Hopping magnetoresistance in ion irradiated monolayer graphene

TL;DR

This study investigates the magnetoresistance behavior in ion irradiated monolayer graphene, revealing negative MR in perpendicular fields due to interference suppression and positive MR in parallel fields due to spin effects, with results supporting the orbital model.

Contribution

It provides experimental evidence for the orbital model of MR in VRH graphene and introduces a unified scaling approach for different samples and temperatures.

Findings

Negative MR in perpendicular fields decreases resistivity.

Positive MR in parallel fields is due to spin alignment effects.

The characteristic field B* scales as T^{1/2} and helps estimate localization radius.

Abstract

Magnetoresistance (MR) of ion irradiated monolayer graphene samples with variable-range hopping (VRH) mechanism of conductivity was measured at temperatures down to $T = 1.8$ K in magnetic fields up to $B = 8$ T. It was observed that in perpendicular magnetic fields, hopping resistivity $R$ decreases, which corresponds to negative MR (NMR), while parallel magnetic field results in positive MR (PMR) at low temperatures. NMR is explained on the basis of the "orbital" model in which perpendicular magnetic field suppresses the destructive interference of many paths through the intermediate sites in the total probability of the long-distance tunneling in the VRH regime. At low fields, a quadratic dependence ($|\Delta R/R|\sim B^2$) of NMR is observed, while at $B > B^*$, the quadratic dependence is replaced by the linear one. It was found that all NMR curves for different samples and different temperatures could be merged into common dependence when plotted as a function of $B/B^*$. It is shown that $B^*\sim T^{1/2}$ in agreement with predictions of the "orbital" model. The obtained values of $B^*$ allowed also to estimate the localization radius $\xi$ of charge carriers for samples with different degree of disorder. PMR in parallel magnetic fields is explained by suppression of hopping transitions via double occupied states due to alignment of electron spins.