Hot electrons reveal electronic collisions may raise resistance in twisted graphene

When a material heats up, its electrical resistance often rises. The underlying cause for this increase is twofold. Electrons carrying current may be scattered by vibrations in the material's atomic lattice, called phonons. Alternatively, electrons may collide with each other. A standard method of measuring resistance involves warming the electrons and lattice together, which tangles the two effects and makes it difficult to discern their individual impacts.

A team of researchers from the Institute for Functional Intelligent Materials (I-FIM) at the National University of Singapore (NUS) has now untangled these effects in twisted bilayer graphene. This material is formed by stacking two sheets of graphene at a slight angle. In a study published in Nature Communications on August 13, 2026, the researchers used terahertz radiation to heat the electrons while leaving the surrounding lattice largely untouched.

The resistance of twisted bilayer graphene increases significantly near the so-called magic angle, a twist of approximately 1.1 degrees. At this angle, some electronic energy bands become unusually flat, causing electrons to move more slowly and interact more strongly with one another. This behavior allows the material to exhibit exotic states, such as correlated insulating states and superconductivity. However, it also complicates the measurement of resistance as a function of temperature.

The researchers built tiny bar-shaped devices from twisted graphene and encapsulated them in hexagonal boron nitride, an electrically insulating material that protects the carbon layers. Metal antennas then channeled terahertz radiation, specifically 0.14-terahertz radiation, into the graphene. Each terahertz photon carries only 0.6 millielectron volts of energy, insufficient to directly drive electrons between energy bands.

Instead, the radiation perturbs the existing charge carriers, which redistribute that energy among themselves within femtoseconds (billionths of a second) before they can dissipate much heat into the lattice.

At the highest radiation power, the electrons' temperature was estimated to be about 20 Kelvin above the lattice temperature, which is held near 2 Kelvin (-456°F). By conducting dedicated heat-transport tests, the researchers determined that the maximum rise in lattice temperature remained below 1 Kelvin. Separate electrical noise measurements further supported the conclusion that electrons and lattice remained thermally decoupled.

The researchers then compared the devices' resistance under dark conditions and while exposed to terahertz radiation. Across the metallic regions of devices with different twist angles, heating the electrons resulted in a substantial positive photoresistance—an increase in resistance caused by the radiation. This response grew with the terahertz power before leveling off.

Near the magic angle, the photoresistance reached several kilohms. In contrast, a control device made from a single sheet of graphene exhibited almost no photoresistance when its electrons were heated. This difference points to the electron system's primary influence on the resistance, as opposed to phonon interactions.