Fabrication and properties of MgB2/AlOx/MgB2 tunnel junctions

TL;DR

This study reports the fabrication of MgB2/AlOx/MgB2 Josephson tunnel junctions with substantial supercurrent and a well-defined superconducting gap, analyzing their microstructure and temperature-dependent behavior to understand non-idealities.

Contribution

First demonstration of MgB2/AlOx/MgB2 tunnel junctions with detailed microstructural analysis and temperature-dependent superconducting properties.

Findings

Junctions exhibit supercurrent and a superconducting gap of 2.2-2.3 mV.

Ic approaches 2.0 mV at 50 mK, but drops rapidly with temperature.

Poor crystallinity and amorphous layers likely cause non-ideal behaviors.

Abstract

Sandwich-type MgB2 Josephson tunnel junctions (MgB2/AlOx/MgB2) have been fabricated using as-grown MgB2 films formed by molecular beam epitaxy. The junctions exhibited substantial supercurrent and a well-defined superconducting gap (D=2.2~2.3mV). The superconducting gap voltage (D) agrees well with those of the smaller gap in the multi-gap scenario. The IcRN product is 0.4-1.3 mV at 4.2K, but approaches to 2.0 mV at 50 mK. The Ic has peculiar temperature dependence far from the Ambegaokar-Baratoff formula. It rapidly decreases with temperature, and disappears above T = 20 K, which is much lower than the gap closing temperature. Interface microstructure between AlOx and MgB2 were investigated using cross-sectional transmission electron microscopy to clarify the problems in our tunnel junctions. There are poor-crystalline MgB2 layers and/or amorphous Mg-B composite layers of a few nanometers between AlOx barrier and the upper MgB2 layer. The poor-crystalline upper Mg-B layers seem to behave as normal metal or deteriorated superconducting layers, which may be the principal reason for all non-idealities of our MgB2/AlOx/MgB2 junctions.