Colossal magnetoresistance from spin-polarized polarons in an Ising system

Recent experiments suggest a new paradigm toward novel colossal magnetoresistance (CMR) in a family of materials EuM[Formula: see text]X[Formula: see text] (M [Formula: see text] Cd, In, Zn; X [Formula: see text] P, As), distinct from the traditional avenues involving Kondo-Ruderman-Kittel-Kasuya-Yosida crossovers, magnetic phase transitions with structural distortions, or topological phase transitions. Here, we use angle-resolved photoemission spectroscopy and density functional theory calculations to explore their origin, particularly focusing on EuCd[Formula: see text]P[Formula: see text]. While the low-energy spectral weight royally tracks that of the resistivity anomaly near the temperature with maximum magnetoresistance ([Formula: see text]) as expected from transport-spectroscopy correspondence, the spectra are completely incoherent and strongly suppressed with no hint of a Landau quasiparticle. Using systematic material and temperature dependence investigation complemented by theory, we attribute this nonquasiparticle caricature to the strong presence of entangled magnetic and lattice interactions, a characteristic enabled by the [Formula: see text]-[Formula: see text] mixing. Given the known presence of ferromagnetic clusters, this naturally points to the origin of CMR being the scattering of spin-polarized polarons at the boundaries of ferromagnetic clusters. These results are not only illuminating to investigate the strong correlations and topology in EuCd[Formula: see text]X[Formula: see text] family, but, in a broader view, exemplify how multiple cooperative interactions can give rise to extraordinary behaviors in condensed matter systems.