Fano interference between collective modes in cuprate high-Tc superconductors

Cuprate superconductors are known for their intertwined interactions and coexistence of competing orders. Here, the authors observe a Fano resonance in the nonlinear THz response of La_2-xSr_xCuO_4, which may arise from a coupling between superconducting and charge-density-wave amplitude fluctuations. Cuprate high- T _c superconductors are known for their intertwined interactions and the coexistence of competing orders. Uncovering experimental signatures of these interactions is often the first step in understanding their complex relations. A typical spectroscopic signature of the interaction between a discrete mode and a continuum of excitations is the Fano resonance/interference, characterized by the asymmetric light-scattering amplitude of the discrete mode as a function of the electromagnetic driving frequency. In this study, we report a new type of Fano resonance manifested by the nonlinear terahertz response of cuprate high- T _c superconductors, where we resolve both the amplitude and phase signatures of the Fano resonance. Our extensive hole-doping and magnetic field dependent investigation suggests that the Fano resonance may arise from an interplay between the superconducting fluctuations and the charge density wave fluctuations, prompting future studies to look more closely into their dynamical interactions.