Two thin layers of a semiconductor were twisted relative to each other by 2.29 degrees, like two sheets of transparent paper with printed grids. Where the lines nearly align, a large pattern of dark and bright spots emerges. Electrons in this pattern behave very differently from those in a regular crystal.
At half filling of this second "superlattice" (a regular structure with a large period), electrons typically get stuck: each one fears occupying a spot where a neighbor already sits. This is a Mott insulator — a substance that should be a conductor but becomes a dielectric due to mutual repulsion of particles. However, the authors observed that along the sample edges, current flows freely, and the resistance remains nearly constant — about 12.9 kilohms. This value matches what single electrons produce in the quantum spin Hall effect, where two channels with opposite spins run along the edge.
Electrons in these channels move strictly in one direction, like cars on a roundabout with no oncoming traffic. If you try to measure the current not directly but slightly to the side, a signal still appears: this indicates that charge carriers go around obstacles along the edges without entering the interior. A magnetic field directed along the plane of the layers enhanced conductivity — another piece of evidence that spin-polarized edge states carry the current.
The temperature dependence of the Hall resistance indicated a characteristic energy of about 1 millielectronvolt (roughly 10 kelvins). This is a hundred times smaller than the thermal energy at room temperature. Thus, such devices require deep cooling, but the very idea, combining strong correlations with topological protection, could lead to conducting channels that fear neither disorder nor magnetic fields.
Twisting two layers by a "magic" angle changes the material's properties more than altering its chemical composition. At 2.29 degrees, electrons move so slowly that their mutual repulsion becomes the dominant force.