Local tunneling based measurements of Coulomb drag potentials

Coulomb drag measurements can reveal both momentum and energy exchange between adjacent electron fluids, providing an effective means of probing particle interactions. Conventional device-scale transport measurements that probe Coulomb drag average over sample disorder and geometric effects, which can inhibit a full understanding of how the two systems are interacting. In this measurement, we show how local tunneling measurements can be used to probe the local drag electrochemical potential in double bilayer graphene (BLG/hBN/BLG) with nanometer resolution. Using a low-temperature STM with open-loop feedback, we drive a dc current in the drive layer and record compensation voltage to null the tunneling current, yielding V_drag(x,y) with μV-level sensitivity. A current-reversal protocol (±I) separates the odd, linear momentum-drag component from the thermoelectric contribution. Gate-controlled carrier density and measurements of sample allow us to understand how V_drag is effected by disorder and sample geometry.