March Meeting 2025, Day 3

Another busy day at the APS Global Physics Summit.  Here are a few highlights:

• Shahal Ilani of the Weizmann gave an absolutely fantastic talk about his group's latest results from their quantum twisting microscope.  In a scanning tunneling microscope, because tunneling happens at an atomic-scale location between the tip and the sample, the momentum in the transverse direction is not conserved - that is, the tunneling averages over a huge range of $\mathbf{k}$ vectors for the tunneling electron.  In the quantum twisting microscope, electrons tunnel from a flat (graphite) patch something like $d \sim$ 100 nm across, coherently, through a couple of layers of some insulator (like WSe2) and into a van der Waals sample.  In this case, $\mathbf{k}$ in the plane is comparatively conserved, and by rotating the sample relative to the tip, it is possible to build up a picture of the sample's electronic energy vs. $\mathbf{k}$ dispersion, rather like in angle-resolved photoemission.  This has allowed, e.g., mapping of phonons via inelastic tunneling.  His group has applied this to magic angle twisted bilayer graphene, a system that has a peculiar combination of properties, where in some ways the electrons act like very local objects, and in other ways they act like delocalized objects.  The answer seems to be that this system at the magic angle is a bit of an analog of a heavy fermion system, where there are sort of local moments (living in very flat bands) interacting and hybridizing with "conduction" electrons (bands crossing the Fermi level at the Brillouin zone center).  The experimental data (movies of the bands as a function of energy and $\mathbf{k}$ in the plane as the filling is tuned via gate) are gorgeous and look very much like theoretical models.
• I saw a talk by Roger Melko about applying large language models to try to get efficient knowledge of many-body quantum states, or at least the possible outputs of evolution of a quantum system like a quantum computer based on Rydberg atoms.  It started fairly pedagogically, but I confess that I got lost in the AI/ML jargon about halfway through.
• Francis M. Ross, recipient of this year's Keithley Award, gave a great talk about using transmission electron microscopy to watch the growth of materials in real time.  She had some fantastic videos - here is a review article about some of the techniques used.  She also showed some very new work using a focused electron beam to make arrays of point defects in 2D materials that looks very promising.
• Steve Kivelson, recipient of this year's Buckley Prize, presented a very nice talk about his personal views on the theory of high temperature superconductivity in the cuprates.  One basic point:  these materials are balancing between multiple different kinds of emergent order (spin density waves, charge density waves, electronic nematics, perhaps pair density waves).   This magnifies the effects of quenched disorder, which can locally tip the balance one way or another.  Recent investigations of the famous 2D square lattice Hubbard model show this as well.  He argues that the ground state of the Hubbard model for a broad range $1/2 < U/t < 8$, where $U$ is the on-site repulsion and $t$ is the hopping term, the ground state is in fact a charge density wave, not a superconductor.  However, if there is some amount of disorder in the form of $\delta t/t \sim 0.1-0.2$, the result is a robust, unavoidable superconducting state.  He further argues that increasing the superconducting transition temperature requires striking a balance between the underdoped case (strong pairing, weak superfluid phase stiffness) and the overdoped case (weak pairing, strong superfluid stiffness), and that one way to achieve this would be in a bilayer with broken mirror symmetry (say different charge reservoir layers above and below, and/or a big displacement field perpendicular to the plane).  (Apologies for how technical that sounded - hard to reduce that one to something super accessible without writing much more.)

A bit more tomorrow before I depart back to Houston.