Superconducting Enhancement and Electronic Nematicity in Substituted BaNi2As2

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Inspired by the frequent presence of nematicity in the high Tc superconducting systems, this thesis is focused on the interplay of nematic and superconducting order in a system that does not possess long range magnetism. Here I describe my measurements of the physical properties of Ba(Ni[1-x]Cox)2As2 and Ba[1-x]SrxNi2As2 intermetallic compounds, characterizing both superconductivity and nematicity in these series.

Thermodynamic, transport, and magnetic properties of single crystals synthesized using a flux growth technique are reported. Using the results of these physical property measurements, I construct the electronic phase diagrams of the Ba(Ni[1-x]Cox)2As2 and Ba[1-x]SrxNi2As2 series. In both substitution series, increasing x smoothly suppresses a tetragonal-triclinic structural phase transition. At the low temperature structural phase boundary, a large enhancement in superconducting Tc is also observed in both systems.

The Ba[1-x]SrxNi2As2 series was further characterized through measurements of symmetry isolated components of the fourth-rank elastoresistivity tensor. I observe a divergence in elastoresistivity over a wide range of temperatures and x values in Ba[1-x]SrxNi2As2 crystals, indicative of electronically driven rotational symmetry breaking in this series. The low temperature elastoresistivity is peaked in the vicinity of optimal Tc, suggesting the enhanced superconducting pairing observed in this region is born from strong nematic fluctuations in the system.