As we bid apparent farewell to LK99, it's important to note that several other pretty exciting things have been happening in the condensed matter/nano world. Here are a few papers that look intriguing (caveat emptor: I have not had a chance to read these in any real depth, so my insights are limited.)
- Somehow I had never heard of Pines' Demon until this very recent paper came out, and the story is told briefly here. The wikipedia link is actually very good, so I don't know that I can improve upon the description. You can have coupled collective modes for electrons in two different bands in a material, where the electrons in one band are sloshing anti-phase with the electrons in the other band. The resulting mode can be "massless" (in the sense that its energy is linearly proportional to its momentum, like a photon's), and because it doesn't involve net real-space charge displacement, to first approximation it doesn't couple to light. The UIUC group used a really neat, very sensitive angle-resolved electron scattering method to spot this for the first time, in high quality films of Sr2RuO4. (An arxiv version of the paper is here.)
- Here is a theory paper in Science (arxiv version) that presents a general model of so-called strange metals (ancient post on this blog). Strange metals appear in a large number of physical systems and are examples where the standard picture of metals, Fermi liquid theory, seems to fail. I will hopefully write a bit more about this soon. One of the key signatures of strange metals is a low temperature electrical resistivity that varies like \(\rho(T) = \rho_{0} + AT\), as opposed to the usual Fermi liquid result \(\rho(T) = \rho_{0} + AT^{2}\). Explaining this and the role of interactions and disorder is a real challenge. Here is a nice write-up by the Simons Foundation on this.
- Scanning tunneling microscopy is a great spectroscopic tool, and here is an example where it's been possible to map out information about the many-body electronic states in magic-angle twisted bilayer graphene (arxiv version). Very pretty images, though I need to think carefully about how to understand what is seen here.
- One more very intriguing result is this paper, which reports the observation of the fractional quantum anomalous Hall effect (arxiv version). As I'd mentioned here, the anomalous Hall effect (AHE, a spontaneous voltage appearing transverse to a charge current) in magnetic materials was discovered in 1881 and not understood until recently. Because of cool topological physics, some materials show a quantized AHE. In 2D electron systems, the fractional quantum Hall effect is deeply connected to many-body interaction effects. Seeing fractional quantum Hall states spontaneously appear in the AHE is quite exciting, suggesting that rich many-body correlations can happen in these topological magnetic systems as well. Note: I really need to read more about this - I don't know anything in depth here.
- On the more applied side, this article is an extremely comprehensive review of the state of the art for transistors, the critical building block of basically every modern computing technology. Sorry - I don't have a link to a free version (unless this one is open access and I missed it). Anyway, for anyone who wants to understand modern transistor technology, where it is going, and why, I strongly encourage you to read this. If I was teaching my grad nano class, I'd definitely use this as a reference.
- Again on the applied side, here is a neat review of energy harvesting materials. There is a lot of interest in finding ways to make use of energy that would otherwise go to waste (e.g. putting piezo generators in your clothing or footwear that could trickle charge your electronics while you walk around).
- In the direction of levity, in all too short supply these days, xkcd was really on-point this week. For condensed matter folks, beware the quasiparticle beam weapon. For those who do anything with electronics, don't forget this handy reference guide.
4 comments:
Any insight into why the strange metal theory paper is "finally" the solution that everyone has been waiting for? Its importance is a bit opaque to me as a naive experimentalist, after all its been pretty clear that incoherent low energy fluctuations and momentum randomizing processes are important purely based on experimental data. Guess I'm not good enough at the math to see it clearly.
Or is it that there hasn't been a convincing model showing these effects that isn't hand wavy?
Anon, I too am a naive experimentalist, so you should take anything I say on this with a grain of salt. I think it’s basically your second point. They write what is meant to be a very general model - electrons interacting (Yukawa-like) with a scalar field (that is itself emergent from electronic degrees of freedom), and include spatial disorder in that interaction strength. The argument is that this gives strange metal phenomenology (linear-T resistivity, T log T specific heat) even in the presence of other kinds of disorder, and that depending on the variation of the coupling, they can get either Planckian dissipation (\alpha around 1) or \alpha << 1 as seen in the q-critical heavy fermion strange metals. The argument is that this is very general. I am not mathematically savvy enough to know whether there are subtle issues here, or to comment with any authority on how fair/reasonable the underlying assumptions are.
Is "Pines demon" basically the normal-state version of a Leggett mode in a superconductor?
Not quite. The two modes do involve out-of-phase bands, but Leggett mode are usually gapped but it seems like Pines demons are gapless.
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