Turbulent times

While I've been absolutely buried under deadlines, it's been a crazy week for US science, and things are unlikely to calm down anytime soon.  As I've written before, I largely try to keep my political views off here, since that's not what people want to read from me, and I want to keep the focus on the amazing physics of materials and nanoscale systems.  (Come on, this is just cool - using light to dynamically change the chirality of crystals?  That's really nifty.)   

Still, it's hard to be silent, even just limiting the discussion to science-related issues.  Changes of presidential administrations always carry a certain amount of perturbation, as the heads of many federal agencies are executive branch appointees who serve at the pleasure of the president.  That said, the past week was exceptional for multiple reasons, including pulling the US out of the WHO as everyone frets about H5N1 bird flu; a highly disruptive freeze of activity within HHS (lots of negative consequences even if it wraps up quickly); and immediate purging of various agency websites of any programs or language related to DEI, with threatened punishment for employees who don't report their colleagues for insufficient reporting of any continued DEI-related activities.

Treating other people with respect, trying to make science (and engineering) welcoming to all, and trying to engage and educate the widest possible population in expanding human knowledge should not be controversial positions.  Saying that we should try to broaden the technical workforce, or that medical trials should involve women and multiple races should not be controversial positions.

What I wrote eight years ago is still true.  It is easier to break things than to build things.  Rash steps very often have lingering unintended consequences.  

Panic is not helpful.  Doomscrolling is not helpful.  Getting through challenging times requires determination, focus, and commitment to not losing one's principles.  

Ok, enough out of me.  Next week (deadlines permitting) I'll be back with some science, because that's what I do.

This week in the arXiv: quantum geometry, fluid momentum "tunneling", and pasta sauce

Three papers caught my eye the other day on the arXiv at the start of the new year:

arXiv:2501.00098 - J. Yu et al., "Quantum geometry in quantum materials" - I hope to write up something about quantum geometry soon, but I wanted to point out this nice review even if I haven't done my legwork yet.  The ultrabrief point:  The single-particle electronic states in crystalline solids may be written as Bloch waves, of the form \(u_{n \mathbf{k}}(\mathbf{r}) \exp(i \mathbf{k} \cdot \mathbf{r})\), where the (crystal) momentum is given by \(\hbar \mathbf{k}\) and \(u_{n \mathbf{k}}\) is a function with the real-space periodicity of the crystal lattice and contains an implicit \(\mathbf{k}\) dependence.  You can get very far in understanding solid-state physics without worrying about this, but it turns out that there are a number of very important phenomena that originate from the oft-neglected \(\mathbf{k}\) dependence of \(u_{n \mathbf{k}}\).  These include the anomalous Hall effect, the (intrinsic) spin Hall effect, the orbital Hall effect, etc.  Basically the \(\mathbf{k}\) dependence of \(u_{n \mathbf{k}}\) in the form of derivatives defines an internal "quantum" geometry of the electronic structure.  This review is a look at the consequences of quantum geometry on things like superconductivity, magnetic excitations, excitons, Chern insulators, etc. in quantum materials.

Fig. 1 from arXiv:2501.01253

arXiv:2501.01253 - B. Coquinot et al., "Momentum tunnelling between nanoscale liquid flows" - In electronic materials there is a phenomenon known as Coulomb drag, in which a current driven through one electronic system (often a 2D electron gas) leads, through Coulomb interactions, to a current in adjacent but otherwise electrically isolated electronic system (say another 2D electron gas separated from the first by a few-nm insulating layer).  This paper argues that there should be a similar-in-spirit phenomenon when a polar liquid (like water) flows on one side of a thin membrane (like one or few-layer graphene, which can support electronic excitations like plasmons) - that this could drive flow of a polar fluid on the other side of the membrane (see figure).  They cast this in the language of momentum tunneling across the membrane, but the point is that it's some inelastic scattering process mediated by excitations in the membrane.  Neat idea.

arXiv:2501.00536 - G. Bartolucci et al., "Phase behavior of Cacio and Pepe sauce" - Cacio e pepe is a wonderful Italian pasta dish with a sauce made from pecorino cheese, pepper, and hot pasta cooking water that contains dissolved starch.  When prepared well, it's incredibly creamy, smooth, and satisfying.  The authors here perform a systematic study of the sauce properties as a function of temperature and starch concentration relative to cheese content, finding the part of parameter space to avoid if you don't want the sauce to "break" (condensing out clumps of cheese-rich material and ruining the sauce texture).  That's cool, but what is impressive is that they are actually able to model the phase stability mathematically and come up with a scientifically justified version of the recipe.  Very fun.