Magnetic polaron bound to the positive muon in SmS: Exchange-driven formation of a mixed-valence state

Abstract

Muon spin-rotation experiments (supported by magnetization measurements) have been carried out in the canonical $4f$ mixed-valence narrow-gap semiconductor SmS from 10 to 900 K in magnetic fields up to 3.5 T. A bound state of an electron around a positive muon is found to form up to about 800 K. This state is a magnetic polaron: the electron wave function is confined within $R\ensuremath{\approx}0.5\text{ }\text{nm}$ (the first two coordination spheres) due to its exchange interaction with Sm magnetic moments. As such, it may serve as a model system for the hypothetical bound state suggested to account for a transition from divalent ${\text{Sm}}^{2+}$ to trivalent ${\text{Sm}}^{3+}$, which is invoked to explain the transformation of SmS from a paramagnetic insulator into a magnetic metal at high pressure.