Nematic phase of Iron Based Superconductors probed by Nernst effect

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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.