Dirac fermion reflector by ballistic graphene sawtooth-shaped npn junctions

We have realized a Dirac fermion reflector in graphene by controlling the ballistic carrier trajectory in a sawtooth-shaped npn junction. When the carrier density in the inner p-region is much larger than that in the outer n-regions, the first straight np interface works as a collimator and the collimated ballistic carriers can be totally reflected at the second zigzag pn interface. We observed clear resistance enhancement around the np+n regime, which is in good agreement with the numerical simulation. The tunable reflectance of ballistic carriers could be an elementary and important step for realizing ultrahigh-mobility graphene field effect transistors utilizing Dirac fermion optics in the near future

グラフェン量子輸送現象の電子光学的制御

学位の種別: 課程博士審査委員会委員 : （主査）東京大学准教授 町田 友樹, 東京大学教授 樽茶 清悟, 東京大学准教授 長田 俊人, 東京大学教授 荒川 泰彦, 東京大学教授 勝本 信吾University of Tokyo(東京大学

Imaging Bulk and Edge Transport near the Dirac Point in Graphene Moiré Superlattices.

Van der Waals structures formed by aligning monolayer graphene with insulating layers of hexagonal boron nitride exhibit a moiré superlattice that is expected to break sublattice symmetry. Despite an energy gap of several tens of millielectronvolts opening in the Dirac spectrum, electrical resistivity remains lower than expected at low temperature and varies between devices. While subgap states are likely to play a role in this behavior, their precise nature is unclear. We present a scanning gate microscopy study of moiré superlattice devices with comparable activation energy but with different charge disorder levels. In the device with higher charge impurity (∼1010 cm-2) and lower resistivity (∼10 kΩ) at the Dirac point we observe current flow along the graphene edges. Combined with simulations, our measurements suggest that enhanced edge doping is responsible for this effect. In addition, a device with low charge impurity (∼109 cm-2) and higher resistivity (∼100 kΩ) shows subgap states in the bulk, consistent with the absence of shunting by edge currents.This work was partly supported by EPSRC EP/L020963/1, JST CREST Grant Numbers JPMJCR15F3 and JSPS KAKENHI Grant Numbers JP25107003, JP25107004, JP26248061, JP15H01010, JP16H00982