Postdoctoral opportunity in materials

The Rice Advanced Materials Research Institute is having its 2025-2026 competition for prestigious postdoctoral fellowships - see here:  https://rami.rice.edu/rami-postdoctoral-fellowship-program  .

If you are interested and meet the criteria, I'd be happy to talk.  I have some ideas that lean into the materials for electronics direction, and other possibilities are welcome.  

2025 Physics Nobel: Macroscopic quantum tunneling

As announced this morning, the 2025 Nobel Prize in Physics has been awarded to John Clarke, Michel Devoret, and John Martinis, for a series of ground-breaking experiments in the 1980s that demonstrated macroscopic quantum tunneling. 

For non-experts: "Tunneling" was originally coined to describe the physical motion of a quantum object, which can pass through a "classically forbidden" region.  I've written about this here, and here is an evocative picture. Suppose there is a particle with a certain amount of total energy in the left region.  Classically, the particle is trapped, because going too far to the left (gray region) or too far to the right (gray region) is forbidden:  Putting the particle inside the shaded regions is "classically forbidden" by conservation of energy.  The particle bounces back and forth in the left well.  If the particle is a quantum object, though, it is described by a wave function, and that wave function has some non-zero amplitude on the far side of barrier in the middle.  The particle can "tunnel" through the barrier, with a probability that decreases exponentially with the height of the barrier and its width.

Fig. 2 from here

Clarke, Devoret, and Martinis were working not with a single particle, but with electrons in a superconductor (many many electrons in a coherent quantum state).  The particular system they chose was a Josephson junction made from an oxide-coated Nb film contacted by a PbIn electrode with a dc current flowing through it.  Instead of an x coordinate of a particle, the relevant coordinate in this system is the phase difference \(\delta\) of the superconducting wave function across the junction.  There is an effective potential energy for this system called a "washboard" potential, \(U(\delta)\), as in this figure.  At the particular DC current, which tilts \(U(\delta)\), the system can transition from one state (\(\delta\) bopping around a constant value, no voltage across the junction) to a state where \(\delta\) is continuously ramping (corresponding to a nonzero voltage across the junction).  The system can get thermally kicked from the zero voltage state to the nonzero voltage state (thermal energy doinks it over the barrier), but the really interesting thing is that the system can quantum mechanically tunnel "through" the barrier as well.

This idea, that a macroscopic (in the sense of comprising many many electrons) system could tunnel out of a metastable state like this, had been investigated by Amir Caldeira and Tony Leggett in this important paper, where they worried about the role of dissipation in the environment.  People tried hard to demonstrate this, but issues with thermal radiation and other noise in the experiments were extremely challenging.  With great care in experimental setup, the three laureates put together a remarkable series of papers (here, here, here) that showed all the hallmarks, including resonantly enhancing tunneling with tuned microwaves (designed to kick the system between the levels shown in panel (d) of the figure above).  

This was an impressive demonstration of controllable, macroscopic quantum tunneling, and it also laid the foundation for the devices now used by the whole superconducting quantum computing community.  

ACS National Nanotechnology Day webinar, Thursday Oct 9

Time for a rare bit of explicit self-promotion on this blog.  This coming Thursday, October 9, as part of the American Chemical Society's activities for National Nanotechnology Day (Why October 9?  In US convention, Oct 9 = 10/9, and 10^-9 m = 1 nm.  Look, it wasn't my idea....), I'm speaking in a free webinar titled "Illuminating the Nano Frontier", with Prof. Dongling Ma of INRS in Quebec.  The event is 11am-12:30pm EDT, and there will also be a recording for people who are unable to watch it live.  Should be a fun event. 

Update:  Here is the link to the webinar recording.  It's free and open-access.

Annual Nobel speculation thread

It’s that time of year again.  The physics Nobel will be announced next Tuesday, and the chemistry prize on Wednesday.  Who will it be this time?  Please speculate in the comments.  As is my annual futile tradition, I will put forward that the physics prize could be Aharonov and Berry for geometric phases in physics (even though Pancharatnam is intellectually in there and died in 1969).  This is a long shot, as always.  Last year was neural networks.  Astro is probably “due”, but who knows.  On the chem side, last year was computational protein design and AlphaFold.  

Fluid mechanics of electrons

Condensed matter physicists often speak of the "electronic fluid" in conductors, and we use a lot of vocabulary that makes analogies between the motion of liquids and the flow of charge (current, vortices, Fermi liquids, and in the UK vacuum triodes were called "valves").   In recent years, there has been a lot of talk about electron flow in the hydrodynamic regime (e.g. this highly cited paper and this recent review and this preprint).  In this regime, electron-electron scattering is strong, and it makes sense to think less about a single-electron picture and more like the electrons acting as a viscous fluid, which can have vortices and a velocity profile transverse to a channel.  (The boundary conditions at the channel walls are a subtle question for sure.)  

Adapted from Fig. 1 of this preprint

This preprint from last week is a great example of pushing this similarity to new limits.  Through a combination of electronic transport measurements and scanning Kelvin probe microscopy, the authors observe what really looks like a hydraulic jump in the flow of electrons through a constriction.  I've written about hydraulic jumps before, here and here.  They're the incompressible flow analog of a standing shockwave, when local fluid velocity goes from supersonic (relative to some sound-like excitation) to subsonic.  Very cute.

(I hope to write more soon, about the role of fluctuations in condensed matter and nanoscale physics.)

H-1B visas and academia

Sorry to talk politics.

