Josephson junctions and SQUIDs created by focused helium ion beam irradiation of YBa${}_2$Cu${}_3$O${}_7$

Abstract

By scanning with a $30\mathrm{keV}$ focused He ion beam (He-FIB) across YBa${}_2$Cu${}_3$O${}_7$ (YBCO) thin film microbridges, we create Josephson barriers with critical current density $j_{\mathrm{c}}$ adjustable by irradiation dose $D$. The dependence $j_{\mathrm{c}}(D)$ yields an exponential decay. At $4.2\mathrm{K}$, a transition from flux-flow to Josephson behavior occurs when $j_{\mathrm{c}}$ decreases below $\approx 2MA/c{\mathrm{m}}^2$. The Josephson junctions exhibit current-voltage characteristics (IVCs) that are well described by the resistively and capacitively shunted junction model, without excess current for characteristic voltages $V_{\mathrm{c}}\lesssim 1\mathrm{mV}$. Devices on MgO and LSAT substrates show non-hysteretic IVCs, while devices on SrTiO${}_3$ show a small hysteresis. For all junctions an approximate scaling $V_{\mathrm{c}}\propto j_{\mathrm{c}}^{1/2}$ is found. He-FIB irradiation with high dose produces barriers with $j_{\mathrm{c}}=0$ and high resistances of $10\mathrm{k\Omega }\dots 1\mathrm{G\Omega }$. This provides the possibility to write highly resistive walls or areas into YBCO using a He-FIB. Transmission electron microscopy reveals an amorphous phase within the walls, whereas for lower doses the YBCO stays crystalline. We have also ``drawn'' superconducting quantum interference devices (SQUIDs) by using a He-FIB for definition of the SQUID hole and the junctions. The SQUIDs show high performance, with flux noise $<500\mathrm{n}{\mathrm{\Phi }}_0/H{\mathrm{z}}^{1/2}$ in the thermal white noise limit for a device with $19\mathrm{pH}$ inductance.

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