Intermodulation Distortion and Nonlinearity in MgB2: Experiment and Theory

The nonlinear surface impedance and intermodulation distortion (IMD) products of MgB₂ thin films have been measured as a function of temperature. The measurements use a stripline-resonator technique, following an approach that was developed and extensively applied to similar measurements of YBCO. The films were grown using the deposition technique of reactive evaporation onto LaAlO₃ and sapphire substrates. The low-power R_S(T) is comparable to that of sputtered Nb films on sapphire, and lower than that of YBCO at the corresponding reduced temperatures. The rf-magnetic-field dependence at T < 20 K follows a moderate slope for H_rf < 500 Oe. At higher rf magnetic fields the increase is larger. The IMD values are larger than those of the best quality YBCO by more than 10 dB. The theoretical analysis assumes uncoupled pi and sigma channels (no interband scattering) and intrinsic nonlinearity to the lowest nonlinear order in the radiation field. Under these assumptions, the calculated IMD indicates dominance of the pi-channel contribution in the clean limit, with a temperature dependence that is consistent with the measured curve. This result indicates that the observed IMD may be consistent with an intrinsic origin for the nonlinearity.