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

Abstract

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.

Figures (3)

• Figure 1: Resistivity in TBG under THz excitation.a, Cartoon schematic of a typical TBG device, showing THz-induced electron heating in the device’s channel. THz radiation funneling into the device is polarized along the channel. b, Optical image of a typical TBG device. Metallic contacts are highlighted in yellow, and the TBG channel exhibiting a THz-induced change in resistivity is highlighted in green. c, Longitudinal resistivity $\rho$ of $2\degree$ TBG as a function of temperature and carrier density $n$. Inset: $\rho(n)$ at representative temperatures. Arrows indicate scaling regimes with $\rho \sim T^\alpha$, where $\alpha = 1$ (orange) and $\alpha = 2$ (green). Thermometers indicate that both electron and lattice temperatures are varied simultaneously. d,$\rho(n)$ measured in the dark and under continuous-wave illumination at 0.14 THz. Inset: schematic showing THz-driven electron heating. Thermometers indicate that THz radiation increases the electron temperature while the lattice remains intact.
• Figure 2: Interaction-driven photoresistance in TBG.a,$\Delta \rho_\mathrm{THz}$ as a function of $n$, plotted for radiation powers $P$ = 0.03 (blue), 0.09, 0.19, 0.37, 0.57, 1 (orange) in units of $P_\mathrm{max}$, for $2\degree$ TBG. Inset: $\Delta \rho_\mathrm{THz}$ close to the CNP for zero and maximum $P$. b, Same as (a) but for MATBG1 device ($\theta = 1.05\degree$). c, Longitudinal resistivity $\rho$ as a function of temperature $T$ for MATBG1 device at filling factor $\nu = 1.7$. The slope $d\rho/dT = 92~\mathrm{\Omega/K}$ is consistent with the Planckian bound in MATBGPhysRevLett.124.076801. d,$\Delta \rho_\mathrm{THz}$ as a function of $P$ for given $n$ measured in MATBG2 device ($\theta = 1.04\degree$). Dashed lines - guides to the eye. e,$\Delta\rho_\mathrm{THz}$, plotted against $\nu$ at $T_\mathrm{L}$ for MATBG2. f,$\Delta \rho_\mathrm{THz}$ in monolayer graphene, normalized to the dark resistivity $\rho$, as a function of magnetic field $B$. Inset: Zoomed-in view highlighting a finite $\Delta \rho_\mathrm{THz}$ response arising from $T_\mathrm{e}$-sensitive Shubnikov–de Haas oscillations (SdHO). Arrow labeled "BM" marks the position of the Bernstein mode resonance.
• Figure 3: Anomalous $T^2$ resistivity in TBG.a, Longitudinal resistivity $\rho$ as a function of filling factor $\nu$ at characteristic temperatures in $1.58\degree$ TBG. Gradient shading highlights a smooth crossover from $T^2$ to linear-$T$ and back to $T^2$ temperature dependence across different $\nu$. b-c, Temperature-dependent change in resistivity, $\Delta\rho = \rho - \rho_0$, shown for representative $\nu$ values near the CNP (b) and superlattice gaps (c). d, Hall carrier density $n_\mathrm{H}$ versus $\nu$ for the same device. Arrows labeled “U” indicate the filling ranges within which intervalley umklapp scattering is allowed. Blue and orange arrows mark the onset of $T^2$ scaling in the hole-side miniband. Purple and green arrows mark the onset of $T^2$ scaling in the electron-side miniband. e, Fermi surfaces at $\nu = 0.1$ (solid circles), where $\rho\sim T^2$ onsets, and $\nu = 0.45$ (dashed circles), where intervalley umklapp is allowed, plotted within the $1.58\degree$ mini Brillouin zone with high-symmetry points $\kappa$, $\kappa'$, and $\gamma$. Blue and orange colours indicate Fermi surfaces of the bands associated with K and K' symmetry points of the original BZ. f, Same as (e), but for fillings in the vicinity of the electron-side superlattice gap. Solid and dashed lines mark the Fermi surfaces at $\nu = 3.8$ and $\nu = 3.45$, respectively.