Superconducting-plasma resonance along the c axis in various copper oxide superconductors

Superconducting plasma along the c axis in the far-infrared region are observed in various high-Tc cuprates including electron-doped and Bi cuprates. It is shown that the sphere resonance of the powder samples occurs near the frequency where the plasma along the c axis exists, since it is only sensitive to the c-axis dielectric constant in the far-infrared region due to the metal-like property in the ab plane. This method provides an alternative to the reflectivity measurement of single crystals for investigating the plasma along the c axis. By measuring down to 7.5 cm-1, the resonance is observed in Bi1.85Pb0.35Sr2Ca2Cu3.1Oy, La1.82Ca1.18Cu2O6+δ, (Nd0.66Sr0.205Ce0.135)2CuO4 (T* phase), (Ba0.56Sr0.44)2Cu1.1O2.2+δ(CO3)0.9, Pr1.85Ce0.15CuO4, and Nd2CuO4-xFx. Among the Bi cuprates, however, no resonance is observed in Bi2Sr2CaCu2Oy and Bi2Sr1.6La0.4CuOy in this frequency region, which suggests that the resonance exists below 7.5 cm-1. The resonance frequencies are discussed based on the Josephson-coupled layer model, and the c-axis penetration depth λc are estimated from the resonance frequencies. It appears that λc depends on the insulating nature of the barrier layer between CuO2 planes. Substitution effects on the resonance are also investigated in La2-xSrx(Cu1-zZnz)O4, Nd1.85Ce0.15(Cu1-zZnz)O4, and YBa2(Cu1-zCoz)3O6+δ, and compared to the results of YBa2(Cu1-zZnz)3O6+δ