Negative thermal expansion

Some interesting science results recently, but I wanted to talk about one a little off the beaten path.  Most people have some exposure to the concept of thermal expansion, the idea that solids tend to increase in size as temperature is increased.  This is why people suggest running a stuck (metal) lid on a glass jar under hot water to make it easier to open - the idea is that the metal expands more with increasing temperature than the glass.  This is why there are flexible joints between sections of concrete road, rather than trying to cast the road in one giant section.  Thermal expansion of the pavement would otherwise buckle the roadway.  

Vibrating H2 molecule, electron density
from DFT, by Dr. Or Cohen.

Where does thermal expansion originate?  In a toy model, we can think of the bound atoms in a solid like balls and springs.  The springs in this case model forces between the atoms that result from the electrons involved in the chemical bonds that hold the solid together.  (We usually think of the nuclei as slow and the electrons as fast, so you can consider the nuclear positions, somehow solving for the electron density given those positions, and figuring out the net force on the nuclei.  There is a whole subfield now in shortcutting these calculations with machine learning.)  In an ideal harmonic oscillator, the potential energy is perfectly symmetric around its minimum position.  Giving the oscillator larger and larger amounts of kinetic energy therefore does not change the time average separation of the atoms. 

When dealing with interatomic potentials, though, the potential is anharmonic - the effective spring is softer in extension than compression.  Another way to put it:  at small separations, the "steric interactions" caused by the Pauli principle give the "hard core repulsion" that tends to keep atoms from overlapping.  As a result, the potential looks like the cartoon (red dashed parabola = harmonic approximation that is good near the equilibrium position).  Now, if you give the atoms more kinetic energy, their time-average separation gets larger.  This is the conventional origin of the usual positive thermal expansion.  (Fun historical note.  In 1910, Lindemann, Churchill's friend ("the prof") and science advisor during WWII, put forward what is now called the Lindemann melting criterion: monatomic solids melt roughly when the root mean square thermal vibration displacement is about 10% of the interatomic distance.  This paper is hard to find online, btw.  Lindemann, Frederick A. "Über die berechnung molekularer eigenfrequenzen" Phys. Z 11, 609-612 (1910).),

Interestingly, some materials have negative thermal expansion - as temperature is increased, the materials shrink!  How does that work?  It seems to fly directly counter to intuitive expectations.  Negative thermal expansion often involves materials with lots of open volume in their structure, built out of rigid subunits (e.g. tetrahedra or octahedra of atoms).  As temperature increases, the subunits can deform a bit and also can rotate in ways that allow them to pack more efficiently.  An example of a material like this is zirconium tungstate.   That brings me to this article in JACS, which reports colossal negative thermal expansion in a metal organic framework compound, with a fractional change in volume of around -0.0006 per Kelvin near around 50 degrees C.  This negative thermal expansion coefficient is six times larger than the previous record, and seems to result from distortion of Zr6/oxygen tetrahedra.  Pretty neat, and these kinds of motifs could lead to materials with more designer thermal structural properties.

NSF, spending, and the end of the fiscal year

We are less than one month away from the end of the federal fiscal year, and traditionally there are internal deadlines for agencies to allocate their final spending by around September 9. Right now, the NSF is on track to issue about 4000 fewer (!!) awards in FY26 than it did annually back in FY21-FY24, and 2000 fewer than it did in the incredibly tumultuous FY25 (with its government shutdowns and mass cutbacks in agency personnel). This is dire, if like me you are a supporter of the agency and its vital role in the US research ecosystem.  

Perhaps even more distressing, the NSF is on track to underspend its FY26 budget appropriation (congressionally approved, presidentially signed) by between $1.25-1.5B, or 15-18%. This is essentially unprecedented - in the past, the NSF has always spent ~ 99% of its appropriation in a given fiscal year. Some large portion of this is from the mid-FY clawbacks that were reported in Science and Nature, supposedly squirreled away to support an as-yet unannounced OSTP "grand challenges" program.  

