Nash, CR; (2017) Circuit elements for quantum phase-slip devices. Doctoral thesis , UCL (University College London).

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Abstract

If proved to exist, the phenomenon of quantum phase slips (QPS) allows us to provide a new standard for current. In order to investigate this effect a QPS circuit needs to be constructed with a superconducting nanowire connected in series with an inductor and two thin film resistors. It was found that it was possible to control the low temperature resistance of chromium oxide films by increasing the oxygen pressure at the time of deposition, meaning chromium oxide films could be fabricated for a large resistance range of thin film resistors, from values in the hundreds of ohms to megaohms. These films were found to be amorphous when measured with XRD and displayed extremely low magnetoresistance (with a peak magnetic moment of 5x10 3 μB per atom, compared to 2 μB per atom in bulk ferro- magnetic Cr2O3). The contact resistance of joining these chromium oxide resistors to gold or niobium-silicon was measured using a transmission-line model and this revealed that a gold interlayer provided a much lower contact resistance than direct contact of the circuit components (a contact resistivity of 0.15 m Ohm cm2 for chromium oxide to gold compared to 65 m Ohm cm2 for chromium oxide to niobium silicon). Several methods were used to restrict nanowire dimensions. It was found that combining a HSQ fabrication method with a neon mill resulted in the creation of wires below the coherence length of niobium nitride (30 nm), which made these wires suitable for QPS measurements.

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