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

Abstract

A scale for magnetic field resilience of a superconductor is set by the paramagnetic limit. Comparing the condensation energy of the Bardeen–Cooper–Schrieffer (BCS) singlet ground state with the paramagnetically polarized state suggests that for an applied field [Formula: see text] (in SI), singlet pairing is not energetically favorable. Materials exceeding or approaching this limit are interesting from fundamental and technological perspectives. This may be a potential indicator of triplet superconductivity, Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) pairing, and other mechanisms involving topological aspects of surface states and may also allow Cooper pair injection at high magnetic fields. We have analyzed the microscopic composition of such a material arising from an unexpected source. A microjet of an organo-metallic gas, [Formula: see text], can be decomposed by a gallium ion-beam, leaving behind a track of complex residue of gallium, tungsten, and carbon with remarkable superconducting properties, like an upper critical field, [Formula: see text], above its paramagnetic limit. We carried out atomic probe tomography to establish the formation of nano-crystalline tungsten carbide (WC) in the tracks and the absence of free tungsten. Supporting calculations show that for Ga distributed on the surface of WC, its s,p-orbitals enhance the density of states near the Fermi energy. The observed variation of [Formula: see text] does not show features typical of enhancement of critical field due to granularity. Our observations may be significant in the context of some recent theoretical calculation of the band structure of WC and experimental observation of superconductivity in a WC-metal interface.

Superconducting Materials and Devices Grown by Focused Ion and Electron Beam Induced Deposition

“…The value of T c , however, presents a non-monotonic dependence with the current, first increasing with increasing current, up to a significant figure of 6.2 K at 50 pA, and then decreasing again when the ion beam current is further increased. Whereas W-C deposits were first described within s-wave type-II superconductivity [73] some authors have recently reported anisotropic superconductivity features [74].…”

New precursors for direct writing of nanostructures using charged particle beams

“…Since the sputtering rate not only depends on the mass but also on the bond strength, preferential sputtering of light atoms could be (partially) suppressed if strong chemical bonds are formed. Deposits derived from W(CO)6 by Ga + -FIBID for example, have a very low (0 at% 135,202 to 8 at% 203 ) oxygen content while the C:W ratio is close to one. This indicates the effective formation of tungsten carbide, which was also confirmed by atom probe tomography showing only signals corresponding to WC while no atomic tungsten ions were observed.…”

“…This indicates the effective formation of tungsten carbide, which was also confirmed by atom probe tomography showing only signals corresponding to WC while no atomic tungsten ions were observed. 135 A potential explanation is that carbon will be bound strongly to tungsten, drastically reducing its sputtering rate while oxygen is still removed preferentially due to its lower mass. A similar trend is observed in the case of Mo(CO)6.…”