A busy and contentious week in condensed matter physics

There were a couple of interesting and controversial things afoot this week in the condensed matter world.

• 
  There was a new preprint from the group of Prof. Hemley at the University of Illinois Chicago featuring electronic transport measurements in samples of the putative room temperature superconductor made from Lu-N-H, samples synthesized by the group of Ranga Dias.  This was mentioned as a potential confirmation of the room temperature superconductivity result by the New York Times.  Plotting the full raw data that goes with the new preprint, however, certainly gives many people (including me) pause.  The raw resistance vs temperature sweep traces have unphysically narrow (in temperature) drops to and rises from zero, as shown.  Obviously I don't know with complete certainty, but this looks exactly like what would be seen if one of the contacts was bad.  Time will tell, but the raw data surely look like a flaky contact rather than some weird re-entrant and thermally hysteretic superconductivity.
• Meanwhile, Physical Review did something quite unusual, as they explain in this editorial that ran in Phys Rev B.  They allowed the Microsoft Quantum group to publish their latest report about looking for Majorana fermions in superconductor/semiconductor hybrid structures, without giving readers all of the necessary parameters and information necessary for reproducing the work.  The rationale is that the community is better served by getting this result into the peer-reviewed literature now even if all of the details aren't going to be made available publicly until the end of 2024.  I don't get why the researchers didn't just wait to publish, if they are so worried about those details being available.  There has been enough controversy about data availability in the Majorana arena that I don't understand why anyone would invite more discussion about transparency on this. Meanwhile, another group reports related phenomenology, though they argue that due to disorder they are not seeing Majoranas in their devices.  A review about the experimental hunt for Majoranas in condensed matter systems also came out this week in Science. 

I'm at a workshop this week, so posting and commenting may be a bit thin.

Food and (broadly speaking) fluid mechanics - great paper!

This paper (author's website pdf here, arxiv version here) is just a spectacularly good review article about fluid mechanics (broadly defined to include a bit about foams and viscoelastic systems) and food/drink.  The article is broadly structured like a menu (drinks & cocktails for multiphase flows; soups & starters for complex fluids; hot entrees for thermal effects; desserts for viscous flows; coffee for granular effects; tea for suspensions and mixing; and dishwashing for flows at interfaces).  

I know I'm a particular niche demographic, in that I'm a scientist who likes cooking and actually had mech-e training in fluid mechanics, but trust me:  this article is just excellent, touching on a ton of interesting phenomena that you can play with in your own kitchen, while making connections to cutting-edge ongoing research.  

Update:  APS Physics has a Q&A with the first author here.

Some recent papers of interest

A couple of recent papers that seem interesting and I need to read more closely:

• This paper in Nature, a collaboration between folks at Ohio University and Argonne, is a neat combination of scanning tunneling microscopy and (synchrotron-enabled) resonant x-ray absorption.  The authors bring an STM tip (an extremely sharp metal tip) down to within a nm of the sample surface, so that electrons can tunnel quantum mechanically from the sample to the tip.  Then bang the sample with x-rays that are resonant with core levels of particular atoms in the sample.  In this case, one sample consisted of iron-containing molecules.  The x-rays could kick electrons out of the iron atoms where they are then detected by the tip, allowing atomic-resolution mapping of the desired atoms.  (It's a bit more subtle than that - see Fig. 2j - but that's basically the gist.)
• This paper in Science is also very cool (arxiv version here).  People are generally used to the idea that photons are quantum objects.  Indeed, photons are often discussed when talking about standard examples of quantum "weirdness".  A 50/50 beam splitter can put a photon in a superposition of going down two different paths, for example.  There is a whole approach to quantum information processing based on these properties.  This new paper demonstrates a beam splitter for individual phonons, specifically surface acoustic waves.  This opens the possibility of a solid-state phonon-based version of that approach to quantum computing.  Very neat.
• Lastly for now, this paper in Nature Materials (arxiv version here) uses STM to look at how superconductivity goes away in a cuprate superconductor as the doping level is increased way beyond the level that optimizes superconductivity.  The decrease in transition temperature and superfluid density with increasing doping has been a mystery.  This paper shows that the system breaks up into superconducting puddles surrounded by metallic regions, and that instead of the superconducting energy gap closing (implying a weakening of the interaction that pairs up the electrons), it "fills in".  Lots to ponder.

