体相YSi₂及YSi₂/Si超晶格中因化学扁平化效应实现最高20.6 K的超导转变温度
Superconducting $T_\mathrm{c}$ up to 20.6 K in bulk YSi$_2$ and YSi$_2$/Si superlattices due to chemical flattening
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中文总结 AI 辅助
本研究提出与硅晶格匹配的AlB₂型超导体YSi₂，通过第一性原理计算发现其因化学扁平化效应实现最高20.6 K的超导转变温度，还提出YSi₂/Si超晶格结构以增强其稳定性。
中文摘要 AI 辅助
当前，主流量子计算机的基本构建单元是约瑟夫森结，其核心通常为硅片上的超导铝（Al）。然而，体相铝的超导转变温度（$T_\text{c}$）约为1.1 K，低于液氦的沸点（约4.2 K），这是其广泛应用面临的挑战之一。本研究提出一种与硅晶格匹配的AlB₂型超导体YSi₂，作为铝的有前景替代材料。通过求解含（不含）非简谐性的各向异性（各向同性）Migdal-Eliashberg方程，得到其$T_\text{c}$约为20.6 K（17.2 K），该值在硅化物中处于最高水平。其优异的超导性主要归因于硅蜂窝结构，该结构因钇（Y）原子的“化学扁平化”效应而呈平面状，而非多数硅化物中的褶皱状。本研究通过第一性原理计算测试了其热力学、动力学、动力学及力学稳定性：特别地，当硅蜂窝结构被压缩至阈值以下时，即使不考虑零点能，通过常规方法计算得到的负弹性刚度常数$C_{66}$也会变为正值；该应变还可使计算得到的晶格常数与实验值一致。本研究提出了实现该应变的结构，即YSi₂(0001)/Si(111)超晶格，其在维持$T_\text{c}$高于7.0 K的同时，还可增强YSi₂的整体稳定性。
英文摘要
Currently, the fundamental building blocks of leading quantum computers are Josephson junctions, whose core is usually the superconducting Al on Si wafers. However, the transition temperature $T_\mathrm{c}$ of bulk Al ($\sim1.1$ K) is below the boiling point of liquid helium ($\sim4.2$ K), which is one of the challenges to its widespread application. Here, we propose a Si-matched AlB$_2$-type superconductor YSi$_2$ as a promising alternative to Al. The solution of anisotropic (isotropic) Migdal-Eliashberg equation without (with) anharmonicity gives $T_\mathrm{c}\sim20.6$ K ($17.2$ K), which is at the highest level in silicides. Its excellent superconductivity can be attributed mainly to the Si honeycombs, which become plane here due to the 'chemical flattening' effects of the Y atoms instead of being buckled in most silicides. We tested its thermodynamical, kinetic, dynamical, and mechanical stability by first-principles calculations. Particularly, the negative elastic stiffness constant $C_{66}$ calculated by usual methods turns positive even without the zero-point energy once the Si honeycombs are compressed below a threshold. This strain can also make its calculated lattice constants agree with the experimental ones. We propose structures to realize this strain, i.e., YSi$_2$(0001)/Si(111) superlattices, which can also strengthen the overall stability of YSi$_2$ while maintaining its $T_\mathrm{c}$ above $7.0$ K.
