Penetration depth studies of unconventional superconductors

We have setup a tunnel-diode oscillator based self-resonating technique operating at 26 MHz to probe the temperature-dependence of the magnetic penetration depth λ. Using this high resolution setup with a noise level of 2 parts-per-billion (ppb), we have measured and present in this thesis penetration depth data down to ∼0.4 K for the following superconducting samples - the Pd-Bi based structurally isomeric single crystalline superconductors α-PdBi2 and β-PdBi2, the Chevrel phase-based single crystalline superconductor CsMo12S14, single crystals of the quasi-one-dimensional superconductor Tl2Mo6Se6, and polycrystalline samples of the filled skutterudite superconductor Pr1−xCexPt4Ge12 for x = 0, 0.02, 0.04, 0.06, 0.07 and 0.085. For both α-PdBi2 and β-PdBi2, analysis of the penetration depth as well as the superfluid density data points towards a conventional single-gap moderate-coupling symmetry of the superconducting order parameter, suggesting similar pairing mech- anism in this class of materials. For the superconductor CsMo12S14, our data show strong signatures of multi-gap superconductivity. We attribute weak-interband coupling between the band-specific superconducting gaps to be responsible for two separate critical transition temperatures, clearly visible in the overall penetration depth and superfluid density data. Our claim for multi-band superconductivity is supported by thermodynamic critical field data, and also by electronic band structure calculations. Our penetration depth measurements on Tl2Mo6Se6 show an exciting two-step superconducting transition, originating from the dimensional crossover from a longitudinally coherent one-dimensional superconducting phase to a globally coherent three-dimensional superconducting ground state. We show that electrical transport measurements on the same sample show a similar two-step transition as well. Finally, for the skutterudite superconductor Pr1−xCexPt4Ge12, our data suggest multi-gap superconductivity with one unconventional gap with point node and one nodeless conventional gap. Preliminary analysis of the superfluid density show that as the Ce doping concentration increases from the optimal value of x = 0, the contribution of the nodal gap decreases monotonically.​Doctor of Philosophy (SPMS