Asymmetric quantum Hall effect and diminished $ν=0$ longitudinal resistance in graphene/InSe heterostructures

Abstract

We investigate quantum transport in graphene/InSe heterostructures and find major asymmetries in the longitudinal resistance ($R_{xx}$) and vanishing $R_{xx}$ peaks at high magnetic fields, particularly at the charge-neutrality point. Our Landauer-Buttiker analysis and numerical simulations show that a monotonically varying density gradient combined with a full equilibration mechanism can explain these phenomena. Our results also suggest the presence of trivial long-range chiral edge current and offer a broadly applicable way to engineer transport properties in quantum Hall systems.

Figures (4)

• Figure 1: Graphene/InSe heterostructure Device D1. (a) Optical micrograph and measurement setup. The red dashed line outlines the graphene. (b) Hall resistance $R_{yx1}$ as a function of back-gate voltage $V_g$ measured at different temperatures. (c) Gate dependence of the calculated carrier density $n_{\mathrm{2D}}$ (red) and the corresponding $R_{yx1}$ at $B = 2\,\mathrm{T}$ (blue). (d) Trivial edge states and Landau level bending. Schematic showing the bending of the zeroth Landau level (zLL) at a trivial edge, allowing the Fermi energy ($E_F$) to create conductive edge channels while the bulk is localized.
• Figure 2: Magnetotransport data from device D1. (a) Longitudinal resistances ($R_{xx1}$, $R_{xx2}$) at $B = +9\,\mathrm{T}$. (b) Corresponding Hall resistances ($R_{yx1}$, $R_{yx2}$) at $B = +9\,\mathrm{T}$. (c) Longitudinal resistances ($R_{xx1}$, $R_{xx2}$) at $B = -9\,\mathrm{T}$. (d) Corresponding Hall resistances ($R_{yx1}$, $R_{yx2}$) at $B = -9\,\mathrm{T}$. (e, f) Landau fan diagrams mapping (e) $R_{xx1}$ and (f) $R_{yx1}$ as functions of back-gate voltage $V_g$ and magnetic field $B$.
• Figure 3: Landauer-Buttiker analysis of two regions with mismatched densities. (a) Global $\nu = -6$ state with insulating bulk and chiral edge states. (b) Mixed $\nu = -6$ and $\nu = -2$ phase with an additional interior edge channel. (c) Global $\nu = -2$ state. (d) Fully equilibrated counter-propagating edge channels at the interface between $\nu = -2$ and $\nu = 2$ regions. (e)Non-equilibrated channels between $\nu = -2$ and $\nu = 2$ regions preserve original potentials, yielding longitudinal voltage equal to source-drain voltage. (f) Global $\nu = 2$ state. (g) Mixed $\nu = 2$ and $\nu = 6$ phase. (h) Global $\nu = 6$ state.
• Figure 4: Tight-binding simulation of quantum Hall transport. (a) Chemical potential profile with Gaussian random disorder ($W=0.03$, $\xi=150a$). (b) Chemical potential under linear gradient (left potential $V_\text{left}=-0.05$, right potential $V_\text{right}=0.05$) with added disorder ($W=0.02$, $\xi=2a$). (c) Resistances for random disorder at positive (top panels) and negative (bottom panels) fields. (d) Resistances for gradient with disorder at positive (top panels) and negative (bottom panels) fields. Gold segments indicate electrodes in (a,b).