## Doctoral Dissertations

8-2012

Dissertation

#### Degree Name

Doctor of Philosophy

Physics

Pengcheng, Dai

#### Committee Members

Pengcheng Dai, Hanno Weitering, Adriana Moreo, Takeshi Egami

#### Abstract

Presented within are neutron scattering studies detailing the spin dynamics of BaNi$_{x}$Fe$_{2-x}$As$_{2}$ for x = 0 (parent), 0.04 (underdoped), and 0.1 (optimal) dopings, and FeSe$_{x}$Te$_{1-x}$ for x = 0 (parent), 0.3 (underdoped), and 0.4 (optimal) dopings. These recently discovered Fe-based superconducting compounds are strikingly similar, in many respects, to the cuprate class of unconventional superconductors and share qualitatively similar phase diagrams consisting of a long range ordered magnetic ground state in the parents which, upon doping, is supplanted in favor of superconductivity. The dopings discussed herein allow us to tune through the phase diagram, beginning with long range ordered parents and ending with optimally doped superconductors with short range magnetic correlations.

For BaFe$_{2}$As$_{2}$, the excitations in the ordered state are strongly damped and persist up to 300meV. Low energies excitations are centered around Q$_{\text{AMF}}$ and disperse towards the zone boundary with increasing energy. Only scattering above 100meV is effected when warming above T$_{N}$. In underdoped x = 0.04 BaNi$_{x}$Fe$_{2-x}$As$_{2}$, we find an order of magnitude reduction in the coupling between layers and a corresponding crossover from 3D to 2D magnetism. In coauthor work on optimal doped x = 0.1 BaNi$_{x}$Fe$_{2-x}$As$_{2}$ we establish the existence of a 3D resonance mode in the superconducting state. Excitations at optimal doping above the resonance are very similar to the paramagnetic scattering observed in the parent and consists of diffuse scattering below 100meV while above this threshold the signal has similar dispersion, linewidths, and intensity as the ordered state.

For FeTe, I discuss our existing efforts and data collection aimed at addressing issues associated with calculating the effective moment from Q,E-integrated data. Tuning through the phase diagram to the x = 0.3 underdoped FeSe$_{x}$Te$_{1-x}$ system we find filamentary superconductivity with magnetic spectral weight sitting at both the AFM and nesting vector. Upon reaching x = 0.4 optimal doping, the scattering completely transfers over to the nesting vector and a 2D resonance mode appears below T$_{c}$.

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