While technically the funds don't go away at the end of September, this kind of underspending raises the possibility of a pocket rescission. OMB and the executive branch have been pushing for massive cuts to the agency; Congress has disagreed. It sure looks like all the "see, don't worry, Congress didn't allow big cuts to the NSF" palliative statements don't hold up very well to scrutiny, if the majority party is content to just give up Article I power to the executive branch. 

In this period of complete flood-the-zone craziness, the mainstream news media seemingly doesn't have the bandwidth or interest to report on this; they seem to have judged that it's too obscure, it doesn't play in Peoria, the public doesn't really care. This kind of disruption will have ripple effects that last for many years and affect US scientific and economic competitiveness, and it's happening without much notice.

This week's news about an agreement between NIH and DOD to funnel NIH funds for infectious disease to DOD (or, in the official statement, to work together on projects of mutual interest), is at least getting some public attention.  Agencies agreeing to pass around at minimum hundreds of millions of dollars outside congressional oversight or what the appropriations acts say is another example of an Article I crisis, when the majority party basically hands over what are supposed to be congressional powers to executive branch.

(An additional sciencey blog post coming soon!)

Lab safety - seriously, be careful out there

This past week was a genuinely horrific reminder of the importance of lab safety, especially in the realm of hazardous chemicals.  

First, a graduate student at Hokkaido University was killed due to some kind of large-scale exposure to hydrofluoric acid.  For those who don't know, HF is used at some rate in semiconductor-related work, because it's a way to etch SiO\(_2\) from silicon surfaces and leave a hydrogen-terminated surface.  (Usually this is done using buffered oxide etch, which is less concentrated than the pure acid but still must be handled with great care and appropriate personal protective equipment.)  Accidental exposure to small amounts of HF is not always immediately obvious, because it is not that aggressive in damaging human skin (unlike, say, nitric or sulfuric acid).  Rather, it attacks the calcium in bones (as well as screwing up many other biological processes).  The topical treatment is calcium gluconate gel, which can help by being a much more readily accessible source of calcium ions to bind with the fluoride ions.  There are no real details out yet about how someone had a massive amount of HF splash on their head/face, but that sure sounds like a terrible case of poor storage and handling practices.  

Then it came out that this past Wednesday a doctoral student at MIT had been exposed to dimethyl mercury.  Here is a reddit discussion thread in r/mit, and here is another one on r/chemistry.  Apologies for the reddit links, but there doesn't seem to be any news reporting about this yet.  From the MIT announcement in those threads, it was in building 18, and decontamination of the space is ongoing.  Any scientist of my generation knows about dimethylmercury because of the horrifying death of Dartmouth chemistry professor Karen Wetterhahn in 1997.  She was exposed to tiny drops of this stuff, which diffused through her latex gloves (which she did not realize at the time).  Prior to her death, people still occasionally used dimethylmercury as a standard in NMR measurements.  Organic mercury compounds are widely recognized as incredibly dangerous because tiny amounts can lead to mercury crossing the blood-brain barrier, leading to terrible neurological systems and death.  Once mercury is into organic tissues, it is very difficult to chelate the metal ions.  Again, there is a shortage of official information about this incident, but MIT's announcement made it clear that any synthesis or use of this compound is not permitted and was unauthorized.  

Update:  the latest from MIT’s emergency response page raises the possibility that there may not have been any exposure or dimethylmercury present.  Fingers crossed that this turns out to be a false alarm.

Update 2:  The always excellent Derek Lowe with a further discussion of what seems now to have (thankfully, hopefully) been a false alarm.

To students reading this:  PLEASE be careful in the lab.  Know the hazards of what you're doing, and use appropriate procedures and protective equipment.  If you ever have questions about safety, for goodness' sake please ask.  Your PI and your environmental health and safety team would far rather have you ask questions and be cautious then to do something dangerous.  No PI should ever make students feel like thinking about safety is unnecessary or overly cautious, and no PI should ever be hesitant about supplying or letting students purchase PPE.  