In now-classic fashion, there was a just-before-close-of-business-Friday announcement by the executive branch yesterday of a "proclamation" that would seriously up-end the H-1B visa program.  (I have no idea why some of these things are called "executive orders" and some are called "proclamations".)  The short version (see this summary by an immigration law firm):  After midnight EDT tonight, for at least the next year, getting a new H-1B visa for someone outside the US will require the employer to pay a $100K fee.  If someone is a current H-1B visa holder and they are presently outside the US after tonight, it sounds like that fee may need to be paid to get back into the US, if this stands. Update:  The USCIS has issued a memo this evening (!) clarifying that the $100K fee only applies to new H-1B petitions, not present visa holders or those who have already filed.  

The history of this is complicated.  A standard concern is that this program can be and has been abused by tech companies, for example, who have been able to bring skilled workers in certain sectors into the US and pay them comparatively low wages.  Thus, runs the argument, these visas depress wages for domestic workers and make it harder for domestic workers to be competitive for such jobs.   While these are non-immigration visas, certainly a decent fraction of H-1B holders have gone down the path of applying for permanent residency or citizenship.  Personally, I think that the overall economic and cultural benefit to the US is hugely positive, though this being the internet I'm sure someone will argue this in the comments.

Like a large fraction of the Friday afternoon proclamations or executive orders, this is a chaos grenade for academia.  The large majorities of articles in the media about this issue do not point out that the H-1B process is widely used by universities to bring international scholars (faculty members, postdocs, research scientists) into the US.   If every H-1B for someone presently outside the US is now going to cost the sponsor $100K up front, this would be extremely disruptive.  The situation in academia is distinctly different from that in high tech industry - the arguments about wage suppression are not nearly as relevant. 

For the last 70+ years, the US has reaped enormous economic, societal, and national security benefits from being a global destination for top scientists, engineers, and scholars.  No system is perfect, but destroying all of this without any realistic plan to replace it is just self-defeating.  When the secretary of commerce says "the gold [$2M for sponsorship] and platinum [$5M for sponsorship] cards would replace employment-based visas that offer paths to citizenship, including for professors, scientists, artists and athletes", that's disconnected from reality for the world of professors and scientists.  It's not hard to envision that a dedicated visa class (like the F1 visas for students) exempt from crazy fees could be created specifically for PhD-level scholars and researchers, but this would require an actual plan from Congress as well as support for the idea.  

As I've said about other topics, don't panic just yet.  It seems certain that there will be lawsuits filed about this bright and early on Monday morning.  Like many other research-related issues (e.g. slashed indirect cost rates), this will very likely be tied up in court for years, and the biggest fees in the near term will go to lawyers.  Still, it's unclear what the status quo will be while those legal arguments are waged, and the executive branch does have a lot of latitude in the US system.  Stay tuned, and if you are in a position to do so, make your voices heard to your legislators.

(US budget update:  the actual spending bills before congress largely ignore the presidential budget recommendation and its brutal cuts.  Of course, there is a showdown brewing about a continuing resolution since it's unclear whether these bills can pass congress.  It's also unclear whether agencies would actually spend their appropriated budgets.  As before, this is a marathon, not a sprint.)

DOE Experimental Condensed Matter Physics PI Meeting 2025 - Day 3 and wrap-up

 A few more interesting tidbits from the concluding half-day of the DOE ECMP PI meeting:

• Dmitri Basov showed some of the remarkable experiments enabled by layers of MoOCl2, which in the IR is an intrinsically hyperbolic optical material.  This material has unusual plasmonic properties considering its high resistivity.  These include peculiar cavity effects such as modifying superfluid density of a proximally coupled superconductor.
• Leonid Butov explained some remarkable evidence for superfluidity of indirect excitons excited in the moire bilayer of MoSe2/WSe2.  Low temperature mean free paths of these objects can exceed hundreds of microns (!).
• Cui-Zu Chang showed evidence that truly stoichiometric FeTe is actually a superconductor with a critical temperature of about 13.5 K, rather than the usual thinking that it is an antiferromagnetic metal. Apparently an extra 2% of interstitial iron is enough to kill superconductivity and induce AFM order.
• James McIver presented an example of how nonlinear optical effects in an optically driven (Floquet) Weyl semimetal seem to vary linearly with driving field - anomalously strong.
• Dmytro Bozhko showed a really neat technique, using Brillouin light scattering to map out the dispersion of phonons and magnons in YIG, and to extend this approach with a special hollow-core optical fiber to low temperatures with the motivation of probing magnon superfluidity in a particular antiferromagnetic insulator.
• Ray Ashoori used his characteristically pretty quantum capacitance measurement technique to examine the density+displacement field+magnetic field phase diagram of 5-layer rhombohedral graphene, revealing some surprising fractional Chern insulator states.
• Claudia Ojeda-Aristizabal discussed some mesoscopic transport measurements in bilayer graphene, where an adsorbed layer of spin-containing CuPc molecules seems to affect both decoherence and the trigonal warping contribution to it (related to intervalley scattering). 
• Feng Wang and You Zhou both discussed recent measurements looking at Wigner crystals and their properties in 2D TMDs, through a variety of means.
• Liuyan Zhao showed some very rich physics obtained in studies that moiré stack bilayers of the van der Waals insulating magnet CrI3.  

Unfortunately I missed the last talk because of the need to head to the airport.  Overall, the meeting was very good.  Program PI meetings can tend to become less about telling coherent scientific stories and more about trying to show everything someone has done in the last three years.  This meeting avoided that, with clear talks that generally focused on one main result, and that made it much more engaging.  As good as tools for virtual gatherings have become, there really is no substitute for an in-person event when you can just talk to someone by the coffee about some new idea.