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.
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