Friday, June 28, 2019

Magic hands, secret sauce, and tricks of the trade

One aspect of experimental physics that I've always found interesting is the funny, specialized expertise that can be very hard to transcribe into a "Methods" section of a paper - the weird little tricks or detailed ways of doing things that can make some processes work readily in one lab that are difficult to translate to others.   This can make some aspects of experimental work more like a craft or an art, and can lead to reputations for "magic hands", or the idea that a group has some "secret sauce". 

An innocuous, low-level example:  My postdoctoral boss had basically a recipe and routine for doing e-beam lithography on an old (twenty+ years), converted scanning electron microscope, plus thermal evaporation of aluminum, that could produce incredibly fine, interdigitated transducers for surface acoustic waves.  He just had it down cold, and others using the same kind of equipment would have had a very tough time doing this at that resolution and with that reliability, even with all the steps written down, because it really was a skill.

Another possible example:  I was talking today with an atomic physics colleague, and he mentioned that there is a particular isotope that only one or two AMO groups in the world have really been able to use successfully in their ultracold atom setups.  The question was, how were they able to get it to work, and work well, when clearly other groups had tried and decided that it was too difficult? 

Any favorite examples out there from readers? 

Thursday, June 20, 2019

The physics subject GRE and grad school

As I've mentioned before, there is a lot of discussion lately about the physics subject GRE.  The exam is intended to cover a typical undergrad physics curriculum in terms of content, and is in the format of about 100 multiple-choice questions in about 170 minutes.  The test is put together with input from a committee of physics faculty, and there is presently a survey underway by ETS to look at undergrad curriculum content and subscores as a way to improve the test's utility in grad admission.  The issue out there is to what extent the test should be a component in admissions decisions for doctoral programs. 

The most common argument for requiring such a test is that it is a uniform, standardized approach that can be applied across all applicants.  Recommendation letters are subjective; undergraduate grades are likewise a challenge to normalize between different colleges and universities.  The subject exam is meant to allow comparisons that avoid such subjectivity.  ETS points to studies (e.g., this one) that argue meaningful correlations between subject test scores and first-year graduate GPA. 

At the same time, there has been a growing trend away from emphasizing the test.  The astronomy and astrophysics community has been moving that way for several years - see here.  There have been recent studies (e.g. this one, with statistics heavily criticized here and relevant discussion here) arguing that the test scores are not helpful in actually predicting success (degree completion, for example) in doctoral programs.  In our own graduate program, one of my colleagues did a careful analysis of 17 years worth of data, and also found (to the surprise of many) basically no clear correlation between the subject test score and success in the program.  (Sampling is tricky - after all, we can only look at those students that we did choose to admit.)  At the same time, the tests are a financial obligation, and as mentioned here scores tend to be systematically lower for women and underrepresented minorities due to educational background and access to opportunities. 

Our program at Rice has decided to drop the physics subject GRE.  This decision was a result of long consideration and discussion, and the data from our own program are hard to argue.  It all comes down to whether the benefits of the test outweigh the negatives.  There is no doubt that the test measures proficiency at rapidly answering those types of questions.  It seems, however, that this measurement is just not that useful to us, because many other factors come into play in making someone an effective doctoral student.   Similarly, when people decide to leave graduate school, it is rare that the driving issue is lack of proficiency in what the test measures. 

I'm on a mailing list of physics department chairs, and it's been very interesting to watch the discussion back and forth on this topic and how much it mirrored our own.  It takes years to see the long term effects of these decisions, but it will definitely be something to watch. 

Friday, June 07, 2019

Round-up of various links

I'll be writing more soon, but in the meantime, some items of interest:

  • A cute online drawing utility for making diagrams and flowcharts is available free at
  • There is more activity afoot regarding the report of possible Au/Ag superconductivity.  For example, Jeremy Levy has a youtube video about this topic, and I think it's very good - I agree strongly with the concerns about heterogeneity and percolation. The IIS group also has another preprint on the arxiv, this one looking at I-V curves and hysteresis in these Au/Ag nanoparticle films.  Based on my prior experience with various "resistive switching" systems and nanoparticle films, hysteretic current-voltage characteristics don't surprise me when biases on the scale of volts and currents on the scale of mA are applied to aggregated nanoparticles.  
  • Another group finds weird effects in sputtered Au/Ag films, and these have similar properties as those discussed by Prof. Levy.  
  • Another group finds apparent resistive superconducting transitions in Au films ion-implanted with Ag, with a transition temperature of around 2 K.  This data look clean and consistent - it would be interesting to see Meissner effect measurements here.  
  • For reference, it's worth noting that low temperature superconductivity in Au alloys is not particularly rare (pdf here from 1984, for example, or this more recent preprint).   
  • On a completely different note, I really thought this paper on the physics of suction cups was very cute.
  • Following up, Science had another article this week about graduate programs dropping the GRE requirement.
  • This is a very fun video using ball bearings to teach about crystals - just like with drought balls, we see that aspects of crystallinity like emergent broken symmetries and grain boundaries are very generic.