Date of Award

5-2010

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Pengcheng Dai

Committee Members

Pengcheng Dai, Jaime A. Fernandez-Baca, Takeshi Egami, Norman Mannella, James R. Thompson

Abstract

We carried out systematic neutron scattering experiments to investigate the magnetic properties and their relationship to the high-$T_c$ superconductivity, when the materials are tuned from their antiferromagnetic (AF) parent compounds to the superconducting regime.

We observed resonance mode in the electron doped cuprate Nd$_{1.85}$Ce$_{0.15}$CuO$_4$, demonstrating that the resonance is a general phenomenon in cuprate superconductors regardless of hole- or electron-doping. In Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$, the local susceptibility displays two distinct energy scales that are broadly consistent with the bosonic modes revealed by scanning tunneling microscopy experiments. These results indicate the presence of very strong electron spin excitations couplings in electron doped cuprates.

Shortly after the discovery of high-$T_c$ superconductivity in the Fe pnictides, we discovered that the magnetic phase diagram of CeFeAsO$_{1-x}$F$_x$ is remarkably similar to that of the cuprates. Besides CeFeAsO, similar magnetic and lattice structures are also observed in PrFeAsO and SrFe$_2$As$_2$ systems. Neutron scattering measurements show that in SrFe$_2$As$_2$, the spectrum of magnetic excitations consists of a Bragg peak at the elastic position, a spin gap, and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimated the effective magnetic exchange coupling using a Heisenberg model.

In order to study the nature of the exchange interactions in the parent compound of Fe pnictides, we studied the high energy spin-wave excitations in CaFe$_2$As$_2$. Although the spin waves in the entire Brillouin zone can be described by an effective three-dimensional anisotropic Heisenberg Hamiltonian, the magnetism in this system is neither purely local nor purely itinerant; rather it is a complicated mix of the two.

When the Fe pnictide is tuned into superconducting regime with doping, the low energy spin fluctuation is dominated by a resonance mode. In the optimally electron doped BaFe$_{1.9}$Ni$_{0.1}$As$_2$, application of a magnetic field that suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results suggest that the energy of the resonance is proportional to the electron pairing energy, and thus indicate that spin fluctuations are intimately related to the mechanism of high $T_c$ superconductivity.

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