Approaching the depairing critical current in superconducting YBa2Cu3O7−x nanowires

Doktorsavhandling, 2013

The microscopic origin of High critical Temperature Superconductivity is still an open issue in condensed matter physics. In these complex oxides electrons self-organize in ways qualitatively different from those of conventional metals and insulators. The study of these materials at the nanoscale can help to shed light on various ordering and phase transitions giving new hints into the origin of high critical temperature superconductivity. In this thesis a systematic study of the transport properties of nanowires made of High critical Temperature Superconductor (HTS) YBa2Cu3O7−δ (YBCO) is presented. A soft nano-patterning technology for the fabrication of these nanowires has been implemented to preserve a homogeneous character of YBCO nano-structures. Two interesting observations are reported regarding the critical current density Jc carried by the nanowires: 1) Its value increases by reducing the nanowire width w. 2) For the smallest wires with w ≃ 40 nm the value of Jc approaches for the first time the theoretical depairing limit indicating nanostructure with properties very close to as grown films. The behavior of the critical current density as a function of width has been explained in terms of current crowding at the inner corners at the connection between the nanowires and the wide electrodes . YBCO nanowire based nanoSQUIDs have been also fabricated. The nano-SQUIDs can work in the whole temperature range from 300mK to 80K. Critical current modulation as a function of an externally applied magnetic field at different temperatures has been studied and the screening parameter βL has been extracted. The experimental and simulated values of βL are in good agreement indicating a Josephson-like behavior in the whole temperature range.