Recent superconductivity results + open positions at Rice

Much as I feel like I should write about the latest developments in US science policy, instead I want to point out two exciting recent superconductivity results.  Below I will also append a couple of other items, including open positions at Rice.

• 

  After Fig. 2b from here

  In this paper, researchers demonstrated high temperature superconductivity in a monolayer of Bi\(_2\)Sr\(_2\)CuO\(_{6+\delta}\) (Bi-2201).  The monolayer contains just a single CuO\(_2\) plane, and remarkably, the superconducting transition is only suppressed about 10% from the bulk value of around 35 K.  The authors were able to explore the phase diagram by tuning the oxygen content in situ, using vacuum annealing to drive out oxygen and ozone exposure to (seemingly gently) put it back in.  This allows them to examine a large swath of temperature/doping/magnetic field parameter space, showing evidence of critical scaling of the resistance near the transition as well as an anomalous metallic state.  There's a lot to digest here.  The mapped out zero-field phase diagram in a single device (shown here) is extremely impressive.  Studies like this can hopefully give new insights into what physics is truly essential to achieve high temperature superconductivity.
• In this paper, investigators placed exfoliated NbSe\(_2\) encapsulated by hBN in a split-ring resonator cavity, and they observed enhanced critical temperature (by 0.15 K out of 6.53 K, or an increase of 2.3%), critical field, and critical current when the resonance frequency of the cavity is such that it apparently couples to superconducting fluctuations in the material on the spatial scale of the cavity.  There is a ton of interest in using electromagnetic cavities to modify the properties of quantum materials - see this review.  As far as I know, this is the first time that coupling to the vacuum mode of a cavity has actually enhanced superconducting properties.  Exciting times.

It's worth noting that both of these papers come out of groups in China - Changgan Zeng at USTC and Yuanbo Zhang at Fudan.   

In other news:

• 
  The NSF is going to make about half the number of awards this year as it did in The Before Times (2021-2024), according to this news article in Nature.  Figure 1 (shown here) is striking.  The claim is that the NSF leadership is taking clawed-back FY26 funding of around $1B and saving it for some as-yet unspecified, unannounced OSTP "grand challenges" project.  
  
• NSF also announced "new" funding opportunities here.  As described in that article linked above, these are not exactly new - it's essentially a reorganization/rebranding of much of the NSF's portfolio now that they've eliminated divisions and retired older funding solicitations.  Noteworthy is that the amount of funding mentioned in these solicitations is all considerably lower than what the aggregate of the older solicitations used to have.  As a non-expert, it looks a lot like these solicitations are being prepared as if the presidential budget requested funding levels (you know, the ones that want to cut NSF by more than half) are the baseline.

Meanwhile, at Rice we have some faculty searches underway:

• The Rice Advanced Materials Institute is searching for an assistant professor with an expertise in computational materials (including AI/ML).  See here.
• Our chemistry department is searching for an assistant professor position with an emphasis including physical chemistry.  See here. 
• There will also be an AMO physics position posted shortly - I'll update with the link when that becomes available.

Finally, Nano Letters is having a seed grant competition for grad students.  It's not much money, but it is good experience and can inspire graduate student creativity. (Full disclosure: I'm an associate editor for the journal.)

NSF - public comment, APS feedback opportunity

In case you weren't aware, in addition to the OMB proposed rule changes which just concluded their public comment period, the NSF has put forward proposed changes in their proposal procedures for public comment, and the last day for feedback is August 24.  The proposal and award policies and procedures guide (PAPPG) is to be replaced with the revised document called the "Guide for Financial Assistance" (GFA).

