Complex dispersion lines in gapped bilayer graphene: Analytical expressions and shear-displacement effects on monolayer--bilayer--monolayer junction conductance

TL;DR

This paper develops an analytical theory for evanescent states in gapped bilayer graphene, incorporating skew interlayer hoppings and shear displacement effects, providing insights into conductance behavior in monolayer-bilayer junctions.

Contribution

It introduces a comprehensive analytical model that includes skew interlayer hoppings and shear displacement effects, extending previous models limited to weak bias and vertical hopping.

Findings

Skew interlayer hoppings, especially γ3, are crucial for electronic properties.

Shear displacement δ_y significantly influences junction conductance.

Analytical results match numerical calculations, validating the model.

Abstract

Analytical treatments of tunneling in bilayer graphene have typically relied on minimal models including only the vertical interlayer hopping $\gamma_1$ and have been restricted to the weak interlayer bias regime $2\varepsilon \ll \gamma_1$. These simplifications limit the ability of analytical theories to describe lattice deformations and strong electric-field effects. In this work, we present an analytical theory of evanescent states in electrically gapped bilayer graphene that overcomes both limitations. Specifically, our approach explicitly incorporates the skew interlayer hoppings $\gamma_3$ and $\gamma_4$ and remains valid even when the interlayer bias $2\varepsilon$ is comparable to $\gamma_1$. Focusing on low-energy electronic states near the charge neutrality point, we analytically derive the complex longitudinal wave numbers, the gap width, and the sublattice pseudospin inside the electric-field-induced gap, and systematically analyze their dependence on the interlayer shear displacement $\vec{\delta}=(\delta_x,\delta_y)$. The analytical expressions quantitatively reproduce exact numerical calculations, demonstrating that skew interlayer hoppings, in particular $\gamma_3$, play an essential role. Taking the zigzag direction as the longitudinal (transport) $x$ direction, the wave vector becomes complex along $x$ while remaining real along the transverse $y$ direction. For a monolayer--bilayer--monolayer junction with transport along this direction, we find that $\delta_y$ has a significantly stronger impact on the conductance than $\delta_x$. This anisotropic response is quantitatively explained by the analytical expressions. Furthermore, we identify a shear-induced phase proportional to $\delta_y$ that appears universally in the analytical expressions for the gap width, the sublattice pseudospin, and the decay length.