The Scientia Institute at Rice sponsors series of public lectures annually, centered around a theme.  The intent is to get a wide variety of perspectives spanning across the humanities, social sciences, arts, sciences, and engineering, presented in an accessible way.  The youtube channel with recordings of recent talks is here. This past year, the theme was "democracy" in its broadest sense.  I was honored to be invited last year to contribute a talk, which I gave this past Tuesday, following a presentation by my CS colleague Rodrigo Ferreira about whether AI has politics.  Below I've embedded the video, with the start time set where I begin (27:00, so you can rewind to see Rodrigo).   Which (macroscopic) states of matter to we see?  The ones that "win the popular vote" of the microscopic configurations.

(A post summarizing recent US science-related events will be coming later.  For now, here is my promised post about multiferroics, inspired in part by a recent visit to Rice by Yoshi Tokura.) Electrons carry spins and therefore magnetic moments (that is, they can act in some ways like little bar magnets), and as I was teaching undergrads this past week, under certain conditions some of the electrons in a material can spontaneously develop long-range magnetic order.  That is, rather than being, on average, randomly oriented, instead below some critical temperature the spins take on a pattern that repeats throughout the material.  In the ordered state, if you know the arrangement of spins in one (magnetic) unit cell of the material, that pattern is repeated over many (perhaps all, if the system is a single domain) the unit cells.  In picking out this pattern, the overall symmetry of the material is lowered compared to the non-ordered state.  (There can be local moment magnets, when the electrons with the magnetic moments are localized to particular atoms; there can also be itinerant magnets, when the mobile electrons in a metal take on a net spin polarization.)  The most famous kind of magnetic order is ferromagnetism, when the magnetic moments spontaneously align along a particular direction, often leading to magnetic fields projected out of the material.    Magnetic materials can be metals, semiconductors, or insulators. In insulators, an additional kind of order is possible, based on electric polarization, \(\mathbf{P}\).  There is subtlety about defining polarization, but for the purposes of this discussion, the question is whether the atoms within each unit cell bond appropriately and are displaced below some critical temperature to create a net electric dipole moment, leading to ferroelectricity.  (Antiferroelectricity is also possible.) Again, the ordered state has lower symmetry than the non-ordered state.  Ferroelectric materials have some interesting applications.   BiFeO3, a multiferroic antiferromagnet, image from here. Multiferroics are materials that have simultaneous magnetic order and electric polarization order.  A good recent review is here.  For applications, obviously it would be convenient if both the magnetic and polarization ordering happened well above room temperature.  There can be deep connections between the magnetic order and the electric polarization - see this paper, and this commentary.   Because of these connections, the low energy excitations of multiferroics can be really complicated, like electromagnons.  Similarly, there can be combined "spin textures" and polarization textures in such materials - see here and here.   Multiferroics raise the possibility of using applied voltages (and hence electric fields) to flip \(\mathbf{P}\), and thus toggle around \(\mathbf{M}\).  This has been proposed as a key enabling capability for information processing devices, as in this approach.  These materials are extremely rich, and it feels like their full potential has not yet been realized.

Here are a couple of neat papers that I came across in the last week.  (Planning to write something about multiferroics as well, once I have a bit of time.) The idea of directly extracting useful energy from the rotation of the earth sounds like something out of an H. G. Wells novel.  At a rough estimate (and it's impressive to me that AI tools are now able to provide a convincing step-by-step calculation of this; I tried w/ gemini.google.com) the rotational kinetic energy of the earth is about \(2.6 \times 10^{29}\) J.  The tricky bit is, how do you get at it?  You might imagine constructing some kind of big space-based pick-up coil and getting some inductive voltage generation as the earth rotates its magnetic field past the coil.  Intuitively, though, it seems like while sitting on the (rotating) earth, you should in some sense be comoving with respect to the local magnetic field, so it shouldn't be possible to do anything clever that way.  It turns out, though, that Lorentz forces still apply when moving a wire through the axially symmetric parts of the earth's field.  This has some conceptual contact with Faraday's dc electric generator.   With the right choice of geometry and materials, it is possible to use such an approach to extract some (tiny at the moment) power.  For the theory proposal, see here.  For an experimental demonstration, using thermoelectric effects as a way to measure this (and confirm that the orientation of the cylindrical shell has the expected effect), see here.  I need to read this more closely to decide if I really understand the nuances of how it works. On a completely different note, this paper came out on Friday.  (Full disclosure:  The PI is my former postdoc and the second author was one of my students.)  It's an impressive technical achievement.  We are used to the fact that usually macroscopic objects don't show signatures of quantum interference.  Inelastic interactions of the object with its environment effectively suppress quantum interference effects on some time scale (and therefore some distance scale).  Small molecules are expected to still show electronic quantum effects at room temperature, since they are tiny and their electronic levels are widely spaced, and here is a review of what this could do in electronic measurements.  Quantum interference effects should also be possible in molecular vibrations at room temperature, and they could manifest themselves through the vibrational thermal conduction through single molecules, as considered theoretically here.  This experimental paper does a bridge measurement to compare the thermal transport between a single-molecule-containing junction between a tip and a surface, and an empty (farther spaced) twin tip-surface geometry.  They argue that they see differences between two kinds of molecules that originate from such quantum interference effects. As for more global issues about the US research climate, there will be more announcements soon about reductions in force and the forthcoming presidential budget request.  (Here is an online petition regarding the plan to shutter the NIST atomic spectroscopy group.)  Please pay attention to these issues, and if you're a US citizen, I urge you to contact your legislators and make your voice heard.

