibiting charge density waves, strong spin-orbit effects, complex band structure, etc.).  Molecular beam epitaxy of these materials with high quality is generally very hard.  For example, Mo and W are extremely refractory, requiring electron beam evaporation at temperatures exceeding 2500 C, and sticking at the sample surface without much diffusion.  Whoever really gets layer-by-layer, large-area growth working with diverse materials is going to make a big impact.
  • I saw Heinrich Jaeger give a great talk about granular materials by design.  These are entirely classical systems, but they are extremely challenging.  If you think about it, they are not crystalline (no long-range symmetries to exploit in modeling), they are non-ergodic (the constituent grains are kinetically limited, and can't explore all possible configurations), and nonlinear (the interactions between particles are short-ranged and very strong).  Very interesting.
  • I caught two talks in the session looking at silicon-based quantum information processing.  It's possible to create and manipulate dangling bonds on the Si surface (localized states that can trap electrons) and look at how those bonds interact with each other.  Very neat.  Looking at particular individual impurities, with the right system (erbium in Si), you can couple a single impurity to a single-electron transistor charge sensor.  Then, you can manipulate that impurity with optical techniques and use the charge detection to determine its state.  Very impressive.
  • The session on secrecy in science was very good.  The ability to manufacture viruses by design is genuinely frightening (though it loses some menace when the words "Pandemic - millions of deaths?" are projected in Comic Sans).  The discussion of intellectual property was great and the role of universities merits its own blog post.  Lastly, I was unaware of the WATCHMAN project, which is a very interesting neutrino physics experiment that as an added bonus should allow the international community to detect rogue nuclear reactors meant for weapons development.


Amado Guloy said...

The transition metal dichalcogenides more often than not have the layered structures that result in CDWS, right? That is pretty hot right now.

Also, what do you think of the biexciton news? Would you really consider it a "new state of matter" like how many people seem to be reporting on it?

Douglas Natelson said...

Hi Amado - You are referring to the "dropleton", right? As in here: http://www.nature.com/nature/journal/v506/n7489/full/nature12994.html

Dropletons seem to be multiexcitons bound together in a rather complex way through many-body interacting physics. They were unexpected, which is nice, and I think they are attractive to theorists b/c they involve comparatively few electrons and holes such that they may be computationally tractable. Calling them a new state of matter seems to be going a bit far for my tastes.