Properties and challenges of hot-phonon physics in metals: MgB$_2$ and other compounds

TL;DR

This paper reviews the conditions under which hot phonons can form in metals, focusing on MgB$_2$, and discusses how anisotropic electron-phonon coupling enables non-thermal phonon distributions with potential experimental signatures.

Contribution

It provides a detailed analysis of hot-phonon physics in metals with anisotropic electron-phonon coupling, highlighting MgB$_2$ as a case study and proposing new avenues for observation in other compounds.

Findings

Hot phonons can form in metals with anisotropic el-ph coupling.

MgB$_2$ exhibits conditions conducive to hot-phonon formation.

Observable signatures of hot phonons are identified and discussed.

Abstract

The ultrafast dynamics of electrons and collective modes in systems out of equilibrium is crucially governed by the energy transfer from electronic degrees of freedom, where the energy of the pump source is usually absorbed, to lattice degrees of freedom. In conventional metals such process leads to an overall heating of the lattice, usually described by an effective lattice temperature $T_{\rm ph}$, until final equilibrium with all the degrees of freedom is reached. In specific materials, however, few lattice modes provide a preferential channel for the energy transfer, leading to a non-thermal distribution of vibrations and to the onset of {\em hot phonons}, i.e., lattice modes with a much higher population than the other modes. Hot phonons are usually encountered in semiconductors or semimetal compounds, like graphene, where the preferential channel towards hot modes is dictated by the reduced electronic phase space. Following a different path, the possibility of obtaining hot-phonon physics also in metals has been however also recently prompted in literature, as a result of a strong anisotropy of the electron-phonon (el-ph) coupling. In the present paper, taking MgB$_2$ as a representative example, we review the physical conditions that allow a hot-phonon scenario in metals with anisotropic el-ph coupling, and we discuss the observable fingerprints of hot phonons. Novel perspectives towards the prediction and experimental observation of hot phonons in other metallic compounds are also discussed.