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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.
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.