We present measurements of the temperature-dependent frequency shift of five niobium superconducting coplanar waveguide microresonators with center strip widths ranging from 3 μm to 50 μm, taken at temperatures in the range 100-800 mK, far below the 9.2 K transition temperature of niobium. These data agree well with the two-level system (TLS) theory. Fits to this theory provide information on the number of TLS that interact with each resonator geometry. The geometrical scaling indicates a surface distribution of TLS, and the data are consistent with a TLS surface layer thickness of order a few nm, as might be expected for a native oxide layer.
@article{arxiv.0802.4457,
title = {Experimental evidence for a surface distribution of two-level systems in superconducting lithographed microwave resonators},
author = {Jiansong Gao and Miguel Daal and Peter Day and Benjamin Mazin and Henry LeDuc and Anastasios Vayonakis and Shwetank Kumar and Bernard Sadoulet and Jonas Zmuidzinas},
journal= {arXiv preprint arXiv:0802.4457},
year = {2009}
}