Neutron Scattering Studies On The Iron-based Superconductors And Transport Studies On The Topological Semimetals

Understanding the exotic properties of quantum materials has been the focus of con-densed matter physics research for decades.On one hand,they represent fundamen-tally novel topological state of matter;on the other hand,they hold promising future for applications with their fascinating properties.In this thesis,I investigate the exotic properties and microscopic mechanism of several different kinds of materials,includ-ing iron-based superconductors and topological semimetals.In Chapter ?,I will introduce conventional superconductors and unconventional superconductors first,especially the iron-based superconductors.Then I will briefly introduce the newly discovered topological semimetals,including Dirac,Weyl and n-odal line semimetals.In Chapter ?,I will describe the methods for single crystal growth,and experi-mental techniques involved in this thesis,including neutron scattering and transport.In Chapter ?,by performing resistivity and inelastic neutron scattering measure-ments on Cu-doped iron-based superconductors 11 system-Fe0.98-zCu2Te0.5Se0.5,it is found that with increasing Cu doping the sample behaves as an insulator.Howev-er,in contrast to expectations that replacing Fe with Cu would suppress the magnetic correlations,the low-energy(<12 meV)magnetic scattering is enhanced in strength,with greater spectral weight and longer dynamical spin-spin correlation lengths.Such enhancements can be a consequence of either enlarged local moments or a slowing down of the spin fluctuations.In either case,the localization of the conduction states induced by the Cu doping should play a critical role.Our results suggest that the rigid band shift model is insufficient in describing the doping effect.In Chapter ?,? show results from neutron scattering and transport measurements on KxFe2-ySe2 single crystals.From the inelastic neutron scattering measurements,both the spin-wave excitations resulting from the block antiferromagnetic order,and spin resonance mode that is tied with the superconductivity are observed,demonstrat-ing that these two orders coexist.From the temperature dependence of the intensity of the magnetic Bragg peaks,we find that well before entering the superconducting state,the development of the magnetic order is interrupted,at?42 K.A low-energy mode at 8 meV at low temperatures,in addition to the known resonant mode of 14.5 meV,is also found.The wave-vector and temperature dependence of this new mode are consis-tent with it being a second magnetic resonance originating from the superconducting phase.In Chapter ?,? show that single crystals of a recently predicted Dirac nodal-line semimetal SrAs3 have been grown.By carrying out magnetotransport measurements and performing first-principle calculations,the nontrivial ? Berry phase by analyzing the Shubnikov-de Haas quantum oscillations,and a robust negative magnetoresistance induced by the chiral anomaly have been observed.These results indicate that SrAs3 is a Dirac nodal-line semimetal.In Chapter ?,? will give a summary of this thesis,and point out the underlying relationship between different systems described in this work,which may motivate further research on topological materials.