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.

DOE Experimental Condensed Matter Physics PI Meeting 2025 - Day 2

It was another very full day.   I had to pop in and out to attend to some things so I didn't get everything, but here are some physics items I learned:

• Dillon Fong introduced me to a technique I didn't know about before, x-ray photon correlation spectroscopy (see this paper).  You can look at time correlations of x-ray speckle near a particular Bragg spot and learn about dynamics and kinetics of transitions and materials growth.  Very cute.
• Charles Ahn presented work on high magnetic field superconductivity in Nd(1-x)Eu(x)NiO2, and I learned about the Jaccarino-Peter effect, in which an external magnetic field can counter the interaction between magnetic dopants and the conduction electrons.  This leads to "reentrant" superconductivity at high magnetic fields. 
• Danny Phelan showed that you can have two different crystal structures for La3Ni2O7, one that is stacked bilayers ("2222"), and one that is stacked monolayer/trilayer ("1313").  
• Ian Fisher talked about using the elastocaloric effect (rapidly and therefore adiabatically stretch or compress a material, leading to a change in its temperature) to identify phase transitions, since the effect is proportional to \( (\partial S/\partial \epsilon)_{T}\), the change in entropy with strain.
• Dan Dessau presented an interesting analysis of data in cuprates suggesting a form for the electronic self-energy that is called a power law liquid, and that this analysis implies that there is not a quantum critical point under the middle of the superconducting dome.
• Jak Chakhalian showed that epitaxially growing an iridate Weyl semimetal directly on top of insulating Dy2Ti2O7 spin ice leads to a dramatic anisotropic magnetoresistance at high in-plane fields that identifies interesting previously unknown physics.
• Daniel Rhodes showed some pretty work on superconductivity in T_d-MoTe2.  This material is extremely air-sensitive, and all of the device fabrication has to be done with great care in a glovebox.  This led to the following exchange.  Audience question: "It is notoriously difficult to make electrical contact to this material.  How did you do this?"  Answer: "Through tears and blood."  This was followed by a serious answer that concluded "The glovebox is always the problem."

DOE Experimental Condensed Matter Physics PI Meeting 2025 - Day 1

That was a full day.  Here are some things I learned, beyond the fact that the ballroom here is clearly kept at about 15°C by default.  (Apologies for not getting everything....)

• About 40% of the DOE ECMP program is related to 2D materials these days.
• Long Ju showed some interesting work trying to understand rhombohedral (ABC-stacked) 5-layer graphene encapsulated by hBN.  Trying to get rid of moiré effects from the hBN/graphene interfaces leads not to more robust quantum anomalous Hall response, but instead leads to very peculiar superconductivity that survives up to very large in-plane and moderately large out-of-plane magnetic fields. This happens in the same regime of charge and gate that would otherwise show QAH.  Looks like some kind of chiral superconductivity that may be topological.
• Andrea Young, meanwhile, in fewer layer rhombohedral systems, showed experiments pointing to superconductivity happening at the verge of a canting transition, where spins are reorienting.
• Eva Andrei gave a nice talk looking at the variety of states one can get when interfacing moiré systems with other moiré systems, and explaining what is meant by intercrystals.  
• Gleb Finkelstein showed how a measurement intended to look at shot noise instead became a very cute noise thermometry probe of thermal transport at the boundary between (graphene) quantum Hall currents and a superconducting electrode.
• Xiao-Xiao Zhang showed a really cute experiment, where the resonance of a drumhead made from an atomically thin film of MnPS3 convey information about magnetic transitions in that material as a function of magnetic field.
• Dan Ralph gave a nice talk about the challenges of electrically generating currents of properly oriented spins to drive magnetic switching in films magnetized perpendicular to the film plane, for spin-orbit torque memories (and fundamental understanding).
• Philip Kim gave a great overview of some remarkable results in electronic interferometers made on graphene, in which telegraph noise shows signatures
• Lu Li spoke about recent measurements showing magnetic oscillations and specific heat signatures of possible neutral fermions in a kagome lattice Mott insulator.
• Xavier Roy talked about CeSiI, a 2D material that is also a heavy fermion metal.  This and its related compounds look like a fascinating family of (unfortunately extremely air sensitive) materials. 
• Harold Hwang gave a great overview of recent work in nickelate superconductors, highlighting the similarities to the cuprates as well as the profound differences (like how electronic configurations other than d9 can also lead to superconductivity).

DOE experimental condensed matter PI meeting, + other items

This week I am attending the every-two-years DOE Experimental Condensed Matter Physics PI meeting.  Previously I have written up highlights of these meetings (see here, here, here, here, here), though two years I was unable to do so because I was attending virtually.  I will do my best to hit some high points (though I will restrict myself to talking only about already published work, to avoid any issues of confidentiality).  

In the meantime, here are a couple of topics of interest from the last couple of weeks.  

• I just learned about the existence of Mathos AI, an AI product that can function as a math solver and calculator, as well as a tutor for students.  It is pretty impressive.
• I liked this historical piece about Subrahmanyan Chandrasekhar (he of the “Chandrasekhar limit”, which describes the degeneracy physics + gravitation that limits the upper size of compact stellar objects like white dwarfs and neutron stars before they collapse into black holes) and his interactions with Stephen Hawking.  It's pretty humanizing to see an intellectual giant like Chandra sending a brief letter to Hawking in 1967 asking for advice on what to read so that Chandra can understand Hawking’s work on singularities in cosmology.  Hawking’s handwritten response is clear and direct.
• In an online discussion about what people will do if Google decides to stop supporting Google Scholar, I was introduced to OpenAlex.  This seems like an interesting, also-free alternative.  Certainly worth watching.  There is no obvious reason to think that Google Scholar is going away, but Alphabet has retired many free products, and it’s far from obvious how they are making any money on this.  Anyone from Google who reads the blog, please chime in.  (Note to self:  keep regularly backing up this blog, since blogger is also not guaranteed future existence.)