AC Response Across the Metal Insulator Transition of YBCO Josephson Junctions Fabricated with a Helium Ion Beam

Abstract

Using focused helium ion beam (FHIB) irradiation, we fabricated in-plane, high-Tc YBCO Josephson junctions. By varying the dose of the irradiation, we tune the junction barriers from metallic (SNS) to insulating (SIS) and investigate how this transition affects microwave-driven dynamics. As the barrier transitions from metallic to insulating, the oscillatory response of the Shapiro steps to the RF power changes dramatically. On either side of the metal-insulator transition, the devices exhibit clean integer Shapiro steps without half-integer features, demonstrating that the current--phase relation is dominated by the first harmonic and that the excess current is minimal. The current-voltage response is well-described by the resistively, capacitively shunted junction model assuming a single-harmonic current--phase relation. This behavior indicates well-controlled junction properties suitable for a wide range of superconducting electronics, including detectors, mixers, and high-density integrated circuits.

Figures (7)

• Figure 1: Microscope image of a 100 $\times$ 100 $\mu$m region showing twenty 4-$\mu$m-wide YBCO bridges where the Au layer was selectively removed to expose the underlying YBCO prior to focused helium-ion-beam irradiation of the junctions (red lines). The inset shows the full 5 $\times$ 5 mm chip containing the four-point bridge structure.
• Figure 2: Transport measurements of FHIB-fabricated Josephson junctions illustrating metallic and insulating barrier characteristics. (a) I--V characteristics of a SNS junction measured at 30, 35, and 50 K created with 2.0 $\times$10$^{16}$ ions/cm$^2$. (b) $I_C^{1/2}$ , $R$ and their product vs temperature for the SNS junction. (c) I--V characteristics of a SIS junction measured at 1.6, 15, 20, and 30 K created with 3.0 $\times$10$^{16}$ ions/cm$^2$. (d) $I_C$, $R$ and their product vs temperature for the SIS junction.
• Figure 3: I--V characteristics of an SNS (a) at 30 K and an SIS (b) at 1.6 K junctions, measured with 18 GHz irradiation (red), and without (black). (c) and (d) show the I--V characteristics of the same SNS (30 K) and SIS (1.6 K) junctions, respectively, for different applied microwave powers. These data illustrate that the step heights go to zero at the minima eg. n=1 at 4.2 mW and 8.5 mW for the SNS junction.
• Figure 4: Measured step heights of the zeroth through fourth order Shapiro steps as a function of applied microwave source power for the SNS junction (a) with $I_CR$ 54 $\mu V$ at 30 K and the SIS junction (b) with $I_CR$ 150 $\mu V$ at 1.6 K, showing Bessel-like oscillations.
• Figure 5: Shapiro step height versus microwave source power for junctions on the same chip written with doses 1.6 (a) and, 2.0 (b) are measured at 60 K, 2.4 (c) at 30 K, and 2.8 (d), 3.0 (e), and 3.4 $\times$10$^{16}$ ions/cm$^2$ (f) all measured at 2 K .