Selective Excitation of Collective Modes in Multiband Superconductor MgB_{2}.

Area of Science:

• Condensed Matter Physics
• Superconductivity
• Terahertz Spectroscopy

Background:

• Nonlinear and nonequilibrium terahertz (THz) spectroscopies advance superconductor research.
• Distinguishing Higgs, Leggett modes, and BCS charge fluctuations in MgB2 remains debated.

Purpose of the Study:

• To selectively excite and identify collective modes in the two-band superconductor MgB2.
• To resolve controversies regarding Higgs and Leggett mode identification in MgB2.

Main Methods:

• Utilized multicycle terahertz (THz) pump-probe spectroscopy with 2D scans.
• Employed single-cycle THz pulses for nonadiabatic excitation.
• Systematically adjusted THz pump pulse characteristics for selective mode excitation.

Main Results:

• Identified the π-band Higgs mode via resonant enhancement when pump frequency matched the π-band gap.
• Observed a damped oscillation attributed to the Leggett mode under nonadiabatic conditions.
• Demonstrated mode-selective responses in MgB2 through tailored THz excitation.

Conclusions:

• The study provides an experimental method to discriminate between collective modes in MgB2.
• Findings help reconcile existing controversies in superconductor research.
• Highlights the importance of interband coupling for observing collective modes in multiband superconductors.