Neutron Scattering Study On Unconventional Superconductors

Superconductivity has been the focus of condensed matter physics since its discovery in the early 20th century.Inspired by the rich physics and tempting applications,the research on superconductivity has always been rather active,benefitting the understanding of strongly-correlated electrons systems.The discovery of iron-based superconductors has boosted the research on high-Tc superconductivity.Studying the similarity between iron-based superconductor and cuprates in phase diagram,crystal structure,interplay between magnetism and superconductivity will be of much help to understand the mechanism of high-Tc superconductivity.The topological material has shown us a brand new dimension to control the material properties.The promising prospect in the applications on quantum computation is also much attractive.In this dissertation,we will study the magnetic excitations in iron-based superconductors,and phonon behaviours in topological superconductor candidates,respectively.The contents and conclusions in this dissertation are organised as follows:1.Introduction to the basic knowledge of superconductivity and experimental methods.We have included the phenomena of superconductivity and brief introduction to BCS theory in the background.Then we move to unconventional superconductivity and focus on iron-based superconductors,with a brief review on the research of topological superconductors.The fundamental elements of neutron scattering techniques are mentioned later,with useful formulas and detailed introduction to instruments.2.Substitution effects of transition metal Ni on the transport properties and lowenergy magnetic excitations in Fe0.98Te0.5Se0.5.We have performed systematic resistivity and inelastic neutron scattering measurements on Nidoped samples to study the impact of Ni substitution on the transport properties and the low-energy magnetic excitations.It is found that,with increasing Ni doping,both the conductivity and super-conductivity are gradually suppressed;in contrast,the low-energy magnetic spectral weight changes little.Comparing with the impact of Co and Cu substitution,we find that the effects on conductivity and superconductivity for the same degree of substitution grow as the atomic number of the substituent deviates from that of Fe.The impact of the substituents as scattering centers appears to be greater than any contribution to carrier concentration.The fact that low-energy magnetic spectral weight is not reduced by increased electron scattering indicates that the existence of antiferromagnetic corre-lations does not depend on electronic states close to the Fermi energy.3.Substitution effects of transition metal Cu on magnetic excitations in the whole energy range in Fe0.98Te0.5Se0.5.From the inelastic scattering result of magnetic excitation in the whole energy range,the low-energy magnetic scattering of copper-doped sample is enhanced in strength,with greater spectral weight and slightly different patterns.The high-energy spectrum weight does not change but just behaves as spinwave excitation,similar to that of electron-doping system in 122 systems.We also calculated local susceptibility and momentum,with S?1/2,which is less that S=1 in the parent compound.Furthermore,the local susceptibility of Cu doped sample is likely to be greater.By systematic investigations of Ni and Cu doping effects,we then conclude the substitution effects of transition metal in 11 system iron-based superconductors.4,Pairing mechanism in the topological superconductor candidate Sr0.1 Bi2Se3.By performing inelastic neutron scattering measurements on a superconducting single crystal of Sr0.iBi2Se3,a prime candidate for realizing topological superconductivity,we find that there exists highly anisotropic phonons,with the linewidths of the acoustic phonons increasing substantially at long wavelengths,but only for those along the[001]direction.This observation indicates a large and singular electron-phonon coupling at small q,which we propose to give rise to the exotic p-wave nematic superconducting pairing in the MxBi2Se3(M=Cu,Sr,Nb)superconductor family.Therefore,we show these superconductors to be the example systems where electron-phonon interaction can induce more exotic superconducting pairing than the s-wave,consistent with their topological superconductivity.