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Heating freezes electrons in twisted bilayer graphene

Electrons usually move more freely at higher temperatures. But they have now been observed to ‘freeze’ as the temperature rises, in a system consisting of two stacked, but slightly misaligned, graphene sheets.
By
  1. Biao Lian

    1. Biao Lian is in the Department of Physics, Princeton University, Princeton, New Jersey 08544, USA.

Particles in a substance randomly jiggle about more vigorously at higher temperatures, causing solids to melt into liquids above a critical temperature. In thermodynamics, higher temperatures favour the formation of states that have larger amounts of entropy, a measure of disorder. The liquid state of a substance typically has a larger entropy than has the solid state, because the movement of atoms is more disordered. However, an exception occurs for helium-3, which freezes into a solid as the temperature rises1. This behaviour is known as the Pomeranchuk effect, and occurs because solid 3He has a larger entropy than does the liquid form — a phenomenon associated with the fluctuation of the spin (angular momentum) of 3He atoms. Writing in Nature, Saito et al.2 and Rozen et al.3 now describe a similar effect in a graphene system, in which electrons are found to ‘freeze’ as the temperature increases.

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Nature 592, 191-193 (2021)

doi: https://doi.org/10.1038/d41586-021-00843-0

References

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