Investigation of microwave properties of superconducting Nb0.4Ti0.6

A superconducting Nb0.4Ti0.6test cavity was constructed to measure the surface losses as function of frequency in the range from 2 to 8 GHz and as a function of temperature below 4.2 K. The temperature-dependent part of the surface resistance agrees essentially with the BCS theory if the material parameters Tc= 9.8 K, Δ(0) = 1.73 kBTc, ξo= 380 Å, λL= 310 Å, and to 8 Å are taken. The magnitude and frequency dependence of the temperature-independent residual resistance observed in the niobium titanium cavity suggests no difference in the behavior of type I and II superconductors. The highest Qo= 2.9 × 10⁹was measured in the TE011mode at 3.9 GHz. A coaxial cavity with removable sample was constructed for an in-depth study of rf breakdown and its relationship with metallurgical parameters as characterized by ac loss, magnetization, and penetration depth measurements. Peak fields in solid type-II superconducting cavities are limited by their low thermal conductivity suggesting the use of films, 0.01 to 0.1 mm thick, on pure niobium or copper. The highest magnetic field of 120 G was measured on a recrystallized sample suggesting that the lower critical field Hc1(for this sample 75 G) does not prevent the application of high-Tctype-II superconductors to microwave devices.