Driving the Higgs oscillation in cuprates: unveiling collective modes or interactions coupled to Cooper pairs

The Higgs mode of a superconductor is a collective excitation of the amplitude of the superconducting order parameter. It can be coherently driven at a frequency 2ω by a multicycle terahertz pulse of frequency ω. This then leads to third harmonic generation (THG) as a result of sum frequency generation between the oscillating condensate and the terahertz driving pulse. We applied such experimental scheme to different families of cuprate high-Tc superconductors. By phase-resolving the THG response with respect to the terahertz drive, we uncover the universal anti-resonance of the driven Higgs oscillation in all samples. This may be understood in terms of an additional collective mode coupled to the Higgs mode. Careful analysis of the anti-resonance feature as a function of hole doping with the help of a coupled oscillators model suggests that the coupled mode could be the magnetic resonant mode. Our findings may shed light on the pairing mechanism in d-wave superconductors.