[Paper Review] Phonon scattering dominated electron transport in twisted bilayer graphene

Hryhoriy Polshyn, Matthew Yankowitz+6 more|arXiv (Cornell University)|Feb 2, 2019

Graphene research and applications32 references91 citations

TL;DR

The paper shows that electron-phonon scattering dominates transport in twisted bilayer graphene across a wide temperature range, with a pronounced T-linear resistivity whose slope tracks the flat-band twist angle and is explained by acoustic phonon scattering and reduced Fermi velocity.

ABSTRACT

Twisted bilayer graphene (tBLG) has recently emerged as a platform for hosting correlated phenomena, owing to the exceptionally flat band dispersion that results near interlayer twist angle $θ\approx1.1^\circ$. At low temperature a variety of phases are observed that appear to be driven by electron interactions including insulating states, superconductivity, and magnetism. Electrical transport in the high temperature regime has received less attention but is also highly anomalous, exhibiting gigantic resistance enhancement and non-monotonic temperature dependence. Here we report on the evolution of the scattering mechanisms in tBLG over a wide range of temperature and for twist angle varying from 0.75$^\circ$ - 2$^\circ$. We find that the resistivity, $ρ$, exhibits three distinct phenomenological regimes as a function of temperature, $T$. At low $T$ the response is dominated by correlation and disorder physics; at high $T$ by thermal activation to higher moiré subbands; and at intermediate temperatures $ρ$ varies linearly with $T$. The $T$-linear response is much larger than in monolayer graphenefor all measured twist angles, and increases by more than three orders of magnitude for $θ$ near the flat-band condition. Our results point to the dominant role of electron-phonon scattering in twisted layer systems, with possible implications for the origin of the observed superconductivity.

Motivation & Objective

• Investigate how transport in twisted bilayer graphene (tBLG) evolves with temperature and twist angle (0.75°–2.0°).
• Identify the dominant scattering mechanisms across temperature regimes and their relation to the band structure of tBLG.
• Assess whether acoustic phonon scattering can account for the large T-linear resistivity observed near the flat-band condition.
• Explore the connection between high-temperature activation to higher moiré subbands and transport properties.

Proposed method

• Measure four-terminal resistivity ρ(T) in tBLG devices with twist angles from 0.75° to 2.02° over broad temperatures.
• Analyze high-T regime with activation to higher moiré subbands and extract activation gaps Δ from Arrhenius fits.
• Model the T-linear resistivity using acoustic phonon scattering with ρ = (π F D_A^2)/(g e^2 ħ ρ_m v_F^2 v_ph^2) k_B T and determine v_F(θ) from quantum oscillations.
• Determine v_F(θ) from temperature dependence of Shubnikov–de Haas oscillations and relate to theoretical linear dependence on θ.
• Compare dρ/dT across angles and densities to assess the role of band structure (flat-band proximity) on phonon coupling.
• Use activation and band-gap considerations to interpret the high-T crossover temperature T_H (and its relation to bandwidth and subband gaps).

Experimental results

Research questions

• RQ1 What are the dominant scattering mechanisms governing ρ(T) in tBLG across different temperatures and twist angles?
• RQ2 How does the T-linear resistivity slope dρ/dT depend on twist angle and carrier density, and can acoustic phonon scattering account for it quantitatively?
• RQ3 What is the role of thermal activation to higher moiré subbands in high-temperature transport?
• RQ4 How does the Fermi velocity v_F(θ) influence transport in the flat-band regime, and can experimental v_F(θ) explain observed trends?
• RQ5 Do observed high-T transport features support a phonon-mediated mechanism for superconductivity in tBLG?

Key findings

• Resistivity in tBLG shows three distinct temperature regimes: low-T correlation/disorder dominated, intermediate-T linear-in-T, and high-T activation-dominated behavior.
• The T-linear regime is ubiquitous across twist angles (0.75°–2.02°) and carrier densities within the lowest moiré subband, with dρ/dT substantially larger than in monolayer graphene and peaking near the flat-band angle (~1.1°).
• The high-T peak temperature TH tracks where thermal activation to dispersive higher moiré subbands becomes significant, correlating with bandwidth and gaps isolating higher subbands.
• The observed T-linear slope is well described by acoustic phonon scattering with Eq. ρ ∝ D_A^2/(v_F^2 v_ph^2), and quantitative agreement improves if D_A/v_ph is enhanced relative to monolayer graphene, consistent with reduced v_F near the magic angle.
• Fermi velocity v_F(θ) grows roughly linearly with θ away from the flat-band angle, and experimental extraction supports v_F(θ) ≈ (0.37±0.12)×(θ−1.05°)×10^6 m/s.
• Activation gaps to higher moiré subbands are typically 30–90 meV, with smallest gaps near the magic angle, facilitating high-T transport features.
• The estimated transport electron-phonon coupling λ_tr, inferred from dρ/dT, can be of order unity under reasonable parameter choices, suggesting phonons may contribute to superconductivity in tBLG.