[论文解读] Debye temperature in LaHx-LaDy superconductors

Talantsev, E. F.|arXiv (Cornell University)|Apr 7, 2020

Iron-based superconductors research被引用 33

一句话总结

本研究通过Bloch-Grüneisen方程从LaHx-LaDy超导体的温度依赖电阻数据推导德拜温度（Tθ），并利用比值Tc1/Tc2 = Tθ1/Tθ2检验电子-声子耦合机制。实验测得的Tθ比值与第一性原理计算预测值之间存在显著差异，挑战了近室温超导体中电子-声子机制的有效性。

ABSTRACT

The Debye temperature, T${_ heta}$=(h/2${\pi}$)/k${_B}$$\omega$$_ heta$, where the Debye frequency $\omega$$_ heta$ is integrated characteristic frequency of full phonon spectrum, $\alpha$$^2$($\omega$)F($\omega$). In the BCS theory, T${_ heta}$ in conjunction with electron-phonon coupling strength parameter, $\lambda$$_{e-ph}$, determines the superconducting transition temperature, Tc. Despite a fact that more accurate theory of electron-phonon mediated superconductivity requires the knowledge of full phonon spectrum, $\alpha$$^2$($\omega$)F($\omega$), which can be very accurately computed by first principles calculation technique, there is no experimental technique which can measure $\alpha$$^2$($\omega$)F($\omega$) in highly-compressed near-room-temperature (NRT) superconductors. Thus, $\omega$$_ heta$ remains to be the only measurable parameter of full phonon spectrum, $\alpha$$^2$($\omega$)F($\omega$), which can be deduced by the fit of experimental temperature-dependent resistance data, R(T), to Bloch-Gr\"uneisen equation. Taking in account that within electron-phonon mediated theory of superconductivity two isotopic counterparts (or, in case of NRT superconductors, the same superconducting phase at different pressures), designated by subscripts of 1 and 2, should be obey the relation of T${_ heta}_1$/T${_ heta}_2$=T${_c}_1$/T${_c}_2$, there is a way to reaffirm/disprove the electron-phonon mechanism of NRT superconductivity. In this paper, we perform the analysis for R3m-phase of H3S at different pressure, as well as for several superconductors in LaHx-LaDy system and show that there is a large disagreement between experimental data and T${_ heta}_1$/T${_ heta}_2$=T${_c}_1$/T${_c}_2$. Taking in account that similar disagreement has recently reported in H3S-D3S system, it can be concluded that primary origin for NRT superconductivity remains to be discovered.

研究动机与目标

• 通过高度压缩的LaHx-LaDy超导体的电阻数据，实验测定德拜温度（Tθ）。
• 通过验证同位素对（如LaHx与LaDy）的理论关系Tc1/Tc2 = Tθ1/Tθ2，检验电子-声子耦合机制。
• 挑战近室温超导体中基于Allen-Dynes与BCS的电子-声子耦合框架的有效性。
• 识别第一性原理计算与实验测得的Tθ值之间的不一致性，提示可能存在非声子介导的配对机制。

提出的方法

• 将实验测得的温度依赖电阻（R(T)）数据拟合至Bloch-Grüneisen方程，以提取Tθ。
• 对同位素对（如LaHx与LaDy）应用Tc1/Tc2 = Tθ1/Tθ2关系，以检验电子-声子耦合。
• 使用归一化电阻R(T)/Rnorm(T)，并依据Tc = 0.05、0.25或拐点来定义Tc。
• 将实验测得的Tθ与Tc推导出的λe-ph与Errea等人[34]的第一性原理计算结果进行比较。
• 分析多个样品在不同压力（130–180 GPa）及激光退火阶段的一致性。
• 在假设μ* = 0.10的前提下，应用Allen-Dynes形式化结合f2*近似，估算λe-ph,aMcM。

实验结果

研究问题

• RQ1 实验测得的同位素超导体LaHx与LaDy之间的Tθ比值是否满足电子-声子理论预测的Tθ1/Tθ2 = Tc1/Tc2？
• RQ2 从电阻数据推导出的电子-声子耦合强度（λe-ph）是否与第一性原理计算结果一致？
• RQ3 为何第一性原理计算在LaHx-LaDy体系中预测的λe-ph高于实验推导值？
• RQ4 实验与理论λe-ph之间的差异对高Tc氢化物中配对机制有何启示？
• RQ5 考虑到对数频率ωln在Allen-Dynes理论中依赖于ln(ω)且具有频率量纲，其物理有效性是否成立？

主要发现

• 对于LaHx（样品10，P = 178 GPa），Tθ = 721 ± 2 K，Tc = 92 K，对应λe-ph,aMcM = 1.82 ± 0.01，而第一性原理计算结果为λe-ph = 2.06–2.76。
• 对于LaHx（样品10，图b），Tθ = 941 ± 9 K，Tc = 107 K，λe-ph,aMcM = 1.62 ± 0.01，但第一性原理值为2.06–2.76。
• 对于LaD11（样品8，P = 142 GPa），Tθ = 1199 ± 14 K，Tc = 125 K，λe-ph,aMcM = 1.49 ± 0.02，而第一性原理计算得λe-ph = 3.14。
• 比值Tc1/Tc2 = 125 K / 107 K = 1.17与Tθ1/Tθ2 = 1199 K / 941 K = 1.27不一致，违反了电子-声子理论预测。
• 对于LaH10（样品3，Tc = 240 K），实验与第一性原理计算的λe-ph值一致，但此为例外情况。
• 本研究结论为：在大多数LaHx-LaDy体系中，电子-声子机制与实验数据不一致，提示近室温超导性可能存在其他配对机制。