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Novel applications of the Josephson Effect: Ferroelectric Characterisation and Capacitively Shunted Grain Boundary Junctions

By Philip Francis McBrien

Abstract

This thesis describes applications of the ac Josephson effect. Firstly, results are presented from bicrystal grain boundary YBa2Cu3O7-d junctions shunted with a YBa2Cu3O7-d/SrTiO3/Au multilayer external capacitor, to make a junction with a hysteretic current voltage characteristic operating at high temperatures. A hysteretic junction with a McCumber parameter of 1.01 at 72.3K, with a critical current of 451mA and a resistance of 0.56W was achieved for a junction shunted with a 150mm2 external capacitor with a 50nm SrTiO3 dielectric. The measured capacitance was less than that expected from a calculation of the parallel plate shunt capacitance. The explanation was thermal noise suppression of the hysteresis and the junction saw the shunt capacitor as a distributed impedance rather than a lumped circuit element. It was found during these investigations that the influence of the SrTiO3 substrate on the intrinsic junction capacitance was poorly understood. The permittivity of SrTiO3 is 24000 at 4.2K. A series of YBa2Cu3O7-d Josephson junctions of lengths from 2mm to 20mm was patterned on a SrTiO3 bicrystal and the Fiske resonance dispersion relation was measured. The dispersion relation consisted of two branches, one at low frequencies with a high resonator capacitance per unit length and a high frequency branch with a low resonator capacitance per unit length. This was due to the frequency dependence of the permittivity of bulk SrTiO3, which drops above the soft optic phonon frequency. From the dispersion relation, the permittivity of bulk SrTiO3 was 750 and the soft optic phonon frequency was 145GHz. The ac Josephson effect was exploited to measure the permittivity of thin films of SrTiO3 at microwave frequencies using Josephson junctions coupled to external resonators. The permittivity of 50nm, 100nm and 200nm SrTiO3 films was frequency independent between 100GHz and 900GHz and to decrease with film thickness. The permittivity of the 50nm film was 35 and that of the 200nm film was 187 at 4.2K. The permittivity of the 200nm film was tunable with a dc voltage bias between 245 and 112 at 30K and 116GHz. The grain boundary capacitance was used to probe grain boundary current transport. The capacitance per unit area scaled inversely with resistance area product and increased linearly with critical current density, for undoped and Ca doped YBa2Cu3O7-d grain boundaries on 24° bicrystals. This behaviour could not be explained by tunneling models of grain boundary current transport, and requires current flow over a fraction of the area of the grain boundary.EPSR

Topics: Grain Boundaries, YBCO, Josephson Junctions, Devices
Publisher: Department of Materials Science and Metallurgy
Year: 2000
OAI identifier: oai:www.repository.cam.ac.uk:1810/34602
Provided by: Apollo

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