J. C. Fenton

Tunability of the superconductivity of tungsten films grown by focused-ion-beam direct writing

We have grown tungsten-containing films by focused-ion-beam (FIB)-induced chemical vapor deposition. The films lie close to the metal-insulator transition with an electrical conductivity which changes by less than 5% between room temperature and 7 K. The superconducting transition temperature Tc of the films can be controlled between 5.0 and 6.2 K by varying the ion-beam deposition current. The Tc can be correlated with how far the films are from the metal-insulator transition, showing a nonmonotonic dependence, which is well described by the heuristic model of [Osofsky et al., Phys. Rev. Lett. 87, 197004 (2001)]. Our results suggest that FIB direct-writing of W composites might be a potential approach to fabricate mask-free superconducting devices as well as to explore the role of reduced dimensionality on superconductivity.

NbSi nanowire quantum phase-slip circuits: dc supercurrent blockade, microwave measurements, and thermal analysis

We present a detailed report of microwave irradiation of ultranarrow superconducting nanowires. In our nanofabricated circuits containing a superconducting NbSi nanowire, a dc blockade of current flow was observed at low temperatures below a critical voltage Vc, a strong indicator of the existence of quantum phase-slip (QPS) in the nanowire. We describe the results of applying microwaves to these samples, using a range of frequencies and both continuous-wave and pulsed drive, in order to search for dual Shapiro steps which would constitute an unambiguous demonstration of quantum phase-slip. We observed no steps, and our subsequent thermal analysis suggests that the electron temperature in the series CrO resistors was significantly elevated above the substrate temperature, resulting in sufficient Johnson noise to wash out the steps. To understand the system and inform future work, we have constructed a numerical model of the dynamics of the circuit for dc and ac bias (both continuous-wave and pulsed drive signals) in the presence of Johnson noise. Using this model, we outline important design considerations for device and measurement parameters which should be used in any future experiment to enable the observation of dual Shapiro steps at experimentally accessible temperatures and, thus, lead to the development of a QPS-based quantum current standard.

Monte Carlo simulations of thermal fluctuations in moderately damped Josephson junctions: Multiple escape and retrapping, switching- and return-current distributions, and hysteresis

A crossover at a temperature T* in the temperature dependence of the width sigma of the distribution of switching currents of moderately damped Josephson junctions has been reported in a number of recent papers, with positive d sigma/dT and IV characteristics associated with underdamped behavior for lower temperatures T T*. We have investigated in detail the behavior of Josephson junctions around the temperature T* by using Monte Carlo simulations including retrapping from the running state into the supercurrent state as given by the model of Ben-Jacob et al. [Phys. Rev. A 26, 2805 (1982)]. We develop discussion of the important role of multiple escape and retrapping events in the moderate-damping regime, in particular considering the behavior in the region close to T. We show that the behavior is more fully understood by considering two crossover temperatures and that the shape of the distribution and sigma(T) around T*, as well as at lower T< T*, are largely determined by the shape of the conventional thermally activated switching distribution. We show that the characteristic temperatures T* are not unique for a particular Josephson junction but have some dependence on the ramp rate of the applied bias current. We also consider hysteresis in moderately damped Josephson junctions and discuss the less commonly measured distribution of return currents for a decreasing current ramp. We find that some hysteresis should be expected to persist above T*. We highlight the importance, even well below T*, of accounting properly for thermal fluctuations when determining the damping parameter Q.

Ion-beam-induced bending of freestanding amorphous nanowires: The importance of the substrate material and charging

Ion-beam irradiation offers great flexibility and controllability in the construction of freestanding nanostructures with multiple advanced functionalities. Here, we present and discuss the bending of free-standing nanowires, against, towards, and ultimately parallel to a flux of directional ion irradiation. Bending components both along and perpendicular to the incident ion beam were observed, and the bending behavior was found to depend both on the ion beam scanning strategy and on the conductivity of the supporting substrate. This behavior is explained by an ion-irradiation-related electrostatic interaction. Our findings suggest the prospect of exploiting this technique to engineer 3D nanostructures for advanced applications.

