YBa₂Cu₃O₇ nano superconducting quantum interference devices on MgO bicrystal substrates

Area of Science:

• Superconducting quantum devices
• Materials science
• Nanotechnology

Background:

• High-transition temperature cuprate superconductors like YBCO are crucial for advanced superconducting quantum interference devices (SQUIDs).
• Grain boundary Josephson junctions are a common approach for fabricating YBCO SQUIDs.
• Substrate choice significantly impacts SQUID performance and fabrication processes.

Purpose of the Study:

• To fabricate and characterize nanopatterned YBCO SQUIDs on Magnesium Oxide (MgO) substrates.
• To investigate the role of gold (Au) layer thickness in device performance and noise characteristics.
• To compare the performance of YBCO SQUIDs on MgO with those on Strontium Titanate (STO).

Main Methods:

• Epitaxial growth of YBCO films on MgO bicrystal substrates via pulsed laser deposition.
• Nanopatterning using Gallium focused ion beam (FIB) milling with varying Au layer thicknesses.
• Electrical transport and noise measurements at 4.2 K.
• Bias-reversal readout for ultra-low flux noise determination.

Main Results:

• YBCO nanoSQUIDs on MgO exhibit performance comparable to devices on STO.
• The Au layer, while necessary for FIB milling, impacts junction resistance, critical current, and inductance.
• Devices with 65 nm Au showed significantly lower low-frequency excess noise compared to STO or reduced Au devices.
• Ultra-low flux noise of ~175 nΦ0 Hz-1/2 down to 10 Hz was achieved in one MgO device.

Conclusions:

• MgO is a viable substrate for high-performance YBCO nanoSQUIDs, offering potential for reduced intrinsic noise.
• Careful optimization of the Au layer thickness is critical for balancing fabrication integrity and noise performance.
• The demonstrated ultra-low flux noise highlights the potential of these devices for sensitive magnetic field sensing applications.