The dispersion relations and characteristics lengths of surface plasmon polaritions (SPPs) at the superconductor-air interface in THz band with different external magnetic fields are analyzed based on two-fluid model, the changes of excitation frequency and angle in different external magnetic fields by attenuated total reflection (ATR) technique are also discussed. The results show that as the external magnetic field increases, the deviation degree of dispersion curve from light line is increased, and the SPPs propagation length, SPPs normalized wavelength and the penetration depth into air are decreased, but the penetration depth into superconductor is increased. Moreover, the dispersion relations and characteristics lengths also vary with different frequencies. The excitation resonance angle is not sensitive to external magnetic field, but the resonance frequency increases with the increase of the external magnetic field. The changes of SPPs properties are ascribed to the electric state transformation between superconducting and normal states.
