Nematic phase of Iron Based Superconductors probed by Nernst effect

现代凝聚态物理学中一个重要且极具争议性的话题是对所谓的电子有序的理解，它由一种自组织的电子态组成，这种电子态打破了底层晶格的旋转对称性，但保持了平移对称性。在许多强关联材料中，电子有序被认为起着重要的作用，并且在二维电子系统、钛酸锶、铜酸盐超导体和铁基超导体（IBS）中，电子有序已被证实。特别是在过去的几年里，IBS中的电子涨落一直是深入研究的对象，因为它们被认为是非常规超导体中库珀配对机制的关键。由于其相图的丰富性，IBS是研究电子向列性与长程磁序上升、超导性和量子临界性之间关系的理想环境。热电输运性质，即塞贝克效应和能斯特效应，已被指出是研究超导现象的一个非常有力的工具，但它们的系统应用仍然缺乏。我想通过这个项目来填补这个空白。特别是，我想通过使用单轴应变/应力作为能斯特效应测量中的可调参数，采用一种新颖的和原始的方法。事实上，在最近的实验中，单轴压力的应用，以诱导结构扭曲，被发现是一个基本的成分，以调查的相位/波动。特别是非常有趣的和有前途的结果已经通过电阻率测量作为单轴应变的函数。事实上，热电性能被预测为更加敏感，打开全新的见解，这一新的方面的物理学的相关电子systems.My方法是复杂的，因为它需要一个设备的应用单轴应变集成，如压电，在一个国家的最先进的热电测量设置。然而，我不仅期望在IBS的波动领域取得重要成果，而且我还将建立一种有希望解决更多物理问题的技术。例如，能斯特效应被认为是超导波动的敏感探针，其应变导数可以用新设备进行研究。此外，拓扑状态强烈影响磁输运和能斯特效应。因此，在该项目中开发的设备也可以长期用于研究拓扑状态，例如通过调整Weyl点的位置。

An important and very controversial topic in modern condensed matter physics is the understanding of the so-called nematic order, which consists of a self-organized electronic state that breaks the rotational symmetry of the underlying lattice but keeps the translational symmetry. The nematic order is considered to play an important role in many strongly correlated materials and it is a well established fact in two dimensional electron systems, strontium ruthenates, superconducting cuprates and iron based superconductors (IBS). In particular, in the last years, the nematic fluctuations in IBS, have been object of intense investigation since they are claimed to be crucial for the Cooper pairing mechanism in unconventional superconductors. Thanks to the richness of their phase diagram, IBS are the ideal environment to investigate the relationship between the electronic nematicity and the rising of long-range magnetic orders, superconductivity and quantum criticality. Among the possible techniques to investigate the nematic phenomenology, thermoelectric transport properties, namely Seebeck and Nernst effect, have been pointed out as an extremely powerful probe, but their systematic application is still missing. With this project I want to fill this gap. In particular, I want to adopt a novel and original approach by using uniaxial strain/stress as a tunable parameter in the Nernst effect measurements. In fact, in recent experiments, the application of uniaxial pressure to induce structural distortions, revealed to be a fundamental ingredient to investigate the nematic phase/fluctuations. In particular very interesting and promising results have been obtained through electric resistivity measurements as a function of uniaxial strain. Indeed, thermoelectric properties are predicted to be far more sensitive, opening completely new insights into this novel aspect of the physics of correlated electron systems.My method is sophisticated since it requires the integration of a device for the application of uniaxial strain, like a piezoelectric, in a state-of-the-art thermoelectric measurement setup. However, I do not only expect important results in the field of nematic fluctuations of IBS but I will also establish a technique promising for much more physical questions. For example, the Nernst effect is known to be a sensitive probe of superconducting fluctuations whose strain derivatives can be studied with the new device. Moreover, topological states strongly affect magnetotransport and the Nernst effect. Therefore the device developed within this project can, in long-term, also be used to investigate topological states, for example by tuning the position of Weyl points.