12-4 Curious Characters of Iron-Based Superconductors

－R&D for Application Based on Their Unique Features－

Fig.12-8　Models of Josephson junctions

Fig.12-8　Models of Josephson junctions

A conventional Josephson junction is described by motion of a simple pendulum model (a), while a junction with iron-based superconducting materials is described by a coupled pendulum model (b).

 

Fig.12-9　Application potential of iron-based superconductors

Fig.12-9　Application potential of iron-based superconductors

Various potential applications of iron-based superconducting materials, such as material properties probes, voltage definition, wires, emitters, and qubit.

Superconductivity is one of the most remarkable phenomena in condensed matter systems. A typical behavior is the sudden disappearance of electric resistance below the superconducting transition temperature. Such a property not only brings about transportation with no energy loss but also leads to sensitive detectors and devices much superior to standard ones. Presently, in the nuclear R&D field, potential applications of the superconductors are being intensively studied. In fact, a superconducting coil will be indispensable for a nuclear fusion reactor.

A new superconducting material, an iron-based superconductor was discovered in 2008. This type of a material has a very high transition temperature compared to copper-oxide superconductors and a richly varied chemical composition. One of its most curious features is that multiple (at least three) superconducting gaps coexist, although most existing superconductors have a single gap. Recently, we have been studying the device characteristics originating from their multi-gap feature, focusing on the Josephson junction, which is a typical superconducting device.

First, we directed our attention to the fact that a conventional Josephson junction is theoretically equivalent to the motion of a single pendulum, and the behavior of the superconducting tunneling current is well described by such an elementary dynamical model (Fig.12-8). The present task is to reveal how the conventional description alters when one of the superconducting electrodes is replaced by the iron-based superconductor. We successfully demonstrated that the system is described by coupled oscillators, and went on to predict various electrical and magnetic characteristics of iron-based superconducting devices.

The present contribution provides a theoretical foundation of Josephson junctions with iron-based superconductors. We believe that this study clarifies the essence of iron-based superconductors and will lead to further developments in superconducting engineering (Fig.12-9).

This work is supported by CREST “Framework Development for Multiscale and Multiphysics Simulations towards Novel Application of Superconductivity” from Japan Science and Technology Agency.

Reference
Ota, Y. et al., Theory of Heterotic Superconductor-Insulator-Superconductor Josephson Junctions between Single- and Multiple-Gap Superconductors, Physical Review Letters, vol.102, issue 23, 2009, 237003-1－237003-4.

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