Saturday, September 30, 2023

Faculty positions at Rice, + annual Nobel speculation

Trying to spread the word:

The Department of Physics and Astronomy at Rice University in Houston, Texas invites applications for two tenure-track faculty positions, one experimental and one theoretical, in the area of quantum science using atomic, molecular, or optical methods. This encompasses quantum information processing, quantum sensing, quantum networks, quantum transduction, quantum many-body physics, and quantum simulation conducted on a variety of platforms. The ideal candidates will intellectually connect AMO physics to topics in condensed matter and quantum information theory. In both searches, we seek outstanding scientists whose research will complement and extend existing quantum activities within the Department and across the University. In addition to developing an independent and vigorous research program, the successful applicants will be expected to teach, on average, one undergraduate or graduate course each semester, and contribute to the service missions of the Department and University. The Department anticipates making the appointments at the assistant professor level. A Ph.D. in physics or related field is required by June 30, 2024.

Applications for these positions must be submitted electronically at (experimental) and (theoretical). Applicants will be required to submit the following: (1) cover letter; (2) curriculum vitae; (3) statement of research; (4) statement on teaching; (5) statement on diversity, mentoring, and outreach; (6) PDF copies of up to three publications; and (7) the names, affiliations, and email addresses of three professional references. Rice University, and the Department of Physics and Astronomy, are strongly committed to a culturally diverse intellectual community. In this spirit, we particularly welcome applications from all genders and members of historically underrepresented groups who exemplify diverse cultural experiences and who are especially qualified to mentor and advise all members of our diverse student population. We will begin reviewing applications by November 15, 2023. To receive full consideration, all application materials must be received by December 15, 2023. The expected appointment date is July 2024.


In addition, the Nobels will be announced this week.  For the nth year in a row, I will put forward my usual thought that it could be Aharonov and Berry for geometric phases in physics (though I know that Pancharatnam is intellectually in there and died in 1969).  Speculate away below in the comments.  I'm traveling, but I will try to follow the discussion.

Tuesday, September 26, 2023

A few quick highlights

 It's been a very busy time, hence my lower posting frequency.  It was rather intense trying to attend both the KITP conference and the morning sessions of the DOE experimental condensed matter PI meeting (pdf of agenda here).  A few quick highlights that I thought were interesting:

  • Kagome metals of the form AV3Sb5 are very complicated.  In these materials, in the a-b plane the V atoms form a Kagome lattice (before that one reader corrects me, I know that this is not formally a lattice from the crystallographic point of view, just using the term colloquially).  Band structure calculations show that there are rather flat bands (for an explanation, see here) near the Fermi level, and there are Dirac cones, van Hove singularities, Fermi surface nesting, etc.  These materials have nontrivial electronic topology, and CsV3Sb5 and KV3Sb5 both have charge density wave transitions and low-temperature superconductivity.  Here is a nice study of the CDW in CsV3Sb5, and here is a study that shows that there is no spontaneous breaking of time-reversal symmetry below that transition.  This paper shows that there is funky nonlinear electronic transport (apply a current at frequency \(\omega\), measure a voltage at frequency \(2 \omega\)) in CsV3Sb5 that is switchable in sign with an out-of-plane magnetic field.  Weirdly, that is not seen in KV3Sb5 even though the basic noninteracting band structures of the two materials are almost identical, implying that it has something to do with electronic correlation effects.
  • Related to that last paper, here is a review article about using focused ion beams for sample preparation and material engineering.  It's pretty amazing what can be done with these tools, including carving out micro/nanostructured devices from originally bulk crystals of interesting materials.  
  • The temperature-dependent part of the electrical resistivity of Fermi liquids is expected to scale like \(T^{2}\) as \(T \rightarrow 0\).  One can make a very general argument (that ignores actual kinematic restrictions on scattering) based on the Pauli exclusion principle that the inelastic e-e scattering rate should go like \(T^{2}\) (number of electron quasiparticles excited goes like \(T\), number of empty states available to scatter into also goes like \(T\)).  However, actually keeping track of momentum conservation, it turns out that one usually needs Umklapp scattering processes to get this.  That isn't necessary all the time, however.  In very low density metals, the Fermi wavevector is far from the Brillouin zone boundary and so Umklapp should not be important, but it is still possible to get \(T^{2}\) resistivity (see here as well).  Similarly, in 3He, a true Fermi liquid, there is no lattice, so there is no such thing as Umklapp, but at the lowest temperatures the \(T^{2}\) thermal conduction is still seen (though some weird things happen at higher temperatures). 
There are more, but I have to work on writing some other things.  More soon....

