Waßer, Florian Tim (2018). Magnetic Excitations in Iron-based Superconductors. PhD thesis, Universität zu Köln.

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Superconductivity is one of the most mesmerising phenomena in condensed matter physics research, and the most recent material class, where it was discovered in, are iron-based superconductors. These compounds display a prodigious interplay between their lattice/orbital degrees of freedom, magnetic order, superconductivity and electronic nematicity, which gives rise to a rich phase diagram. Today there is a broad consensus that superconductivity is driven by spin fluctuations (paramagnons), whose fingerprint in the excitation spectra (observable by inelastic neutron scattering) is the so-called spin resonance mode. It is directly related to the details of the superconducting pairing mechanism, and its connection to the lattice structure and magnetic order is studied by X-ray, elastic and inelastic neutron scattering experiments in this thesis. The impact of antiferromagnetic order with large moments on the superconducting state is investigated by polarised inelastic neutron scattering in underdoped Ba(Fe1-xCox)2As2, where both orders coexist on a microscopic scale. Since superconductivity emerges in the presence of broad magnetic anisotropy gaps, it is shown that the corresponding spin resonance mode appears only in the two transversal channels, at two distinct energies, and longitudinal contributions are gapped. This situation is in contrast to the one in optimally and overdoped BaFe2As2, where isotropic spin resonance modes are observed. Concluding, this anisotropy is attributed to a band- and orbital-selective pairing mechanism. In Ba1−xNaxFe2As2 the interplay between structure, magnetic order and superconductivity is more intricate. The spins in the already orthorhombic and magnetically ordered phase undergo a second magnetic transition at a lower temperature, which rotates the moments from an alignment within the FeAs-layers to a perpendicular arrangement, by concomitantly suppressing the orthorhombic distortion. This spin reorientation transition highlights the importance of spin-orbit coupling in this material class. Moreover, in a naive picture, low-energy spin fluctuations, which promote superconductivity, rotate concomitantly with the associated static moments. However, polarised inelastic neutron scattering experiments revealed that this is not the case, and thus clearly demonstrate that the polarisation of low-energy spin excitations is uniform in iron-based superconductors. Furthermore, the spin resonance mode in Ba1−xNaxFe2As2 consists of two contributions which display a remarkably different doping dependence. On the one hand, the intensity of the high-energy part traces the shape of the superconducting dome in the phase diagram. While, on the other hand, it is shown by absolute unit calculation, that the intensity of the low-energy part is the accumulated spectral weight, which is removed from the magnetic Bragg peaks, as magnetic order and superconductivity compete for the same electronic states below Tc. For this reason, the low-energy part is most intense close to the magnetic end-point in the associated phase diagram and is strongly reduced in intensity beyond that point. In particular, the low-energy part in Ba0.61Na0.39Fe2As2 displays the strongest resonance mode ever observed in iron-based superconductors, while the high-energy contribution is similar intense as the one in optimally Co-doped BaFe2As2. Consequently, this observation provides a simple explanation on the origin of split spin resonance modes, observed in underdoped iron-based superconductors.

Item Type: Thesis (PhD thesis)
CreatorsEmailORCIDORCID Put Code
Waßer, Florian Timflorian.wasser@web.deUNSPECIFIEDUNSPECIFIED
URN: urn:nbn:de:hbz:38-91829
Date: 27 August 2018
Language: English
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects: Physics
Uncontrolled Keywords:
Iron-based SuperconductorsEnglish
Neutron ScatteringEnglish
Magnetic ExcitationsEnglish
Date of oral exam: 16 October 2018
NameAcademic Title
Braden, MarkusProf. Dr.
Lorenz, ThomasProf. Dr.
Refereed: Yes
URI: http://kups.ub.uni-koeln.de/id/eprint/9182


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