Valley-symmetry-preserved quasi-one-dimensional transport in ballistic graphene with gate-defined carrier guiding

From QCLab
  • Speaker: Minsoo Kim (POSTECH)
  • Date: Thursday March 31, 2016 17:00
  • Place: Jeongho Seminar Room

Graphene nanoribbons, a one-dimensional (1D) graphene system, with zigzag edges are predicted to exhibit interesting electronic properties stemming from its Dirac band structure. However, to date, investigation of the properties is highly limited because of the defects and the roughness at the edges, which mix different valleys in graphene. Recent progress in preparing a high-quality graphene layer enables one to investigate the intrinsic carrier transport nature in the material. Here, we report the signature of conservation of valley symmetry in two types of quasi-1D ballistic transport devices; one is a quantum point contact (QPC) and another is an Aharonov-Bohm (AB) interferometer. Devices were fabricated on monolayer graphene with high carrier mobility, where the graphene was encapsulated between two thin hexagonal boron nitride (h-BN) layers. In measurements, charge carriers were confined in a potential well formed by the dual operation of the bottom and top gates and the four-terminal magnetoconductance (MC) was measured with varying the charge carrier density, dc bias, and temperature. Graphene in the device was in the ballistic regime, exhibiting the conductance quantization in steps of delta G = 4e^2/h starting from G = (2, 6), 10 e^2/h in a constricted conducting channel of QPC-type devices. This behavior is similar to the one observed in zigzag graphene nanoribbons having edge localized channels. Our tight-binding calculation shows that quasi-1D charge flow on a graphene plane acts a zigzag-type nanoribbon, unless it is perfectly aligned along the armchair direction. In the AB interferometry, we observed h/e periodic modulation of MC and the zero-field conductance minimum with a negative MC background. All these results strongly suggest that qausi-1D channels built in our devices preserve the intrinsic Dirac transport nature of carriers with its valley symmetry, which would be conveniently utilized for valleytronics in graphene.