Dynamics of high-angle grain boundary YBCO Josephson junctions

Licentiatavhandling, 2004

This thesis describes experimental investigations of properties of high-angle YBa2Cu3O7−δ(YBCO)bicrystal Josephson junctions and SQUIDs fabricated on SrTiO3-substrates. The main focus of theinvestigation has been on the effects of the predominant d-wave symmetry of the superconductingwavefunction in YBCO on transport properties and dynamics.At a high-angle grain-boundary interface between two high-temperature superconductors Andreevstates can form, the current carried by these states hasπ-periodic component which flows in a direc-tion opposite of the usual Josephson current. Asymmetric high-angle grain boundaries also exhibit acritical current which is four orders of magnitude lower than symmetric lower-angle junctions, thiscan be attributed to the node-lobe arrangement of the the superconducting order parameter.High-angle grain-boundary dc-SQUIDs that have been studied which exhibit unusual dynamicssuch as a relative ”shift” of the positions of the positive and negative modulation and a highly non-sinusoidal dependence on the external field. This behavior vanished when moving to very narrowjunctions. These result are explained using a semi-classical model which assumes the presence ofa2ndharmonic in the current-phase relation. Numerical simulations confirm that this model is inqualitative agreement with experimental results.The properties of sub-micron sized junctions have also been studied. These exhibit some un-usual behavior. These junctions have been used to study the tunnelling spectra since the high normalresistance means that it is possible to study energies close to the gap.Finally, some general properties of high-angle Josephson Junctions are discussed. It is arguedthat some seemingly inconsistent experimental results can be explained using a multi-channel modelwhich accounts for the wiggling and faceting of the interface.