Title:

Investigations into the vortex lattice of Nichalcogenide and Fearsenide superconductors, using smallangle neutron scattering

This work summarises investigations into the superconductivity in 122structure, I4/mmm symmetry materials; the heavyfermion TlNi\(_2\)Se\(_2\), and the ironarsenides (Ba\(_0\).\(_5\)K\(_0\).\(_5\))Fe\(_2\)As\(_2\) and KFe\(_2\)As\(_2\). Smallangle 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\(_2\).
