Fabricating Nanogaps inYBa2Cu3O7−δ

Abstract

The advances of nanotechnologies applied to high-critical-temperature superconductors (HTSs) have recently given a huge boost to the field, opening new prospectives for their integration in hybrid devices. The feasibility of this research goes through the realization of HTS nanogaps with superconductive properties close to the as-grown bulk material at the nanoscale. Here we present a fabrication approach allowing the realization of YBa 2 Cu 3 O 7−δ (YBCO) nanogaps with dimensions as small as 35 nm. To assess the quality of the nanogaps, we measure, before and after an ozone treatment, the current-voltage characteristics and the resistance versus temperature of YBCO nanowires with various widths and lengths, fabricated by using different lithographic processes. The analysis of the superconducting transition with a thermally activated vortex-entry model allows us to determine the maximum damage the nanowires undergo during the patterning which relates to the upper bound for the dimension of the nanogap. We find that the effective width of the nanogap is of the order of 100 nm at the superconducting transition temperature while retaining the geometrical value of about 35 nm at lower temperatures. The feasibility of the nanogaps for hybrid Josephson devices is demonstrated by bridging them with thin Au films. We detect a Josephson coupling up to 85 K with an almost ideal magnetic-field response of the Josephson current. These results pave the way for the realization of complex hybrid devices, where tiny HTS nanogaps can be instrumental to study the Josephson effect through barriers such as topological insulators or graphene.

Josephson effect through YBa2Cu3O7−δ /Au-encapsulated nanogaps

“…To fit the I C (T ) measurement with Eq. (1), we assumed a value ≈ 16.5 meV for the superconducting energy gap at the interface YBCO/Au in agreement with what we have found experimentally [14] and what is reported in literature [30]. As many parameters are involved in the SINI S model, there is not a unique set that can fit our data (see Table I).…”

“…The nanogaps are defined by using a combination of a diamondlike carbon (DLC) mask and soft ion milling [14][15][16]. The slope that the nanogap edges form with the substrate has been extracted by atomic force microscope (AFM) and scanning electron microscope (SEM) inspection of the nanostructures in analogy with previous works [17,18].…”

“…1(a)-1(c). More details on the fabrication process can be found elsewhere [14]. Briefly, to realize our devices, first a nanogap structure is defined in a high-quality, epitaxial c-axis-oriented YBa 2 Cu 3 O 7−δ film grown on a (110) MgO substrate, with a thickness of 50 nm and covered with a 25 nm thick protective layer of Au (deposited in situ).…”

“…This is done by using a lift-off process for a Ti Figure 1(d) shows a SEM image of a typical device. To recover the oxygen loss and damages occurring at the edges of the nanogap, in particular during the ion milling procedure to form the nanogaps, we preformed an ex situ ozone annealing [14,15] after the full process. This procedure has shown to be very effective to recover optimal doping in YBCO nanowires and to restore YBCO gap features at the YBCO/Pt/Au interfaces [14,15].…”

“…To recover the oxygen loss and damages occurring at the edges of the nanogap, in particular during the ion milling procedure to form the nanogaps, we preformed an ex situ ozone annealing [14,15] after the full process. This procedure has shown to be very effective to recover optimal doping in YBCO nanowires and to restore YBCO gap features at the YBCO/Pt/Au interfaces [14,15]. However, we cannot exclude that this procedure may lead to a formation of an oxide layer on the Ti adhesion film as we will discuss later.…”