  The concept of quasi-particles forms the theoretical basis of our microscopic
understanding of emergent phenomena associated with quantum mechanical
many-body interactions. However, quasi-particle theory in disordered materials
has proven difficult, resulting in the predominance of mean-field solutions.
Here we report first-principles phonon calculations and inelastic x-ray and
neutron scattering measurements on equiatomic alloys (NiCo, NiFe, AgPd, and
NiFeCo) with force constant dominant disorder - confronting a key 50-year-old
assumption in the Hamiltonian of all mean-field quasi-particle solutions for
off-diagonal disorder. Our results have revealed the presence of a large, and
heretofore unrecognized, impact of local chemical environments on the
distribution of the species-pair-resolved force constant disorder that can
dominate phonon scattering. This discovery not only identifies a critical
analysis issue that has broad implications for other elementary excitations
such as magnons and skyrmions in magnetic alloys, but also provides an
important tool for the design of materials with ultra-low thermal conductivity.
