Investigation Of Electronic Nematicity In EuFe₂(As_1-xP_x)₂ And Large Upper Critical Field In Nb₂PdS₅

Since the discovery of high-Tc iron pnictide superconductors,lots of experimental groups from all over the world study superconducting samples of this system in different ways which make us understanding of iron-based superconductor more deeply than before.Recent years,with the breakthroughs and deeply in experiment,electronic nematics were observed in many iron pnictide superconductor samples.(Electronic nematics:an electron orientational order which breaks the underlying rotational symmetry)And the electronic nematic phase is found that in close proximity to the supeconducing phase which has aroused our great interest.In this paper,we use torque magnetometry as a probe to study the electronic nematics in EuFe2(As1-xPx)2 without external pressure.The nematic phase which measued through four different content of P doped samples is found to proliferate well above the structural transition Ts and to persist into the superconducting regime at optimal doping,after which it becomes absent or very weak.Then,our experiment results are also discussed and compared with the experimental group in Germany which investigate the electronic anisotropy in a way of thermoelectric power(TEP).At last,we according to the data and phase diagram analysis on the relationship between electronic nematic phase and superconducting phase.Chapter five in this paper introduct the research on the large upper critical field of the newly discovered superconducor Nb2PdS5.We study the evolution of its superconducting properties with doping,focusing on the behavior of the upper critical field Hc2.In contrast to the previous results of Se doping on S sites,superconductivity is found to be rather robust against the Pt and Ni dopants on the one-dimensional Pd chains.Most strikingly,the reduced Hc2,i,e.,the ratio of Hc2/Tc,is seen to be significantly enhanced by the heavier Pt doping but supressed in the Ni-doped counterparts,distinct from the nearly constant value in the Se-doped samples.Our findings therefore suggest that the upper critical field of this system can be modified in a tunable fashion by chemical doping on the Pd chains with elements of varying mass number.The spin-orbit coupling on the Pd sites should play an important role on the large upper critical field beyond the pauli pair-breaking field.The last,we also dicussed the influence from other factors on the upper critical field.