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


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

Kovalev, S.; Dong, T.; Shi, L.-Y.; Reinhoffer, C.; Xu, T.-Q.; Wang, H.-Z.; Wang, Y.; Gan, Z.-Z.; Germanskiy, S.; Deinert, J.-C.; Ilyakov, I.; van Loosdrecht, P. H. M.; Wu, D.; Wang, N.-L.; Demsar, J.; Wang, Z.

We report on time-resolved linear and nonlinear terahertz spectroscopy of the two-band superconductor MgB2
with a superconducting transition temperature Tc ≈ 36 K. Third-harmonic generation (THG) is observed below
Tc 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 = 2Delta π (T ) fulfilled, for the dirty-limit superconducting gap
2Delta π . In contrast, for f = 0.6 and 0.7 THz with 2 f > 2Delta π (T → 0) = 1.03 THz, the THG intensity increases
monotonically with decreasing temperature. Moreover, for 2 f < 2Delta π (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 π -band superconducting gap, pointing to the importance of scattering
for observation of the Higgs mode in superconductors.

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Publ.-Id: 33496