Realization of YBa2Cu3O{7-\delta} nanostructures with a focused Helium ion beam

Abstract

Helium ion microscopy is a leading edge imaging technology that uses a beam of helium ions to acquire high-resolution images of a wide range of materials and specimens down to the nanoscale level. Beside the excellent imaging quality, not least because of the contrast mechanisms, the heliom ion microscope (HIM) can be simultaneously operated as a tool for nanostructuring. One approach, which was pursued in the context of this thesis, is the local modification of superconducting properties of YBa2Cu3O7 (YBCO) thin films by irradiation with a focused helium ion beam created by a helium ion microscope. This technique allows for versatile and more complex circuit designs compared to state-of-the-art Josephson junctions (JJs) based on grain boundaries in YBCO.

The first part deals with the characterization of JJs and superconducting quantum devices (SQUIDs), created by helium ion beam irradiation and their evolution with time when stored at room temperature. Through the choice of the irradiation dose, the electrical properties of these devices can be adjusted. When irradiated with a sufficiently high dose, the material can be driven into an insulating state with completely suppressed superconductivity, enabling to define geometries without removal of material.

Furthermore, by the same principle, this technique is capable of creating artificial pinning centers for Abrikosov vortices. In previous work, pinning arrays exhibited matching effects, i.e., effects of commensurability between the vortex lattice and the pinning array, only in the vicinity of the critical temperature of the thin film, due to limitations in the nearest neighbor distance of artificial pinning sites obtained with the used fabrication techniques. By utilizing a focused helium ion beam beam, it becomes possible to fabricate pinning arrays with spacings down to 20 nm and first matching fields (where the densities of pinning centers and vortices coincide) up to 6 T over a wide temperature range significantly below the critical temperature, confirming strong pinning of the vortices at the artificially engineered pinning centers.