Hot electrons reveal electronic collisions

A team from the National University of Singapore has directly shown that collisions between electrons can significantly increase electrical resistance in twisted bilayer graphene. Their experiment 2026, used terahertz radiation to selectively heat the electrons while the material's atomic lattice remained near 2 Kelvin.

Conventional measurements heat both the electrons and the lattice together, making it hard to separate their individual effects on resistance. Assistant professor Denis Bandurin, who led the study, explained the goal: "We wanted to separate those two temperatures and ask what the electrons themselves were doing." The team's approach provided a new control knob.

The puzzle of twisted graphene

Twisted bilayer graphene is created by stacking two atom-thin sheets of carbon at a slight angle. Near a twist of about 1.1 degrees, known as the magic angle, the electronic energy bands become very flat. This dramatically slows the electrons and enhances their interactions, leading to exotic states like superconductivity. It also complicates the interpretation of standard resistance-versus-temperature data, which can show trends attributed to either electron collisions or lattice vibrations.

"In twisted bilayer graphene, the same temperature dependence can have more than one plausible microscopic origin," said Artur Shilov, the study's first author. The team needed a method to isolate the electronic contribution.

A selective heating technique

The researchers fabricated small devices from twisted graphene, protected by layers of hexagonal boron nitride. They applied 0.14-terahertz radiation through metal antennas. Each photon carried only 0.6 millielectron volts of energy, enough to stir the existing charge carriers but not enough to excite electrons between major energy bands.

The electrons rapidly shared this energy among themselves, reaching an estimated temperature about 20 Kelvin above the cold lattice before significant heat could transfer. Separate measurements confirmed the lattice temperature rose by less than 1 Kelvin, keeping the two systems thermally decoupled.

The team then measured the change in resistance under illumination. In metallic regions of magic-angle devices, they observed a large positive photoresistance-the resistance increased due to the radiation. The effect grew with terahertz power and leveled off, reaching several kilohms. A control device made from a single layer of graphene showed almost no such photoresistance, confirming the effect is tied to the unique electronic structure of the twisted system.

"When the lattice stays cold and the resistance still rises sharply, the response is tied primarily to the hotter electronic system," Shilov said.

An unconventional source of resistance

A second finding emerged in devices twisted farther from the magic angle. Here, the team observed resistance growing with the square of the temperature (a T² dependence) at charge densities as low as 100 billion carriers per square centimeter. This is puzzling because, in a conventional clean metal, electron-electron collisions that conserve total momentum should not dissipate electrical current.

Twisted graphene's electronic structure is not conventional. Its Dirac-like bands break Galilean invariance, and the moiré pattern creates multiple electronic valleys. The researchers propose that collisions between electrons in different valleys can change their velocities enough to reduce net current, even if total momentum is conserved. Their theoretical calculations predicted a T² resistance coefficient between 0.005 and 0.5 ohms per Kelvin squared. The measured value of about 0.1 ohms per Kelvin squared fell within this range.

The team notes this agreement makes intervalley scattering a plausible mechanism, though it does not rule out other contributions. The work demonstrates a direct pathway for electron interactions to generate resistance in a system where the lattice plays almost no thermal role.