Sunday, September 17, 2023

Meetings this week

This week is the 2023 DOE experimental condensed matter physics PI meeting - in the past I’ve written up highlights of these here (2021), here (2019), here (2017), here (2015), and here (2013).  This year, I am going to have to present remotely, however, because I am giving a talk at this interesting conference at the Kavli Institute for Theoretical Physics.  I will try to give some takeaways of the KITP meeting, and if any of the ECMP attendees want to give their perspective on news from the DOE meeting, I’d be grateful for updates in the comments.

Thursday, September 07, 2023

Things I learned at the Packard Foundation meeting

Early in my career, I was incredibly fortunate to be awarded a David and Lucille Packard Foundation fellowship, and this week I attended the meeting in honor of the 35th anniversary of the fellowship program.  Packard fellowships are amazing, with awardees spanning the sciences (including math) and engineering, providing resources for a sustained period (5 years) with enormous flexibility.  The meetings have been some of the most fun ones I've ever attended, with talks by incoming and outgoing fellows that are short (20 min) and specifically designed to be accessible by scientifically literate non-experts.  My highlights from the meeting ten years ago (the last one I attended) are here.  Highlights from meetings back when I was a fellow are here, herehere, here.

Here are some cool things that I learned at the meeting (some of which I'm sure I should've known), from a few of the talks + posters.  (Unfortunately I cannot stay for the last day, so apologies for missing some great presentations.)   I will further update this post later in the day and tomorrow.

  • By the 2040s, with the oncoming LISA and Cosmic Explorer/Einstein Telescope instruments, it's possible that we will be able to detect every blackhole merger in the entire visible universe.
  • It's very challenging to have models of galaxy evolution that handle how supernovae regulate mass outflow and star formation to end up with what we see statistically in the sky
  • Machine learning can be really good at disentangling overlapping seismic events.
  • In self-propelled/active matter, it's possible to start with particles that just have a hard-shell repulsion and still act like there is an effective attractive interaction that leads to clumping.
  • There are about \(10^{14}\) bacteria in each person, with about 360\(\times\) the genetic material of the person.  Also, the gut has lots of neurons, five times as many as the spinal cord (!).  The gut microbiome can seemingly influence concentrations of neurotransmitters.
  • Bees can deliberately damage leaves of plants to stress the flora and encourage earlier and more prolific flowering.
  • For some bio-produced materials that are nominally dry, their elastic properties and the dependence of those properties on humidity is seemingly controlled almost entirely by the water they contain.  
  • It is now possible to spatially resolve gene expression (via mRNA) at the single cell level across whole slices of, e.g., mouse brain tissue.  Mind-blowing links here and here.
  • I knew that ordinary human red blood cells have no organelles, and therefore they can't really respond much to stimuli.  What I did not know is that maturing red blood cells (erythrocyte precurors) in bone marrow start with nuclei and can participate in immune response, and that red blood cells in fetuses (and then at trace level in pregnant mothers) circulate all the different progenitor cells, potentially playing an important role in immune response.
  • 45% of all deaths in the US can be attributed in part to fibrosis (scarring) issues (including cardiac problems), but somehow the uterus can massively regenerate monthly without scarring.  Also, zero common lab animals menstruate, which is a major obstacle for research; transgenic mice can now be made so that there are good animal models for study. 
  • Engineered cellulose materials can be useful for radiative cooling to the sky and can be adapted for many purposes, like water harvesting from the atmosphere with porous fabrics.

Thursday, August 31, 2023

What is the thermal Hall effect?

One thing that physics and mechanical engineering students learn early on is that there are often analogies between charge flow and heat flow, and this is reflected in the mathematical models we use to describe charge and heat transport.  We use Ohm's law, \(\mathbf{j}=\tilde{\sigma}\cdot \mathbf{E}\), which defines an electrical conductivity tensor \(\tilde{\sigma}\) that relates charge current density \(\mathbf{j}\) to electric fields \(\mathbf{E}=-\nabla \phi\), where \(\phi(\mathbf{r})\) is the electric potential.  Similarly, we can use Fourier's law for thermal conduction, \(\mathbf{j}_{Q} = - \tilde{\kappa}\cdot \nabla T\), where \(\mathbf{j}_{Q}\) is a heat current density, \(T(\mathbf{r})\) is the temperature distribution, and \(\tilde{\kappa}\) is the thermal conductivity.  

