PhD Thesis: High-frequency phenomena in small Bi2Sr2CaCu2O8+x intrinsic Josephson junctions
Wednesday 20 May 2015
to 16:00 at
Holger Motzkau (Stockholm University, Department of Physics)
In this thesis, the tunneling between individual atomic layers in structures of Bi2Sr2CaCu2O8+x based high-temperature
superconductors are experimentally studied employing the intrinsic Josephson effect. A special attention is paid to the
fabrication of small mesa structures using micro and nanofabrication techniques.
In the first part of the thesis, the periodic Fraunhofer-like modulation of the critical current of the junctions as a
function of in-plane magnetic field is investigated. A transition from a modulation with a half flux quantum to a flux
quantum periodicity is demonstrated with increasing field and decreasing junction length. It is interpreted in terms of the
transformation of the static fluxon lattice of stacked, strongly coupled intrinsic Josephson junctions and compared with
theoretical predictions. A fluxon phase diagram is constructed.Numerical simulations have been carried out to complement
the experimental data.
In the second part of the thesis, different resonant phenomena are studied in the dynamic flux-flow state at high magnetic
fields, including Eck-resonances and Fiske steps. Different resonant modes and their velocities, including superluminal
modes, are identified.
In the third part, different experiments attempting to detect radiation from small mesa structures using different setups
based on hot-electron bolometer mixers and calorimeters are described. No distinct radiation with emission powers higher
than about 500pW could be detected. Furthermore, the interaction with external GHz-radiation is studied. Resonances
attributed to an induced flux-flow are observed, and the reflectivity of the sample can be tuned by switching mesas between
the superconducting and quasiparticle state.
In the last part, the resistive switching of mesas at high bias is studied. It is attributed to a persistent electrical doping of
the crystal. Superconducting properties such as the critical current and temperature and the tunneling spectra are analyzed
at different doping states of the same sample. The dynamics of the doping is studied, and attributed to two mechanisms;
a charge-transfer effect and oxygen reordering.