Friday, October 27, 2023

Reading material - orders of magnitude and difficult times

Over the past couple of weeks (and more) I have found a number of things to read that I wanted to pass on.  First, if you'd like a break from the seemingly continual stream of bad news in the world and enjoy good "think like a physicist"/dimensional analysis/order of magnitude estimate/Fermi problem discussions, I suggest:

On a much more sobering note, I was saddened to learn of the grave illness of Prof. Jan Zaanen, who has terminal cancer.  A colleague brought my attention to an essay (link here) that Prof. Zaanen has written in the hopes that it will be widely read, and I pass it along.  

More soon.

Wednesday, October 18, 2023

Scientific travel

Particularly in these post-pandemic, climate-change-addled, zoom-enabled times, I appreciate the argument that it's always worth asking, "Is this trip really necessary?"  We are in the age of remote work and zoom seminars that are attended by people from all over the world.  Is there sufficient benefit to in-person visits to justify travel for work?  I just got back from my first really lengthy science trip in a number of years, and it was definitely very valuable, with experiences and knowledge transfer that just could not have happened nearly as readily any other way.  

I was fortunate enough to be able to attend and speak at a (beginning of October) summer school at ISTA, which is a large and growing scientific institute in Klosterneuberg, outside of Vienna, and I was also able to visit my collaborator's lab at TU Wien as well as the Vienna MicroKelvin Laboratory.  I spend the following week visiting the Laboratoire de Physique des Solides in Orsay, hanging out with the quantum electronics group.  Many thanks to my hosts for helping to organize these trips and for making me feel so welcome.

In-person visits allow for longer, extensive, interactive conversations - standing at a whiteboard, or having coffee, or pointing at actual apparatus.  It's a completely different experience than talking to someone over zoom or over the phone.  I think I did a better job explaining our work, and I definitely think that I learned a lot more about diverse topics than if I'd only had brief virtual interactions.   As an experimentalist, it can be very valuable to learn details about how some measurements are actually done, even including which bits of equipment and instrumentation are preferred by others.  (LPS has a ton of Rohde and Schwarz equipment, which I've really not seen to that extent in the US.  I'd also never heard of mycryofirm and their closed cycle cryostats.)

As an added bonus, I got to visit the Musée Curie in Paris and see Marie Curie's lab.  Here is a photograph of a Geiger counter that they'd made c. 1930.  Hand-soldered, uninsulated wires.  The biggest tube is the actual Geiger-Müller tube which produces current pulses when ionizing radiation zips through it.  The other two tubes make an amplifier to crank up the current pulses enough to turn an electric motor that drives a mechanical counter on the far right outside the box.

Hopefully I will finish up some writing and be posting more soon.

Thursday, October 05, 2023

The Nobels, physics and chemistry

As you undoubtedly know, the 2023 Nobel in physics has been awarded to Pierre Agostini, Ferenc Krausz, and Anne L'Huillier, for the development of techniques associated with attosecond-scale optical pulses.  Here is the more popular write-up about this (including a good handwave of how attosecond pulses can be made) from the Nobel Foundation, and here is the more technical version.  A number of people (including friends and relatives) have asked me in the last couple of days about this, including what discoveries have these techniques led to, and how is this work different than preceding Nobel prizes (like the 1999 chemistry prize for femtosecond chemistry, the 2005 prize in physics for frequency combs, and the half of the 2018 physics prize for femtosecond pulsed lasers).  This isn't really my area of expertise, but my impression from talking with people is that the attosecond work is thus far more of a technical achievement than a technique that has led to a series of groundbreaking scientific results or technologies.  

Scientifically, the attosecond regime is very fast compared to the dynamics of, e.g., solids.  That said, attosecond techniques have been used to characterize condensed matter systems, as described here. Crudely speaking, the relevant energy scale associated with 100 as is \(h/10^{-16} s \sim\) 40 eV, the kind of energy (in the deep ultraviolet range) associated with photoemission.  It makes sense that some of the results highlighted in the Nobel citation have to do with using these methods to measure time delays associated with photoemission - like seeing that 4f electrons take longer to photoemit than s and p electrons in other bands.  If readers can point to a great explanation that goes deeper than this, please leave it in the comments.

The 2023 chemistry prize has been awarded to Moungi Bawendi, Louis Brus, and Alexey Ekimov, for the discovery and development of semiconductor nanocrystals now popularly called quantum dots.  These systems are absolutely great platforms to demonstrate quantum confinement.  By taking a bulk semiconductor and carving it up into pieces so small that the electronic wavefunctions get squeezed by the boundaries - this generally increases the energy spacings between levels, including the energy associated with the gap between the valence band and the conduction band. That is, a semiconductor that might fluoresce in the red in the bulk can be chopped into pieces that fluoresce in the green or the blue (higher energies).  The story of these materials (their growth, how to make them uniform and stable without bad defects at their surfaces) is very cool.  Quantum dots are now widely used as luminescent materials in display devices, and they are also broadly employed as fluorophores for biological imaging and related applications.  (Louis Brus is a Rice alumnus - huzzah!  I've never met Dr. Ekimov, but in my experience, Brus and Bawendi are both very nice, down-to-earth people whose groups write clear, non-hype-ridden papers.)