Anomalous Terahertz Nonlinearity in Disordered s-Wave Superconductor Close to the Superconductor-Insulator Transition

Area of Science:

• Condensed matter physics
• Superconductivity
• Nonlinear optics

Background:

• The Higgs mode in superconductors is crucial for understanding emergent phenomena.
• Nonlinear terahertz spectroscopy offers a powerful tool for probing these modes.
• Disorder effects in superconductors, especially near the superconductor-insulator transition, are complex and not fully understood.

Purpose of the Study:

• To investigate the influence of disorder on the nonlinear terahertz response and Higgs mode detection.
• To explore the behavior of the Higgs mode in NbN thin films with varying disorder levels.
• To understand the origin of anomalous signals in disordered superconductors.

Main Methods:

• Utilizing nonlinear terahertz spectroscopy to probe NbN thin films.
• Systematically varying the disorder level (Ioffe-Regel parameter, kFl) in NbN films.
• Analyzing the third-harmonic generation (THG) signal above and below the critical temperature (Tc) and in magnetic fields.

Main Results:

• An anomalous normal-state THG signal was observed in strongly disordered films above Tc, persisting in high magnetic fields.
• Below Tc, THG intensity sharply increased, indicating a dominant contribution from the driven Higgs mode.
• The THG spectrum in disordered samples showed a broadened, multipeak structure attributed to quantum path interference.

Conclusions:

• Disorder significantly tunes the nonlinear terahertz response and the Higgs mode in superconductors.
• A strong coupling exists between normal-state electrons and the superconducting Higgs mode in strongly disordered samples.
• The observed phenomena provide new insights into electron behavior and emergent phenomena in disordered superconducting systems.