We investigate the behavior of the Higgs (amplitude) mode when superconductivity emerges on a preexisting charge-density-wave state. We show that the weak overdamped square-root singularity of the amplitude fluctuations in a standard BCS superconductor is converted in a sharp, undamped power-law divergence in the coexisting state, reminiscent of the Higgs behavior in Lorentz-invariant theories. This effect reflects in a strong superconducting resonance in the Raman spectra, both for an electronic and a phononic mechanism leading to the Raman visibility of the Higgs. In the latter case, our results are relevant to the interpretation of the Raman spectra measured experimentally in NbSe2.
Comment on: Calculation of an Enhanced $A_{1g}$ Symmetry Mode Induced by Higgs Oscillations in the Raman Spectrum of High-Temperature Cuprate Superconductors
Quantum fluctuations and the emergence of in-gap Higgs mode in superconductors
Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；The University of Tokyo（东京大学）；RIKEN Center for Emergent Matter Science (CEMS)（理化学研究所凝聚体物质科学中心）；Trans-Scale Quantum Science Institute, The University of Tokyo（东京大学跨尺度量子科学研究院）
Terahertz pulse-driven collective mode in the nematic superconducting state of Ba$_{1-x}$K$_x$Fe$_2$As$_2$
利用太赫兹光研究了铁基超导体Ba $_{1-x}$ K $_x$ Fe $_2$ As $_2$.在超导状态下，观察到太赫兹克尔信号，并将其分配给超导自由度的非线性太赫兹耦合。太赫兹克尔信号的偏振依赖性对双阶的共存非常敏感。在没有三阶的情况下，太赫兹克尔信号的$C_4$对称偏振依赖性与超导凝聚体的Higgs振幅模式的耦合是一致的。在共存的超导态和超导态的信号成为纯粹的超导与消失的$C_4$对称分量，信号的出现，一个新的超导集体模式激活的向列性。
