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 $T_{\mathrm{ph}}$, 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 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 also recently prompted in literature, as a result of a strong anisotropy of the electron–phonon (el-ph) coupling. In the present paper, taking MgB₂ 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.

失衡系统中电子和集体模式的超快动力学主要受从电子自由度（通常吸收泵浦源的能量）到晶格自由度的能量转移控制。在传统金属中，这种过程导致晶格整体加热，通常由有效晶格温度描述${吨}_{\mathrm{酸碱度}}$，直到达到所有自由度的最终平衡。然而，在特定材料中，很少有晶格模式为能量转移提供优先通道，导致振动的非热分布和热声子的开始，即晶格模式比其他模式具有更高的人口数。热声子通常出现在半导体或半金属化合物（如石墨烯）中，其中朝向热模式的优先通道由减小的电子相空间决定。由于电子 - 声子（el-ph）耦合的强各向异性，最近在文献中也提出了在金属中获得热声子物理学的可能性。在本文中，取 MgB ₂作为一个有代表性的例子，我们回顾了在具有各向异性 el-ph 耦合的金属中允许热声子场景的物理条件，并讨论了热声子的可观察指纹。还讨论了其他金属化合物中热声子的预测和实验观察的新观点。