Investigations into the vortex lattice of Ni-chalcogenide and Fe-arsenide superconductors, using small-angle neutron scattering

Jellyman, Erik (2018). Investigations into the vortex lattice of Ni-chalcogenide and Fe-arsenide superconductors, using small-angle neutron scattering. University of Birmingham. Ph.D.

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Abstract

This work summarises investigations into the superconductivity in 122-structure, I4/mmm symmetry materials; the heavy-fermion TlNi$$_2$$Se$$_2$$, and the iron-arsenides (Ba$$_0$$.$$_5$$K$$_0$$.$$_5$$)Fe$$_2$$As$$_2$$ and KFe$$_2$$As$$_2$$. Small-angle neutron scattering (SANS) was used to study the vortex lattice (VL) of single crystal samples of these superconductors under temperature, field and angle modulation. We observe a linear response of the form factor to temperature variation for TlNi$$_2$$Se$$_2$$ and KFe$$_2$$As$$_2$$, concurrent with nodal and unconventional superconductivity. Weak VL anisotropy and no VL morphology were observed in TlNi$$_2$$Se$$_2$$ and (Ba$$_0$$.$$_5$$K$$_0$$.$$_5$$)Fe$$_2$$As$$_2$$ respectively, whereas KFe$$_2$$As$$_2$$ experiences strong VL anisotropy. Observations of (Ba$$_0$$.$$_5$$K$$_0$$.$$_5$$)Fe$$_2$$As$$_2$$ confirm Pauli paramagnetic effects (PPE) above 0.1H$$_c$$$$_2$$ . PPE are seen near Tc for KFe$$_2$$As$$_2$$, but not for TlNi$$_2$$Se$$_2$$. Literature review regarding the Fermi surface and electron bands confirms van Hove singularities (vHs) near the Fermi level (E$$_F$$ ) for all three materials. This provides some explanation of the variation in pairing symmetry for similarly structured superconductors. The tuning of the $$_v$$Hs with respect to E$$_F$$ determines the symmetry of the gap function as well as the effective mass of the electrons for TlNi$$_2$$Se$$_2$$, (Ba$$_1$$$$_-$$$$_x$$K$$_x$$)Fe$$_2$$As$$_2$$ and KFe$$_2$$As$$_2$$.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Supervisor(s):
Supervisor(s)EmailORCID
Blackburn, ElizabethUNSPECIFIEDUNSPECIFIED
Colclough, Mark SUNSPECIFIEDUNSPECIFIED
Licence:
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Physics and Astronomy
Funders: None/not applicable
Subjects: Q Science > QC Physics
URI: http://etheses.bham.ac.uk/id/eprint/8513

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