YBa$_2$Cu$_3$O$_7$ nano-constriction Josephson junctions and SQUIDs fabricated by focused helium-ion-beam irradiation

TL;DR

This work tackles the fabrication bottleneck of ultra-narrow YBCO constriction Josephson junctions by employing focused helium-ion-beam irradiation to create resistive regions that suppress superconductivity without amorphization, achieving constriction widths down to the He-FIB spot size. The authors demonstrate RSJ-like IVCs for a 6 nm constriction, observe THz-induced Shapiro steps in cJJs integrated with antennas, and realize a dc nanoSQUID with two cJJs, supported by a Langevin-based model that accounts for skewed current-phase relations. Key findings include a characteristic voltage around a few millivolts, robust operation up to tens of kelvin, and significant flux focusing in planar devices, underscoring the practicality of this approach for high-Tc nanoelectronics. Overall, the methodology provides a scalable route to compact, high-performance YBCO-based superconducting components with potential impact on THz detection and superconducting metrology.

Abstract

By focused $30\,\mathrm{keV}$ He ion beam irradiation, epitaxially grown YBa$_2$Cu$_3$O$_7$ (YBCO) thin films can be driven from the superconducting to the insulating state with increasing irradiation dose. A properly chosen dose suppresses superconductivity down to $4\,\mathrm{K}$, while crystallinity is still preserved. With this approach we create areas of normal-conducting YBCO that can be used to define resistively shunted constriction-type Josephson junctions (cJJs) on the nanometer scale. We also demonstrate that the fabricated cJJs can be incorporated in direct current superconducting quantum interference devices and can be used as detector junctions in THz antennas.

Figures (9)

• Figure 1: (a) Sketch of an YBCO microbridge with He-FIB-irradiated area of length $\ell$ and width $W$. The YBCO film is $t = 30 \,\mathrm{nm}$ thick. (b) $R(T)$ curves of microbridges that contain areas ($\ell=30\,$nm, $W=2\,\mathrm{\mathrm{\mu m}}$) irradiated with different areal doses $D_\mathrm{a}$ (color indicates $D_\mathrm{a}$ in $\,\mathrm{ions/nm^2}$). $R$ is measured at constant bias current $I = 1 \,\mathrm{\mu A}$. The right vertical axis shows $R\,W / l$, which below $T_\mathrm{c,0}$ corresponds to the sheet resistance $R_\square$ of the irradiated region.
• Figure 2: YBCO cJJ: (a) Color plot $D_\mathrm{a,eff}(x,y)$ calculated for a constriction with $w=6\,\mathrm{nm}$ and for a He-FIB with $\sigma=3\,\mathrm{nm}$ in the $40 \times 40\,\mathrm{nm^2}$ region around the constriction. The dashed lines indicate the nominal edges of the irradiated areas (triangles) with the design distance $w = 6 \,\mathrm{nm}$ between the tips. The $D_\mathrm{a,eff}(0,y)$ profile indicated by the orange dashed line is shown in (b). The yellow area indicates the cJJ width $w$. (c) The orange IVC describes a cJJ irradiated with $D_\mathrm{a} = 40 \,\mathrm{ions/nm^2}$ and $w = 6 \,\mathrm{nm}$, measured at $T = 4.2 \,\mathrm{K}$. The blue IVC illustrates a RSJ-fit with a critical current $I_\mathrm{c} = 10.8 \,\mathrm{\mu A}$ and a junction resistance $R_\mathrm{n} = 240 \,\mathrm{\Omega}$. The inset is a sketch of the bridge geometry, where the yellow regions indicate the He-FIB-irradiated resistive regions (100 nm wide) that define the banks of a cJJ with the nominal constriction width $w$.
• Figure 3: IVCs (at $T = 4.2\,$K) of YBCO cJJs with different constriction width $w$ all irradiated with $D_\mathrm{a} = 40 \,\mathrm{ions/nm^2}$.
• Figure 4: Critical current $I_c(w)$ and junction resistance $R_\mathrm{n}(w)$ for YBCO cJJs irradiated with $D_\mathrm{a} = 40 \,\mathrm{ions/nm^2}$, extracted from IVCs measured at $T = 4.2 \,\mathrm{K}$.
• Figure 5: YBCO dc SQUID with cJJs: (a) Calculated effective dose $D_\mathrm{a,eff}(x,y)$, assuming a Gaussian distribution of the He-FIB intensity with $\sigma=3 \,\mathrm{nm}$. (b) Circuit diagram of the dc SQUID used for simulations. (c) Experimental (dots) and simulated (lines) IVCs of the nanoSQUID at zero field (black) and at $B=10.6 \,\mathrm{mT}$ (orange). (d) Experimental (black) and simulated (orange) field modulation of positive and negative critical current. (e) Normalized current phase relations of the weak links, as extracted from simulations. (f) Voltage vs. applied magnetic field dependence at different bias currents through the nanoSQUID from $I = -6.33 \,\mathrm{\mu A}$ (dark blue) to $I = 6.35 \,\mathrm{\mu A}$ (dark red)). All measurements were performed at $T\approx 4.2\,\mathrm{K}$.