Anomalous Physical Properties In Quasi-2D FeSe-based Superconductors And A Derived CuAs-based Compound

As a second high-Tc superconductor,the iron-based superconductors not only provide a new platform to help solve the long-standing mysteries in high-Tc cuprate superconductors,but also attract widespread attention on account of their distinct structural characteristics and abundant physical properties.In the past three decades,there has been considerable progress on the research of high-Tc cuprate superconductors,which spawned a series of new concepts,new methods and new theories.However,there are still many basic problems such as the origin of pseudogap and its relation with superconductivity to be resolved.The discovery of iron-based superconductors may shed light on these issues.In addition,the iron-based superconductor is a huge family,which has a large number of materials.In some specific systems,there tends to be nontrivial topological properties closely related to the structural features,thus showing great significance for the research of topological superconductivity.In this dissertation,in order to explore the pseudogap behavior and related topological properties in iron-based superconductors,we focused on two systems:one is the organic-ion-intercalated FeSe-based superconductors with quasi-2D nature,and the other is the 112-type Dirac semimetal YCuAs₂ which owns a derived structure with ironbased superconductors.By means of nuclear magnetic resonance,electrical transport,magnetic susceptibility and heat capacity measurements,we systematically studied the anomalous physical properties in these systems.There are six chapters as follows in the dissertation:1.IntroductionIn this chapter,we elaborate the related research background from five aspects.Firstly,we briefly review the overall history of the development of superconductors,and introduce the basic properties of two kinds of high-Tc superconductors.Secondly,we focus on the 11-type iron-based superconductors,that is,the FeSe-based superconductors,and introduce their basic physical properties.Thirdly,started from the 112-type iron-based superconductors,we introduce the basic properties of a series of 112-type materials with characteristic square-net layer as structure units.Fourthly,we introduce the dominant mechanisms of the anomalous physical property—negative magnetoresistance.And finally,we elaborate the research content and significance of this dissertation.2.Experimental methodsIn this chapter,we introduce two dominant experimental methods involved in our research,including the electrochemical intercalation method and the nuclear magnetic resonance technology.3.Universal pseudogap behavior in quasi-2D FeSe-based superconductorsIn this chapter,we systematically studied three kinds of quasi-2D organic-ionintercalated FeSe-based superconductors in two steps.Firstly,by employing the nuclear magnetic resonance measurement,together with electrical transport,magnetic susceptibility,Nernst effect and scanning tunneling microscopy measurements,we unambiguously reveal a pseudogap behavior arising from superconducting fluctuations in the normal state of（TBA）_xFeSe and（CTA）_xFeSe.Such a result confirms that strong phase fluctuation is also an important character in the quasi-2D layered iron-based superconductors as widely observed in high-Tc cuprate superconductors.And then we further synthesized a new kind of layered FeSe-based superconductor（PY）_xFeSe,which owns smaller interlayer distance but persistent quasi-2D nature.By performing combined nuclear magnetic resonance and anisotropic electrical transport measurements,we definitely reveal an almost identi cal pseudogap behavior being of preformed Cooper pairing nature.Such a result demonstrates that the pseudogap behavior is a universal behavior in these layered FeSe-based superconductors with quasi-2D nature,and suggests a crucial role of 2D electronic structure on the high-Tc superconductivity in FeSe-based superconductors.4.Exotic transport properties in quasi-2D FeSe-based superconductors under hydrostatic pressureIn this chapter,we focused on the quasi-2D FeSe-based superconductor（CTA）_xFeSe,and preliminarily studied its electrical transport properties under hydrostatic pressure.We found that,there is a pressure-induced "two-step drop behavior" during the superconducting transition,and the two resistance drops exhibit 2D and 3D features respectively.Such a behavior is possibly analogous to the pressureinduced crossover from 2D to 3D superconducting states observed in optimally doped Bi2Sr2CaCu2O8+δ superconductor.Further analysis reveals that the 2D superconducting state exhibits different pressure dependence with the 3D superconducting state.And in the meanwhile,the pressure phase diagram of（CTA）_xFeSe is quite similar to the hole-doping phase diagram of La2-xBaxCuO4,which hints possible existence of other ordered states in（CTA）_xFeSe.In addition,arising from the pseudogap behavior,there exist-paired electrons without long-range phase coherence in the 2D superconducting state.Therefore,we reasonably speculate that the exotic transport behavior of（CTA）_xFeSe under hydrostatic pressure may indicate the existence of new quantum phases such as pair-density waves.5.Giant negative magnetoresistance in 112-type material YCuAs₂ with derived structureIn this chapter,we systematically studied the 112-type Dirac semimetal YCuAs₂,which owns a derived structure with iron-based superconductors.In collaboration with the theoretical research group,we firstly demonstrated that YCuAs₂ is a Dirac semimetal with significant spin-orbit interactions.Experimentally,we observed a giant negative magnetoresistance up to 40%under 9 T at 2 K,which is not restricted to the specific orientation between magnetic field and electric current but with considerable anisotropy.By employing careful sample characterization,together with magnetic susceptibility,electrical transport and nuclear magnetic resonance measurements,we found that the giant negative magnetoresistance in YCuAs₂ is tightly connected with the local moments induced by Cu vacancies.For such a nonmagnetic（paramagnetic）Dirac semimetal,we speculate that the strong spin-charge interplay in YCuAs₂ may be attributed to the unique topological structure and Cu vacancies in our sample.6.Summary and prospectsIn the last chapter,we make a comprehensive summary of the present work and look ahead to the future research.