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

高T_c氧化铜超导体的结构是约瑟夫森结的超晶格，即沿晶体学c轴沿着无限排列的约瑟夫森结。这导致出现在太赫兹频率区域中的集体激发模式，即所谓的约瑟夫森等离子体模式。然而，约瑟夫森等离子体模式是纵向激发，并且不直接与THz辐射耦合。为了产生光学活性等离子体模式，需要对约瑟夫森耦合强度进行空间调制。在本项目中，我们成功地通过调制约瑟夫森耦合强度来操纵光学约瑟夫森等离子体模（1）利用化学/结构操作合成了T^* 型高T_c铜氧化物，其在CuO_2平面的一侧具有顶端氧原子，从而形成具有不同耦合强度的约瑟夫森结交替阵列，（2）外加磁场平行于CuO_2面，磁场在CuO_2面之间形成Josephson涡旋，从而调制Josephson耦合强度。

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.