Materials Science Laboratory
For the electrical application of superconductor, it is
essential to make a Josephson junction, where the two superconductors are
connected by the thin insulating layers, but a junction using high-Tc superconductors
has not been fabricated yet. Recently, it has been revealed that some kinds
of high-Tc superconductors behave as the Ā~Ā~Ā~S/I/SĀ~Ā~Ā~ (Ā~Ā~Ā~superconductor/insulator/superconductorĀ~Ā~Ā~)-type
Josephson junction array by themselves, which is the so-called intrinsic
Josephson effect. However, the relation between the types of high-Tc superconductors
and the types of Josephson junction arrays has not been clarified.
Among the various kinds of high-Tc superconductors, there are so-called multilayers which have more than one block layer per unit cell. We have shown that the T*-phase, which is a typical example of multilayers, behaves as the Ā~Ā~Ā~S/I/S/IĀf/S/I/SĀ~Ā~Ā~ (Ā~Ā~Ā~superconductor/ insulator1/superconductor/insulator2/superconductorĀ~Ā~Ā~ )-type Josephson junction array (Fig. 1). This was confirmed by observing the double Josephson plasma resonance, which corresponds to the plasma at the rocksalt-type (La,Sr)2O2 block layer (S/I1/S) junction and the fluorite-type Sm2O2 block layer (S/I2/S) junction (Fig. 2) . A comparison to the results for other high-Tc superconductors revealed that the high-Tc superconductors are related to many types of Josephson junction arrays, such as the Ā~Ā~Ā~superconductor /metal/superconductor Ā~Ā~Ā~ and Ā~Ā~Ā~superconductor/superconductorĀf/superconductorĀ~Ā~Ā~-type junction arrays, by their crystal structures . We obtained these results by using a newly developed measurement method (sphere resonance method), which does not need to use single crystals, and constructing a THz-region (150-600 GHz) measurement system. These results give a new understanding of the electronic state of high-Tc superconductors along the c-axis as well as show the possibility of fabricating new kinds of devices using the intrinsic Josephson effect.
 H. Shibata and T. Yamada, Phys. Rev. Lett. 81(1998) 3519.
 H. Shibata and A. Matsuda, Phys. Rev. B59(1999) 11672.
Fig. 1: Crystal structure of T*-phase, Ā~Ā~Ā~S/I/S/IĀf/S/I/SĀ~Ā~Ā~-type Josephson junction array, and its equivalent circuit.
Fig. 2: Absorption spectra of T*-phase.