While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration depth in the electron-doped iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$ compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic-magnetic transition above, and even somewhat below, $T_c$, the former is a significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transition above $T_c$. Thus our observation that such distinct systems display remarkably similar penetration depth peaks, combined with the theoretical result that a QPT alone does not ensure the existence of a peak, unveils a puzzling and seemingly universal manifestation of quantum fluctuations in the iron pnictides.

中文翻译：

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Ba(Fe1-xCox)2As2 超导圆顶内的量子相变来自基于金刚石的光学磁力法

虽然不寻常的常态特性，例如电阻率的非费米液体行为，通常与强量子涨落有关，但其存在于超导圆顶内的证据却少得多。在本文中，我们使用基于金刚石中 NV 中心的灵敏微创光学磁力计来探测电子掺杂铁基超导体 Ba(Fe$_{1- x}$Co$_x$)$_2$As$_2$。发现了在推定的磁性 QPT 附近具有明显峰值的非单调演化。这种行为让人想起以前在等价取代的 BaFe$_2$(As$_{1-x}$P$_x$)$_2$ 化合物中看到的行为，尽管这两个系统之间存在显着差异。而后者是一个非常干净的系统，显示节点超导性和单个同时发生的一阶向列磁跃迁高于甚至低于 $T_c$，而前者是一个明显更脏的系统，具有完全有隙的超导性和分裂的二阶$T_c$ 以上的向列和磁跃迁。因此，我们观察到这些不同的系统显示出非常相似的穿透深度峰，结合理论结果，即 QPT 本身并不能确保峰的存在，揭示了铁核中量子涨落的令人费解且看似普遍的表现。前者是一个明显更脏的系统，具有完全带隙的超导性和高于 $T_c$ 的分裂二阶向列和磁跃迁。因此，我们观察到这些不同的系统显示出非常相似的穿透深度峰，结合理论结果，即 QPT 本身并不能确保峰的存在，揭示了铁核中量子涨落的令人费解且看似普遍的表现。前者是一个明显更脏的系统，具有完全带隙的超导性和高于 $T_c$ 的分裂二阶向列和磁跃迁。因此，我们观察到这些不同的系统显示出非常相似的穿透深度峰，结合理论结果，即 QPT 本身并不能确保峰的存在，揭示了铁核中量子涨落的令人费解且看似普遍的表现。