Broadband electron spin resonance from 500 MHz to 40 GHz using   superconducting coplanar waveguides
  We present non-conventional electron spin resonance (ESR) experiments using microfabricated superconducting waveguides. We show that a very broad frequency range, from 0.5 to 40 GHz, becomes accessible with a high frequency accuracy of less than 1 MHz at low temperatures down to 1.6 K and in fields up to 1.4 T. This allows an accurate inspection of the ESR absorption position in the frequency domain, in contrast to the more common observation in the field domain. We demonstrate the applicability of frequency-swept ESR on Cr3+ atoms in ruby as well as on organic radicals of the Nitronyl-nitroxide family (using the 2-(4'-methoxyphenyl)-4,4,5,5-tetra-methylimidazoline-1-oxyl-3-oxide, a.k.a. NitPhOMe). Measurements between 1.6 and 30 K reveal a small frequency shift of the ESR and a resonance broadening below the critical temperature, which we both attribute to a modification of the magnetic field configuration due to the appearance of shielding currents. 
C. Clauss, D. Bothner, D. Koelle, R. Kleiner, L. Bogani, M. Scheffler,   and M. Dressel
cond-mat.mes-hall