Fabrication of Josephson junctions using MgB2

Kenji Ueda, and Toshiki
MakimotoMaterials Science Laboratory

 The superconducting transition temperature (Tc) of magnesium diboride (MgB2) is 39K, the highest among metallic compounds [1]. The mechanism of superconductivity of MgB2 has been determined to be multi-gap superconductivity, and much research on MgB2 is investigating superconducting electronic applications.
 Superconducting tunnel junctions (Josephson tunnel junctions) are the key components of devices for superconducting electronic applications such as high-sensitive magnetic sensors (SQUID) and electromagnetic wave detectors. Nb(Tc=9K) tunnel junctions are mainly used in these superconducting devices, but cooling down devices using Nb requires liquid helium. MgB2, on the other hand, has higher Tc and superconducting devices using MgB2 tunnel junctions can work without using liquid helium. There have been many attempts to fabricate Josephson tunnel junctions that work above liquid nitrogen temperature (77 K) using high-Tc superconductors (HTS). However, Josephson tunnel junctions using HTS have not been fabricated yet, although it has been almost twenty years since the discovery of HTS. Therefore, MgB2 is expected for materials fabricating Josephson tunnel junctions.
 We have succeeded in fabricating Josephson tunnel junctions (MgB2/AlOx/MgB2) using as-grown MgB2 films fabricated by molecular beam epitaxy (MBE) [2] and observed, for the first time, supercurrent at 20 K [3]. This temperature is the highest among artificially fabricated tunnel junctions and can be easily reached by using compact commercial cryocoolers.
 Even though the properties of our tunnel junctions still have to be improved further, we believe that these results will enlarge the area of the superconducting electronic applications.

[1] J. Nagamatsu et al., Nature 410 (2000) 63.
[2] K. Ueda, M. Naito, Appl. Phys. Lett. 79 (2001) 2046.
[3] K. Ueda, S. Saito, K. Semba, T. Makimoto, and M. Naito, Appl. Phys. Lett. 86 (2005) 172502.

Fig. 1:
Typical current-voltage characteristics of the MgB2 tunnel junction measured at 4.2 K.
Fig. 2:
Temperature dependence of supercurrent of the MgB2 tunnel junction. The inset shows current-voltage characteristics measured at 20K.

[back] [Top] [Next]