Nanoscale YBa2Cu3O7-δ devices -Searching for a subdominant order parameter using a single electron transistor
The microscopic origin of High critical Temperature Superconductivity (HTS) is still an open issue in condensed matter physics. It is believed that by exlporing the quasiparticle energy spectrum one can learn about the mechanism promoting the superconducting state.
In this thesis we have developed a nanoscale spectroscopic tool, an all superconducting YBa2Cu3O7−δ (YBCO) Single Electron Transistor, allowing us to obtain information from the quasiparticle spectrum of an entire nanometer scale island. In this experiment we find a fully gapped superconductivity which strongly depends on the externally applied magnetic
field. This finding shows that the order parameter is not purely
dx2−y2 with nodes, instead it has an additinal subdominant imaginary component which
lifts the zero energy quasiparticles.
The realization of the transistor has required the engineering of nanaoscale
YBCO Josephson grain boundary (GB) junctions with stringent demands
on the transport properties. Part of the work in this thesis has been devoted
to the development and characterization of two methods to fabricate
nanoscale GB junctions. A conventional method based on e-beam lithography
and ion milling and a new soft nanopatterning technique. The new
method is based on the phase competition between superconducting YBCO
and insulating greenphase at the grain boundary. This has allowed the
creation of junctions with minimal damage in the fabrication process. Together,
the two methods create grain boundaries that span a large range of
critical current densities and normal resistivities, which can be employed
in various applications.
single electron transistors
high temperature superconductivity
Kollektorn, Institutionen för Mikroteknologi och Nanovetenskap, Chalmers tekniska högskola
Opponent: Prof. Reinhold Kleiner, Physikalisches Institut, University of Tübingen, Germany