Anomalous thermopower oscillations in graphene-NW heterostructures

Being sensitive to the electron-hole asymmetry, thermopower is an excellent tool for probing the behaviour of local charge puddles near the charge neutrality point in graphene heterostructures. In this work, we present anomalous thermoelectric results measured in Monolayer graphene (MLG)-NW heterostructures. Our devices consist of InAs nanowire (NW) and graphene vertical heterostructure, electrically isolated by thin (~ 10 nm) hexagonal boron nitride (hBN) layers. In contrast to conventional thermoelectric setup, where a heater is placed on one side of a sample, we use the InAs NW as a local heater placed in the middle of the graphene channel. The thermoelectric voltage across bilayer graphene (BLG) in NW-BLG devices shows a change in sign around the Dirac point as predicted by Mott's formula. In contrast, the thermoelectric voltage measured across MLG in NW-MLG devices shows anomalous large-amplitude oscillations around the Dirac point, unrelated to the Mott response estimated from the electrical conductivity data. Using this unique heating geometry, where the temperature gradient is maximum at the centre of the graphene channel, thermopower could capture the local modification in the density of states in graphene, which is manifested as oscillations in thermopower.