High frequency properties of YBCO thin films diagnosed by time-domain terahertz spectroscopy

We measure the complex conductivity σ and the surface impedance of c-axis oriented 100-nm-thick YBa₂Cu₃O_7-δ (YBCO) thin films deposited on (100)MgO substrates (0.5 mm thick) at frequencies between 0.5 and 1.5 THz, using time-domain terahertz spectroscopy. The real part of σ decreases with increasing frequency and its value is about 2.4×10⁵ (S/m) at 1 THz. The imaginary part of σ increases with increasing frequency and then monotonically decreases at around 750 GHz. The real part of σ of YBCO thin films increased with decreasing temperature and decreases below 60 K. The imaginary part of σ increased with decreasing temperature and the trend is enhanced below 60 K. The surface resistance of YBCO thin film is of the order of 10^-1Ω and increases with increasing temperature. The surface reactance of YBCO thin film is independent of the frequency in this THz region, and its value is less than 10^-3Ω at 40 K.