Abstract
The concept of quasi-particles forms the theoretical basis of our microscopic understanding of emergent phenomena associated with quantum-mechanical many-body interactions. However, the 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 ultralow thermal conductivities.
Highlights
Quasi-particle elementary excitations, including electron quasiparticles, phonons, magnons, plasmons, excitons, etc., along with skyrmions,[1,2] Majorana fermions,[3] and their mutual interactions, represent remarkably successful theoretical descriptions of emergent phenomena associated with quantum-mechanical many-body interactions
In contrast to fully ordered crystals that can be described within the Bloch theorem, the broken translational symmetry in alloys associated with substitutional chemical disorder has long challenged the development of analogous robust quasi-particle theories of configurationally averaged observables that inherently lead to the finite quasi-particle lifetimes measured experimentally —even at zero temperature
Phonon dispersion and linewidth measurements for NiCo and NiFe samples were performed at room temperature by using highresolution inelastic X-ray scattering (IXS) and inelastic neutronscattering (INS) measurements along the [001] and [011] reciprocal lattice directions
Summary
Quasi-particle elementary excitations, including electron quasiparticles, phonons, magnons, plasmons, excitons, etc., along with skyrmions,[1,2] Majorana fermions,[3] and their mutual interactions, represent remarkably successful theoretical descriptions of emergent phenomena associated with quantum-mechanical many-body interactions. Density functional theory (DFT) was employed to calculate the force constants of NiCo, NiFe, AgPd, and NiFeCo alloys with chemical disorder modeled by the supercell method.
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