그래핀 소자에서의 양자 간섭과 1차원 전도특성

Abstract

In low-dimensional mesoscopic systems, measurements of transport properties such as quantum confinement effects and phase-coherent quantum interferences of conduction electrons allow one to understand the detailed physics of condensed matter systems. Graphene is such a good low-dimensional platform where one can study the relativistic quantum mechanics, taking into account its linear energy-momentum dispersion accompanied with the valley degeneracy and the easy gate tunability of its carrier doping. The thesis aims to examine the low-dimensional phase-coherent transport properties of graphene. It consists of two main parts. The first one is about the Andreev interference in diffusive graphene devices. The other one deals with the symmetry-preserved one-dimensional (1D) transport and related quantum interferences in ballistic graphene.

In the first part, locality of a phase-coherent Andreev interference is investigated in a graphene-based Andreev interferometer. Interplay between the superconductivity and the Dirac-fermionic nature of electronic states of graphene leads to unique phase-coherent transport, when graphene is in proximity contact with superconducting electrodes. We report on gate-tuned locality of superconductivity-induced phase-coherent magnetoconductance (MC) oscillations in Andreev interferometers consisting of a superconducting Al ring in contact with two ends of a T-shaped mono-layer graphene bar. The conductance oscillations arise from the flux change through the superconducting Al loop, with a gate-dependent Fraunhofer-type modulation of the envelope, which is independent of the sample-specific impurity configuration in the graphene sheet. We confirm a transitional change in the character of the pair coherence, between local and nonlocal, in the same device as the effective length-to-width ratio of the device was modulated by tuning the pair-coherence length in the graphene layer.

In the second part, symmetry-preserved 1D transport properties of ballistic graphene was investigated for the first time. A zigzag graphene nanoribbon is predicted to exhibit intrinsic electronic properties stemming from its Dirac band structure. To date, however, investigation of them is highly limited because of the defects and the roughness at the edges, with different valley properties intermixed. Here, we report the signature of conservation of valley symmetry in two types of 1D ballistic graphene devices; one is a quantum point contact (QPC) and another is an Aharonov-Bohm (AB) interferometer. Carrier confinement was realized by the dual-gate operation along with large difference in the sheet conductance between different gated regions of high-mobility graphene. Constricted conducting channel of a QPC device exhibits the conductance quantization in steps of 4$e^2/h$ starting from 10$e^2/h$ at zero magnetic field, a behavior similar to the one observed in zigzag graphene nanoribbons of edge-confined channels. Our tight-binding calculation shows that quasi-1D charge flow on a graphene plane acts as a zigzag-type nanoribbon. In an AB interferometer, we observed $h/e$ periodic modulation of MC and the zero-field conductance minimum with a negative MC background. All these results signify valley-symmetry-preserved 1D transport. Next, magneto-electric effects in this quasi-1D channels of graphene are investigated to build the quantum Hall interferometry based on graphene. In the quantum-Hall regime, we demonstrate that the QPC arranged on a graphene sheet acts as electronic analogue of a beam splitter that splits incident beams from a source into two beams.