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Register a new userSuperconducting MoSi nanowires
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We have fabricated disordered superconducting nanowires of molybdenium silicide. A molybdenium nanowire is first deposited on top of silicon, and the alloy is formed by rapid thermal annealing. The method allows tuning of the crystal growth to optimise, e.g., the resistivity of the alloy for potential applications in quantum phase slip devices and superconducting nanowire single-photon detectors. The wires have effective diameters from 42 to 79 nm, enabling the observation of crossover from conventional superconductivity to regimes affected by thermal and quantum fluctuations. In the smallest diameter wire and at temperatures well below the superconducting critical temperature, we observe residual resistance and negative magnetoresistance, which can be considered as fingerprints of quantum phase slips.
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Vortex dynamics in superconductors have received a great deal of attention from both fundamental and applied researchers over the past few decades. Because of its critical role in the energy relaxation process of type-II superconductors, vortex dynamics have been deemed a key contributor to the response rate of the emerging superconducting single photon detector (SSPD). With the support of electrical transport measurements under external magnetic fields, vortex dynamics in superconducting a-MoSi thin films are investigated in this work. It is ascertained that the vortex state changes from pinned to flux flow under the influence of the Lorentz force. The critical vortex velocity v* and quasi-particle inelastic scattering time {tau}* under different magnetic fields are derived from the Larkin-Ovchinnikov model. Under high magnetic fields, the v* power law dependence (v*~B-1/2) collapses, i.e., v* tends to zero, which is attributed to the obstruction of flux flow by the intrinsic defects, while the {tau}* increases with the increasing magnetic field strength. In addition, the degree of vortex rearrangement is found to be enhanced by photon-induced reduction in potential barrier, which mitigates the adverse effect of film inhomogeneity on superconductivity in the a-MoSi thin films. The thorough understanding of the vortex dynamics in a-MoSi thin films under the effect of external stimuli is of paramount importance for both further fundamental research in this area and optimization of SSPD design.
We use a scanning nanometer-scale superconducting quantum interference device (SQUID) to image individual vortices in amorphous superconducting MoSi thin films. Spatially resolved measurements of the magnetic field generated by both vortices and Meissner screening satisfy the Pearl model for vortices in thin films and yield values for the Pearl length and bulk penetration depth at 4.2 K. Flux pinning is observed and quantified through measurements of vortex motion driven by both applied currents and thermal activation. The effects of pinning are also observed in metastable vortex configurations, which form as the applied magnetic field is reduced and magnetic flux is expelled from the film. Understanding and controlling vortex dynamics in amorphous thin films is crucial for optimizing devices such as superconducting nanowire single photon detectors (SNSPDs), the most efficient of which are made from MoSi, WSi, and MoGe.
We have measured the resistance vs. temperature of more than 20 superconducting nanowires with nominal widths ranging from 10 to 22 nm and lengths from 100 nm to 1050 nm. With decreasing cross-sectional areas, the wires display increasingly broad resistive transitions. The data are in very good agreement with a model that includes both thermally activated phase slips close to Tc and quantum phase slips (QPS) at low temperatures, but disagree with an earlier model based on a critical value of R_n/Rq. Our measurements provide strong evidence for QPS in thin superconducting wires.
We analyze the effect of different types of fluctuations in internal electron energy on the rates of dark and photon counts in straight current-carrying superconducting nanowires. Dark counts appear due to thermal fluctuations in statistically independent cells with the effective size of the order of the coherence length; each count corresponds to an escape from the equilibrium state through an appropriate saddle point. For photon counts, spectral broadening of the deterministic cut off in the spectra of the detection efficiency can be phenomenologically explained by local thermal fluctuations in the electron energy within cells with the same effective volume as for dark counts.
When a ferromagnet is placed in contact with a superconductor, owing to incompatible spin order, the Cooper pairs from the superconductor cannot survive more than one or two nanometers inside the ferromagnet. This is confirmed in the measurements of ferromagnetic nickel (Ni) nanowires contacted by superconducting niobium (Nb) leads. However, when a thin copper (Cu) buffer layer (3 nm, oxidized due to exposure to air) is inserted between the Nb electrodes and the Ni wire, the spatial extent of the superconducting proximity range is dramatically increased from 2 to a few tens of nanometers. Scanning transmission electron microscope images verify the existence of Cu oxides and the magnetization measurements of such a 3 nm oxidized Cu film on a SiO2/Si substrate and on Nb/SiO2/Si show evidence of ferromagnetism. One way to understand the long-range proximity effect in the Ni nanowire is that the oxidized Cu buffer layer with ferromagnetism facilitates the conversion of singlet superconductivity in Nb into triplet supercurrent along the Ni nanowires.
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