Fabrication And Electronic Transport Transition Properties Of Graphene/YBCO Junction

Graphene has attracted enormous interest and intense research activities due to its particular two-dimensional structure and electrical properties. The proximity effects in graphene based superconducting devices were first demonstrated in graphene/Al junctions at sub-kelvin temperature. Recently, superconducting contacts with higher transition temperatures such as W, Pb, Nb, ReW, and NbN, have been studied. However, all those works adapted conventional superconductor as the contact which has a relatively low critical temperature normally less than15K, and the super-current through graphene is always observed below2K. These low Tc studies provide the evidence for existence of interactions in graphene, enabling the possibility of inducing proximity superconductivity with high Tc materials.Up to now, electrical properties of graphene in contact with the high-temperature superconductors, such as YBCO, have not been explored experimentally. Different from conventional low Tc superconductor, PE in an anisotropic high Tc superconductor strongly depends on the crystallographic orientation of the superconductor surface at the N-S interface. One example is that the superconducting coherent penetration depth along the a-b plane (transport into the a-b plane) is much longer than that along the c axis (transport perpendicular to the a-b plane). As such, the study on graphene/YBCO junction will be very meaningful from the scientific point of view. In addition, confirming the existence of the proximity effect in graphene/YBCO junction will boost the development of novel high Tc superconductor SQUID.In this work, we study the electronic transport properties of graphene/YBCO junction. First, we introduced the method to synthesize graphene films by chemical vapor deposition on Cu; Following that, we introduced the processes to transfer large-area graphene synthesized on metal substrates like Cu by wet etching methods. Then, confocal raman spectroscopy, atomic force microscopy, optical microscopy and PPMS were performed to evaluate the quality of transferred graphene.Second, we introduced the method to synthesize YBCO films. The YBCO film was deposited on SrTiO3single crystal substrates by means of on-axis pulsed-laser deposition (PLD). The growth parameters are optimized to achieve the right compromise between high crystal temperature Tc and smoothness of the film surface. Then, the main we described the main fabrication steps to introduce the devices of graphene/YBCO contacts. Finally, we simply described the system to test the electrical properties of the graphene/YBCO junction.Then, we measured the junction. In order to ensure the superconductivity of the YBCO materials, the electrical transport properties of YBCO hall bar were measured in advance. Following that, we studied the superconducting proximity effect of the junction. The result indicated that, the transport temperature of YBCO film is85～95℃. When the temperature is lower than the Tc of YBCO, the transition of differential resistance from a non-zero value to zero at high bias across graphene/YBCO indicates the tunneling of Cooper from YBCO to graphene. The transition of differential resistance can be suppressed by the magnetic field or heating of the sample.Finally, we made a summary and prospect to our work.