Approaching the depairing critical current in superconducting YBa2Cu3O7−x nanowires
Doctoral thesis, 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