A century has passed since the discovery of superconductivity in 1911. In these 100 years, generations of scientists have spent tremendous effort to reveal the secrets behind this fascinating physical phenomenon. However, the deeper they dig, the more mysteries they discover. Nowadays, superconductivity is no longer merely the disappearance of electrical resistivity but a macroscopic quantum mechanical phenomenon that is linked to unknown mechanisms and new physics. In this thesis, unconventional superconductivity in a bulk transition-metal dichalcogenide single crystal and heterostructure materials are studied.

In the first part, we briefly go through the history and developments of the research on superconductivity. Then, the typical behaviors of superconductors and the related theore...[ Read more ]

A century has passed since the discovery of superconductivity in 1911. In these 100 years, generations of scientists have spent tremendous effort to reveal the secrets behind this fascinating physical phenomenon. However, the deeper they dig, the more mysteries they discover. Nowadays, superconductivity is no longer merely the disappearance of electrical resistivity but a macroscopic quantum mechanical phenomenon that is linked to unknown mechanisms and new physics. In this thesis, unconventional superconductivity in a bulk transition-metal dichalcogenide single crystal and heterostructure materials are studied.

In the first part, we briefly go through the history and developments of the research on superconductivity. Then, the typical behaviors of superconductors and the related theoretical work are reviewed.

Then, we present the results of the study on the superconductivity in a bulk transition-metal dichalcogenide, NbS₂, in which torque magnetometry, AC calorimetry, and thermal expansion measurements were conducted to measure the upper critical fields of the sample in various temperature and applied field conditions. In this piece of work, we provided firm evidence for the presence of non-Pauli-limited superconductivity in a parallel field, which may be attributed to the development of a Fulde-Ferrell-Ovchinnikov (FFLO) state.

Finally, we move to our study on topological insulator-superconductor heterostructures. Using focus-ion beam etching, we fabricated nanowire structures on the surfaces of heterostructure samples, which might host Majorana fermions at the ends when subject to a uniaxial magnetic field. Based on the results of the point contact spectroscopy measurements, we believed that we have successful captured the signals of the superconducting gap in Bi₂Te₃/FeTe heterostructure, which superimposed with the normal state signal. Also, zero-bias anomalies in the point contact conductance spectra were detected, which might be contributed by Majorana fermions.