▎ 摘　　要

Ballistic graphene samples in a multimode regime show the sub-Sharvin charge transport, characterized by the conductance reduced by a factor of pi/4 compared to standard Sharvin contacts in two-dimensional electron gas, and the shot-noise power enhanced up to F approximate to 1/8 (with F the Fano factor) [Phys. Rev. B 165413 (202)]. Here we consider the disk-shaped (Corbino) setup in graphene, with inner radius r(1) and outer radius r(2), finding that the multimode conductance is slightly enhanced for any r(1) < r(2), reaching (4-pi) approximate to 0.8684 of the Sharvin value for r(1) << r(2) . At the same limit, the Fano factor is reduced, approaching (9 pi - 28)/(12 - 3 pi) approximate to 0.1065 < 1/8. Closed-form approximating expressions for any r(1)/r(2) ratio are derived, supposing incoherent scattering of Dirac fermions on an asymmetric double barrier and compared with exact numerical results following from the mode-matching method. Sub-Sharvin values are restored in the narrow-disk limit r(1)/r(2) -> 1. For experimentally accessible radii ratios 0.5 << r(1)/r(2) << 0.8, both the conductance and the Fano factor are noticeably closer to the values predicted for the r(1) << r(2) limit, yet still differ from standard Sharvin transport characteristics. The system behavior upon tuning the electrostatic potential barrier from a rectangular to parabolic shape is studied numerically and the crossover from the sub-Sharvin to standard Sharvin transport regime is demonstrated. Implications for a finite section of the disk are also discussed.