This thesis describes applications of the ac Josephson effect. Firstly, results are presentedfrom bicrystal grain boundary YBa2Cu3O7-d junctions shunted with a YBa2Cu3O7-d/SrTiO3/Aumultilayer external capacitor, to make a junction with a hysteretic current voltage characteristicoperating at high temperatures. A hysteretic junction with a McCumber parameter of 1.01 at72.3K, with a critical current of 451mA and a resistance of 0.56W was achieved for a junctionshunted with a 150mm2 external capacitor with a 50nm SrTiO3 dielectric. The measuredcapacitance was less than that expected from a calculation of the parallel plate shuntcapacitance. The explanation was thermal noise suppression of the hysteresis and the junctionsaw 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 theintrinsic junction capacitance was poorly understood. The permittivity of SrTiO3 is 24000 at4.2K. A series of YBa2Cu3O7-d Josephson junctions of lengths from 2mm to 20mm waspatterned on a SrTiO3 bicrystal and the Fiske resonance dispersion relation was measured. Thedispersion relation consisted of two branches, one at low frequencies with a high resonatorcapacitance per unit length and a high frequency branch with a low resonator capacitance perunit 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, thepermittivity 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 ofSrTiO3 at microwave frequencies using Josephson junctions coupled to external resonators.The permittivity of 50nm, 100nm and 200nm SrTiO3 films was frequency independentbetween 100GHz and 900GHz and to decrease with film thickness. The permittivity of the50nm film was 35 and that of the 200nm film was 187 at 4.2K. The permittivity of the 200nmfilm 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. Thecapacitance per unit area scaled inversely with resistance area product and increased linearlywith 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 boundarycurrent transport, and requires current flow over a fraction of the area of the grain boundary.
Materials Science and Metallurgy, University of Cambridge 2000
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