Quantum point contact in graphene: transmission and shot noise

We have performed shot noise experiments on chiral edge modes partitioned at a quantum point contact (QPC) deposited on monolayer graphene. With the bulk filling-factor fixed at ν_b=-2, we have holes occupying two chiral edge modes. When the modes reach the QPC, their transmission across its saddle-point potential is dictated by the QPC split-gate voltage V_sg, which allows us to transmit both modes, reflect only the inner mode, or both. The map of the transmission in the V_bg-V_sg plane obtained agrees well with that in Refs. (Zimmermann et al.). With the magnetic field fixed at 5 T or 13 T, we measured the power spectral density S(ω) at 300 mK versus the bias voltage V for selected values of V_sg. We find that when the outer edge mode is being partitioned, S(ω) obeys the conventional Landauer-Büttiker expression. However, when the inner mode is partitioned, the profiles of S(ω) vs. V are anomalous. We will discuss the anomalous features and speculate on the role of strong interaction between copropagating modes.