2/ My working definition of a "boring material" is a solid material that has been known for a long time, was studied for a while during the previous century, and then ultimately abandoned because it was considered to be sufficiently well-understood or sufficiently boring.
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3/ But it turns out that there are a lot of fascinating things yet to be discovered or understood in boring materials. What follows is a very incomplete laundry list of unsolved mysteries and recently-discovered surprises in boring materials.
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4/ Let's start with the flagship boring materials: the elemental metals. IRON is something we do not understand, in a very basic sense. We have theories of magnetism (how a material can become a magnet), and we have theories of metals (how a material can conduct electricity).
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5/ But we have essentially no understanding of how the same electrons that produce magnetism can carry electric current. (In technical jargon, we have no theory of "itinerant magnetism") There are similar issues in our understanding of iron oxide (RUST, or MAGNETITE).
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6/ CHROMIUM is a shiny metal that we understand well enough, but if you add just a little bit of rhodium it becomes a superconductor, through some mechanism that we don't understand at all but which looks similar to the famous high-Tc superconductors. https://en.wikipedia.org/wiki/High-temperature_superconductivity …
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7/ LIQUID METALS, like gallium or mercury, are very cool and very mysterious. All our traditional concepts of electron and phonon bands have to be thrown out. But there are still simple systematic dependences of the electric resistance on temperature and magnetic field.
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8/ These simple dependencies imply a simple theory that we don't have yet. A question that I am especially tantalized by is: could there be a liquid superconductor??
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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).
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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_.
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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 …
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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.
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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.
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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)
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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??
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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.
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17/ The poster child for the value of reconsidering old semiconductors is the crystal STRONTIUM TITANATE ("STO"). STO is a synthetic gemstone (you can buy it cheap online), which happens to have an enormous dielectric constant.
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18/ This huge dielectric constant arises from a ferroelectric phase transition (a buckling-type instability within the atomic cell) that _almost_ happens, but is aborted by quantum fluctuations of the atom positions.
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19/ This aborted transition was understood in the 1960s, but returning to this material in the 2010s revealed more crazy things. First, STO can be a fairly robustsuperconductor with only a miniscule amount of added electrons. This is well beyond the purview of any theory we know.
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20/ Second, STO can act like a metal even when the apparent mean-free-path of electrons (the distance electrons can travel before scattering off something) is even shorter than the inter-atomic distance. This is a violation of both quantum mechanics and common sense.
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21/ Finally, I'll end with the most studied-to-death material on earth: SILICON. We can do just about everything with silicon at this point, but there is one big thing we don't understand: its transition from insulating to conducting as a function of added ("dopant") atoms.
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22/ Roughly speaking, this "Mott transition" happens when the concentration of dopants becomes high enough that it is easier energetically for electrons to be hybridized (in a quantum sense) between atoms than it is for them to each sit on one atom.
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23/ Near the transition point electrons are somehow like a thick liquid: technically flowing, but poorly, and with their properties somehow arising from an interplay between their repulsion and the quantum hybridization between atom-centered wavefunctions.
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24/ Modern experiments are getting better and better about probing this soupy-liquid at nanometer length scales and femtosecond time scales. These better "movies" might finally allow us to figure out what happens, in a huge class of "strongly correlated electron" situations.
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25/25 Thank you for coming to my TED Talk. Here is a grainy photo of me haranguing other workshop participants about the virtue of boring materials.pic.twitter.com/8fqz7yEQu2
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