Two dimensional materials, as represented by graphene and transition metal dichalcogenides (TMDCs), have recently attracted intensive studies because of their intriguing physical properties and potential electrical and optical applications. Niobium diselenide (NbSe₂) is one type of TMDCs that possesses superconducting phase with the bulk T_c ∼ 7K. The layered structure of NbSe₂ enables the study of superconductivity in atomically thin systems through nanofabrication techniques and transport measurements. In this thesis, quantum transport behaviors regarding superconductivity are studied in the atomically thin NbSe₂ nanostructures, where the three main topics are the electronic phase transition in NbSe₂ nanochannels, the Ising superconductivity in few-layer NbSe₂ and the quantum transpo...[ Read more ]

Two dimensional materials, as represented by graphene and transition metal dichalcogenides (TMDCs), have recently attracted intensive studies because of their intriguing physical properties and potential electrical and optical applications. Niobium diselenide (NbSe₂) is one type of TMDCs that possesses superconducting phase with the bulk T_c ∼ 7K. The layered structure of NbSe₂ enables the study of superconductivity in atomically thin systems through nanofabrication techniques and transport measurements. In this thesis, quantum transport behaviors regarding superconductivity are studied in the atomically thin NbSe₂ nanostructures, where the three main topics are the electronic phase transition in NbSe₂ nanochannels, the Ising superconductivity in few-layer NbSe₂ and the quantum transport in graphene/NbSe₂ heterojunction.

First, the electrical transport behavior of NbSe₂ nanochannels is studied with the variation of disorder level and magnetic field. With the increase of disorder level accompanying the shrinking of the channel width, NbSe₂ channels transit from superconducting phase to dirty metal, and further to insulating state. Fluctuation induced tunneling (FIT) and variable range hopping (VRH) models are used to analyze the temperature dependent resistance of the channel in insulating regime. It is inferred that upon the increase of disorder, superconductivity is gradually suppressed as the system turns into large conduction areas separated by small insulating barriers (FIT model) and then into localization centers with the hopping conduction (2D VRH model). The electronic phase transition in the presence of magnetic field yields a different scenario. By applying the scaling theory, an exponential product zν = 0.7 is determined, which is in stark contrast with the quantum phase transition theory of superconductor-insulator transition (SIT) where zν > 1. Possible reasons for this product value are discussed, while it requires further identification.

Next, by thinning down the NbSe₂ to few-layer, the Ising superconductivity in the system is revealed by transport measurement. The in-plane upper critical field determined by the I-V measurement is largely enhanced. Strong spin-orbital coupling and Ising pairing in few-layer NbSe₂ are accounted for the enhancement. Also, the Berezinskii–Kosterlitz–Thouless (BKT) transition is observed with the critical temperature T_ϕ ∼ 3.2K, which illustrates the fluctuation effect in the 2D superconducting system.

Finally, the transport behavior at the interface of few-layer graphene (FLG)/few-layer NbSe₂ is systematically studied mainly through the differential conductance spectroscopy. The dI/dV curves exhibit nonlinearity with two sets of coherence peaks and strong dependence on the gate voltage. In the vicinity of charge neutrality point (CNP) of FLG, the differential conductance shows a central dip feature. While away from the CNP, the central dip gradually transits into a conductance peak. Through fitting the differential conductance curves with Blonder–Tinkham–Klapwijk (BTK) theory, two gaps, their temperature dependency and the interface transparency are extracted. The gate dependency of the differential conductance is well understood. The origin of the two gaps and the possible reasons for the large gap to T_c ratios are analyzed and discussed.