Tuning current flow in superconducting thin film strips by control wires. Applications to single photon detectors and diodes

TL;DR

The paper addresses edge crowding and Pearl screening limitations in thin-film superconducting strips used for detectors by introducing current-carrying control wires and bilayer configurations to sculpt the supercurrent profile $J(x)$. Through London and Ginzburg-Landau analyses, it demonstrates that an inverted or flattened current profile with edge dips can be achieved in strips wider than the Pearl length, mitigating lithographic defects and delaying vortex entry, while enabling nonreciprocal, diode-like behavior and in-situ tunability. The work further shows that bilayer geometries and control currents can eliminate Pearl crowding and enable dense arrays of wide straight-strip detectors, and analyzes vortex dynamics to establish ultimate sensitivity limits set by vortex-antivortex unbinding. Overall, the approach offers a practical route to extending SNSPDs to larger areas, enabling tunable detector performance and nonreciprocal superconducting devices, with implications for quantum sensing and beyond.

Abstract

It is shown that integration of a thin film superconducting strip with current-carrying control wires enables one to engineer a profile of supercurrent density $J(x)$ with no current crowding at the edges of a strip wider than the magnetic Pearl length $Λ$. Moreover, $J(x)$ in a strip can be tuned by control wires to produce an inverted $J(x)$ profile with dips at the edges to mitigate current crowding at lithographic defects and block premature penetration of vortices. These conclusions are corroborated by calculations of $J(x)$ in a thin strip coupled inductively with side control wires or in bilayer strip structures by solving the London and Ginzburg Landau equations in the thin film Pearl limit. Thermally-activated penetration of vortices from the edges and unbinding of vortex-antivortex pairs in inverted $J(x)$ profiles are evaluated. It is shown that these structures can be used to develop single-photon strip detectors much wider than $Λ$. Such detectors can be tuned {\it in situ} by varying current in control wires to reach the ultimate photon sensitivity limited by unbinding of vortex-antivortex pairs. The structures considered here also exhibit a non-reciprocal current response and behave as superconducting diodes.

Figures (22)

• Figure 1: Factors limiting the maximum current in superconducting thin strips: (a) edge roughness causing local current crowding and premature penetration of vortices. The dashed line depicts a boundary of a layer in which local variations of current density are essential. (b) The Pearl current crowding at the edges of a bare strip (blue). An inverted current density profile $J(x)$ in a strip tuned by control wires (red) deactivating both the edge defects and the Pearl current crowding.
• Figure 2: A strip of width $w$ between two control strip wires of width $l$. The coordinate axes are shifted downward for clarity.
• Figure 3: Magnetic field lines calculated from Eqs. (\ref{['A2']}) - (\ref{['Q1']}) for $l=0.15w$, $k=0.03$, $k_1=50k$, $b=h=0.05w$.
• Figure 4: Sheet current density $J(x)$ calculated for different strip widths $w$ and normalized control currents $I_1/I$ for W$_{0.8}$Si$_{0.2}$ films with $d=4$ nm, $\Lambda = 245,\mu$m wsi, $k_1=200k$ and $l=0.15w$: (a) $w\approx 20\,\mu$m, $I_1/I=2.5, 5.03, 7.58, 10.12$, (b) $w\approx 100\,\mu$m, $I_1/I=0.79, 1.62, 2.46, 3.31$, (c) $w\approx 1$ mm, $I_1/I=0.64, 0.82, 1.01, 1.21$.
• Figure 5: Two long thin film strips electrically connected at one end and carrying equal antiparallel currents $I$. A dielectric interlayer of thickness $h$ is shown in black.