C-axis Josephson plasma resonance observed in Tl₂Ba₂CaCu₂O₈ superconducting thin films by use of terahertz time-domain spectroscopy

We have unambiguously observed the c-axis Josephson plasma resonance (JPR) in high-critical-temperature (T_c) cuprate (Tl₂Ba₂CaCu₂O₈) superconducting thin films, employing terahertz time-domain spectroscopy in transmission as a function of temperature in zero magnetic field. These are believed to be the first measurements of the JPR temperature dependence of a high-T_c material in transmission. With increasing temperature, the JPR shifts from 705 GHz at 10 K to ∼170 GHz at 98 K, corresponding to an increase in c-axis penetration depth from 22.4 ± 0.6 μm to 94 ± 9 μm. The linewidth of the JPR peak increases with temperature, which indicates an increase in the quasi-particle scattering rate. We have probed the onset of the c-axis phase coherence to ∼0.95 T_c. The JPR vanishes above T_c as expected.