Higgs Amplitude Mode in BCS Superconductors Nb$_{1-x}$Ti$_{x}$N induced by Terahertz Pulse Excitation (1305.0381v2)

Abstract: Ultrafast responses of BCS superconductor Nb1-xTixN films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BCS gap energy, indicating the appearance of the theoretically-anticipated collective amplitude mode of the order parameter, namely the Higgs amplitude mode. Our result opens a new pathway to the ultrafast manipulation of the superconducting order parameter by optical means.

• The paper demonstrates nonadiabatic dynamics by showing that intense THz pulses trigger the Higgs amplitude mode aligned with the BCS gap energy.
• It employs THz pump-THz probe spectroscopy and advanced Ti:sapphire laser systems to capture ultrafast oscillatory responses in NbTiN films.
• The study reveals that modulating the superconducting order parameter via terahertz excitation opens new avenues for quantum optical control.

Terahertz-Induced Higgs Amplitude Mode in BCS Superconductors

The paper presents an in-depth paper of the nonadiabatic excitation dynamics of niobium titanium nitride (Nb$_{1-x}$Ti$_x$N) superconductors, focusing on the observation of the Higgs amplitude mode facilitated by terahertz (THz) pump-THz probe spectroscopy. The paper involves superconductors in the BCS regime and uses intense monocycle THz pulses to excite the system. The observed transient oscillations in the electromagnetic response of Nb$_{1-x}$Ti$_x$N coincide with the superconducting BCS gap energy, suggesting the emergence of the Higgs amplitude mode, a theoretically postulated collective excitation of the order parameter.

The research utilizes a sophisticated experimental setup involving a regenerative amplified Ti:sapphire laser system, operating at an 800-nm center wavelength. The system produces beams for THz pulse generation and electro-optic sampling of the transmitted probe THz pulse. The choice to utilize THz frequency was strategic, as it resonates with the BCS gap, ensuring nonadiabatic conditions necessary for these observations and overcoming limitations of visible optical pulses that introduce excess energy and disrupt superconductivity.

The experiments performed with varying compositions and thicknesses of Nb$_{1-x}$Ti$_x$N films (samples A, B, and C) reveal that the oscillation frequency matches the BCS gap energy, aligned with the Higgs mode predictions. Numerical analyses confirm that the probe E-field at the fixed gate accurately reflects the temporal changes in the order parameter following THz pulse excitation.

The results delineate a clear oscillatory response characterized by an initial fast rise within 2 ps followed by an oscillation in the probe E-field, with a power-law decay indicative of the Higgs mode. This transient behavior is enhanced with increasing pump intensity, albeit with a decrease in oscillation amplitude, reflecting a pumping-induced modulation of the order parameter. It confirms the theoretical conjectures regarding collective nonequilibrium dynamics in metallic BCS superconductors.

Furthermore, the work identifies intriguing correlations regarding the order parameter's behavior and the phonon-induced thermalization process, as described by slow changes in probe E-field measurements. The strong agreement between experimental observations and theoretical frameworks underscores the validity of using THz pulses to drive and paper ultrafast collective modes in superconductors.

Implications and Future Directions

The observation of the Higgs amplitude mode in metallic BCS superconductors through THz excitation opens substantial avenues for ultrafast control and manipulation of superconducting states via optical methods. This ability to modulate order parameters could pioneer developments in quantum optical applications within the THz regime.

Moreover, extending this experimental approach to investigate other types of superconductivity, including unconventional superconductors or those with anisotropic symmetry, may yield important insights into nonadiabatic dynamics and symmetry-broken states in quantum many-body systems. This research provides a robust experimental foundation for future exploratory and applied work aimed at unraveling the complex interplay between excitation energy, order parameter dynamics, and superconductivity.