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 $_2$ .