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.

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