Far-infrared plasma and electromagnetic waves in anisotropic superconductors

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A plasma edge appearing in the c-axis polarized far-infrared reflectivity spectrum of high-T_C superconductors, which was first discovered for La-cuprates by our group, is now recognized as a general phenomenon of cuprate superconductors. For Bi cuprates the plasma frequency is located in the microwave region because of their extremely large anisotropy. This plasma is now called as Josephson plasma, since its appearance is connected to Josephson coupling between superconducting CuO_<> layrs.We have pursued, mainly on La_2-_XSr_XCuO_4 and YBa_<a2>Cu_3O_<7-y1>, how the Josephson plasma frequency varies with doping, Zn-substitution, and applied magnetic fields. In addition, using thin LSCO crystals, the propagation and dissipation of the plasma wave have been investigated. Major results and findings are :1.The Josephson plasma frequency (omega_p) is determined predominantly by the value of the c-axis normal-state resistivity just above T_c. The anisotropy of the high-T_c superconductors becomes weaker and thus the c-axis resistivity becomes lower as doping proceeds. It is demonstrated that the decrease in anisotropy leads to an increase in omega_p.2.It is shown that the high-T_c superconductors become transparent for c-axis polarized light with frequency higher than omega_p. This is explained by the propagation of a polariton which is a coupled electromagnetic wave between light and Josephson plasma. The polariton is found to suffur weak damping which arises from the gaplessness of high-T_c superconductivity, intrinsic of cooper pairing with d-wave symmetry.3.While omega_p in Bi cuprates is very sensitive to externally applied magnetic fields, that in La cuprates is found to be insensitive. This difference is ascribable to the difference in the vortex pinning strength, and it turns out that the pinning becomes stronger with increase of doping in any cuprate system.