Angle-dependent chiral tunneling in biased twisted bilayer graphene

TL;DR

This study explores how perpendicular interlayer bias influences chiral tunneling in twisted bilayer graphene, revealing angle-dependent transmission suppression, symmetry breaking, and modulation of resonant tunneling features.

Contribution

It introduces a dual-gated model to analyze bias effects on electron tunneling in TBLG, highlighting the control of transmission properties via interlayer bias and twist angle.

Findings

Bias suppresses normal-incidence transmission by opening a band gap.

Interlayer bias induces direction-dependent and valley-specific asymmetries.

Bias modulates Fabry--Pérot resonances, shifting and altering transmission peaks.

Abstract

In twisted bilayer graphene (TBLG), chiral tunneling can be tuned by parameters such as the twist angle, barrier height, and Fermi energy. This differs from the tunneling behavior observed in monolayer and Bernal bilayer graphene, where electrons either pass completely through or are fully blocked due to the Klein paradox. Here we investigate the effect of a perpendicular interlayer bias on electron tunneling through electrostatic barriers in TBLG. Using a dual-gated model, which controls the carrier density and interlayer potential difference independently, we compute the transmission and reflection probabilities of electrons at different angles and energies for representative twist angles of $\theta = 1.8^{\circ}$, $3.89^{\circ}$, and $9.43^{\circ}$. We find that a moderate bias suppresses normal-incidence transmission by opening a band gap in the low-energy spectrum. Our results show this leads to near-total reflection at low energy, with transmission starting to increase just above the gap due to twist-dependent conducting channels. The applied bias breaks the system's effective inversion symmetry, resulting in pronounced direction-dependent and valley-specific asymmetries in the angular distribution of transmitted electrons. We show that electrons incident at different angles show notable variations in transmission under bias. Furthermore, interlayer bias modulates Fabry--P\'{e}rot--like resonances in the TBLG barrier, shifting the energies of transmission peaks and altering their intensity.