Hot Electrons Reveal Electronic Collisions Increase Resistance in Twisted Graphene

Isolating Electron Collisions in Twisted Bilayer Graphene

When materials heat up, electrical resistance typically climbs, but figuring out the exact cause has frustrated physicists for years. The electrons carrying an electrical current can scatter off phonons, which are vibrations of the material’s atomic lattice, or they can collide directly with one another. Conventional temperature tests usually warm both the electrons and the surrounding lattice at the same time, making it difficult to separate their individual contributions to resistance measurements. To solve this problem, researchers at the Institute for Functional Intelligent Materials built specialized bar-shaped devices. According to the study, they encapsulated twisted bilayer graphene sheets in hexagonal boron nitride to protect the carbon layers. Graphite gates controlled the charge carrier density inside the material. Instead of relying on standard thermal heating, the team used metal antennas to channel 0.14-terahertz radiation directly into the graphene layers. Each terahertz photon carried roughly 0.6 millielectronvolts of energy. That energy level was intentionally kept too low to push electrons between different energy bands, but it was enough to stir up the existing charge carriers. Conventional transport measurements heat the electrons and the lattice together, so their fingerprints are superimposed, said Assistant Professor Denis Bandurin, a Principal Investigator at the Institute for Functional Intelligent Materials who led the study. We wanted to separate those two temperatures and ask what the electrons themselves were doing.

Magic Angle Physics and Terahertz Heating

Twisted bilayer graphene is created by stacking two single layers of carbon atoms—arranged in a honeycomb lattice—at a slight rotational angle. When offset, the overlapping structures form a larger repeating moiré superlattice that alters how electrons travel. Near a twist of roughly 1.1 degrees, known as the magic angle, certain electronic energy bands flatten out. This causes electrons to slow down and interact much more strongly with each other, giving rise to exotic states like superconductivity and correlated insulators. This unique environment makes standard resistance measurements hard to interpret. Fermi liquid theory dictates that resistance from electron interactions scales with the square of temperature, while linear temperature dependence often points to phonon scattering or strange metal behavior. Because twisted bilayer graphene exhibits multiple trends depending on the twist angle and charge carrier count, simple temperature curves allow for several competing explanations. In twisted bilayer graphene, the same temperature dependence can have more than one plausible microscopic origin, said Artur Shilov, a PhD student at the Institute for Functional Intelligent Materials and first author of the paper. We needed another control knob.

Measured Resistance Spikes and Lattice Independence

By injecting terahertz radiation, the research team forced the charge carriers to redistribute energy among themselves within femtoseconds. This process created a hot electronic state before the heat could transfer to the atomic lattice. At peak radiation power, the electrons reached temperatures roughly 20 kelvin above the lattice, which remained stable near 2 kelvin. Dedicated heat-transport tests confirmed that the maximum rise in the lattice temperature stayed below 1 kelvin. Despite the lattice remaining cold, the material’s electrical resistance climbed by several kilo-ohms in devices twisted close to the magic angle. This sharp increase proved that electronic collisions alone can drive substantial resistance, highlighting a dominant electronic contribution in regimes previously attributed primarily to phonon activity.