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Electrical Transport in Graphene-Supercoductor Junction

Electrical Transport in Graphene-Supercoductor Junction
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Electrical properties of graphene have been investigated intensively since it was firstly discovered. One interesting thing in graphene is that a graphene flake exhibits very different electrical properties in accordance with the number of layers. Mono-layer graphene is characterized by a relativistic Dirac fermion of charge carriers due to its linear energy-momentum relation with the conduction and valence band intersecting at p=0. As a consequence of the band structure, a quantum state of mono-layer graphene carriers has an additional quantum number called a pseudospin which is coupled to the momentum direction. On the other hand, bi-layer graphene consisting of two graphene mono-layers exhibits totally different energy-momentum relation because two mono-layer graphenes are weakly coupled by an interlayer hopping of electrons. Although non-Dirac-like and parabolic dispersion relation are intermediate properties between mono-layer graphene and bulk graphite, bi-layer graphene system is still chiral due to the sublattice symmetry. In addition, electrical field-induced band-gap opening makes bi-layer graphene unique by itself compared with mono-layer graphene and bulk graphite. Quantum nature of charge carriers becomes significant as the length scale of a device is comparable with the phase-coherent length. The Berry phase associated with chiral nature of mono-layer graphene, π, is revealed in the half-integer quantum-Hall effect, anti weak-localization and Fabry-perot type interference experiments. Superconductivity, characterized by a superconducting order parameter, can enhance quantum interference effects of a mesocscopic device. When a non-superconducting material is placed between two superconductors, a Josephson junction will be formed. In this thesis, we have investigated electrical transport properties of mono- and bi-layer graphene Josephson junctions. The chiral nature of mono- and bi-layer graphene in a Josephson junction is not observed because our devices are in the diffusive region, thus the momentum coupled pseudospin is not maintained in the whole device. However, two-dimensional structure of graphene layer and relative low carrier density enable one to modulated carrier density of graphene by an external electric field. In the first part, we demonstrate realization and measurement of Al and Pb0.93In0.07 mono-layer graphene Josephson junction. The often-adopted superconducting electrode material, Al, shows unsatisfactorily low superconducting transition temperature (1 K) and energy gap (125 µeV). To overcome the disadvantage, we have fabricated and measured proximity-coupled superconducting junctions consisting of a mono-layer graphene sheet in a contact with Pb0.93In0.07 electrodes. A much higher superconducting transition temperature (TC ~ 7.0 K) and a large superconducting energy gap (∆PbIn ~ 1.1 meV) of PbIn alloy allow the observation of the Josephson supercurrent for temperature as high as 4.8 K with a large value of the ICRN product of ~255 µeV, an order of magnitude higher than that for Al-based graphene Josephson junction. Magnetic-field and microwave responses of the junction yield direct evidences for genuine Josephson coupling through mono-layer graphene. It is also revealed that the subgap structure of differential conductance (dI/dV) induced by the multiple Andreev reflection. Moreover, gate-dependence of IC and IC/IR is well explained by theory for diffusive and long Josephson junction. In the second part, we report electronic transport measurement on superconducting proximity effect in dual-gated bilayer graphene Josephson junction with PbIn electrodes. The effect of band gap opening is confirmed by increase of resistance as we modulated top and back gate voltage away from zero-gap charge neutral point. The resistive states near the charge neutral point shows insulating behavior in R-T curve regardless of superconducting transition of PbIn electrode below TC. However, highly doped regime shows metallic R-T behavior and junction becomes superconducting at T < TC. Moreover, magnetic-field-induced Fraunhofer-pattern like supercurrent modulation and microwave irradiation indicate formation of genuine Josephson junction at the superconducting state. The transition from superconducting state to insulating state is related with normal state conductance of the junction and the transition occurs at Gsq,c ~ 7e^2/h.
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