Signature of magnon polarons in electron relaxation on terbium revealed by comparison with gadolinium

Terbium and gadolinium have eight and seven 4$f$ electrons, respectively. This one-electron difference sparks surprisingly different behaviors. Here, the authors use spin-, angle-, and energy-dependent photoemission to investigate the relaxations of the photo-hole in the surface states of terbium and gadolinium metal. They find distinct, spin-dependent linewidths for gadolinium, in contrast to spin-independent relaxation for terbium. This peculiar difference is explained by strong magnon-phonon interaction in terbium leading to magnon polarons that couple to both spin-decay channels, in contrast to gadolinium.