NanoSQUIDs from YBa$_2$Cu$_3$O$_7$/SrTiO$_3$ superlattices with bicrystal grain boundary Josephson junctions

TL;DR

This paper demonstrates the fabrication of nanoSQUIDs using YBa₂Cu₃O₇/SrTiO₃ superlattices with grain boundary Josephson junctions, showing reduced low-frequency flux noise compared to single-layer devices.

Contribution

It introduces a novel superlattice-based nanoSQUID fabrication method with significantly lower flux noise, enhancing device performance.

Findings

Superlattice nanoSQUIDs have comparable transport properties to single-layer devices.

They exhibit over ten times less low-frequency flux noise.

Improved microstructure at grain boundaries reduces noise.

Abstract

We report on the fabrication and characterization of nanopatterned dc SQUIDs with grain boundary Josephson junctions based on heteroepitaxially grown YBa$_2$Cu$_3$O$_7$ (YBCO)/ SiTrO$_3$ (STO) superlattices on STO bicrystal substrates. Nanopatterning is performed by Ga focused-ion-beam milling. The electric transport properties and thermal white flux noise of superlattice nanoSQUIDs are comparable to single layer YBCO devices on STO bicrystals. However, we find that the superlattice nanoSQUIDs have more than an order of magnitude smaller low-frequency excess flux noise, with root-mean-square spectral density $S_\Phi^{1/2}\sim 5-6\,\mu\Phi_0/\sqrt{\rm Hz}$ at 1 Hz ($\Phi_0$ is the magnetic flux quantum). We attribute this improvement to an improved microstructure at the grain boundaries forming the Josephson junctions in our YBCO nanoSQUDs.