标题： 无序超导体在窄带和宽带极限下的二维相干光谱学 显示英文标题

标题： Two-dimensional coherent spectroscopy of disordered superconductors in the narrow-band and broad-band limits

摘要： 我们理论分析了无序超导体在两个极限下的二维相干光谱(2DCS)信号：一个是窄带极限，使用正弦脉冲波，另一个是宽带极限，使用δ函数脉冲。 虽然窄带极限下的2DCS信号与三阶非线性极化率$\chi^{(3)}(3\Omega; \Omega, \Omega, \Omega)$（三次谐波产生）和$\chi^{(3)}(\Omega; \Omega, \Omega, -\Omega)$（交流Kerr效应）有关，但我们发现，在宽带极限下，二维频率空间中对角线和水平线上信号与另一种非线性极化率$\chi^{(3)}(\Omega; \Omega, 0, 0)$（直流Kerr效应）有关。 我们基于BCS平均场理论和杂质的自洽玻恩近似，对超导体的格点模型中的这些极化率进行了数值评估。 窄带和宽带极限下的2DCS信号分别在超导能隙频率处表现出阈值和共振行为，其物理起源结合准粒子和希格斯模激发进行了讨论。 显示英文摘要

摘要： We theoretically analyze two-dimensional coherent spectroscopy (2DCS) signals for disordered superconductors in two limits: One is the narrow-band limit with sinusoidal pulse waves, and the other is the broad-band limit with delta-function pulses. While the 2DCS signal in the narrow-band limit is related to the third-order nonlinear susceptibilities $\chi^{(3)}(3\Omega; \Omega, \Omega, \Omega)$ (third harmonic generation) and $\chi^{(3)}(\Omega; \Omega, \Omega, -\Omega)$ (ac Kerr effect), we find that in the broad-band limit the signal along the diagonal and horizontal lines in the two-dimensional frequency space is related to another nonlinear susceptibility $\chi^{(3)}(\Omega; \Omega, 0, 0)$ (dc Kerr effect). We numerically evaluate those susceptibilities for a lattice model of superconductors based on the BCS mean-field theory and self-consistent Born approximation for impurities. The 2DCS signals in the narrow-band and broad-band limits show threshold and resonance behaviors at the superconducting-gap frequency, respectively, whose physical origin is discussed in light of quasiparticle and Higgs-mode excitations.