The proposed revised document is here: https://www.regulations.gov/document/NSF-2026-OTR-0001-0003 

The place to file a comment is here: https://www.regulations.gov/docket/NSF-2026-OTR-0001/document?withinCommentPeriod=true

The place read already-filed comments is here: https://www.regulations.gov/docket/NSF-2026-OTR-0001/comments

The Federation of American Societies for Experimental Biology has a brief summary of the proposed changes here.  There are a number of (IMO) problematic issues with the would-be revisions, not the least of which is they presumptively assume that the OMB changes are going into effect, including the very controversial ones that would allow termination of awards at any time, for any reason (including not aligning with presidential priorities), with no appeal.  It's a good idea to be aware of what's being proposed and to take advantage of the opportunity to provide feedback.  

Update:  The APS has also put together a guide to the proposed changes.

Speaking of feedback, the American Physical Society is doing a broad survey (beyond just physicists) asking for input about up to three things that people think the NSF does well and up to three things that the NSF should do differently.  It's open to everyone, and the response spaces are brief (500 char ea.). Again, if this is something that matters to you, I encourage you to contribute - it's quick.  

Reproducibility in materials research, and an anecdote

Yesterday I attended the 40th annual summer research colloquium of the Smalley-Curl Institute at Rice, a fun internal conference that provides a great opportunity for undergrads (including visitors), graduate students, and a few postdocs to present their work.  The keynote speaker was our EVPR, Prof. David Sholl, who gave a very informative talk about reproducibility in the chemical engineering/materials literature.  We hear a lot these days about crises of reproducibility in scientific research, and Prof. Sholl rightly points out that in some fields the expectation of reproducible results is high - no one would spend $1B on a chemical engineering plant if they weren't very sure that the catalytic processes were going to work as expected at scale.  Keys to reproducibility include, unsurprisingly, repeated results and independent replication.  One metaresult that was interesting is this paper, looking at the literature on metal-organic frameworks and how often there are published replications of syntheses; not as often as you would think or want!  

A truly surprising (to me, anyway) result is this one.  The Brunauer–Emmett–Teller (BET) (yes, that Teller) method is a long-established technique that uses gas adsorption measurements to infer the surface area of porous materials.  Many research groups were given identical raw adsorption isotherms and asked to calculate the specific surface areas, resulting in a surprisingly large spread of results (Fig 1 of the paper).  Clearly not everyone had the same analysis procedures even for a technique developed in the 1930s!

Some take-away lessons from this are encapsulated here, in an article titled "Five easy ways to make your research more reproducible".  Good stuff.  The talk raised a number of questions relevant to our present era of huge enthusiasm about AI-based materials research and "self-driving" labs.  If the AI models are all trained on the literature, and the literature is not representative of complete and reproducible procedures, that's a problem.  

One personal anecdote about reproducibility and its challenges in materials synthesis.  Twenty years ago (!), I was working with a colleague who had a postdoc who was synthesizing Fe3O4 (magnetite) nanoparticles via wet chemistry methods (see here). We did some fun electronic transport experiments bridging very closely spaced electrodes with such nanoparticles, and we saw some very dramatic hysteretic response kick in as \(T\) was reduced below about 120 K.  That's the temperature of the Verwey transition in magnetite, where the material enters a more insulating low temperature phase.  Basically all of the devices we made with that batch of nanoparticles showed this phenomenon.  Then the postdoc took up a faculty position and a senior grad student came in and took over the synthesis, and for several months, subsequent batches of nanoparticles just didn't seem to show the effect.  The key issue is oxygen stoichiometry.  Get a little oxygen rich, and you form nanoparticles that include some \(\gamma\)-Fe2O3, which doesn't have the Verwey physics and in nanoparticle form looks really similar in x-ray diffraction to the desired magnetite.  Anyway, we started working with a collaborator who could grow epitaxial Fe3O4 films, and in those devices the electronic effect was there all the time.  All this led to this publication and subsequent papers, and I still think it's a cool set of results about a nonequilibrium transition in a correlated material.  In the end, after several months the chemistry grad student did get back to making nanoparticle batches that showed the transition. It turns out that at some point he had changed the length of a piece of tubing in the gas manifold, and unexpectedly that had altered the reaction kinetics just a little.  Changing it back got the synthesis to be reliable again.  This is an example of how finicky materials synthesis can be!

