Quantum fluctuations in superconducting nanostructures

Modern nanofabrication technology enableTfabrication of very narrow quasi-1-dimensional superconducting nanowires demonstrating finite resistivity within the range of experimentally obtainable temperatures. The observations were reported in ∼10 nm nanowires of certain superconducting materials. The effect has been associated with quantum phase slip process - the particular manifestation of quantum fluctuations of the order p arameter. In titanium, the phenomenon can be observed already at dimensions ∼35 nm where the fabrication is well reproducible and the dimensions of samples can be characterized with high accuracy. We have performed systematic study of the size dependence of transport properties for superconducting ultra-narrow titanium nanowires utilizing method of gradual size reduction by low energetic ion beam milling. The method enables the study of the same nanowire with 1 nm effective diameter reduction between measurement cycles. The experiments demonstrated clear crossover to fluctuation dominated regime when the dimensions are reduced below a certain threshold. All available at our disposal microscopic analyses indicate no damage or contamination of the bulk of the nanowire due to the ion milling.
The next stage of the work was to demonstrate experimentally the quantum duality between the physics of Josephson junction and superconducting nanowire governed by quantum fluctuations. We showed that sufficiently narrow nanowires embedded in high-impedance environment demonstrate the insulating state - Coulomb blockade. The system can be considered as a single Cooper pair transistor without any dielectric barriers. Irradiation of the nanowire with external RF drive leads to formation of the Bloch steps on the IV characteristics - the phenomenon dual to the well-known Shapiro effect, currently used as quantum standard of electric voltage. We have performed experiments demonstrating the corresponding current singularities which confirm the hypothesis. The observation is of significant importance for both scientific and metrological communities.