UC Santa Barbara

A successful program for improving the quality of graphene based 2DEGs has been the steady removal of charged impurities near the graphene surface. This was first accomplished by using hBN as a substrate and encapsulant, and then further improved by using atomically uniform graphite as a gate electrode instead of amorphous evaporated metal, resulting in the first all `van der Waals' heterostructure. These improvements lead to the robust observation of even-denominator fractional quantum Hall states in bilayer graphene, as well as many other interesting correlated ground states at zero magnetic field such as superconductivity and ferromagnetism in both twisted and un-twisted graphene multi-layers. Moreover, as many of these states occur at an electron density near charge neutrality, they may be accessed within a single device simply by field-effect gating. However, engineering more complex mesoscopic devices which are designed to interface these exotic phases of electronic matter require fabricating spatially varying potential profiles on the order of the correlation lengths in these systems, typically around 100nm. Additionally, these manufactured potentials need to be energetically uniform, i.e., they must not introduce uncontrolled disorder potentials on the order of the energy gaps of the systems of interest, again typically around 1meV, on the requisite length scales. Here we report on a novel fabrication technique, AFM based local anodic oxidation gate lithography, to pre-pattern graphite gates with critical feature sizes smaller than 100nm which does not introduce any unwanted contaminants or damage to the underlying van der Waals heterostructure. We then use this technique to fabricate two types of mesoscopic devices operated in the fractional quantum Hall regime, a quantum point contact and an edge-state Fabry-Pérot interferometer. We show that our quantum point contact nearly perfectly mimics a simple model which characterizes electron tunneling between nu = 1/3 and nu = 1 edge modes at any tunneling strength. This indicates that these junctions are nearly disorder-free validating our novel fabrication technique. Moreover, we use our nearly defect free junctions to create an edge-state Fabry-Pérot interferometer operating in nu = 1/3. Here we observe sharp phase jumps in the interference pattern consistent with the expected braiding phase of anyons in the nu = 1/3 state. These results taken together show that our method for fabricating mesoscopic devices within the graphene based van der Waals heterostructure platform, developed over the last four years, makes these devices comparable to the most modern III-V semiconductor quantum wells, which took over 30 years of refinement.