Ah, once again I am at the conclusion that yes, in fact, science is difficult and hard.

It’s a double dose of reality today. We’re starting our next batch of experiments and the first day is never easy. I expect that I will be here through most of the night as we struggle to set things up and start doing science.

But we also got back a set of comments on a paper we recently submitted. The editor seemed ok with it, but one of the referees was quite unhappy. And that is just part of it. I can complain and be upset that one of my favorite papers did not meet with rapt approval. But that is just part of science, and despite my moaning, if we can answer the questions of the referee it will probably make for a better paper. That said, even a perfect paper that is sound can still face rejection. Peer review does not always work. But it’s better than anything else we’ve tried so far (and wading through the arXiv servers for a few hours every day where many of the papers contain gross errors is not a better alternative).

Some of the referee questions are benign and easily answered. Those are easy as we can make small clarifications or corrections to the text. Some of the questions reflect more of a misunderstanding on the part of the referee. Those are also good things to fix because it’s not just a little textual offense, but rather indicates a genuine lack of communication on our part. Lastly there are the questions raised that are tough. These are things that perhaps we ourselves did not fully appreciate. They are things that we must first make certain we understand and then likely include in the paper.

I suppose the main point of this is to say that it does not matter how much I like the paper, or how clever I think our experiment, what matters is how well it can hold up under scrutiny. Or in other words, it must be science.

So, it’s several days into the conference and I feel reasonably comfortable rating the various aspects of the conference as usual. Again, there’s no review given here of the science. That’s great as always. Instead these are just my view of the various other experiences.

• Weather : Pleasant
• Electrical Outlets in Convention Center : modest, but accessible.
• Number of Bathrooms : Far too inadequate. And it’s.... ugly. Our behavior seems to devolve upon entering.
• Internet Access in Convention Center : Terrible inside any room. Crying in frustration is not uncommon.
• Food Quality On-Site : Terrible
• Food Off-Site : High
• Convention Staff : Amiable, helpful, and personable
• APS Staff : Nice as always
• Proximity of Hotels/Restaurants : Nearby and walking distance.

Internet Off-Site : Did Dallas just get internet access a week ago? The entire downtown suffers from terrible access. It’s not just slow. We’re dropping ~ %20 of all packets.

Every year the American Physical Society holds the March Meeting. This year we’re all in Dallas TX. It is (I believe) the single largest yearly meeting of physicists in the world and it is devoted to topics within condensed matter physics, materials science, and similar systems. Let me state that in a different way... Our one section has a larger participation than the other sections of physics combined! Well, maybe it’s not that big if you put ALL of them together... but this is still quite a large meeting.

It’s quite fun to attend for a variety of reasons.

First, most people that attend are presenting something. That is always a great deal of fun and a great chance to help create your presence within the community. Telling people about what I’m doing is, for me at least, one of my favorite activities. Call me a narcissist if you must.

Second, you get to attend talks by all these other people on a huge variety of subjects. I quite excited to learn about what others are doing within my active areas of research (surface science), but also to hear a wide variety of things such as topological insulators, complex oxides, magnetism and much, much more. At any given time I can always find several different talks that I’m interested in seeing (hmmm... what’s the difference between hearing a talk and seeing a talk?). In fact, that’s one of the frustrating things. Very often you really wish you were in 2 or 3 different places at once.

Lastly, and very importantly, one of the reasons to have these meetings is to see your friends and collaborators, both past and present. Every year I attend I am greeted warmly by friends and former professors (Oscar Vilches being among my favorites!), many of whom I’ve not seen since the last annual meeting. It is sad that so many of us live so far apart as it would truly be great to talk with everyone face to face more often. Some times the discussion is very much related to ongoing work, possible collaborations, or finishing that languishing paper from the past each of us carries around. But very often it’s also a chance to see and hear how your friends have been fairing in the world. This in and of itself is reason enough to attend.

This year there is the added benefit of the meeting being held in Dallas TX. This is a nice change for me to see some friends and family, even if only briefly.

As an aside, it’s kind of funny to see all these physicists and other scientists running around in a very small area. For lunch and dinner we spill out onto the streets and into view of the public. There are several stereotypes (many of them not positive) that can readily be seen.

It’s easy to get wrapped up in one’s own little world. In fact, it is not too often that I realize just how large our “little” group is.

The Synchrotron Radiation Studies group recently had a good meeting and we ended up taking a group photo at the end. Not everyone is pictured, but most of us are there. I’m over on the far left.

Amid all the terrible news that’s been going around lately, I figured I’d post something nice.

A couple of weeks ago we submitted another manuscript and it’s currently with referees. In it we discuss our observation of step-flow motion occurring on a platinum surface at very high temperatures. We actually do it with a new trick as well (and that’s actually the most important part). I’ll try to explain it below in terms that I hope are understandable.

