In-plane electron-phonon coupling anisotropy and multiple charge density wave orders in the superconductor Bi₂Rh₃Se₂

The ternary chalcogenide superconductor Formula Presented was discovered to have charge density wave (CDW) order and electron-phonon coupling (EPC). However, it is still debated whether the phase transition at Formula Presented is a CDW one. Here, we systematically performed Raman scattering investigations on single-crystal Formula Presented. Angle-resolved polarized Raman spectroscopic measurements reveal the in-plane anisotropic EPC in Formula Presented, which inhibits the Raman intensity. Temperature-dependent x-ray diffraction and Raman measurements indicate the existence of the incommensurate (ICCDW) and commensurate (CCDW) CDW orders below Formula Presented and Formula Presented K, respectively, in Formula Presented. The appearance of zone-folded phonon modes at Formula Presented K confirms the CCDW-induced Formula Presented reconstruction of the Brillouin zone. Semiquantitative analysis based on Landau theory of second-order phase transitions of CDW-correlated and uncorrelated phonon modes reflects extensive coupling between the phonons and CDW orders. Our findings elucidate the existence of CCDW and ICCDW and confirm the critical role of EPC in CDW transitions in Formula Presented.