Novel Pd and Chalcogen Based Low-Dimensional Superconductors and Electronic Properties of Type-I Weyl Semimetals

This thesis investigates a new family of Pd based transition metal chalcogenides which displays unconventional superconducting behavior and also the newly discovered Weyl physics in transition metal monopnictides. The superconductors investigated are Nb₂Pd₀.₈₁S₅ (NPS215), Nb₃PdxSe₇ (NPSe317), Ta₄Pd₃Te₁₆ (TPT) and chemically substituted compounds. The Nb based samples displayed relatively low superconducting transition temperatures, but extremely high upper critical fields Hbc2 that exceeded the weak coupling Pauli limiting fields by a factor considerably greater than two. For a NPS215 crystal with Tc ∼ 6.7 K, Hbc2 (T ➝ 0K) = 37 T. For NPSe317, we found crystals with Tc = 1.87 K and 3.5 K, Hbc2 (T ➝ 0K) = 14.1 T and 28 T. Both compounds also displayed unconventional behavior, in the case of NPS215, Hc₂'s do not saturate even for temperatures down to T = 0.4 K and H⊥bc2 remained linear in the whole temperature range. For NPSe317, H⊥bc2s are well described by the Ginzburg-Landau expression where they saturated at the lowest temperatures. H‖bc2 in the samples with Tc ≅ 3.5 K showed a -T½ dependence, such behavior was only observed in the two-dimensional superconductivity of atomically thin NbSe₂. The high superconducting anisotropy together with Fermi surface calculations indicate that these systems are low dimensional superconducting systems. The TPT samples showed linear in T non-saturating Hc₂s for all three orientations, but their magnitudes are comparatively small and the system seems to be orbital limited. Quantum oscillations revealed that the Weyl semimetals, (Nb,Ta)(P,As), have small Fermi surfaces with a non-trivial Berry phase. The chiral anomaly was observed in the form of negative magnetoresistivity when H‖j. The SdH and dHvA oscillations observed at low magnetic fields experience changes in amplitude and in periodicity upon reaching the quantum limit of the systems. Coupled with field induced effective mass renormalization points towards electronic/topological phase transition(s) in these semimetals. Anomalous Hall effect was observed in these systems and there appears to be nearly quantized Hall plateaus that are nearly linearly quantized..