Observation of Raman active phonon with Fano lineshape in quasi-one-dimensional superconductor K$_2$Cr$_3$As$_3$

We study the polarization-resolved phononic Raman scattering in the recent discovered quasi-one-dimensional superconductor K$_2$Cr$_3$As$_3$. With support from first-principles calculations, we characterize several phonons, among which one mode has a Fano lineshape, indicative of an electron-phonon coupling. While the common expectation of an electron-phonon coupling is the conventional superconducting mechanism, we show that this mode is related to the in-plane Cr vibration, which modulates the exchange coupling between the first nearest Cr neighbors. Our result support the presence of magnetic fluctuations coupled to the electrons $via$ the lattice. We acknowledge MOST (2010CB923000, 2011CBA001000, 2011CBA00102, 2012CB821403 and 2013CB921703), NSFC (11004232, 11034011/A0402, 11234014, 11274362 and 11474330) of China and by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, Grant No. XDB07020100.