Transport measurements in twisted bilayer graphene: Electron-phonon coupling and Landau level crossing

Ting-Fung Chung, Yang Xu, and Yong P. Chen
Phys. Rev. B 98, 035425 – Published 20 July 2018
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Abstract

We investigate electronic transport in twisted bilayer graphene (tBLG) under variable temperatures (T), carrier densities (n), and transverse magnetic fields, focusing on samples with small twist angles (θ). These samples show prominent signatures associated with the van Hove singularities (VHSs) and superlattice-induced minigaps (SMGs). Temperature-dependent field-effect measurement shows that the difference between temperature-dependent resistivity and residual resistivity, ρxx(T,n)ρ0(n), follows Tβ for n between the main Dirac point (DP) and SMG. The evolution of the temperature exponent β with n exhibits a W-shaped dependence, with minima of β0.9 near the VHSs and maxima of β1.7 toward the SMGs. This W-shaped behavior can be qualitatively understood with a theoretical picture that considers both the Fermi surface smearing near the VHSs and flexural-acoustic phonon scattering. In the quantum Hall regime, we observe only Landau level crossings in the massless Dirac spectrum originating from the main DP but not in the parabolic band near the SMG. Such crossings enable the measurement of an enhanced interlayer dielectric constant, attributed to a reduced Fermi velocity. Moreover, we measure the Fermi velocity, interlayer coupling strength, VHS energy relative to the DP, and gap size of SMG, four important parameters used to describe the peculiar band structure of the small-θ tBLG.

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  • Received 2 May 2018
  • Revised 13 June 2018

DOI:https://doi.org/10.1103/PhysRevB.98.035425

©2018 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Ting-Fung Chung1,*, Yang Xu1, and Yong P. Chen1,2,3,†

  • 1Department of Physics and Astronomy and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA
  • 2School of Electrical and Computer Engineering and Purdue Quantum Center, Purdue University, West Lafayette, Indiana 47907, USA
  • 3WPI-AIMR International Research Center on Materials Science, Tohoku University, Sendai 980-8577, Japan

  • *Corresponding author: tfchung@lbl.gov
  • Corresponding author: yongchen@purdue.edu

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Issue

Vol. 98, Iss. 3 — 15 July 2018

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