Fig. 1: Nonlinear terahertz spectroscopy of d-wave superconductors.
From: Fano interference between collective modes in cuprate high-Tc superconductors
a An illustration of the amplitude oscillation of the superconducting order parameter (2ω) driven by electromagnetic radiation (ω) and its coupling to another collective mode (black pendulum) at temperatures above and below Tπ, i.e., the anti-resonance temperature. b The generalized Fano resonance model describes the interference between a driven damped (continuum) mode and an underdamped (discrete) mode, here represented as oscillators 1 and 2, respectively. The Fano resonance/interference is characterized by the asymmetrical line shape of the amplitude response (blue), and also by the negative π jump in the phase response (red), a.k.a. the anti-resonance. Upper panel: in typical spectroscopy experiments, the electromagnetic driving frequency (ωdrive) is swept. Lower panel: in our experiment the driving frequency is fixed, but the resonance frequency of oscillator 1 (ωosc1) is swept. Here, oscillator 1 represents the superconducting fluctuations, whose energy scale is a function of temperature. By sweeping temperature from 0 towards Tc, the energy of the superconducting fluctuations decreases from maximum to zero. We chose the direction of the ωosc1 axis as shown so that it corresponds to a temperature axis that increases to the right. Note, however, that sweeping temperature is not entirely equivalent to sweeping driving frequency, as additional parameters like the damping constant may depend on temperature. Details of the generalized Fano resonance model can be found in Supplementary Materials S8.
