Resonant detection of electronic nematicity in correlated electron systems

在普通金属中，电子态仅受它们与原子晶格的相互作用所支配，因此电子系统从主晶格继承其对称性。然而，如果电子之间的相互作用足够强，这个简单的范例肯定不再适用，可能会出现比母晶体更低对称性的有趣的强关联状态：电子向列态破坏旋转对称性，而晶体的平移对称性保持不变，这类似于液晶中的向列相。这种状态的一个特点是对称相关方向之间明显的散射时间各向异性。小的外部晶格扭曲影响了向列相电子系统的择优方向，导致电阻与单轴应变有很大的依赖关系。电子向列性被认为是铜基和铁基高温超导体中强电子关联的重要因素。向列性是一种相当罕见的现象，只在少数几个系统中观察到，它可能出现在两个已知的高T_c类中，自然会引出一个基本问题：向列相涨落是否有助于库珀配对，或者向列性是一种相互竞争的现象，对高温超导来说是无关的，甚至是不利的？我们打算定量测量大范围的磷灰石超导体的向列相磁化率，并将其与转变温度T_c相关联。由于可用的单晶尺寸较小，许多类别的砷化铁在传统的测量技术中是无法获得的。我们提出了一种基于聚焦离子束(FIB)雕刻单晶微谐振器的高灵敏共振向列线检测方法。通过在单晶上雕刻100-10微米长的机械振荡器，我们利用材料在弯曲模式周期中的压缩和膨胀来直接测量向列相磁化率，与现有的方法相比，具有更高的精密度和准确度。同时，悬臂装置的共振频率将提供系统弹性常数的直接测量。这将使我们能够将向列相的磁化率与离子晶格的热力学性质联系起来，这是识别向列相对对称性破坏的贡献程度的关键比较。本项目的目的是对不同结构类别的磷灰石超导体的向列性进行全面的比较，并研究其在超导中的作用。一旦这项新技术被开发出来，它将成为研究铁基超导体以外的各种其他材料中向列相现象的通用工具。

In normal metals, the electronic states are governed merely by their interaction with the atomic lattice, and as a result the electronic system inherits its symmetries from the hosting lattice. However, if the interactions between the electrons themselves are sufficiently strong, this simple paradigm must not hold true anymore and intriguing strongly correlated states of lower symmetry than the parent crystal may arise: An electronic nematic state breaks a rotational symmetry while the translational symmetry of the crystal remains intact, named in analogy to the nematic phase in liquid crystals. A hallmark of such states is a pronounced scattering time anisotropy between symmetry related directions. Small external lattice distortions influence the preferred direction of the nematic electronic system, yielding a substantial dependence of the resistance on uniaxial strain. Electronic nematicity has been argued to be of importance for the strong electron correlations in both copper- and iron-based high-temperature superconductors. Nematicity is a rather rare phenomenon, observed only in a handful of systems, and its potential appearance in both known high-Tc classes naturally leads to a fundamental question: Do nematic fluctuations contribute to Cooper pairing, or is nematicity a competing phenomenon irrelevant or even disadvantageous for high-Tc superconductivity? We intend to quantitatively measure the nematic susceptibility in a wide range of pnictide superconductors and correlate it with the transition temperature Tc. Many classes of iron-arsenides are inaccessible to traditional measurement techniques due to the small size of available single crystals. We propose to develop a highly-sensitive resonant nematicity detection method based on Focused Ion Beam (FIB) carved single crystal microresonators. By carving 100-10 micrometer long mechanical oscillators out of pnictide single crystals, we exploit the compression and expansion of the material during a bending mode cycle to directly measure the nematic susceptibility with higher precision and accuracy compared to existing methods. At the same time, the resonance frequency of the cantilever devices will provide a direct measure of the elastic constants of the system. This would enable us to correlate the nematic susceptibility with a thermodynamic property of the ionic lattice, which is a key comparison to identify the degree of the nematic contribution to the symmetry breaking. This project is aimed to provide a comprehensive comparison of nematicity in the various structural classes of pnictide superconductors, and investigate its role in superconductivity. Once this new technique has been developed, it will be a versatile tool to research nematic phenomena in a variety of other materials beyond iron-based superconductors.