Giant Magnetodrag in Graphene at Charge Neutrality

We report experimental data and theoretical analysis of Coulomb drag between two closely positioned graphene monolayers in a weak magnetic field. Close enough to the neutrality point, the coexistence of electrons and holes in each layer leads to a dramatic increase of the drag resistivity. Away from charge neutrality, we observe nonzero Hall drag. The observed phenomena are explained by decoupling of electric and quasiparticle currents which are orthogonal at charge neutrality. The sign of magnetodrag depends on the energy relaxation rate and geometry of the sample.

Figure 1

Mechanism of magnetodrag at charge neutrality. Upper panel: In an infinite system quasiparticle currents ${\mathit{P}}_i$ in the two layers flow in the same direction, leading to positive drag ${\rho }_{xx}^D=V_2/j_1>0$. Lower panel: In a thermally isolated system no net quasiparticle flow is possible (leading to inhomogeneities in the quasiparticle density); quasiparticle currents in the two layers have opposite directions yielding negative drag.

Figure 2

(a) Longitudinal drag resistivity in magnetic field as a function of the top ($V_T$) and bottom ($V_B$) gate voltages. Lines track positions of maxima in single-layer resistivity in top (open symbols) and bottom (solid symbols) layers. (b) Magnetodrag for $n_1=n_2=n$ at $T=160\mathrm{K}$. Solid symbols represent the experimental data. The error bars for the data are within the symbol size. The theoretical lines show solutions to Eqs. (6). (c) Magnetodrag for $n_1=n_2=n$ at $T=240\mathrm{K}$. Solid symbols: experimental data; lines: theory (6). (d) Map of Hall drag resistivity as a function of $V_T$ and $V_B$. The white diagonal area corresponds to vanishing Hall drag for $n_1=-n_2$. The lines are the same as in (a). (e) Experimental data (blue squares, left axis) and theory (red solid line, right axis) for the Hall drag resistivity for $n_1=n_2=n$. The theoretical curve is calculated on the basis of the microscopic theory of Ref. 5. Note the sign change at $n\approx \pm 2\times {10}^{11}{\mathrm{cm}}^{-2}$. Inset: schematics of Hall drag measurements in double-layer system. (f) Hall drag resistivity for $n_1=n_2=n$. The data (blue squares) are identical to those in (e); the red solid line represents solutions to Eq. (6).

Figure 3

The magnetic field dependence of the longitudinal drag resistivity at the neutrality point. The positive sign of the magnetodrag in weak fields corresponds to the limit $W\gg {\ell }_{\mathrm{ph}}$, where ${\ell }_{\mathrm{ph}}\approx 1.2\mu \mathrm{m}$ for the parameters of the plot. The magnetic-field dependence of scattering rates is disregarded in the plot.