Tuning topological disorder in MgB$_{2}$

TL;DR

This study investigates how topological disorder, induced by neutron irradiation and Al doping, alters the Raman spectra of MgB$_2$, revealing disorder-induced violations of selection rules and contributions from non-zero momentum phonons.

Contribution

It demonstrates that topological disorder causes high frequency spectral features in MgB$_2$ by breaking selection rules, supported by experimental Raman data and theoretical phonon density calculations.

Findings

Disorder induces high frequency spectral structures.

Violations of selection rules explain spectral modifications.

High frequency features originate from non-zero momentum phonons.

Abstract

We carried out Raman measurements on neutron-irradiated and Al-doped MgB$_2$ samples. The irradiation-induced topological disorder causes an unexpected appearance of high frequency spectral structures, similar to those observed in lightly Al-doped samples. Our results show that disorder-induced violations of the selection rules are responsible for the modification of the Raman spectrum in both irradiated and Al-doped samples. Theoretical calculations of the phonon density of states support this hypothesis, and demonstrate that the high frequency structures arise mostly from contributions at ${\bf q}\not=0$ of the E$_{2g}$ phonon mode.