d-wave Josephson Junctions

Doctoral thesis, 2005

The properties of d-wave YBCO Josephson junctions and dc-SQUIDs with high misorientation angles have been studied experimentally . The predominant d-wave symmetry of the pairing wavefunction in high-Tc superconductors has, to a large extent, been a hinderance in applications, but it also makes it possible to fabricate devices with novel properties that may be suitable for applications in the quantum regime; one particulary interesting property is a strong second harmonic component in the current-phase relation of junctions with the electrodes oriented in a node-lobe arrangement; so-called 0-45 junctions. The work has been focused on the properties of grain-boundary 0-45 dc-SQUIDs: Their dc-properties as well as the effects of a strong second harmonic component on the dynamics have been studied experimentally and modelled numerically. The experimental results can be explained by taking into account the meandering of the grain boundary which in combination with a strong orientational dependence of the transport properties due to the d-wave symmetry results in interfaces with properties that change dramatically over a length scale of 50-100 nm. The properties of grain-boundary YBCO 0-45 junctions have been studied in the quantum regime. Macroscopic quantum tunnelling has been demonstrated in these junctions and microwave spectroscopy is here used to verify the presence of discrete energy levels. The quality factor of the junction has been experimentally determined to 40+-10, a much higher value than previously believed to be possible in high-Tc structures. Finally, the possibility of using high-T$_c$ structures to build qubits is discussed. There is now a renewed interest in this possibility, especially in light of our recent results which demonstrate that the inherent dissipation mechanisms are much less detrimental to coherent phenomena than previously feared.