Researchers for Functional Intelligent Materials (I-FIM) at the National University of Singapore have, for the first time, separated the effects of electrons and atomic lattice vibrations on electrical resistance in twisted bilayer graphene. Using 0.14-terahertz radiation to selectively heat electrons while keeping the surrounding lattice nearly unchanged, the team observed resistance increases of several kilo-ohms in devices twisted near the “magic angle.” “Conventional transport measurements heat the electrons and the lattice together, so their fingerprints are superimposed,” said Assistant Professor Denis Bandurin, a Principal Investigator at I-FIM, who led the study. Published August 13, 2026, in Nature Communications, this technique reveals a strong electronic contribution to resistance even when phonon scattering is typically dominant.
Terahertz Radiation Isolates Electronic Resistance in Twisted Graphene
Researchers achieved this isolation by selectively heating electrons while maintaining a relatively unchanged lattice temperature, a feat previously unattainable with conventional transport measurements. This technique allowed for a focused examination of electron behavior within the quantum material, revealing insights into the origins of resistance at the “magic angle” where intriguing properties emerge. 0.14-terahertz radiation, delivering only 0.6 millielectronvolts of energy per photon, is insufficient to directly shift electrons between energy bands, but provides ample energy to stir existing charge carriers.
These carriers rapidly redistributed energy amongst themselves, establishing a hot electronic state before significant heat transfer to the lattice could occur. This carefully controlled energy input allowed the researchers to probe the electronic system in isolation.
The ability to distinguish electronic and vibrational contributions to resistance is important for understanding the complex behavior of twisted bilayer graphene. “We wanted to separate those two temperatures and ask what the electrons themselves were doing,” Bandurin said.
The findings suggest that, at low carrier densities, standard mechanisms for resistance, like umklapp scattering or inter-band collisions, are insufficient to explain the observed behavior. The unique band structure of twisted graphene, deviating from the typical parabolic energy band relationship, appears to be important in this process.
“The microscopic picture is not complete,” the researchers note, “The method helps us rule out some explanations and shows us where the remaining question lies.” Further investigation, extending measurements to larger twist angles is planned to refine this understanding and explore the limits of this technique in other moiré and low-density quantum materials. Following how the electron-electron and electron-phonon contributions change enables us to work towards explaining the crossover between quadratic and linear-in-temperature resistance.
Conventional transport measurements heat the electrons and the lattice together, so their fingerprints are superimposed.
Assistant Professor Denis Bandurin, a Principal Investigator at I-FIM, who led the study
I-FIM Separates Electron and Lattice Temperatures in Bilayer Graphene
This selective heating maintained a near-constant lattice temperature while significantly impacting the electronic system, a feat previously unattainable with standard measurement techniques. “When the lattice stays cold and the resistance still rises sharply, the response is tied primarily to the hotter electronic system,” explained Assistant Professor Denis Bandurin, a Principal Investigator at I-FIM who led the Nature Communications study. The team’s approach provides a new way to probe the complex interplay between electrons and the lattice, moving beyond inferences based solely on temperature-resistance curves.
In twisted bilayer graphene, the same temperature dependence can have more than one plausible microscopic origin.
Artur Shilov, a PhD student at I-FIM and first author of the paper
Magic Angle Graphene Displays T-Squared Resistance at Low Carrier Density
Resistance in twisted bilayer graphene exhibits a unique temperature dependence, scaling with the square of temperature (T-squared) even at low carrier densities of 100 billion charge carriers per square centimetre, a concentration previously thought insufficient to support such behavior. The team’s approach isolated the electronic contribution to resistance, revealing a phenomenon not attributable to lattice vibrations. Devices twisted near the magic angle demonstrated a substantial increase in resistance, confirming the dominance of electronic processes.
A control sample of single-layer graphene showed minimal photoresistance under similar heating, validating the method’s ability to differentiate between electronic and phonon-driven resistance. The team acknowledges that a complete microscopic understanding remains elusive, but this work narrows the possibilities and offers a new pathway for exploring electron interactions in quantum materials.
We are proposing a mechanism that is compatible with both the band structure and the size of the observed effect.
Dmitrii Maslov, Professor from the University of Florida
Dirac Bands and Intervalley Scattering Explain Current Limits in Twisted Graphene
The unusual resistance observed in twisted bilayer graphene stems not from lattice vibrations, but from interactions between electrons in distinct energy valleys within the material, according to new analysis. Researchers demonstrated that even with minimal heat transfer to the graphene lattice, substantial resistance remained, indicating a predominantly electronic origin. This finding challenges conventional models where lattice scattering typically dominates resistance at higher temperatures and carrier densities.
Calculations suggest that collisions between electrons residing in these distorted valleys within the band structure can alter their velocities, effectively impeding current flow while conserving overall momentum. This mechanism circumvents the need for umklapp scattering, a process reliant on lattice momentum absorption, and explains resistance even when phonon contributions are minimized. The predicted coefficient for this T-squared resistance, a measure of how resistance scales with temperature, falls between 0.005 and 0.5 ohms per kelvin squared, depending on how strongly other charges screen the interaction.
This work unifies observations of both linear and quadratic temperature dependencies in twisted graphene, suggesting that electronic interactions consistently limit current regardless of the twist angle. The team employed 0.14-terahertz radiation delivering 0.6 millielectronvolts of energy to selectively excite electrons, maintaining a nearly unchanged lattice temperature and allowing for isolation of electronic contributions to resistance.
“The microscopic picture is not complete,” but this approach helps narrow the possibilities and identify the remaining questions, according to the researchers. The technique developed could extend to other moiré systems and low-density quantum materials, providing a pathway to better understand and model charge transport in correlated materials.
A next step is to extend these measurements to larger twist angles, where the graphene layers increasingly approach the decoupled limit.
Denis Bandurin, Asst Prof
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