Imagine a perfect surface and we’ll shine a beam of light (x-rays in this case) on that surface. Let’s say something like this, with a diagram of the surface we’re scattering from (on the left) and a calculation of what would then be observed in our detector (on the right):



The illuminated area has no structure, therefore the scattering we observe is a single bright region with a few fringes, all related to the size and shape of the incident light (I’ll ask that you forgive that the simulation is from a square beam and not the circular one shown).

Now, we’re going to add a couple of complications. First, we’re going to look at one of those edges instead the middle of a terrace. Next, realize that there is a small addition distance if you reflect from the lower side than from the upper side. Now, remember that we’re using x-rays. And what I’ve also not told you is that we’ve got the angles of everything set such that the extra distance adds up to 1/2 the wavelength of the photons. If it’s exactly 1/2, then the two regions can cancel each other and you see something like this.



The single spot of scattered light splits into 2 regions, with zero in the middle.

Recall that I said the platinum was hot. It’s not just hot, it’s really hot, usually around 1800 K. At these temperatures the surface will slowly sublimate (go directly from solid to vapor). Very often atoms at the step edges (particularly at kinks in the step edge) and diffuse around on the surface. At these temperatures the atoms will often then leave the surface through sublimation and never return. What you find is a net loss of atoms occurring from the step edges. As the atoms leave, the position of the step shifts (in this case to the right).

I suppose the only other piece of the puzzle missing (and it’s an important piece) is that the beam of photons we’re using is highly coherent over the illuminated area. Practically what that means is that a single photon can interfere with itself with contributions coming from the entire (almost) illuminated area. That’s important. If the “coherence” was smaller (as is usually the case in scattering experiments), then you’d have lots of photons that only see the top step or the bottom step even when the step-edge runs through the middle of the illuminated area.

So, we’ve got steps traveling from left to right and we’re sensitive to changes over the entire illuminated area. Occasionally there is a step in view, occasionally there is no step. When the average distribution of the steps in uniform (think of a staircase, though with very small vertical steps!), then what you’d expect is that the observed scattering pattern oscillate between the two patterns shown above. And not just oscillate, but do so in a rather uniform fashion. This is exactly what we find!

On the left we have the “perfect” surface with a single spot more or less in the image. On the right is an example for a few tens of seconds later where single peak has split and is now 2 peaks. If we continue to wait, the image will eventually resemble the first image again. This cycle repeats for many minutes before becoming noisy again (ie more structure and dynamics than just single steps moving).

We can then analyze (it’s a kind of averaging mostly) the images to produce a nice signal. I’ll show you 2 examples, though we’ve measured 10 or so.

These are a couple of the signals we are capable of detecting (I’ll spare you the details of how we get to this point). The important thing is that you can see a fairly clear signal, an oscillation. The period of oscillation (how long it takes to go from a peak to another peak) corresponds to the time it takes for a single step to move across our field of view. What can you get from this? Well, first, at higher temperatures the oscillation is faster. That makes sense as the higher the temperature then the faster atoms should leave the surface and hence the faster the steps will move. Also notice that the higher temperature signal appears “more ordered” than the one at lower temperature. This indicates that as the temperature increases step-flow motion becomes more important than other motion (such as step-meandering). There’s a third and fourth thing in this signal that take us in a different direction and relate to something we’re working on for a different paper (they’re related to a more general behavior of the surface even when it has several steps/defects).

“Ok,” you say, “That’s nice, but how can you be sure what you’re seeing is real?” Great question. The rate of change of the oscillation frequency with temperature can tell us how much energy it takes for atoms to sublimate. In this case we measure 5.4 (.9) eV. That can be compared to the known value of 5.9 eV. So, to within our ability to measure, the temperature dependence of the frequency ties exactly to a known quantity. That’s a pretty strong indication that we’re doing things correctly. (I’ll spare you all the details of how we repeated the experiments twice with difference samples/configurations to help convince ourselves)

I’m pretty sure this is the first observation of step-flow motion on a platinum surface. But more importantly, this is a different way of measuring such processes than through the usual means of electron microscopy. Since this is an x-ray based technique, we can do such studies in environments that are not just vacuum measurements (like this). Instead we can measure in harsh chemical environments where electron microscopy has difficulty. And to give a hint, we’ve already been doing this! Additionally this serves as a nice connection between x-ray photon correlation spectroscopy (which measures dynamics) and imaging structure.

In case anyone is really interested, a pre-print of the paper is available on the arXiv servers, cond-mat > arXiv:1103.0263 Persistent Oscillations of X-ray Speckles: Pt (001) Step Flow. I’ll be presenting this along with some newer work next week at the American Physical Society March Meeting in Dallas.