Topics: Condensed Matter Physics, Materials Science, Modern Physics, Superconductors
It’s one of the most stubborn open questions of modern physics: What’s the mechanism of high-temperature superconductivity? All superconductors need some way of binding their electrons, which are fermions, into quasiparticles called Cooper pairs, which act as bosons. The low-temperature superconductivity in metals is well described by the Bardeen-Cooper-Schrieffer theory, which states that the pairs are held together by phonons. But in 1986, cuprate ceramics were discovered to superconduct at a much higher temperature via a different, unknown mechanism. Despite four decades of research and the discovery of many other unconventional superconducting materials, their mechanism remains a mystery.
So the condensed-matter physics community took note when, in 2018, superconductivity was found in magic-angle graphene: two or more layers of the atomically thin carbon material stacked with a relative twist of 1.1°. Its allure is in its tunability: With a single graphene device, researchers can explore regions of the superconducting phase diagram that otherwise would require the synthesis of several new materials. But despite that advantage, magic-angle graphene has until now resisted a basic measurement: the size of the hole in the density of states called the superconducting gap, a measure of how much energy is needed to break apart a Cooper pair.
It’s not that the density of states couldn’t be measured. That could be done using tunneling spectroscopy, a technique related to scanning tunneling microscopy. The trouble lay in confirming that the gap being measured was really a superconducting gap. Other phases of matter—for example, insulators—also have gaps in their densities of states, and magic-angle graphene hosts a rich array of phases that all lie close to one another in parameter space and thus could be easily confused. (For details, see the 2024 PT feature article “Twisted bilayer graphene’s gallery of phases,” by B. Andrei Bernevig and Dmitri K. Efetov.)
Energy scales of superconducting graphene come into focus, Johanna L. Miller, Physics Today
Comments