Microwave Response in Superconducting Nanowires
We fabricate superconducting wires using molecular templating technique to study their transport properties when exposed to microwave radiation (MW), at cryogenic temperatures. Our wires have a diameter on the order of the zero temperature coherence length, which makes them quasi-one-dimensional at all temperatures below the superconducting critical temperature, T C. When our wires are exposed to MW of sufficient power we observe the formation of a finite voltage, dynamic superconducting that we do not see without MW. We identify this dynamic superconducting state as a microwave induced phase slip center (PSC) and study its properties in depth in this dissertation. The jump-wise transition, observed at high bias currents, from the superconducting state into the PSC and the transition from the superconducting state to the normal state are both observed to be stochastic in nature. We attribute this switching to the occurrence of a single quantum phase slip (QPS) which in the presence of a high bias current creates enough heat to warm the wire to a temperature above TC, or, in the presence of MW, triggers the PSC dynamic state. We present a model for the increase of the rate of QPS with the increase in MW power and show that it is consistent with our observations. The AC Josephson effect is observed in the PSC state (but not in the normal state, to which the wire transits at high bias when no MW is applied) and modeled using a multi-valued current phase relationship. With our model we are able to reproduce the observed fractional Shapiro steps.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign