Anisotropic electron-phonon coupling on a two-dimensional circular Fermi contour

Angle-resolved photoemission reveals that the electron-phonon coupling (EPC) around the circular Fermi contour of the Be(0001) Gamma surface state is strongly anisotropic. The mass enhancement parameter lambda is found to range from values of similar to 1, along high-symmetry directions, to similar to 0.6 in between in stark contrast to the theoretical calculations. Given the simplicity of this system it furnishes an ideal prototype for unraveling the origin of anisotropic EPC in two-dimensional materials.