Monday, November 27, 2023

Noise in a strange metal - pushing techniques into new systems

Over the holiday weekend, we had a paper come out in which we report the results of measuring charge shot noise (see here also) in a strange metal.   Other write-ups of the work (here and especially this nice article in Quanta here) do a good job of explaining what we saw, but I wanted to highlight a couple of specific points that I think deserve emphasis.  

In thermal equilibrium at some temperature \(T\), there are current and voltage fluctuations in a conductor - this is called Johnson-Nyquist noise - and it is unavoidable.  Shot noise in electrical current results from the granularity of charge and, as shown in its original incarnation (pdf is in German), from the statistical variation in the arrival times of electrons.   Shot noise is an "excess" noise that appears in addition to this, only when a conductor is driven out of equilibrium by an applied voltage and carries a net current.

While the idea of shot noise is tunnel junctions and vacuum tubes had been worked out a long time ago (see the above 1918 paper by Schottky), it was in the 1990s when people really turned to the question of what one should see in noise measurements in small metal or semiconductor wires.  Why don't we see shot noise in macroscopic conductors like your house wiring?  Well, shot noise requires some deviation of the electrons from their thermal equilibrium response - otherwise you would just have Johnson-Nyquist noise.  The electrons in a metal or semiconductor are coupled to the vibrations of the atoms (phonons) - the clearest evidence for this is that the decrease in scattering of the electrons by the phonons explains why metals become more conductive as temperature is decreased.  In conductors large compared to the (temperature-dependent) electron-phonon scattering length, the electrons should basically be in good thermal equilibrium with the lattice at temperature \(T\), so all that should be detected is Johnson-Nyquist noise.  To see shot noise in a wire, you'd need the wire to be small compared to that e-ph length, typically on the order of a micron at low temperatures.  In the 1980s and 1990s, it was now possible to make structures on that scale.

Fig. 4 from the paper
The theory of what should be seen was worked out in a couple of different ways, initially assuming that it is safe to describe the conductor as a Fermi gas (ignoring electron-electron interactions).  One approach started from the conduction-as-wave-transmission picture of Landauer (see here and here for two examples).  A complementary approach (see here) calculated noise from the electronic distribution functions and got the same answer for non-interacting electrons, that the current noise should be 1/3 of the classic Schottky result.  That factor of 1/3 is called the Fano factor, \(F\).   If electron-electron interactions are "turned on", allowing the electrons to exchange energy amongst themselves but not lose energy to the lattice, the noise is actually a bit larger, \(F \rightarrow \sqrt{3}/4\).   It turns out that these values were verified in experiments in gold wires (see here and here, though one has to be careful in experimental design to see  \(F \rightarrow \sqrt{3}/4\)).  This confirmation is a great triumph of our understanding of physics at these mesoscopic scales.  (Interestingly, similar results are expected even with a non-degenerate electron gas - see here and here.)

We applied these same experimental approaches to nanowires we made from exquisite films of a strange metal, YbRh2Si2, and we found that the noise is much reduced from the usual result seen in Au wires (which we also confirmed).  We tested whether phonons could be responsible for the noise suppression, applying the same approach as had been done in the '90s (measurements on wires tens of microns long, where e-ph scattering should be important), and found (in addition to further confirming the e-ph energy loss results in Au from the '90s) that energy loss to phonons can't explain what we see in YbRhsSi2.  

Some further points of interest:

  • Until recently there really has not been much attempt to push the theoretical analysis of these kinds of measurements beyond the 1990s/early 2000s results.  My colleague Qimiao Si and his group have looked at whether strong Fermi liquid corrections affect the expected noise, and the answer is "no".  Of course, there are all kinds of additional complications that one could imagine.
  • This work was only possible because of the existence of high quality thin films of the material, and the ability to fabricate nanostructures from this stuff without introducing so much disorder or chemical change as to ruin the material.  My collaborator Silke Bühler-Paschen and her group have spent years learning how to grow this and related materials, and long-term support for materials growth is critically important.  My student, the lead author on the study, did great work figuring out the fabrication.  It's really not trivial.  
  • I think it's worthwhile to consider pushing older techniques into new regimes and applying them to new materials systems.  The heyday of mesoscopics in the 1990s doesn't need to be viewed as a closed, somewhat completed subfield, but rather as a base from which to consider new ways to characterize the rich variety of materials and phases that we have to play with in condensed matter.  

Thursday, November 16, 2023

Faculty positions at Rice - follow-up

I had mentioned about 6 weeks ago that my department at Rice is searching in the quantum/AMO space for experiment and theory.   Now I want to put the larger context of this out there - Rice has four quantum-related searches going on right now:

Quantum experiment (PHYA):
Quantum engineering (ECE):
Quantum materials (MSNE):

Interested candidates, we hope you will apply!  It's an exciting time here, and our quantum initiative folks can help make sure applications end up in the right place.  

Postdoctoral opportunities at Rice

I will be sending some emails shortly, but I wanted to point out postdoctoral opportunities here at Rice University.

The Smalley-Curl Institute is having a competition for two two-year postdoctoral fellow slots.  Click on the link for the details.  The requirements for a candidate:

  • Nomination by current SCI faculty member
  • Ph.D. in a field related to an SCI focus areas
  • Successful Ph.D. thesis defense before start of appointment
  • Ph.D. completed no more than three years before the start of the appointment
I would be happy to work with an interested, competitive candidate on this, and the deadline for applying is December 31.  Research areas in my lab these days include:  nanostructure-based studies of correlated quantum materials, including noise-based measurements; studies of spin transport and thermally driven spin effects in insulating magnets, from basic science to applications in low-power electronics; plasmon-based nanophotonic light sources and plasmonic junctions for physical chemistry.  If you're a student finishing up and are interested, please contact me, and if you're a faculty member working with possible candidates, please feel free to point out this opportunity. 

