Chu, Jiun-HawShi, Yue2019-02-222019-02-222018Shi_washington_0250O_19520.pdfhttp://hdl.handle.net/1773/43409Thesis (Master's)--University of Washington, 2018The iron-based superconductors (FeSCs) is one of the only two high temperature superconductors. Soon after its discovery, mounting experiments performed on several families of FeSCs demonstrate the existence of an electronic driven broken rotational symmetry phase, the nematic phase. As an integral part of the phase diagram, understanding the nature of the nematic phase is a key step to unveil the origin of high temperature superconductivity. The underlying charge or spin degree of freedom responsible for the nematic transition might be the same degree of freedom that gives rise the superconductivity and magnetic order. In this thesis, we use the elastoresistance technique to probe the nematic fluctuations in iron chalcogenide superconductors FeTe1-xSex, which is one of the prototypes of FeSCs with the simplest crystal structure. Specifically, two unique features of FeTe1-xSex distinguish it from the well-studied iron pnictides. First, the bicollinear antiferromagnetic order in FeTe is different from the collinear antiferromagnetic order in iron pnictides. Second, the nematic phase in FeSe forms without magnetic ordering, which is not the case in iron pnictides. Consequently, a systematic study of the FeTe1-xSex family will provide important insights into the universal role played by the nematic fluctuations. In my thesis, I first describe the synthesis and annealing procedures of FeTe1-xSex crystals. Then I introduce the theoretical background and experimental techniques of elastoresistance measurement for B2g symmetry channel of Tetragonal symmetry point group. Finally, I describe the measurement of strong nematic fluctuation in FeTe1-xSex. Our results show that the nematic fluctuations increase as the Se doping concentration increase for FeTe1-xSex (x = 0 - 0.5). The sign of elastoresistivity coefficient 〖2m〗_66 flips between the FeTe0.8Se0.2 and FeTe0.7Se0.3 from negative to positive. The bare nematic critical temperature T^* extracted from the Curie-Weiss fitting of 〖2m〗_66 are negative for all components measured in our experiments. With the Se doping concentration increase, the bare nematic critical temperature T^* approaches to zero, which implies the existence of nematic quantum criticality at the optimally doping.application/pdfen-USCC BYelastoresistance measurementFeTe1-xSexiron-based superconductorsnematic susceptibilityMaterials ScienceMaterials science and engineeringThesis