AgSnSe$_2$ 中的超导电性、价态跳跃与拓扑晶体金属
Superconductivity, valence-skipping and topological crystalline metal in AgSnSe$_2$
AI总结：
该研究通过第一性原理计算发现AgSnSe$_2$不存在价态跳跃，但具有非平庸拓扑和镜面对称保护的表面态，表明其为拓扑晶体金属候选材料，并预测其单能隙超导临界温度约7K，与实验值5K相符。
摘要：
Kataria等人[Phys. Rev. B 107, 174517 (2023)]最近提出的价态跳跃现象驱动Ag掺杂SnSe中双能隙超导电性的建议，再次将这一长期存在的问题凸显出来。然而，利用第一性原理密度泛函理论计算得到的AgSnSe$_2$电子性质证实了不存在晶体学不等价的Sn位点，这似乎并未为该体系中的价态跳跃提供有力支持。有趣的是，避免能带交叉（包含自旋轨道耦合）和非零镜面陈数（$n_{\mathcal{M}}$）的特征证实了非平庸拓扑性。沿镜面存在的镜面对称保护的表面态表明，AgSnSe$_2$可能是拓扑晶体金属（TCMs）的潜在候选材料。此外，我们利用Migdal-Eliashberg理论计算了AgSnSe$_2$的电子-声子耦合和各向异性超导性质，得到了临界温度$T_c \approx 7$K的单能隙超导电性，与实验值5K一致。这种三维材料中拓扑与超导电性的相互作用显得相当有趣，可能为探索超导电性和拓扑性提供新的见解。
英文摘要：
The recent suggestion of valence-skipping phenomenon driving a two-gap superconductivity in $Ag$-doped SnSe, by Kataria, \textit{et al.} [Phys. Rev. B 107, 174517 (2023)], has brought to the fore a long-standing issue once again. The absence of crystallographically inequivalent Sn cites corroborated by electronic properties of AgSnSe$_2$, calculated using first-principles density functional theory, however, does not appear to provide a strong support in favor of valence-skipping in this system. Interestingly, the signature of avoided band-crossings (with the inclusion of SOC) and non-zero \textit{mirror} Chern number ($n_{\mathcal{M}}$) confirm a non-trivial topology. The presence of mirror symmetry-protected surface states along the mirror planes indicates that AgSnSe$_2$ could be a potential candidate for topological crystalline metals (TCMs). Moreover, our calculation of electron-phonon coupling and anisotropic superconducting properties of AgSnSe$_2$, using Migdal-Eliashberg theory, gives a single-gap superconductivity with critical temperature $T_c \approx 7$K, consistent with the experimental value of $5$K. The interplay of topology and superconductivity in this three-dimensional material appears quite intriguing and it may provide new insights into the exploration of superconductivity and topology.
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