Strong Dirac Photoconductance in Charge Neutral Graphene-Boron Nitride-Graphene Heterostructures

Photoexcited hot charge carriers may shed light on the intriguing dynamics of electrons and holes near charge neutrality in very clean graphene. When two graphene layers are separated by thin hexagonal boron nitride (hBN), interlayer photocurrent can occur due to the exponential tail of the hot carrier distribution extending into the valence band of the hBN.[1] Using an ultrafast pulsed laser we excite a graphene-boron nitride-graphene (G-BN-G) heterostructure with photon energy less than the gap of the hBN and observe strong and highly nonlinear interlayer photocurrent as a function of the interlayer voltage. In a fully encapsulated heterostructure (BN-G-BN-G-BN), we find that the differential photoconductance significantly increases when both graphene layers are near charge neutrality. Strikingly, as intralayer voltage is applied near charge neutrality, strong negative differential photoconductance is observed as a function of interlayer voltage. This unusual effect is enhanced at intermediate temperatures ranging from 35 to 70 K. We speculate that this effect is a result of highly anomalous energy transport near the Dirac point. [1] Nature Physics 12, 455–459 (2016)