Gate-controlled supercurrent effect in dry-etched Dayem bridges of non-centrosymmetric niobium rhenium

Abstract

The application of a gate voltage to control the superconducting current flowing through a nanoscale superconducting constriction, named as gate-controlled supercurrent (GCS), has raised great interest for fundamental and technological reasons. To gain a deeper understanding of this effect and develop superconducting technologies based on it, the material and physical parameters crucial for the GCS effect must be identified. Top-down fabrication protocols should also be optimized to increase device scalability, although studies suggest that top-down fabricated devices are more resilient to show a GCS. Here, we investigate gated superconducting nanobridges made with a top-down fabrication process from thin films of the non-centrosymmetric superconductor niobium rhenium with varying ratios of the constituents (NbRe). Unlike other devices previously reported and made with a top-down approach, our NbRe devices systematically exhibit a GCS effect when they were fabricated from NbRe thin films with small grain size and etched in specific conditions. These observations pave the way for the realization of top-down-made GCS devices with high scalability. Our results also imply that physical parameters like structural disorder and surface physical properties of the nanobridges, which can be in turn modified by the fabrication process, are crucial for a GCS observation, providing therefore also important insights into the physics underlying the GCS effect.

High-Performance Gate-Controlled Superconducting Switches: Large Output Voltage and Reproducibility

“…Most of the samples have been deliberately made with w S much larger than the typical w S (up to ∼200 nm) reported in previous studies on the GCS. 1,[3][4][5][6][7][8][9][10]12,20,22,25,27,28 This has been done to verify the argument that a GCS is easier to observe as w S approaches the coherence length ξ of the S. Our measurement results instead show that GCS can also occur in devices with w S of 550 nm (Figure 1e), which is ∼40 times larger than the ξ of Nb in the diffusive regime (ξ < 15 nm; ref 44). We have not tested devices with larger w S , which may in principle still show GCS.…”

“…The control of a superconducting current (supercurrent) via the application of a gate voltage ( V G ), currently known as gate-controlled supercurrent (GCS), has become a subject of great interest, as evidenced by the number of experimental − and theoretical studies − reported on it. Among the main motivations behind the interest in the GCS is its potential for the development of voltage-controlled superconducting logics that would intrinsically have low-energy dissipation (since based on superconductors) and better performance than other superconducting logics already available. − Compared to these, GCS-based logics would offer several advantages including higher device density (due to the smaller device size), larger number of devices connectable in series (i.e., higher fan out), stronger resilience to magnetic noise and easier interfacing with conventional metal-oxide semiconductor (CMOS) circuits to form hybrid superconducting/semiconducting computing architectures .…”

“…A lower V G,offset would also lead to an increase in the fan out, since the output voltage V out of a GCS device, which depends on its characteristic voltage I c0 R N ( R N being the normal-state resistance and I c0 being I c at V G = 0), can be fed as input signal to the gate (i.e., used as the V G ) of another GCS device connected downstream . In most GCS devices reported to date, V G,offset is typically of few tens of volts, ,,,,,,, while I c0 R N is typically of few millivolts or tens of millivolts, ,,,,,,, meaning that these two voltages differ by several orders of magnitude. This large difference between I c0 R N and V G,offset currently makes the connection of GCS devices in series not feasible.…”

“…In terms of the fabrication process, it is necessary to optimize protocols for high scalability of devices and to achieve high reproducibility in their realization of a GCS. Several groups have already tried to scale up the fabrication of GCS devices using subtractive patterning, ,, which involves lithographic patterning of a device into a negative resist layer used then as mask to etch an underlying S thin film, other than by additive patterning involving lithographic patterning of a polymer mask followed by S material growth and lift off. Some of these studies, however, suggest that devices made by subtractive patterning do not show a GCS unlike devices made by additive patterning, unless unconventional Ss that can be grown with a small grain size (e.g., Nb 0.18 Re 0.82 ) are used, in combination with a nontrivial surface chemistry activated by the etching process …”

“…Some of these studies, however, suggest that devices made by subtractive patterning do not show a GCS unlike devices made by additive patterning, 26 unless unconventional Ss that can be grown with a small grain size (e.g., Nb 0.18 Re 0.82 ) are used, in combination with a nontrivial surface chemistry activated by the etching process. 27 Achieving high reproducibility in the behavior of GCS-based devices, which is also essential for a better fundamental understanding of the effect as explained above, remains another crucial objective. All the studies on the GCS reported to date are in fact based on the characterization of few devices (typically one or two devices in each study), across which several parameters (e.g., substrate, S type, gate-to-channel distance, device geometry) are often varied, even within the same study.…”