Date of Award

5-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Pengcheng Dai

Committee Members

Hanno Weitering, Takeshi Egami, Adriana Moreo, Haidong Zhou

Abstract

Presented within are neutron scattering studies of several different high temperature superconducting materials: BaFe1.9Ni0.1As2 [Barium Iron Nickel Arsenic], BaFe1.85Ni0.15As2 [Barium Iron Nickel Arsenic], Ba0.67K0.33Fe2As2 [Barium Potassium Iron Arsenic], and Pr0.88LaCe0.12CuO4-y [Praseodymium Lanthanum Cerium Copper Oxide]. The main focus is on the magnetic excitations within the systems.

For BaFe1.9Ni0.1As2 [Barium Iron Nickel Arsenic], we measured the intensity of its magnetic excitations and compared the results with excitations in antiferromagnetic non-superconducting BaFe2As2 [Barium Iron Arsenic]. We find electron-doping only affects spin excitations below 100 meV while the total size of the magnetic moment and the energy distribution do not change much. It shows that the magnetic moments in both materials are similar to insulating copper oxides, an indicator of the importance of strong electron correlations in high temperature superconductivity.

For both BaFe1.85Ni0.15As2 [Barium Iron Nickel Arsenic] and Ba0.67K0.33Fe2As2 [Barium Potassium Iron Arsenic], we use polarized inelastic neutron scattering to study their low-energy spin excitations and their spatial anisotropy. Our neutron polarization analysis reveals that magnetic excitations are isotropic for the in-plane and out-of-plane components in both the normal and superconducting states for BaFe1.85Ni0.15As2 [Barium Iron Nickel Arsenic], while in Ba0.67K0.33Fe2As2 [Barium Potassium Iron Arsenic] large difference in spin gaps were found. A comparison of these results with those of undoped BaFe2As2 [Barium Iron Arsenic] and optimally electron-doped BaFe1.9Ni0.1As2 [Barium Iron Nickel Arsenic] suggests that the spin anisotropy observed Ba0.67K0.33Fe2As2 [Barium Potassium Iron Arsenic] are likely due to their proximity to their parent compound, where spin anisotropy exists below TN [Neel Temperature], while the neutron spin resonance is isotropic in the overdoped regime, consistent with a singlet to triplet excitation.

For as-grown and optimal superconducting Pr0.88LaCe0.12CuO4-y [Praseodymium Lanthanum Cerium Copper Oxide] (PLCCO), we measured their magnetic excitations over a wide energy range, and compared their corresponding results. The spectra is considerably larger throughout the whole zone in as-grown PLCCO, than in the optimal superconducting PLCCO, which is very different from the BaFe2As2 [Barium Iron Arsenic] system.

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