Preserving the Josephson Coupling of Twisted Cuprate Junctions via Tailored Silicon Nitride Circuits Boards

TL;DR

This work tackles the reproducibility challenge in fabricating twisted BSCCO Josephson junctions by introducing a dry, cryogenic assembly workflow that combines silicon nitride nanomembranes with the cryogenic stacking technique. By tailoring NMB geometry and performing contact formation in an inert, low-temperature environment, the authors achieve high-quality, twist-angle dependent Josephson coupling with magnitudes rivaling the best devices reported to date. The study demonstrates that asymmetric membrane designs suppress wire-bonding–induced disorder, yielding sharp, hysteretic I–V characteristics and robust $I_cR_N$ across twist angles from $0^{ ing}$ to $45^{ ing}$. Overall, the approach enhances interface preservation and contact integrity, providing a scalable platform for superconducting van der Waals heterostructures and future quantum devices.

Abstract

Controlled fabrication of twisted van der Waals heterostructures is essential to unlock the full potential of moire materials. However, achieving reproducibility remains a major challenge, particularly for air-sensitive materials such as $Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}$ (BSCCO), where it is crucial to preserve the intrinsic and delicate superconducting properties of the interface throughout the entire fabrication process. Here, we present a dry, inert and cryogenic assembly method that combines silicon nitride nanomembranes (NMBs) with pre-patterned electrodes and the cryogenic stacking technique (CST) to fabricate high-quality twisted BSCCO Josephson junctions (JJs). This protocol prevents thermal and chemical degradation during both interface formation and electrical contact integration. We also find that asymmetric membrane designs, such as a double cantilever, effectively suppress vibration-induced disorder due to wire bonding, resulting in sharp and hysteretic current-voltage characteristics. The junctions exhibit a twist-angle-dependent Josephson coupling with magnitudes comparable to the highest-performing devices reported to date, but achieved through a straightforward and versatile contact method, offering a scalable and adaptable platform for future applications. These findings highlight the importance of both interface and contact engineering in addressing reproducibility in superconducting van der Waals heterostructures.

Figures (5)

• Figure 1: Schematic of the dry, cryogenic assembly process combining silicon nitride nanomembranes (NMBs) with the cryogenic stacking technique (CST) to fabricate twisted BSCCO Josephson junctions (JJs). Circular insets display optical images corresponding to each fabrication step. White scale bars represent 100 $\mu$m. I) Mechanical exfoliation of BSCCO flakes on SiO$_2$/Si substrates. II) Cooling the stage to -90 $^\circ$C and realization of the twisted JJ. III) Slowing warming the stage to -40 $^\circ$C and removing the PDMS. IV) Landing a previously picked-up membrane onto the JJ. V) Warming to room temperature and removing the shaped PDMS, leaving in place the membrane on the JJ.
• Figure 2: (a)–(c) Optical microscopy images of the three compared (covered, with hole, double cantilever) untwisted BSCCO devices, with close-up views of the junction regions. Pink dashed lines outline the edges of the nanomembranes. Scale bars represent 100 $\mu$m for the full images and 20 $\mu$m for the zoomed-in areas. (d) Representative current density–voltage (J–V) characteristics measured at 10 K for junctions fabricated with a fully covering membrane, a membrane with a hole, and the membrane with the double cantilever design. Arrows indicate the direction of the current sweep. All three samples exhibit a critical temperature of approximately $T_c \approx$ 84 K.
• Figure 3: Normalized differential resistance ($dV/dI$)/$R_N$ as a function of bias current density $J$ and temperature $T$ from 5 K to 85 K of the compared untwisted JJs shown in Figures \ref{['fig2:untwisted']}(a)-(c).
• Figure 4: (a) Optical microscopy images of the four twisted devices. On the top left of each images, a zoom-in of the region of the junction while on the top right the angle of the twist. Scale bars represent 100 $\mu$m for the full images and 20 $\mu$m for the zoomed-in areas. (b) Temperature-dependent electrical resistance normalized at 300 K obtained through the interface of the twisted BSCCO junctions. Inset: Angle dependence of $T_c$ of the corresponding JJs. The blue-shaded area is around the mean value of the critical temperatures and its width is two times the standard deviation. The black dashed line indicates the $T_c$ of an optimally doped bulk BSCCO crystal.
• Figure 5: (a) Normalized differential resistance ($dV/dI$)/$R_N$ as a function of normalized bias current $IR_N$ and temperature $T$ from 5 K to 88 K of the twisted JJs shown in Figure \ref{['fig4:twisted']}(a). The twist angle is displayed on top of each color plot. The current is swept from negative to positive bias. (b) Normalized bias current–voltage ($IR_N-V$) characteristics for all JJs at 10 K. Each curve is shifted along the y-axis for better visualization. the red triangles highlight the position of the $I_cR_N$ value were the voltage jumps occur in the $IR_N-V$ curve. (c) Comparison of the angular dependence of $I_cR_N$ between this work (10 K), the work of Zhao et al. (12 K) and Martini et al. (5 K). The red dashed line follows the $cos(2\theta)$ curve, which is the expected angular dependence in first approximation for tunneling between $d-$wave superconductors. The error bars show the uncertainty on the value of $R_N$.