Effect of metal/substrate interfaces on radio-frequency loss in superconducting coplanar waveguides

Microscopic two-level systems (TLSs) are known to contribute to loss in resonant superconducting microwave circuits. This loss increases at low power and temperatures as the TLSs become unsaturated. We find that the loss is dependent on both the substrate-superconductor interface and the roughness of the surfaces. A native, oxide-free interface reduced the loss due to TLSs. However, a rough surface in the CPW gap did not cause more TLS loss, but the overall loss was significantly increased for the roughest surfaces.