Brian Skinner

9/ Let's move on from metals to semimetals: materials where both electron- and hole-type carriers coexist, and both carry current. A prominent poster-child for the value of reconsidering old semimetals is samarium hexaboride (SmB6).

10/ SmB6 was studied in the 1960s, and there was some puzzle there that was ultimately understood. But going back to this material in the 21st century, we found that the material actually had _topological surface states_.

11/ The "topology" arises out of the strong magnetic interactions between light electrons and heavy electrons. Things got even more interesting in the past few years, as I summarized here:https://twitter.com/gravity_levity/status/1179468901400203264 …

12/ Perhaps the most famous semimetal is GRAPHITE (pencil lead), whose electronic structure was mostly understood in the 1940s. But it turns out that putting graphite in a huge magnetic field changes things dramatically.

13/ Newly-developed magnetic field facilities were used to study graphite in fields up to 90 tesla. In such large fields graphite becomes an _excitonic insulator_: free electrons and holes bind to each other and form composite bosons that can undergo Bose Einstein condensation.

14/ BISMUTH is a semimetal with an ultralow concentration of free charges (one per 10^5 bismuth atoms). It was a big surprise, then to see this become a _superconductor_ at low enough temperature. Actually, bismuth holds the record for the lowest-TC superconductor (~ 0.5 mK)

15/ Maybe even more shocking is amorphous bismuth. This is a pile of bismuth atoms with no discernible order. Somehow this becomes a superconductor at ~7K. i.e., it is a 14,000-fold better superconductor than clean, orderly bismuth. What the heck??

16/ Finally, one can turn to the semiconductors: objects which do not conduct electricity at low temperature in their native state, due to gaps between electron bands.