Perfect Superconducting Diode and Supercurrent Range Controller

TL;DR

The paper presents a theoretical model for a multi-wire superconducting SQUID with linear current-phase relations to realize nonreciprocal superconducting behavior. By computing vorticity stability regions in the current–field plane and analyzing how magnetic field and vortices adjust phase relationships, the authors show how perfect superconducting diodes (PSD) and superconducting range controllers (SRC) can emerge. They identify two PSD realizations: a point-wise PSD in a 2-SQUID and a b-invariant PSD in larger wire-count devices, with disorder enabling SRC by shifting VSR boundaries. The findings offer a pathway to integrate diodes and current-range control into cryogenic superconducting circuits for quantum computing and low-power electronics, using conventional superconductors and nanoscale nanowires.

Abstract

Diodes have a nonreciprocal voltage versus current relationship, produced by breaking the space and time reversal symmetry. However, developing high-end superconducting computers requires a superconducting analogue of the traditional semiconductor diode. Such a superconducting diode exhibits non-reciprocity, or a high asymmetry in its critical currents. We present a model of a perfect superconducting diode based on a superconducting quantum interference device made with multiple superconducting nanowires. The diode predicted by our model has a large positive critical current, while the negative critical current can be exactly zero. This 100\% diode efficiency ($η= 1$) remains stable against small changes of the magnetic field. Another important result is that under certain and quite broad conditions such devices can act as supercurrent range controllers. In such device a supercurrent can flow with zero voltage applied, but only if the supercurrent is contained in some narrow, adjustable range, which excludes zero current.

Figures (7)

• Figure 1: 2-SQUID experimental data with model fits. The model parameter fits can be found in Table \ref{['tab:parameters']}. This experimental data and our model fits validates our vorticity switching assumption, where if one nanowire reaches its critical current, then the vorticity of the device will switch due to a phase slip.
• Figure 2: (a)SEM image of the superconducting 2-nanowire sample The left image is an overview image of the nanowires sample. The right image is a zoomed-in image of one of the fabricated superconducting nanowires. (b)Schematic of the four-probe measurement circuit. A voltage source is put in series with a high-value standard resistor $R_{st}$ (shown 5k$\Omega$ here) compared to the sample resistance. This set-up produces a small (relative to zero) current bias to the nanowire sample. An NI-DAQ card equipped with an analog-to-digital converter simultaneously measures the sample voltage, V, and the voltage across the standard resistor, $V_{st}$. These measurements are recorded in real time as the bias current is swept up and down. The bias current is then calculated using Ohm’s law, $I = V_{st}/R_{st}$. The resulting V-I data points are plotted on the computer screen. Both V and $V_{st}$ signals are amplified by preamplifiers (red triangles) before entering the NI-DAQ card input channels. The critical current is defined as the current at which a sharp jump is observed from $V=0$ to $V>0$.
• Figure 3: Schematic of a two nanowire SQUID (2-SQUID). It involves two superconducting thin-film electrodes connected by two parallel nanowires. The magnetic field is applied perpendicular to the device, i.e., along the z-axis. One of the nanowires is schematically shown wider to indicate that it has a different $I_c$ and/or a different $\phi_c$ parameters.
• Figure 4: A SRC is demonstrated for VSRs generated by (a) a 2-SQUID with critical phases set to $[\pi, 2\pi]$ and critical currents set to $[2/3, 4/3]$. The plotted vortices are $-1$ (red), $0$ (blue), and $1$ (black). And (b) a disordered 3-SQUID with wires at $[0, 0.45, 1]$, critical phases set to $[6, 7, 5]$, and critical currents set to $[1.01, 1.12, 0.87]$. The vortices from left to right are $[2,-1]$ (red), $[3,0]$ (black), $[4,1]$ (blue). The grey lines indicate any other VSRs corresponding to different vorticity states.
• Figure 5: A b-invariant PSD can be constructed by (a) a symmetrical 3-SQUID with $\phi_c=2\pi$ for all wires, setting vorticity states [1,-2] (red) and [2, -1] (blue) and by (b) a non-symmetrical 5-SQUID with wires at $[0,5/8, 2/3, 5/6, 1]$ and critical phases set to $12\pi/5$. The vorticity state is $[3,-1,0,0]$. In both figures, the underlying gray lines indicate other VSRs corresponding to different vorticity states.