ARPA-E Roadshow

Today, Rice hosted the ARPA-E Roadshow, a series of presentations by ARPA-E program officers, MC-ed by the director, Prof. Evelyn Wang.   It was all about the energy transition, and it was pretty fascinating, particularly hearing from leaders of startups who were making commercialization transitions as well as program officers describing highlights of their portfolios.  A few highlights:

• "Hardware is hard." - said by Rita Hansen, quoting a timeworn truth when talking about the challenge of actually building and deploying pathbreaking gadgets in the field.
• "Work for ARPA-E, and you get to design emojis!" - Halle Cheeseman, poking fun at the fact that every project has its own little icon-like logo.
• Carlos Araque of Quaise Energy was part of a panel and spoke about their plans to use enormously powerful microwave sources to drill holes 20 km deep, so that one can have ubiquitous geothermal energy.  (I'll admit, cool as this sounds, I just don't understand how they plan to get vaporized rock out of a many-km-deep bore hole.)
• Joe Zhou of Quidnet Energy was also on the panel (with Araque and Hansen) and spoke about their plan for underground fracking-type pumping to use compressed water for energy storage for solar/wind/etc.  It's more geographically portable than pumping water up a nearby mountain for energy storage, but sounds like it could have some nontrivial challenges.
• Hinetics plans to have an integrated cryocooler in their motors, so that they can use high-Tc superconducting wiring without the need for separate refrigeration or cryogens.  Sounds very clever.
• Veir has plans for compact, evaporative LN2 cooling of high-Tc transmission lines.  This would allow very high current transmission at low voltages, so that utilities could avoid the giant, ugly towers and use a lot less land/narrower rights-of-way.  
• Brimstone is making net-carbon-negative cement based on calcium silicate (instead of traditional calcium carbonate which liberates CO2 when it sets).  This seems like potentially a huge deal if it scales, since concrete accounts for 8% of global CO2 emissions annually.

All of this stuff is far away from what I do for research, but it was certainly thought-provoking, and it showcases how much cleverness there is out there to bring to bear on the challenge of reducing climate impact.

What is a spin glass?

As mentioned previously, structural glasses are materials in which there is no periodic lattice (no long-range spatial order) and the building blocks get "stuck" in some configuration, kinetically unable to get to the true energetic minimum state which would almost certainly be a periodic crystal.  Upon cooling from the liquid state, their viscosity increases by many orders of magnitude (in various ways) until they act like rigid solids.  Distinguishing "glassy" physics includes strongly interacting building blocks, a complicated energy landscape with many local minima, spatial disorder leading to hugely varying interaction strengths and a very broad distribution of relaxation times (so that responses to perturbations aren't simple exponentials in time, but are more slowly decaying functions such as \(-\log t\)).  These slow relaxations are called "aging", and when the system is perturbed (e.g., a sudden stress is applied, or a sudden temperature change is applied and removed), the system's response picks back up ("rejuvenation") before aging again.

Analogs of all of these properties are also seen in spin glasses, which I wrote about a bit in this post about the 2021 Nobel in Physics.  In a spin glass, the degrees of freedom aren't atoms or groups of atoms, but instead are the magnetic moments of particular atoms, such as isolated Fe atoms in a Cu bulk.   The analog of the periodic crystal would be some version of long-range magnetic order.  In a typical spin glass, the magnetic atoms are positioned randomly in a non-magnetic host, so that the magnetic interactions between neighbors are strong, but often random in sign and strength due to disorder.  As a result, the magnetic system has a complicated energy landscape with many minima (corresponding to configurations with similar energies but it would cost significant energy to rearrange the spins to get from one local energy minimum configuration to another).  These systems show aging, rejuvenation, etc.

The universality of glassy dynamics across such microscopically different systems is one of those remarkable emergences that crops up in condensed matter.  Despite the very different microscopic physics, there is some deeper organizing principle at work that leads to these properties.  

Spin glasses have attracted quite a bit of interest for a couple of reasons.  First, they are comparatively easy to study, since magnetic properties and their time evolution are usually easier to measure than detailed microscopic structural arrangements in structural glasses.  Second, it is possible to create models of spin glasses in a variety of systems, including using qubits.  Spin glasses can also be mapped to certain kinds of optimization problems (see this pdf news article).

Interestingly, a recent paper in Nature (arxiv version) by folks at D-Wave has used their 5000 qubit gadget to do a quantum simulation of a spin glass.  They can program the interactions among the qubits and make them random and frustrated as in a spin glass.  In small test configurations, they show that they can see (at short times, anyway) quantum coherent dynamics that agree with calculations.  They can then look at much larger systems, well beyond traditional calculational practicality, and see what happens.  I don't know enough about the system to evaluate this critically, but it looks like a very nice platform.  (They’ve come along way from when their founder used to argue and insult in blog comments.  They now show as anonymous, but the one from Geordie Rose is clear from context.)