## Tuesday, September 17, 2019

### DOE Experimental Condensed Matter PI Meeting, Day 2

Among the things I heard about today, as I wondered whether newly formed Tropical Storm Imelda would make my trip home a challenge:

• In "B20" magnetic compounds, where the crystal structure is chiral but lacks mirror or inversion symmetry, a phase can form under some circumstances that is a spontaneous lattice of skyrmions.  By adding disorder through doping, it is possible to un-pin that lattice.
• Amorphous cousins of those materials still show anomalous Hall effect (AHE), even though the usual interpretation these days of the AHE is as a consequence of Berry phase in momentum space that is deeply connected to having a lattice.  It's neat to see that some Berry physics survives even when the lattice does not.
• There is a lot of interest in coupling surface states of topological insulators to ferromagnets, including using spin-orbit torque to switch the magnetization direction of a ferromagnetic insulator.
• You could also try to switch the magnetization of $\alpha-Fe_{2}O_{3}$ using spin-orbit torques, but watch out when you try to cram too much current through a 2 nm thick Pt film.
• The interlayer magnetic exchange in van der Waals magnets continues to be interesting and rich.
• Heck, you could look at several 2D materials with various kinds of reduced symmetry, to see what kinds of spin-orbit torques are possible.
• It's always fun to find a material where there are oscillations in magnetization with applied field even though the bulk is an insulator.
• Two-terminal devices made using (Weyl superconducting) MoTe2 show clear magnetoresistance signatures, indicating supercurrents carried along the material edges.
• By side-gating graphene structures hooked up to superconductors, you can also make a superconducting quantum intereference device using edge states of the fractional quantum Hall effect.
• In similar spirit, coupling a 2D topological insulator (1T'-WTe2) to a superconductor (NbSe2) means it's possible to use scanning tunneling spectroscopy to see induced superconducting properties in the edge state.
• Just in time, another possible p-wave superconductor.
• In a special stack sandwiching a TI between two magnetic TI layers, it's possible to gate the system to break inversion symmetry, and thus tune between quantum anomalous Hall and "topological Hall" response.
• Via a typo on a slide, I learned of the existence of the Ohion, apparently the smallest quantized amount of Ohio.