Interaction-limited conductivity of twisted bilayer graphene revealed by giant terahertz photoresistance

Identifying the microscopic processes that limit conductivity is essential for understanding correlated and quantum-critical states in quantum materials. In twisted bilayer graphene (TBG) and other twist-controlled materials, the temperature dependence of metallic resistivity follows power-law scaling, with the exponent spanning a broad range, rendering standard transport measurements insufficient to unambiguously identify the dominant scattering processes and giving rise to competing interpretations ranging from phonon-limited transport and umklapp scattering to strange metallicity and heavy fermion renormalization. Here, we use terahertz (THz) excitation to selectively raise the electron temperature in TBG while keeping the lattice cold, enabling a direct separation of electron-electron and electron-phonon contributions to resistivity. We observe a giant THz photoresistance, reaching up to 70% in magic-angle devices, demonstrating that electronic interactions dominate transport even in regimes previously attributed to phonons, including the linear-in-temperature resistivity near the magic angle. Away from the magic angle, we observe coexisting photoresistance and robust quadratic-in-temperature resistivity at extremely low carrier densities where standard electron-electron scattering mechanisms (umklapp and Baber inter-band scattering) are kinematically forbidden. Our analysis identifies the breakdown of Galilean invariance in the Dirac-type dispersion as a possible origin of the interaction-limited conductivity, arising from inter-valley electron-electron collisions. Beyond twisted bilayer graphene, our approach establishes THz-driven hot-electron transport as a general framework for disentangling scattering mechanisms in low-density quantum materials.

💡 Research Summary

In this work the authors investigate the microscopic origin of resistivity in twisted bilayer graphene (TBG) by exploiting terahertz (THz) radiation to selectively heat the electronic system while keeping the lattice temperature low. Conventional transport measurements, which vary both electron temperature (Te) and lattice temperature (TL) together, cannot unambiguously separate electron‑electron (e‑e) from electron‑phonon (e‑ph) scattering. By illuminating TBG devices with a continuous‑wave 0.14 THz beam (photon energy ≈0.6 meV), the authors raise Te up to ~15 K above a 2 K lattice without inducing inter‑band transitions. The resulting change in resistivity, ΔρTHz = ρTHz − ρdark, is therefore a direct probe of how resistivity depends on Te alone.

Across a range of twist angles—from the magic angle (~1.05°) to larger angles (~2–3°)—and for carrier densities spanning the charge‑neutral point to full miniband filling, several key observations emerge. First, in magic‑angle devices the THz‑induced photo‑resistance is enormous, reaching up to 70 % of the dark resistance. Importantly, this large ΔρTHz persists even in regimes where the equilibrium resistivity scales linearly with temperature (ρ ∝ T), a behavior traditionally ascribed to phonon scattering. The data therefore demonstrate that e‑e interactions dominate the temperature‑dependent resistivity even when the apparent scaling is linear.

Second, away from the magic angle the devices still exhibit a positive ΔρTHz that grows with THz power, while the equilibrium resistivity shows a robust T² dependence characteristic of a Fermi‑liquid. Remarkably, the T² scaling survives down to extremely low carrier densities (filling factor ν ≈ 0.1) where conventional current‑relaxing e‑e mechanisms—Umklapp scattering and Baber inter‑band scattering—are kinematically forbidden. This indicates the presence of an alternative current‑relaxation channel that is active at elevated Te.

The authors propose that the breakdown of Galilean invariance in the Dirac‑type miniband dispersion of TBG provides such a channel. In the moiré superlattice, electrons from opposite valleys (K and K′) can scatter with a total momentum change equal to a reciprocal lattice vector of the reduced Brillouin zone, i.e., “valley‑inter‑Umklapp” processes. These processes do not conserve the total electronic momentum and thus relax the charge current even though they are purely e‑e collisions. Because they depend on the electronic distribution, they are strongly enhanced when Te is raised by THz radiation, producing the observed giant photo‑resistance.

Control experiments on monolayer graphene, where transport is known to be phonon‑limited, show only a negligible ΔρTHz unless a magnetic field is applied (which activates Shubnikov‑de Haas oscillations or Bernstein‑mode resonances). This contrast underscores that the large THz photo‑resistance in TBG is not a generic hot‑carrier effect but is tied to its unique band structure and strong e‑e interactions.

Quantitatively, ΔρTHz scales roughly linearly with the incident THz power and saturates at the maximum power, consistent with an electron temperature rise that can be estimated from the damping of Shubnikov‑de Haas oscillations. The temperature derivative dρ/dT in the magic‑angle linear‑T regime reaches ≈92 Ω K⁻¹, a value that matches the Planckian bound (kB/ħ) often invoked in discussions of strange metals.

Overall, the study establishes THz‑driven hot‑electron transport as a powerful, non‑invasive probe for disentangling scattering mechanisms in low‑density, strongly correlated quantum materials. It provides compelling evidence that interaction‑limited conductivity, mediated by valley‑inter‑Umklapp e‑e collisions, governs the resistivity of TBG across a wide range of twist angles and carrier densities, challenging interpretations that rely solely on phonon scattering or conventional Umklapp processes. The methodology and insights are likely applicable to other twist‑controlled two‑dimensional systems and to the broader quest of identifying the origins of non‑Fermi‑liquid behavior in quantum materials.