... well, for the physics world anyways.

Last week our latest paper was published in Physical Review Letter (PRL) and I promised to write more, though I wasn’t quite expecting to say this. Our letter was used as the cover article for that issue. So if you go look at Vol 103, Issue 16, then you’ll see two little hexagons at the top of the page. That’s from our article and is part of Figure 1.

-------------------------------------

Surface X-Ray Speckles: Coherent Surface Diffraction from Au(001)
M. S. Pierce, K. C. Chang, D. Hennessy, V. Komanicky, M. Sprung, A. Sandy, and H. You

Published 15 October 2009
165501  Full Text: PDF (526 kB)  | Buy Article

We present coherent speckled x-ray diffraction patterns obtained from a monolayer of surface atoms. We measured both the specular anti-Bragg reflection and the off-specular hexagonal reconstruction peak for the Au(001) surface reconstruction. We observed fluctuations of the speckle patterns even when the integrated intensity appears static. By autocorrelating the speckle patterns, we were able to identify two qualitatively different surface dynamic behaviors of the hex reconstruction depending on the sample temperature.

-------------------------------------

First, I am ecstatic about this news. In fact, I’m ecstatic just to have another paper published in PRL. Making on the cover is something else! This is my third PRL, but it’s significant for another reason as well. This is solely from my work at Argonne and, while a “coherent x-ray scattering experiment,” it is significantly different from my first two which came from my thesis work. It’s an important step to show that my publication record and potential is not totally tied to just one thing (be it thesis or post-doc). My choice to switch fields so drastically from my thesis work has been a hard decision to bear (both for me as well as those that I work with). It was a gamble, but I believe it has paid off.

Now about that figure. I’ve posted similar versions of it before on this website to help explain some of the features of the surfaces we study.

The figure is a representation of the two possible orientations of the surface atoms on a gold crystal cut along one of the cubic directions. Instead of maintaining the underlying face-centered-cubic structure, the surface atoms rearrange (reconstruct is the term we use) into a hexagonal pattern. Now, if you think about it, you there’s not a single preferred way to lay down a hexagon over a square. Cut out a square and hexagon and see for yourself. If you try to align two of the hexagon sides with two of the square sides, you’ll be left with points of the hexagon protruding over the other two sides of the square. Now, rotate the hexagon by 90 degrees (1 quarter of a turn). The hexagon sides that used to overlap will now have points over the square edges and the sides which used to be points will now be aligned with the square. So you can see, there are two equivalent ways to lay down hexagons over squares. Nature is no fool and so naturally our crystals contain surface regions with each of the different orientations. In a nutshell, that’s what the figure is meant to show (or remind the scientists reading the article).

Perhaps it’s easier to see if I rotate the entire figure by 45 degrees.

The only detracting thing here is that all of the above is not new. In fact, it’s 30 years old more or less. What we did was to observe changes in those hexagons in a clever way. But we were not establishing the existence of the hexagons.

In truth I did lobby a bit for a different figure. We were informed earlier in the week that our paper was being considered for the cover art. “Considered” is still a long shot and I was gratified to have the article be up for consideration, but wasn’t hoping for too much. They requested high resolution copies of our Figure 1 (which I sent). But I also included higher quality versions of some of our other figures which I thought were quite aesthetically pleasing, but also show cased some of the newer results. Figure 1 was just too pedagogical for my tastes. But in the end I was only willing to push so much for fear that it would remove our slim chance.



This is the basic figure I was lobbying to be modified for use. Right now it still has too many labels and numbers to be cover art, but ignore those for a second. The vertical “component” of each is collection of speckles. Where a speckle is bright, the plot is red. Where it is dim (or absent), the figure is blue. The horizontal direction in each of the 3 plots is time. So, if a speckle is bright (red) and unchanging with time, then there will be a red streak in the horizontal direction. If the speckle is changing, then the red will change to some other color and other places, not previously bright will begin to show red. If it’s rapidly changing then there will be only short periods of persistence of the speckles. That’s what you see in the 3 figures. What could be labeled, hot, hotter, and really hot. On the left the speckle is not changing very rapidly. On the right you see very rapid fluctuation. The middle is well, somewhere in the middle.

Anyhow, get rid of the numbers, massage them a bit, and you’d have an aesthetically pleasing image that contains the core of the new science that we did.

That aside, figure 1 is a nice graphic and I’m of course very happy they decided to use it. The “atoms” are raytraced using the open-source POVRAY package. In fact, coupling the graphical rendering power of POVRAY with the mathematical ability of Matlab can make for some very beautiful (and complicated) images. By ray-tracing standards my little pictures are not anything special. In fact, compared to some of the computer rendered artwork out there, these images are plain bland. However, they’re still pretty in their own right, and have some content to them. They’re also much nicer than any of the normal simple chemical rendering programs out there. So thank you to the people that wrote POVRAY.

Wow! I am simply giddy at the moment.