  In this letter, the properties of split-ring resonators (SRRs) loaded with
high-Q capacitors and nonlinear varactors are theoretically analyzed and
experimentally measured. We demonstrate that the resonance frequency $f_m$ of
the nonlinear SRRs can be tuned by increasing the incident power. $f_m$ moves
to lower and higher frequencies for the SRR loaded with one varactor and two
back-to-back varactors, respectively. For high incident powers, we observe
bistable tunable metamaterials and hysteresis effects. Moreover, the coupling
between two nonlinear SRRs is also discussed.
