Band-selective third-harmonic generation in superconducting MgB2: Possible evidence for the Higgs amplitude mode in the dirty limit

We report on time-resolved linear and nonlinear terahertz spectroscopy of the two-band superconductor MgB2 with a superconducting transition temperature T-c approximate to 36 K. Third-harmonic generation (THG) is observed below T-c by driving the system with intense narrow-band THz pulses. For the pump-pulse frequencies f = 0.3, 0.4, and 0.5 THz, the temperature-dependent evolution of the THG signals exhibits a resonance maximum at the temperatures with the resonance conditions 2 f = 2 Delta(pi) (T) fulfilled, for the dirty-limit superconducting gap 2 Delta(pi). In contrast, for f = 0.6 and 0.7 THz with 2 f > 2 Delta(pi) (T -> 0) = 1.03 THz, the THG intensity increases monotonically with decreasing temperature. Moreover, for 2f < 2 Delta(pi) (T -> 0) the THG is found nearly isotropic with respect to the pump-pulse polarization. These results suggest a predominant contribution of the driven Higgs amplitude mode of the dirty-limit pi-band superconducting gap, pointing to the importance of scattering for observation of the Higgs mode in superconductors.