You may or may not have noticed that I’ve been making a concerted effort to do more ResearchBlogging posts explaining notable recent results. I’ve been trying to get at least one per week posted, and coming fairly close to that. I’ve been pretty happy with the fake Q&A format that I’ve settled into, and while they’re time-consuming to write, they’re also kind of fun.
This past week, alas, was kind of brutal, as I was doing a ton of reading in preparation for my DAMOP talk tomorrow, which, in retrospect, is kind of insane, and SteelyKid’s day care being closed for two days didn’t help (though it was a lot of fun running around with her). As a result, I haven’t had time to do any ResearchBlogging type posts for this week– there’ll be some down the road, but not immediately.
Fortunately, the APS has my back, with a very nice Viewpoint in Physics by Joseph Thywissen on some new results from Immanuel Bloch’s BEC emporium. They also provide free access to the relevant article.
X-ray scattering is a powerful tool to investigate the structure of materials. In particular, because x rays have a wavelength comparable to the spacing between ions in a solid, coherent scattering from a crystalline material leads to constructive interference in so-called “Bragg” directions, similar to the diffraction peaks of visible light reflected off a periodic grating. Crystallographers measure these Bragg angles to infer the structure of a solid, while biologists and chemists apply x-ray scattering to crystals of large molecules–most famously, to discover the double helix structure of DNA. Similarly, measurements of the scattering outside the Bragg peaks can reveal deviations from perfect order, or excitations.
In many ways, neutral atoms moving in an optical lattice mimic the behavior of electrons moving in a conventional crystalline solid–only the period of the optical crystal is 1000 times longer than the ionic crystal, and of course atoms are more massive than electrons. For this reason, the energy and temperature scales relevant to neutral atomic systems are a billion times lower than their electronic counterparts, hence the need for “ultracold” conditions. In an article appearing in Physical Review Letters , Christof Weitenberg from the Max Planck Institute for Quantum Optics in Garching, Germany, and colleagues compare angle-resolved scattering and direct microscopy of ultracold atoms (Fig. 1). Their work is one of the most precisely controlled light-scattering experiments to date, and the first to apply crystallographic methods to neutral atoms prepared in an insulating phase. While recent work has demonstrated site-resolved images of such cold-atom lattices [2, 3], in this new work, Weitenberg et al. shift the focus of their microscope away from the lattice and into the far field to measure the angular distribution of light scattered off such atoms. In this way, they are able to distinguish coherent from incoherent scattering, and demonstrate a detection method for magnetic ordering of atomic spins.
One might guess that microscopy would be the best way to learn about structure. Why then is angle-resolved scattering such a widespread technique?
Some of the viewpoint write-ups at Physics end up being pitched a little too high, but this one was really well-done. If Joseph ever starts talking to his dog, I could be in trouble.
So, there’s your substitute ResearchBlogging about physics for the week. I’m on my way to DAMOP for the rest of this week, where I may or may not post anything at all (I’ll try to get some PNAS posts scheduled, but that will depend on how things go travel-wise.)