PhDs - how long a doctorate should take, and a new pilot program

I think it's safe to say that most people who've considered the issue think that a doctoral degree in the sciences and engineering in the US often takes too long.  

How long?  According to the latest data (see here, Table 1-12), the median time to degree in the physical sciences, for example, is 5.7 years after starting the program, while in all of engineering it's 5.3 years.  

Too long for what?  Well, life, basically.  Any decision to go to grad school is inherently a trade-off with opportunity costs.  Graduate stipends remain low compared to expected wages in entry-level (bachelors degree-qualified) positions in the sciences and engineering in industry.  The long duration of doctoral programs is certainly a powerful disincentive for many who might be interested but are under financial pressures.  Family considerations are also a major factor.  From the perspective of basically any career path, thanks to the time value of money and ideas of seniority, it's better to get going earlier if you have the qualifications for the particular job.  Companies would rather hire younger (cheaper) people.

So, there are already strong reasons to think about shortening doctoral programs.  Now, with the proposed change in duration of status of student visas (rule here, with plenty of editorializing; legal challenges very likely forthcoming in September) to four years, there is additional pressure. 

Why do US programs take so long?  Don't they give PhDs in three years in the UK and Europe?  In the UK and Europe, a student enters a doctoral program after already pursuing and receiving a masters degree, with grad level coursework taking place there.  Thus they go directly into research.  In the US, in contrast, it is far more common for students to go directly into the doctoral program.  Likewise, in the US, it is far more common for funding for students to go through PI-written research proposals, while in the UK, the students come funded, so to speak.  

Enter a new pilot program from NSF, the UIDP [University Industry Demonstrated Partnership] Industry-Integrated PhD Scholars Program (I-PhD). The idea is to shorten the doctorate to four years, with at least one of those years on-site at a company.  As the announcement says, "Students' first year of funding will be provided by their universities, with the remaining years covered by NSF. Industry partners will provide matching commitments to cover at least one year of practical experience conducting dissertation research at a company site. Students will be co-advised by academic and industry mentors, equipping them with critical skills for their future careers."  The initial plan is $47M over five years, and there will be a webinar (see here) next week about this.   (Up front, I do want to disagree with the framing that existing PhD programs are geared exclusively for academic careers.  It's well established that the fraction of PhDs in the sciences and engineering who go on to become faculty is low, and most go into industry.  Faculty PIs know this.  Students know this.  The problem solving and analytical skills taught in doctoral programs remain highly valued outside academia, at least until AI replaces us all.)

This is certainly a very interesting pilot program.  There are rumors that the DOE Genesis Mission is going to put something extremely similar in place as well.   The implementation details will be enormously important.  (For example:  Who is eligible?  Who handles the coordination between industry and the university - that is, who does the match-making and how?   At the department-company level and at the particular academic/industrial advisor level?   How will intellectual property be handled?  Publications?  Project design? If there are economic challenges, how committed are the companies?)  Given that this is a form of NSF fellowship, it seems highly likely that it will only be open to US citizens and permanent residents.  Obviously, not every discipline is well-suited to this, in terms of there being a ready supply of companies set to buy in.  Still, it is absolutely worth seeing how this works.

Update:  Thanks to one of my colleagues for pointing out the fine print, which is here.  In brief, as expected this is only open to US citizens and permanent residents.  No indirect costs allowed.  There is a $16K cost-of-education piece that looks like a substitute for grad tuition.  The intellectual property issues have to be ironed out between the university and the company before the start.  Perhaps not unexpectedly, this is most likely to work well for programs and PIs who already have close collaborations with particular companies.  Engineering disciplines are most likely to fit well here, it seems, while basic research farther away from applications will have more challenges.  (Question:  will finance companies or AI materials companies be interested in supporting theorist/computational scientists through this mechanism?)