Electrical Transport Properties in Graphene Josephson Junctions

Abstract

The graphene is rapidly emerging new material from condensed physics because it is a gap-less semiconductor, or semi-metal, in which the Fermi-energy can be tuned continuously from the valence band to the conduction band. As a result of its conical band structure at low energies, electrons behave like massless Dirac particles. This is quite special for a solid-state system and requires a different idea when trying to explain transport experiments in graphene. In this thesis, we investigated interplay between massless Dirac fermions and Cooper pair by using superconductor？Vgraphene？Vsuperconductor (SGS) Josephson junctions at low temperature. We first delicately tuned conventional fabrication process for making very transparency and clean interface between graphene and superconducting material with Aluminum. For removing the radio-frequency noise, we also renewed in filtering set-up in measurement system. In addition, a small magnetic field was applied to cancel out any residual magnetic field in a cryostat for zero-field measurements. In low bias voltage, due to the Josephson effect, we could induce a supercurrent (a current that flows without dissipation) in graphene. The device is effectively a Josephson field-effect transistor (JOFET) in which the supercurrent can be tuned with a gate voltage. Moreover, the supercurrent is bipolar: depending on gate voltage it is carried either by holes or electrons. In addition, we observed the above-gap structure

an anomalous jump of dI/dV occurring at high bias voltage (V) above the superconducting energy gap (2？？/e), where e is an electric charge. The backgate voltage (Vg), temperature, and H dependences of the above-gap structure indicate that the increase of the electron temperature Te in graphene due to Joule heating is responsible for the above-gap structure. The result indicates that a serious consideration is required for the electron heating effect on the quantum electronic transport at very low temperature, in particular, for the graphene nanoribbon containing very small lateral area of the junction.Moreover, we also made a success of demonstration topgate between superconducting electrodes. We could combine both back and top gate operations to obtain p-p, p-n, n-p, n-n junction states in a graphene SGS Josephson junctions. We observed Josephson phenomena completely suppressed near the Dirac point (region I) and very suppressed or vanished in inhomogeneous doped regions (p-n or n-p: region II). We also reproduced the ordinary Josephson phenomena in homogeneous doped regions (n-n or p-p: region III). This work would provide new system of investigating relation between Kline tunneling and Cooper pair and opens a way of turning on and off the supercurrent in the graphene Josephson junction by applying the top- and back-gate voltages, which would be highly essential for the quantum-information device applications of graphene in near future.