Topological semimetals are currently attracting increasing interest due to their potential applications in topological qubits and low-power electronics, which are closely related to their thermal transport properties. Recently, the triply degenerate nodal points near the Fermi level of WC are observed by using angle-resolved photoemission spectroscopy. In this work, by solving the Boltzmann transport equation based on first-principles calculations, we systematically investigate the phonon transport properties of topological semimetals WC and WN. The predicted room-temperature lattice thermal conductivities of WC (WN) along the a and c directions are 1140.64 (7.47) W m⁻¹ K⁻¹ and 1214.69 (5.39) W m⁻¹ K⁻¹. Considering the similar crystal structure of WC and WN, it is quite interesting to find that the thermal conductivity of WC is more than two orders of magnitude higher than that of WN. It is found that, different from WN, the large acoustic-optical (a-o) gap prohibits the acoustic+acoustic → optical (aao) scattering, which gives rise to very long phonon lifetimes, leading to ultrahigh lattice thermal conductivity in WC. For WN, the lack of an a-o gap is due to soft phonon modes in optical branches, which can provide more scattering channels for aao scattering, producing very short phonon lifetimes. Further deep insight can be attained from their different electronic structures. Distinctly different from that in WC, the density of states of WN at the Fermi level becomes very sharp, which leads to destabilization of WN, producing soft phonon modes. It is found that the small shear modulus G and C₄₄ limit the stability of WN, compared with WC. Our studies provide valuable information for phonon transports in WC and WN, and motivate further experimental studies to study their lattice thermal conductivities.

中文翻译：

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软声子模式驱动拓扑半金属WC和WN之间的晶格热导率巨大差异

拓扑半金属由于其在拓扑量子位和低功率电子中的潜在应用而引起了越来越多的兴趣，这与它们的热传输特性密切相关。最近，通过使用角分辨光发射光谱法观察到了WC的费米能级附近的三重简并结点。在这项工作中，通过基于第一性原理计算求解玻尔兹曼输运方程，我们系统地研究了拓扑半金属WC和WN的声子输运性质。沿a和c方向的WC（WN）的室温晶格热导率的预测值为1140.64（7.47）W m ^-1 K ^-1和1214.69（5.39）W m ^-1 K ^-1。考虑到WC和WN的相似晶体结构，发现WC的导热系数比WN的导热系数高两个数量级是非常有趣的。发现与WN不同，较大的声光（ao）间隙阻止了声+声→光学（aao）散射，这导致了非常长的声子寿命，从而导致WC中的超高晶格导热率。对于WN，缺少ao间隙是由于光学分支中的软声子模，其可以为aao散射提供更多的散射通道，从而产生非常短的声子寿命。可以从它们不同的电子结构获得进一步的深刻见解。与WC截然不同的是，费米能级的WN态密度变得非常尖锐，这导致WN不稳定，从而产生软声子模态。与WC相比，G和C ₄₄限制了WN的稳定性。我们的研究为WC和WN中的声子传输提供了有价值的信息，并激发了进一步的实验研究来研究其晶格热导率。