March Meeting 2025, Day 4 and wrap-up

 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.