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

Area of Science:

• Condensed Matter Physics
• Superconductivity
• Terahertz Spectroscopy

Background:

• MgB_{2} is a multigap superconductor with unique electronic properties.
• Terahertz (THz) spectroscopy probes low-energy excitations in materials.
• Understanding superconducting gap dynamics is crucial for novel electronic applications.

Purpose of the Study:

• Investigate the terahertz (THz) nonlinear optical response of MgB_{2}.
• Characterize the superconducting amplitude mode in MgB_{2} using THz two-dimensional coherent spectroscopy (THz 2DCS).
• Compare the THz response of MgB_{2} to single-gap superconductors like NbN.

Main Methods:

• Utilized broadband and narrow-band THz drive fields for excitation.
• Employed THz two-dimensional coherent spectroscopy (THz 2DCS) to measure nonlinear responses.
• Analyzed the temperature dependence of the first (FH) and third harmonic responses.

Main Results:

• Identified a nonlinear response at twice the lower superconducting gap energy (2Δ_{π}) at low temperatures.
• Observed a monotonic increase in FH intensity with decreasing temperature when normalized.
• Found that the superconducting amplitude mode in MgB_{2} is well-defined only at low temperatures and exhibits strong damping with increasing temperature.

Conclusions:

• The observed THz nonlinear response in MgB_{2} suggests a temperature-dependent superconducting amplitude mode.
• Strong damping of the amplitude mode with increasing temperature indicates the significant role of interband coupling in MgB_{2}.
• These findings provide insights into the fundamental properties of multigap superconductivity and THz-driven dynamics.