[论文解读] Temperature and quantum anharmonic lattice effects on stability and superconductivity in lutetium trihydride

Roman Lucrezi, Pedro P. Ferreira+2 more|arXiv (Cornell University)|Apr 13, 2023

Physics of Superconductivity and Magnetism参考文献 60被引用 5

一句话总结

本研究通过在从头算计算中引入温度和量子非谐晶格效应，解决了关于镥三氢化物（LuH₃）的相互矛盾的报告，表明Fm\bar{3}m相在200 K以上具有动力学稳定性，并预测其超导临界温度（Tc）为50–60 K，远低于稳定化温度。结果表明，任何观测到的室温超导性都无法用常规电子-声子配对机制解释。

ABSTRACT

In this work, we resolve conflicting experimental and theoretical findings related to the dynamical stability and superconducting properties of $Fm\overline{3}m$-LuH$_3$, which was recently suggested as the parent phase harboring room-temperature superconductivity at near-ambient pressures. Including temperature and quantum anharmonic lattice effects in our calculations, we demonstrate that the theoretically predicted structural instability of the $Fm\overline{3}m$ phase near ambient pressures is suppressed for temperatures above $200\, ext{K}$. We provide a $p\,\unicode{x2013}\,T$ phase diagram for stability up to pressures of $6\, ext{GPa}$, where the required temperature for stability is reduced to $T>80\, ext{K}$. We also determine the superconducting critical temperature $T_ ext{c}$ of $Fm\overline{3}m$-LuH$_3$ within the Migdal-Eliashberg formalism, using temperature- and quantum-anharmonically-corrected phonon dispersions, finding that the expected $T_ ext{c}$ for electron-phonon mediated superconductivity is in the range of $50$ $\unicode{x2013}$ $60\, ext{K}$, i.e., well below the temperatures required to stabilize the lattice. When considering moderate doping based on rigidly shifting the Fermi level, $T_ ext{c}$ decreases for both hole and electron doping. Our results thus provide evidence that any observed room-temperature superconductivity in pure or doped $Fm\overline{3}m$-LuH$_3$, if confirmed, cannot be explained by a conventional electron-phonon mediated pairing mechanism.

研究动机与目标

• 解决实验报告中关于LuH₃存在室温超导性与理论预测其动力学不稳定的矛盾。
• 研究温度和量子非谐晶格效应对Fm\bar{3}m-LuH₃结构稳定性的影响。
• 通过非谐校正的声子色散关系，精确计算电子-声子耦合介导的超导临界温度（Tc）。
• 评估刚性带 doping 对Fm\bar{3}m-LuH₃中Tc的影响。
• 确定Fm\bar{3}m相在压力和温度变化下实现动力学稳定性的条件。

提出的方法

• 采用密度泛函理论（DFT）结合PBE泛函和ONCV赝势，使用12×12×12 k-网格和100 Ry平面波截断能。
• 应用自洽谐波近似（SSCHA）计算2×2×2超胞中温度和非谐校正的声子色散关系。
• 使用泡状近似计算非谐自由能Hessian矩阵，忽略四阶校正项，因其影响可忽略不计。
• 通过Epw代码应用Migdal-Eliashberg形式化，利用在密集k-和q-网格上插值的Wannier电子-声子矩阵元计算Tc。
• 通过移动费米能级实现刚性带 doping，研究其对Tc的影响。
• 使用三次样条插值对p–T数据进行插值，生成相图并可视化稳定性区域。

Figure 1: Phonon dispersions as a function of temperature. SSCHA phonon dispersion for different temperatures $T$ (solid coloured lines). The calculations have been performed for the structure with $a=$5.040\text{\,}\mathrm{\text{\AA}}$$ and the harmonic phonon dispersions are indicated by dashed bl

实验结果

研究问题

• RQ1 当考虑温度和量子非谐效应时，LuH₃的Fm\bar{3}m相在常压下是否具有动力学稳定性？
• RQ2 在正确考虑非谐声子色散关系的情况下，Fm\bar{3}m-LuH₃的超导临界温度（Tc）是多少？
• RQ3 考虑到预测的Tc值，电子-声子耦合能否解释LuH₃中的室温超导性？
• RQ4 空穴或电子掺杂如何影响Fm\bar{3}m-LuH₃的Tc？
• RQ5 LuH₃中Fm\bar{3}m相稳定性的压力-温度（p–T）相图是怎样的？

主要发现

• LuH₃的Fm\bar{3}m相在200 K以上具有动力学稳定性，解决了以往理论预测因虚频声子模式导致不稳定的矛盾。
• p–T相图显示，该相在最高6 GPa的压力下仍可稳定，且在6 GPa时所需稳定温度降低至T > 80 K。
• 当使用非谐声子色散关系时，Fm\bar{3}m-LuH₃的超导临界温度Tc预测范围为50–60 K。
• 在空穴和电子掺杂下，Tc均下降，表明在此框架下掺杂无法增强超导性。
• 预测的Tc值显著低于稳定晶格所需的温度，意味着LuH₃中的室温超导性无法用常规电子-声子配对机制解释。

Figure 2: Phonon dispersions as a function of pressure. Low-energy part of the phonon dispersions as a function of pressure $p$ at a fixed temperature ( $T=$150\text{\,}\mathrm{K}$$ ). The full dispersion relations for all modes are provided in Supplementary Fig. 2.