I saw a couple of interesting talks this morning before heading out: Alessandro Chiesa of Parma spoke about using spin-containing molecules potentially as qubits, and about chiral-induced spin selectivity (CISS) in electron transfer.  Regarding the former, here is a review.  Spin-containing molecules can have interesting properties as single qubits, or, for spins higher than 1/2, qudits, with unpaired electrons often confined to a transition metal or rare earth ion somewhat protected from the rest of the universe by the rest of the molecule.  The result can be very long coherence times for their spins.  Doing multi-qubit operations is very challenging with such building blocks, however.  There are some theory proposals and attempts to couple molecular qubits to superconducting resonators, but it's tough!   Regarding chiral induced spin selectivity, he discused recent work trying to use molecules where a donor region is linked to an acceptor region via a chiral bridge, and trying to manipulate spin centers this way.  A question in all the CISS work is, how can the effects be large when spin-orbit coupling is generally very weak in light, organic molecules?  He has a recent treatment of this, arguing that if one models the bridge as a chain of sites with large \(U/t\), where \(U\) is the on-site repulsion energy and \(t\) is the hopping contribution, then exchange processes between sites can effectively amplify the otherwise weak spin-orbit effects.  I need to read and think more about this. Richard Schlitz of Konstanz gave a nice talk about some pretty recent research using a scanning tunneling microscope tip (with magnetic iron atoms on the end) to drive electron paramagnetic resonance in a single pentacene molecule (sitting on MgO on Ag, where it tends to grab an electron from the silver and host a spin).  The experimental approach was initially explained here.  The actual polarized tunneling current can drive the resonance, and exactly how depends on the bias conditions.  At high bias, when there is strong resonant tunneling, the current exerts a damping-like torque, while at low bias, when tunneling is far off resonance, the current exerts a field-like torque.  Neat stuff. Leah Weiss from Chicago gave a clear presentation about not-yet-published results (based on earlier work), doing optically detected EPR of Er-containing molecules.  These condense into mm-sized molecular crystals, with the molecular environment being nice and clean, leading to very little inhomogeneous broadening of the lines.  There are spin-selective transitions that can be driven using near telecom-wavelength (1.55 \(\mu m\)) light.  When the (anisotropic) \(g\)-factors of the different levels are different, there are some very promising ways to do orientation-selective and spin-selective spectroscopy.  Looking forward to seeing the paper on this. And that's it for me for the meeting.  A couple of thoughts: I'm not sold on the combined March/April meeting.  Six years ago when I was a DCMP member-at-large, the discussion was all about how the March Meeting was too big, making it hard to find and get good deals on host sites, and maybe the meeting should split.  Now they've made it even bigger.  Doesn't this make planning more difficult and hosting more expensive since there are fewer options?  (I'm not an economist, but....)  A benefit for the April meeting attendees is that grad students and postdocs get access to the career/networking events held at the MM.  If you're going to do the combination, then it seems like you should have the courage of your convictions and really mingle the two, rather than keeping the March talks in the convention center and the April talks in site hotels. I understand that van der Waals/twisted materials are great laboratories for physics, and that topological states in these are exciting.  Still, by my count there were 7 invited sessions broadly about this topic, and 35 invited talks on this over four days seems a bit extreme.   By my count, there were eight dilution refrigerator vendors at the exhibition (Maybell, Bluefors, Ice, Oxford, Danaher/Leiden, Formfactor, Zero-Point Cryo, and Quantum Design if you count their PPMS insert).  Wow.   I'm sure there will be other cool results presented today and tomorrow that I am missing - feel free to mention them in the comments.