Felling of individual freestanding nanoobjects using focused-ion-beam milling for investigations of structural and transport properties

We report that, to enable studies of their compositional, structural and electrical properties, freestanding individual nanoobjects can be selectively felled in a controllable way by the technique of low-current focused-ion-beam (FIB) milling with the ion beam at a chosen angle of incidence to the nanoobject. To demonstrate the suitability of the technique, we report results for zigzag/straight tungsten nanowires grown vertically on support substrates and then felled for characterization. We also describe a systematic investigation of the effect of the experimental geometry and parameters on the felling process and on the induced wire-bending phenomenon. The method of felling freestanding nanoobjects using FIB is an advantageous new technique enabling investigations of the properties of selected individual nanoobjects.

Compact chromium oxide thin film resistors for use in nanoscale quantum circuits

We report on the electrical characterisation of a series of thin amorphous chromium oxide (CrOx) films, grown by dc sputtering, to evaluate their suitability for use as on-chip resistors in nanoelectronics. By increasing the level of oxygen doping, the room-temperature sheet resistance of the CrOx films was varied from 28 Ω/◻ to 32.6 k Ω/◻. The variation in resistance with cooling to 4.2 K in liquid helium was investigated; the sheet resistance at 4.2 K varied with composition from 65 Ω/◻ to above 20 G Ω/◻. All of the films measured displayed linear current–voltage characteristics at all measured temperatures. For on-chip devices for quantum phase-slip measurements using niobium–silicon nanowires, interfaces between niobium–silicon and chromium oxide are required. We also characterised the contact resistance for one CrOx composition at an interface with niobium–silicon. We found that a gold intermediate layer is favourable: the specific contact resistivity of chromium-oxide-to-gold interfaces was 0.14 mΩc...

The radio-frequency impedance of individual intrinsic Josephson junctions

We have measured the response of an array of Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions to irradiation at 3 GHz. By measuring the dependence of the switching current upon the radio-frequency current for five of the junctions in the array we show quantitatively that the junctions have identical impedances at 3 GHz, this impedance being given by the inverse of the slope of the current-voltage characteristics.

Low-Loss Superconducting Nanowire Circuits Using a Neon Focused Ion Beam

We present low-temperature measurements of low-loss superconducting nanowire-embedded resonators in the low-power limit relevant for quantum circuits. The superconducting resonators are embedded with superconducting nanowires with widths down to 20nm using a neon focused ion beam. In the low-power limit, we demonstrate an internal quality factor up to 3.9x10^5 at 300mK [implying a two-level-system-limited quality factor up to 2x10^5 at 10 mK], not only significantly higher than in similar devices but also matching the state of the art of conventional Josephson-junction-embedded resonators. We also show a high sensitivity of the nanowire to stray infrared photons, which is controllable by suitable precautions to minimize stray photons in the sample environment. Our results suggest that there are excellent prospects for superconducting-nanowire-based quantum circuits.

Embedding NbN Nanowires Into Quantum Circuits With a Neon Focused Ion Beam

Parasitic two-level systems are generally present in superconducting circuits as a result of conventional fabrication and processing, and these lead to noise and loss of coherence in quantum systems. We examine the use of a Ne focused ion beam for producing nanowires integrated into superconducting circuits with a minimized density of parasitic two-level systems. We report measurements of nanowires produced by Ne focused-ion-beam milling in NbN resonators. The resonator losses increase after the nanowire is fabricated, with the Q factor decreasing by ~30% to ~10 5, which is a factor 10-100 higher than in the most closely comparable circuits previously measured. This indicates that the Ne focused ion beam is a promising route for creating superconducting-nanowire-based devices with low levels of decoherence.

Superconducting NbN Nanowires and Coherent Quantum Phase-Slips in DC Transport

A quantum current standard dual to the Josephson voltage standard has been trumpeted as a primary application for the phenomenon of coherent quantum phase-slips (CQPS) in superconducting nanowires, but it requires a dc transport geometry, distinct from the ring geometry in which CQPS have been probed by microwave spectroscopy measurements. We present measurements on NbN nanowires with a width of 15 nm and including integrated thin-film resistors to provide a high-impedance environment. Analysis shows that the nanowire behavior is in line with expectations. The results demonstrate that the process is promising both for future dc transport CQPS measurements and for the fabrication of other circuits in which low-width superconducting nanowires are required.