  We have prepared meta-atoms based on radio frequency superconducting quantum
interference devices (RF SQUIDs) and examined their tunability with dc magnetic
field, rf current, and temperature. RF SQUIDs are superconducting split ring
resonators in which the usual capacitance is supplemented with a Josephson
junction, which introduces strong nonlinearity in the rf properties. We find
excellent agreement between the data and a model which regards the Josephson
junction as the resistively and capacitively-shunted junction. A magnetic field
tunability of 80 THz/Gauss at 12 GHz is observed, a total tunability of 56$%$
is achieved, and a unique electromagnetically-induced transparency feature at
intermediate excitation powers is demonstrated for the first time. An RF SQUID
metamaterial is shown to have qualitatively the same behavior as a single RF
SQUID with regards to DC flux and temperature tuning.