We know from experience that the electrical conductivity really has to be a tensor, meaning that the current and the electric field don't have to point along each other.  The most famous example of this, the Hall effect, goes back a long way, discovered by Edwin Hall in 1879.  The phenomenon is easy to describe.  Put a conductor in a magnetic field (directed along \(z\)), and drive a (charge) current \(I_{x}\) along it (along \(x\)), as shown, typically by applying a voltage along the \(x\) direction, \(V_{xx}\).  Hall found that there is then a transverse voltage that develops, \(V_{xy}\) that is proportional to the current.  The physical picture for this is something that we teach to first-year undergrads:  The charge carriers in the conductor obey the Lorentz force law and curve in the presence of a magnetic field.  There can't be a net current in the \(y\) direction because of the edges of the sample, so a transverse (\(y\)-directed) electric field has to build up.  

There can also be a thermal Hall effect, when driving heat conduction in one direction (say \(x\)) leads to an additional temperature gradient in a transverse (\(y\)) direction.  The least interesting version of this (the Maggi–Righi–Leduc effect) is in fact a consequence of the regular Hall effect:  the same charge carriers in a conductor can carry thermal energy as well as charge, so thermal energy just gets dragged sideways.   

Surprisingly, insulators can also show a thermal Hall effect.  That's rather unintuitive, since whatever is carrying thermal energy in the insulator is not some charged object obeying the Lorentz force law.  Interestingly, there are several distinct mechanisms that can lead to thermal Hall response.  With phonons carrying the thermal energy, you can have magnetic field affecting the scattering of phonons, and you can also have intrinsic curving of phonon propagation due to Berry phase effects.  In magnetic insulators, thermal energy can also be carried by magnons, and there again you can have Berry phase effects giving you a magnon Hall effect.  There can also be a thermal Hall signal from topological magnon modes that run around the edges of the material.  In special magnetic insulators (Kitaev systems), there are thought to be special Majorana edge modes that can give quantized thermal Hall response, though non-quantized response argues that topological magnon modes are relevant in those systems.  The bottom line:  thermal Hall effects are real and it can be very challenging to distinguish between candidate mechanisms. 

(Note: Blogger now compresses the figures, so click on the image to see a higher res version.)

Wednesday, August 23, 2023

Some interesting recent papers - lots to ponder

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

Thursday, August 17, 2023

Neutrality and experimental detective work

One of the remarkable aspects of condensed matter physics is the idea of emergent quasiparticles, where through the interactions of many underlying degrees of freedom, new excitations emerge that are long-lived and often can propagate around in ways very different than their underlying constituents.  Of course, it’s particularly interesting when the properties of the quasiparticles have quantum numbers or obey statistics that are transformed from their noninteracting counterparts.  For example, in the resonating valence bond model, starting from electrons with charge \(-e\) and spin 1/2, the low energy excitations are neutral spin-1/2 spinons and charge \(e\) holons.  It’s not always obvious in these situations whether the emergent quasiparticles act like fermions (obeying the Pauli principle and stacking up in energy) or bosons (all falling into the lowest energy state as temperature is reduced).  See here for an example.

Suppose there is an electrically insulating system that you think might host neutral fermionic excitations.  How would you be able to check?  One approach would be to look at the low temperature specific heat, which relates how much the temperature of an isolated object changes when a certain amount of disorganized thermal energy is added.  The result for (fermionic) electrons in a metal is well known:  because of the Pauli principle, the specific heat scales linearly with temperature, \(C \sim T\).  (In contrast, for the vibrational part of the specific heat due to bosonic phonons, \(C \sim T^3\) in 3D.).  So, if you have a crystalline(*) insulator that has a low temperature specific heat that is linear in temperature (or, equivalently, when you plot \(C/T\) vs. \(T\) and there is a non-zero intercept at \(T=0\)), then this is good evidence for neutral fermions of some kind.  Such a system should also have a linear-in-\(T\) thermal conductivity, too, and an example of this is reported here. This connects back to a post that I made a month ago.  Neutral fermions (presumably carrying spin) can lead to quantum oscillations in the specific heat (and other measured quantities). 

This kind of detective work, considering which techniques to use and how to analyze the data, is the puzzle-solving heart of experimental condensed matter physics.  There is a palette of measurable quantities - how can you use those to test for complex underlying physics?

(*) It’s worth remembering that amorphous insulators generally have a specific heat that varies like \(T^{1.1}\) or so, because of the unreasonably ubiquitous tunneling two-level systems.  The neutral fermions I’m writing about in this post are itinerant entities in nominally perfect crystals, rather than the localized TLS in disordered solids.