Fig. 4: Critical response.
a Typical distribution of response function ∣χ∣ on the complex frequency plane at the critical point (take perturbation momentum q/π = 0.5 for example). Inset: the sectional view of χ along the real axis, where the red solid, blue dotted and black dashed curves denote the total, the part arising from the simple Bardeen-Cooper-Schrieffer (BCS) theory, and the part arising from the order-parameter fluctuations of χ, respectively. We see that χ vanishes outside the gapped spectral loops of ω = Ek+q/2 + Ek−q/2 with quasi-energy Ek (see also the shaded regions in the inset of a), whose shapes for different q are shown in (b). As a result, χ always vanishes for ω outside all such spectral loops for any q. Here we set the interaction strength U/ε = 4, hopping coefficient ts/ε = tso/ε = 1 and occupation coefficient ν = 1/4, where tso is the spin-orbit-coupling (SOC) strength.
