Amplitude Mode in a Multigap Superconductor MgB_{2} Investigated by Terahertz Two-Dimensional Coherent Spectroscopy.

We have investigated the terahertz (THz) nonlinear response of the multigap superconductor MgB_{2}, using THz two-dimensional coherent spectroscopy (THz 2DCS). With broadband THz drive fields, we identified a nonlinear response at twice the lower superconducting gap energy 2Δ_{π} at the lowest temperatures. Using narrow-band THz driving pulses, we observed first (FH) and third harmonic responses. The FH intensity shows a monotonic increase with decreasing temperature when properly normalized by the driving field strength. This is distinct from the single-gap superconductor NbN, where the FH signal exhibited a resonant enhancement at temperatures when twice the gap energy 2Δ was resonant with the driving photon energy, which was interpreted to originate from the superconducting amplitude mode. Our results in MgB_{2} are consistent with a well-defined amplitude mode only at the lowest temperatures and indicate strong damping as temperature increases. This likely indicates the importance of interband coupling in MgB_{2} and its influence on the nature of the amplitude mode and its damping.