Congratulations to Andrew Dane, who won the best poster award at the 706th W.E. Heraeus Seminar, titled Superconducting Kinetic Inductances for his work on high-Q superconducting niobium resonators incorporating nanoscale quasiparticle traps.

Details regarding his poster abstract may be found below:

Superconducting Nb Resonators with Gold Nanodot Decorations

Andrew Dane, Omid Noroozian, Emily Barrentine, Di Zhu, Thomas Stevenson, Harvey Moseley, Karl Berggren

While the operation of kinetic inductance detectors (KIDs) is well understood, reports of anomalous behaviors in KIDs fabricated from highly-disordered, thin-film superconductors indicate that a detailed understanding of the superconducting density of states and behavior of excited quasiparticles is necessary to reconcile experimental findings with theory. Anomalous behaviors include: (1) differences in the inductive and dissipative photo-response times, such that the shift in the frequency of the resonator outlasts the reduction in Q by a factor of four, (2) resonant frequency vs temperature that cannot be explained by Mattis-Bardeen theory without ad-hoc modifications. It has been observed experimentally that in highly-disordered thin-film superconductors, spontaneous fluctuations of the superconducting gap can arise. The interaction between excited quasiparticles and a spatially varying gap may explain the anomalous behaviors observed in KIDs, for instance if quasiparticle trap states exist that can temporarily localize excited quasiparticles.

In this work, we report on the fabrication and measurement of Nb CPW resonators, as well as progress towards a physical simulation of the case where the superconducting gap varies along the resonator body. The Nb used for fabrication was 50 nm thick, sputtered onto r-plane sapphire at 500 °C, and was not expected to have significant variation in the gap along the film. In order to produce a spatially varying gap, we lithographically defined arrays of gold nano-dots, on top of the CPW centerline, in an attempt to locally reduce the superconducting gap in the Nb due to the proximity effect. In undecorated resonators at high photon number, an internal Q of greater than one million was measured at 7 mK, while the internal Q of the decorated resonators was substantially less. The resonant behavior of these devices vs temperature and power, and their relevance to high-kinetic inductance materials will be discussed.

Targeted irradiation of nanostructures by a finely focused ion beam provides routes to improved control of material modification and understanding of the physics of interactions between ion beams and nanomaterials. Here, we studied radiation damage in crystalline diamond and silicon nanostructures using a focused helium ion beam, with the former exhibiting extremely long-range ion propagation and large plastic deformation in a process visibly analogous to blow forming. We report the dependence of damage morphology on material, geometry, and irradiation conditions (ion dose, ion energy, ion species, and location). We anticipate that our method and findings will not only improve the understanding of radiation damage in isolated nanostructures, but will also support the design of new engineering materials and devices for current and future applications in nanotechnology.

A complete description of the work may be found here.

Training of deep neural networks (DNNs) is a computationally intensive task and requires massive volumes of data transfer. Performing these operations with the conventional von Neumann architectures creates unmanageable time and power costs. Recent studies have shown that mixed-signal designs involving resistive crossbar architectures are capable of achieving acceleration factors as high as 30 000 × over the state of the art digital processors. These approaches involve utilization of non-volatile memory elements as local processors. However, no technology has been developed to-date that can satisfy the strict device requirements for the unit cell. This paper presents the superconducting nanowire-based processing element as a crosspoint device. The unit cell has many programmable non-volatile states that can be used to perform analog multiplication. Importantly, these states are intrinsically discrete due to quantization of flux, which provides symmetric switching characteristics. Operation of these devices in a crossbar is described and verified with electro-thermal circuit simulations. Finally, validation of the concept in an actual DNN training task is shown using an emulator.

A complete description of the work may be found here.

Superconducting nanowire-based devices are increasingly being used in complex circuits for applications such as photon detection and amplification. To keep up with the growing circuit complexity, nanowire processing is moving from single layer fabrication to heterogeneous multilayer processes. Hydrogen silsesquioxane (HSQ) is the most common choice of negative-tone electron-beam resist for patterning superconducting nanowires. However, HSQ has several limitations, including an inability to be removed without a strong reagent that damages the superconducting film, making it unsuitable for multilayer fabrication. As a result, it is vital to consider alternative resists that can be removed through less harmful solvents. Here, the authors explore the use of ma-N 2400 series deep ultraviolet photoresist as an electron-beam resist for fabricating superconducting nanowire devices. They demonstrate that ma-N can be used to pattern dense lines as narrow as 30 nm and isolated features below 20 nm in width. They also examine the reproducibility of 36 identical superconducting devices by comparing their minimum dimensions and switching currents. Through this analysis, they conclude that ma-N 2400 is a suitable electron-beam resist for fabricating nanoscale devices and has the potential to expand the use of nanowire-based technologies into more advanced applications.

A complete description of the work may be found here.