Formation of tungsten carbide by focused ion beam process: A route to high magnetic field resilient patterned superconducting nanostructures

Abstract

A directed narrow jet of an organo-metallic gas containing a heavy metal can be decomposed by an accelerated beam of gallium ions, leaving behind a track made up of a complex residue of gallium, heavy metal and carbon. The process is highly controllable and in certain cases, the residue has remarkable superconducting properties, like an upper critical field (Hc2 ∼ 10 T) that is higher than the paramagnetic limit. Werthamer-Helfand-Hohenberg (WHH) analysis shows the presence of moderate spin-orbit (SO) scattering and a Maki parameter compatible with unconventional (e.g., FFLO-like) superconducting states. Using a spatially resolved mass spectrometric technique (Atomic probe tomography), we show that the possible origin of the SO effects lies in the formation of nanocrystalline tungsten carbide (WC) with a possible non-centrosymmetric crystal structure. We also show that when Ga is distributed on the surface of nano-crystallite WC, the sp-orbitals of Ga give rise to bands with a significant density of states near the Fermi energy. The superconductor is in the dirty limit where the mean free path (l) is much smaller than the zero temperature coherence length, i.e., l ξ0 ≈ 5 nm. Low-temperature magnetotransport with in-situ rotation of the sample in a magnetic field shows clear anisotropic effects that weaken as the width of the tracks are increased from ∼ 100 nm to ∼ 1 µm [Phys. Rev. B 103, L020504, 2021]. The combination of the transition temperature (Tc ≈ 5 K), the critical field Hc2 ≥ 10 T and nanometer-scale patternability of these tracks make them an attractive component for engineered mesoscopic structures.