Exploration of Possible Enhancement of Superconductivity by Interface, Doping, and Pressure Effects

The superconductivity in CaFe2As2, both at ambient and elevated pressures, remains an open question. Both interface-associated superconductivity and spontaneous phase separation (producing a new phase at low temperature) have been proposed. Thus far, superconductivity has been only observed below 12 K and within a narrow pressure range. As the evidence for filamentary superconductivity, a slight resistivity drop below 10 K has only been detected in a few extraordinary CaFe2As2 samples. In this dissertation we show that superconductivity with a noticeable Meissner effect and a Tc up to 25 K can be induced at ambient pressure. These occur only when the two coexisting phases, i.e., the tetragonal-to-orthorhombic phase P1 and the tetragonal-to-collapsed tetragonal phase P2, are mesoscopically organized in a layer-stacking manner. The X-ray diffraction profile-analysis reveals a possible formation of dense interfaces, which is correlated with the superconductivity. The magnetic data further suggest that the spin-density-wave excitation in the P1 phase is largely suppressed along the interfaces. The microstress along the interfaces, therefore, may play a crucial role in the interface-associated superconductivity. The ambient-pressure superconducting CaFe2As2 sample we obtained unexpectedly demonstrates two superconducting transitions under high pressure from 0 to 17 kbar, and they have distinctive responses to external pressure. The higher transition with Tc ~25 K at ambient pressure can be further enhanced to 30 K under a pressure of 17 kbar. The pressure study further supports that the enhanced Tc is associated with the interfaces between phase P1 and P2. The work presented in this dissertation provides the most direct evidence for interface-enhanced Tc in undoped CaFe2As2 to date. To further explore other possible Tc enhancement in CaFe2As2, doping studies with Sm and Eu were also conducted. Superconductivity has been observed in the samples with Sm doping at ambient pressure and in the samples with Eu doping under high pressures. Related work are also presented as we attempted to search for enhancement of superconductivity in another layered compound beta-PdBi2.