Search and Control of Josephson Plasma Modes in 7^* Type High-Tc Superconductors

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The structure of high-T_c copper oxide superconductors is a superlattice of Josephson junctions, an infinite array of Josephson junctions along the crystallographic c- axis. This gives rise to a collective excitation mode, so-called Josephson plasma mode, appearing in the THz frequency region. Hoever, the Josephson plasma mode is a longitudinal excitation, and does not directly couple with THz radiation. In order to produce optical-active plasma mode, spatial modulation of the Josephson coupling strength is necessary. In the present project, we successfully manipulated the optical Josephson plasma mode by modulating the Josephson coupling strength(1) using chemical/structural operation to synthesize T^* type high-T_c cuprates which have apical oxygen atoms on one side of an CuO_2 plane and hence form an alternating array of Josephson junctions with different coupling strengths, and(2) applying external magnetic field parallel to the CuO_2 planes which penetrate as Josephson vortices between the CuO_2 planes and thus modulate the Josephson coupling strength.