M.S. Raven

C-axis expansion in RF sputtered YBa2Cu3Ox thin films

The c-axis lattice parameter of YBa2Cu3Ox thin films deposited on SrTiO3(100) and MgO(100) has been carefully measured and compared with the critical temperature and normalized resistance of a number of films. The films were prepared in situ using RF magnetron off-axis sputter deposition in the high-pressure regime. The Tc dependence on the c-axis expansion has previously been reported to be much weaker in thin films than in bulk material and considered not to be entirely due to O deficiency in the chain sites. However, the change in c is less than 1% and conflicting results have previously been obtained. By comparing X-ray diffraction peak shifts relative to substrate peaks and also detailed analysis of (001) data we have obtained unambiguous evidence for the c-axis expansion. We find that the Tc versus c-axis expansion tends to follow bulk values for films on MgO(100) but for films sputtered onto SrTiO3(100) the Tc is fairly constant with c-axis expansion. In the latter films we also find a-axis-orientated grains, which account for higher resistance values and the shape of the R/T curves. The stability of Tc with c-axis expansion is considered to be partly due to the presence of the a-axis-orientated grains.

Preliminary measurements of the low frequency impedance of superconducting epitaxial Y-Ba-Cu-O films

The authors have produced epitaxial thin films of superconducting Y-Ba-Cu-O grown onto (110) and (100) oriented SrTiO3 and MgO substrates by in situ RF sputter deposition. The impedance of the films has been measured and the results compared with two-fluid theory. The impedance of the films depended strongly on crystallographic orientation. Between room temperature and the critical temperature Tc the film reactance was inductive for well orientated (100) films and capacitive for (013) orientated films. The films were purely inductive for temperatures below Tc, with a constant value equal to the film inductance after flux exclusion due to the Meissner effect.

Quasi-static impedance and penetration depth of YBCO thin films

Abstract The low frequency complex impedance and penetration depth of high temperature superconducting thin films has been investigated. Analysis of two-dimensional current density and impedance equations shows that when the penetration depth λ=δ/√2 (where δ is the skin depth) there is a crossover from superconductivity to normal conductivity which is observed experimentally. We also include the effects of the normal inductance and displacement current and show that negative (capacitive) reactance may arise if the latter is sufficiently large. The analysis is compared with measurements obtained from thin film YBCO grown epitaxially on SrTiO 3 and MgO substrates by RF Sputtering.

Votage step response of Al(Ga)As GaAs resonant tunnel diodes

Abstract The step response of Al(Ga)As GaAs double barrier resonant tunnel (DBRT) diodes was determined using a semi-empirical approach which uses high frequency small signal conductance measured data rather than static current voltage characteristics. The differential equation arising from the equivalent circuit of the DBRT diode is solved for each value of the measured conductance. This is repeated throughout the negative differential conductance region finally yielding the large signal step response. The results obtained are compared with direct time domain reflectometry measurements using a network analyser.

Hall Effect in YBa2Cu3Ox Thin Films and Contact Misalignment

The sign of the Hall effect provides critical transport information both for the normal state and superconducting state of High Temperature Superconductors. For Hall effect measurements in thin films, a contact misalignment of only a few micrometers produces a significant longitudinal voltage in addition to the transverse Hall voltage. This misalignment voltage is usually cancelled by reversing the current and magnetic field. However problems may arise for normal state transport in anisotropic materials and vortex flow in the mixed state. In this paper the effect of misalignment of the Hall sidearm electrodes is considered in detail for the case of c-axis oriented YBa 2 Cu 3 O x thin films. The longitudinal voltage produced by the sidearm displacement is used with the Hall voltage to determine a localized Hall tangent. For the normal state this value is found to be in good agreement with the average Hall tangent for the whole film. Differences found between the localized and average Hall tangents for the transition region suggests a method for detecting the motion of single vortices.