Influence of Impurity Spin Dynamics on Quantum Transport in Epitaxial Graphene

Experimental evidence from both spin-valve and quantum transport measurements points towards unexpectedly fast spin relaxation in graphene. We report magnetotransport studies of epitaxial graphene on SiC in a vector magnetic field showing that spin relaxation, detected using weak-localization analysis, is suppressed by an in-plane magnetic field $B_{\parallel }$, and thereby proving that it is caused at least in part by spinful scatterers. A nonmonotonic dependence of the effective decoherence rate on $B_{\parallel }$ reveals the intricate role of the scatterers’ spin dynamics in forming the interference correction to the conductivity, an effect that has gone unnoticed in earlier weak localization studies.

Figure 1

(a) Layout of the sample and an optical micrograph [28] showing predominantly monolayer graphene with bilayer inclusions. (b) Magnetoresistance sharpens with decreasing temperature ($B_{\parallel }=0$). (c) The sharpness of the magnetoresistance curve is nonmonotonic in $B_{\parallel }$ ($T=450\mathrm{mK}$).

Figure 2

(a) Dephasing rate as a function of temperature at $B_{\parallel }=0$ extracted from the raw (blue circles) and rescaled (red circles) magnetoresistance. The blue dotted line is the best linear fit to the raw data. The slopes of the dashed lines are given by Eq. (1), and the offset constants are chosen arbitrarily for ease of comparison with the data. The solid red line is the theory developed in this work. (b) Dephasing rate extracted from the rescaled magnetoresistance plotted as a function of $B_{\parallel }$ at $T=450\mathrm{mK}$. Roman numerals denote different scattering regimes. The dotted line is a guide to the eye.

Figure 3

(a) Illustration of the influence of precession of the impurity spin on the weak localization effect. (b) Comparison between the experimental values of the curvature of magnetoconductance at $T=0.45-2.1\mathrm{K}$ with the theoretical values calculated using the parameters obtained using a fit at $T=2.1\mathrm{K}$.