Low-Energy Charge-Density Excitations in MgB$_{2}$: Striking Interplay between Single-Particle and Collective Behavior for Large Momenta

TL;DR

This study reveals a unique interplay between single-particle and collective excitations in MgB₂, observed through high-resolution NIXS, highlighting the material's layered electronic structure and complex charge-density dynamics.

Contribution

First observation of cosine-like charge-density excitation dispersion in MgB₂ using NIXS, supported by theoretical insights into strong coupling effects and local-field influences.

Findings

Periodic energy dispersion of charge excitations along c*-axis

Reappearance of collective mode in higher Brillouin zones

Strong coupling between single-particle and collective behaviors

Abstract

A sharp feature in the charge-density excitation spectra of single-crystal MgB$_{2}$, displaying a remarkable cosine-like, periodic energy dispersion with momentum transfer ($q$) along the $c^{*}$-axis, has been observed for the first time by high-resolution non-resonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-$q$ collective mode residing in the single-particle excitation gap of the B $\pi$ bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB$_{2}$.