Cuprate Superconductivity Analysis via Helium Ion Microscopy

Understanding superconductivity at high temperatures has been an evasive scientific hurdle. This thesis outlines efforts taken by the author to exploit recent advances in nanofabrication through helium focused ion beam (He-FIB) for the advancement of scientific and technical understanding of high-transition temperature superconductivity. He-FIB has recently been shown to be a promising technique for the fabrication of high-quality directly-written planar Josephson junctions (JJs) with tunable parameters in YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO). Herein, it is demonstrated that these He-FIB JJs are fundamental building blocks of common superconducting circuits in novel planar geometries like superconducting quantum interference devices, gradiometers, and single flux quantum logic. Furthermore, it is demonstrated that the He-FIB direct-write technique can be utilized in rare earth based cuprates, such as HoBCO, and exfoliated BSCCO, which may lead to advanced three-dimensional circuits by combining the He-FIB direct-written JJs with the intrinsic $c$-axis BSCCO JJs. I demonstrate that series arrays of closely spaced, planar long JJs are transducers of magnetic flux featuring high-dynamic range and wide-bandwidth and that they are operable at cryogenic nitrogen temperatures. For this application, a robust automated process using FIB nanolithography for layout designs with feature sizes from sub-nanometer to millimeter scales is developed. Additionally, the JJ array’s geometry and properties for magnetic flux sensing are optimized. I present a series array of long JJs fabricated in YBCO containing 2640 JJs with a critical current deviation of 30\% exhibiting a sensitivity of 1.7 mV/$\mu$T and a linear response over a range of 10 $\mu$T at 40 K, resulting in a dynamic range of 100 dB. The exact nature of the order parameter is not completely understood in cuprate materials, which can impact the performance of these devices. Direct-written planar JJs offer a novel way to characterize cuprate superconductors in their $a$-$b$ plane regarded to be the site of superconductivity in these materials. Measurements of the density of states are taken at variable angles to help determine the practical order pairing symmetry. Measurements indicate a significant $s$-wave component mixed with no more than 30\% of the prominent $d$-wave symmetry