Rice also has a university-wide endowed honorific postdoctoral program called the Rice Academy of Fellows.   Like all such things, it's very competitive, and it similarly has a deadline of January 3, 2024.  Again, applicants have to have a faculty mentor, so in case someone is interested in working with me on this, please contact me via email. 

Saturday, November 11, 2023

Scientific publishing - where are we going?

I think it's safe to say that anyone involved in scientific publishing will tell you that it's a mess and the trends are worrisome.  This week, this news release/article came out about this preprint which shows a number of the issues.  In brief (not all of this is in the preprint; some is me editorializing):

Figure 1 from this preprint

  • The number of scientific papers being published is growing at a rate that looks completely unsustainable.  In my opinion, it's problematic on multiple levels.  There aren't enough reviewers (though that doesn't bother all publishers) and the average paper gets smaller and smaller readership (raising the question of why bother to publish papers that no one reads).  Does it make sense that the number of papers is skyrocketing while the number of PhDs granted is falling?
  • Some publishers (especially Frontiers, Hindawi, MDPI) have boosted this by drastically cranking up the number of papers that they publish, through launching specialized journals with "special issues" designed to have super-short review times (assuming that review is even truly part of the process).  Lest you think this is only the provenance of publishers previously accused of being predatory, this week alone I have received five different "special issue" announcements from AIP journals.
  • Why do people do this?  To try to game the impact factor calculations.  I've aired my grievances before about why journal impact factor is a lousy metric.  
  • Why do people want to inflate impact factors?  Because that's how journals keep score, and some countries put in place big-time incentives tied to impact factor.  A publisher worries that if its journal's impact factor falls below some threshold, then the government of China, for example, will no longer view that journal as important, and then thousands of authors will stop submitting....
  • Open access is a complicating factor, with some publishers charging absolutely sky-high charges, while at the same time having very high profit margins.  In the US, at least, those charges can be much larger than what grants will support.
  • Over all of this is the concern that massively inflating the amount of scientific literature lowers its quality and undermines the credibility of science in general.  
Coincidentally, this week we hosted Steinn Sigurðsson for a colloquium.  He is now the scientific director of the arxiv, the preprint archive that went from a quick and dirty preprint sharing site in 1991 to an enormously important part of the global scientific enterprise.  In his talk he hit on some wild numbers.  The arxiv is up to around 20,000 papers per month now (!) (in part because new disciplines like quantitative biology are using the arxiv).  Thankfully the arxiv has recently landed some good support.  Their annual operating budget is around $3.5M, and this is an enormous bargain by any measure.  The arxiv is partnering with volunteer developers who are adding some neat functionality.  Unsurprisingly, generative AI is a serious concern, even more so than for the publishing houses.  

It's a transformative time, for sure.  Maybe what we are seeing is analogous to the fluctuations that happen when approaching a 2nd order phase transition, and we are headed for a real change in the way publishing works.  It's hard to see how the current trends can continue unabated.

Wednesday, November 01, 2023

Strategic planning + departmental reviews

It's been a while since I've written a post about the ways of academia, so I thought it might be time, though it's not exactly glamorous or exciting.  There are certain cycles in research universities, and two interrelated ones are the cycle of departmental strategic planning and the cycle of external departmental reviews.

Strategic planning can be extremely important, as it allows departments to take stock of where things are, what opportunities exist for improvement (in terms of research, teaching, departmental operations), and how the department aspires to move forward.  Often this can involve a hiring plan, based on demographic trends in the department (e.g., how many faculty lines are expected to be available in the next, say, five to seven years?), rising field/school/university research priorities (e.g., there is likely to be enormous investment in AI/ML in the coming years).  Discussions for strategic planning can be frought, since even maintaining departmental faculty size means alloting new hires between different possible research areas in a zero sum.  Still, arriving at a departmental plan is often expected at one level up (that of a School or College, depending on the university's org chart labeling scheme), and having a plan that department members know and understand is helpful in transparency of how decisions get made that shape the future of the department.  It doesn't make sense to do reformulate these plans at too rapid a frequency, since the ability to implement the plan can be strongly perturbed by, e.g., economic events, global pandemics, or big changes in university leadership.

Very often, deans (or provosts) also value periodic reviews of departments by an external visiting committee.  The visiting committee is typically put together with input from the department (research areas that should be represented, suggestions of possible reviewers) and invited to come for a couple of days of interviews and departmental presentations.  These reviews are typically very broad, looking at research, teaching, departmental climate, staffing levels and organization, infrastructure and space needs, etc.  It's important to talk to all stakeholders (departmental leadership, TT and NTT faculty, staff, undergrad and grad students, postdocs, and of course the dean or equivalent who is the intended recipient of the report). The expected output of these visits is a report to the dean (or provost).  Such a report can be very helpful for the department to get feedback on their plans and operations, and to serve as a way of putting priorities forward to the dean/provost level.  Similarly, often deans find these things valuable as a way to make certain arguments up to higher levels.  It seems to be human nature that a statement made by a nominally objective external committee can get more traction than the same statement made by locals.  Like strategic plans, it only makes sense to do external reviews on a timescale sufficiently long that the department would have a chance to address issues raised from the previous visit before the next one.  For both of these things, every five years is on the edge of being too frequent, and every ten years would definitely be too long an interval.

Participating in external visits takes time, but I've found it to be a very valuable experience.  It's allowed me to meet and work with faculty from a variety of places, and it can be very helpful to see how other institutions do things (even at a level of learning about tools like software that can be useful for tracking degree progress, or organizations that work to facilitate career placement at the graduate level).