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| Hauptseite > Workflowsammlungen > Öffentliche Einträge > Interplay between magnetism and superconductivity in iron based high temperature superconductors |
| Hauptseite > Workflowsammlungen > Öffentliche Einträge > Interplay between magnetism and superconductivity in iron based high temperature superconductors |
| Dissertation / PhD Thesis | FZJ-2013-04820 |
2013
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
ISBN: 978-3-89336-921-8
Please use a persistent id in citations: http://hdl.handle.net/2128/5471
Abstract: In this thesis, magnetic properties of a series of different Fe-based superconducting materials have been studied by means of neutron scattering techniques. Magnetic correlations in underdoped Ba(Fe$_{0.95}$Co$_{0.05}$)$_{2}$As$_{2}$ have been investigated for three phases of the phase diagram. It was possible to detect the spin gap and spin resonance signal, two features of the particle hole excitation spectrum at Q=(0.5, 0.5, 0), characteristic for the superconducting phase. The spin wave excitations present in the ordered phase have been analyzed quantitatively in terms of a linear spin wave model, whereas a spin diffusion model was applied to the collective excitations of the paramagnetic phase. However, it was found that both models can be applied to excitations in all three phases. In optimally doped CaFe$_{0.88}$Co$_{0.12}$AsF, a spin resonance signal was detected as part of the spin excitation spectrum at Q=(0.5, 0.5, 0). The observation of the spin resonance signal supports the s± symmetry of the superconducting gap function. In the undoped CaFeAsF compound three dimensional spin wave like excitations of the static Fe-SDW order have been observed at Q$_{AFM}$=(0.5, 0.5, 0.5), for temperatures below T$_{N}$. Above T$_{N}$ and for energies below 20 meV, the spin wave like excitations are replaced by short range two dimensional paramagnetic excitations, which persist up to 270 K. In superconducting FeSe$_{0.5}$Te$_{0.5}$ polarized neutron scattering investigations revealed the magnetic nature of the spin resonance signal and the excitation spectrum at Q=(0.5, 0.5, 0) up to 30 meV. The whole excitation spectrum including the spin resonance signal consists of an isotropic distribution of spin excitations with magnetic moments fluctuating in the ab-plane and perpendicular to the ab-plane, $χ^{"}_{ab}$(Q, ω) $\approx$ $χ^{"}_{c}$(Q, ω). In Eu(Fe$_{1−x}$Co$_{x}$)$_{2}$As$_{2}$ and EuFe$_{2}$(As$_{1−x}$P$_{x}$)$_{2}$ the effect of impurity doping on the static order of the magnetic lattice of the Eu$^{2+}$-moments has been studied by means of polarized and non-polarized neutron diffraction experiments. The introduction of cobalt leads to a helical type structure of the Eusublattice with Eu$^{2+}$-moments oriented parallel to the ab-plane. Whereas, partial replacement of arsenic by phosphorous leads to a ferromagnetic type structure and eventually results in a coexistence of long range ferromagnetic order and superconductivity.
Keyword(s): Dissertation
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