Gate tuneable beamsplitter in ballistic graphene

We present a beam splitter in a suspended, ballistic, multiterminal, bilayer graphene device. By using local bottomgates, a p-n interface tilted with respect to the current direction can be formed. We show that the p-n interface acts as a semi-transparent mirror in the bipolar regime and that the reflectance and transmittance of the p-n interface can be tuned by the gate voltages. Moreover, by studying the conductance features appearing in magnetic field, we demonstrate that the position of the p-n interface can be moved by 1μm. The herein presented beamsplitter device can form the basis of electron-optic interferometers in graphene. 1 ar X iv :1 51 1. 03 04 4v 2 [ co nd -m at .m es -h al l] 1 2 N ov 2 01 5 Semi-transparent mirrors act as beam splitters in optical experiments. They are important building blocks for many interference experiments, be it a Fabry-Pérot, a Michelson or a Mach-Zehnder two path interferometer. In two-dimensional electron gases (2DEGs) such mirrors have been constructed using quantum point contacts in the quantum Hall regime. Thereby, the Mach-Zehnder experiment could be implemented [1] involving, however, strong magnetic fields. Graphene offers the unique possibility to mimic optical systems once transport is ballistic. Due to recent advances in fabrication techniques, ballistic electron transport can be observed on the micrometer scale as demonstrated by magnetic focusing experiments [2–4]. By using p-n interfaces, Fabry-Pérot interferometers have been realized in single-layer [5–7], gapped bilayer [8] and trilayer graphene [9]. Moreover, the observation of electron guiding, snake states [10, 11] or ballistic supercurrents [12–14] highlighted the possibilities of p-n junctions in graphene. P-n interfaces formed in graphene can be reflective, transparent or semi-transparent, depending on the angle of incidence of the charge carriers and the shape of the potential that forms the interface. For smooth p-n junctions, trajectories close to zero incidence angle are transmitted as a result of Klein tunneling, whereas electrons arriving under large angles are reflected. This suggests that by using a tilted p-n interface, where the Klein-tunneling trajectories are not dominating, a partially transparent mirror can be achieved. In fact, measurements on short and tilted p-n interfaces in graphene devices on SiO2 revealed an increase in two-terminal resistance [15, 16]. Here we present the realization of a semi-transparent mirror in suspended graphene, using a bottomgate structure which is tilted with respect to the current flow direction. The presented four-terminal device allows us to measure reflectance and transmission of the mirror in a ballistic, ungapped bilayer sample. We show that in the unipolar regime, the measured currents can be understood within a simple geometrical picture, whereas in the bipolar regime a partially reflective mirror is formed. Moreover, we demonstrate that the transport properties in weak magnetic field can be substantially altered by moving the position of the mirror by distances up to 1μm. Finally we discuss possibilities for the realization of future graphene interferometers based on the present device. In order to measure the reflectance of a bilayer p-n interface we designed a four-terminal sample as shown in Figure 1a. Bilayer graphene is expected to exhibit a more reflective