Carrier-density-dependent momentum shift of the coherent peak and the LO phonon mode in p-type high-$T_c$ superconductors

Systematic Raman scattering experiments were carried out in p-type superconductors YBCO, LSCO, Bi2212, and Bi2201. The superconducting coherent (pair-breaking) peak is anisotropic in k space not only in energy but also in existence itself. The position of the coherent peak in k space changes generally from $(\pi /2,\pi /2)$ to $(\pi ,0),$ as carrier density increases, in correlation with the change of the low-energy electronic density of states. However, the change of the coherent peak in k space is more rapid than that of the electronic density of states, which suggests the existence of an additional pair-enhancing and pair-breaking mechanism. We found from two-phonon Raman scattering that the phonon mode with the strongest electron-phonon interactions changes from the breathing mode at $(\pi ,\pi )$ to the half-breathing mode at $(\pi ,0)$ in good correlation with the change of the coherent peak. It suggests that electron-phonon interactions play an important role to generate the superconductivity in the limited region of k space, even if magnetic interactions are the main origin of the superconductivity.