Amplitude mode in a multi-gap superconductor MgB${}_2$ investigated by terahertz two-dimensional coherent spectroscopy

Abstract

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 $2{\Delta }_{\pi }$ 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 $2\Delta$ 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.

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• Unlocking Quantum Control and Multi-Order Correlations via Terahertz Two-Dimensional Coherent Spectroscopy
• Two-dimensional coherent spectroscopy of disordered superconductors in the narrow-band and broad-band limits
• Third-harmonic generation in superconductors: Role of quantum geometry in the competition between Higgs mode and quasiparticles
• Higgs and Nambu-Goldstone modes in a spin-1 $XY$ model with long-range interactions
• Momentum-resolved two-dimensional spectroscopy as a probe of nonlinear quantum field dynamics
• Two-dimensional THz spectroscopy in electronic systems: a many-body diagrammatic approach