Workshop highlights: Spins, 1D topo materials from carbon, and more

 While virtual meetings can be draining (no breaks to go hiking; no grabbing a beer and catching up, especially when attendees are spread out across a 7 timezones), this workshop was a great way for me to catch up on some science that I'd been missing.  I can't write up everything (mea culpa), but here are a few experimental highlights:

• Richard Berndt's group has again shown that shot noise integrated with STM is powerful, and they have used tunneling noise measurements to probe where and how spin-polarized transport happens through single radical-containing molecules on gold surfaces.
• Katharina Franke's group has looked at what happens when you have a localized spin on the surface of a superconductor.  Exchange coupling can rip apart Cooper pairs and bind a quasiparticle in what are called Yu-Shiba-Rusinov states.  With STM, it is possible to map these and related phenomena spatially, and the states can also be tuned via tip height, leading to very pretty data.

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  Amazing polymers from here.

  Pavel Jelinek gave a talk with some really eye-popping images as well as cool science.  I had not realized before that in 1D conjugated systems (think polyacetylene) it is possible to see a topological transition as a function of length, between a conjugated state (with valence-band-like orbitals filled, and conduction-band-like orbitals empty) and another conjugated state that has an unpaired electron localized at each end (equivalent to surface states) with effectively band inversion (empty valence-band-like states above filled conduction-band-like states) in the middle.  You can actually make polymers (shown here) that show these properties and image the end states via STM.  
  
• Latha Venkataraman spoke about intellectually related work.  Ordinarily, even with a conjugated oligomer, conductance falls exponentially with increasing molecular length.   However, under the right circumstances, you can get the equivalent topological transition, creating resonant states localized at the molecular ends, and over some range of lengths, you can get electronic conduction increasing with increasing molecular length.  As the molecule gets longer the resonances become better defined and stronger, though at even larger lengths the two end states decouple from each other and conductance falls again.
• Jascha Repp did a really nice job laying out their technique that is AFM with single-charge-tunneling to give STM-like information for molecules on insulating substrates.  Voltage pulses are applied in sync with the oscillating tip moving into close proximity with the molecule, such that single charges can be added or removed each cycle.  This is detected through shifts in the mechanical resonance of the AFM cantilever due to the electrostatic interactions between the tip and the molecule.  This enables time-resolved measurements as well, to look at things like excited state lifetimes in individual molecules.

The meeting is wrapping up today, and the discussions have been a lot of fun.  Hopefully we will get together in person soon!