Date of Award

5-2005

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Pengcheng Dai

Committee Members

Janice L. Musfeldt, James R. Thompson, Victor Barzykin

Abstract

We present comprehensive neutron scattering studies on the electron-doped superconductors Nd2-xCexCuO4 and Pr1-xLaCexCuO4 to understand how magnetic properties and phonon are related to superconductivity. For the superconducting Nd1.85Ce0.15CuO4, a c-axis magnetic field enhances the commensurate antiferromagnetic order, while an ab-plane magnetic field induces spin-flop transition from the noncollinear to the collinear magnetic structure. The c-axis field-induced effect is absent in both the parent Nd2CuO4 and the as-grown nonsuperconducting Nd1.85Ce0.15CuO4 samples. These results, combined with those on the hole-doped La2-xSrxCuO4 suggest that antiferromagnetism competes with superconductivity.

In the Pr1-xLaCexCuO4 system, we find that quasi-2 dimensional commensurate antiferromagnetism coexists with superconductivity and 3 dimensional antiferromagnetism in the underdoped region. To determine whether the antiferromagnetic order is the competing order in high temperature superconductors, regardless of hole-doping or electron-doping, we performed field-induced experiments on the electron-doped superconductor Pr0.88LaCe0.12CuO4±δ by applying magnetic fields along the c-direction and in the ab-plane direction. The anisotropy of the field-induced moment demonstrates the competing nature of these two order parameters.

Inelastic neutron scattering has been performed to study the generalized phonon density of states (GDOS) of electron-doped Nd2-xCexCuO4 with various levels of doping. Upon doping, the softening of phonon around 70 meV has been observed. However, most of the softening occurs within a few percent of Ce doping, and it is not related to the doping induced nonsuperconducting-superconducting transition.

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