Quantum Transport Properties in Tungsten Ditelluride Based Devices




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The success of mechanical exfoliation on graphene has paved a new field of research in two-dimensional (2D) materials. As one of the transition metal dichalcogenides (TMDCs), tungsten ditelluride (WTe2) has attracted a mass of interests since a novel non-saturating positive magnetoresistance was discovered in 2014. A lot of researches in this material have been published, such as band structure studies with angle-resolved photoelectron spectroscopy (ARPES), quantum oscillations in transport measurements, superconductivity in WTe2 etc. It is worth mentioning that the topological properties of WTe2 have been verified in both bulk (type-II Weyl semi-metal) and in monolayer (2D topological insulator) forms. The topological properties make WTe2 a potential candidate for hosting Majorana bound state, which is theoretically predicted to arise from the proximity effect between a s-wave superconductor and the surface states of a topological insulator (TI). This dissertation will present quantum transport studies in multi-layer WTe2, which acts as an intermediate between the bulk and monolayer limits. Our goal is to explore the transport properties in WTe2 itself, and investigate its interaction with other quantum materials, especially superconductors. A series of different types of devices based on multi-layer WTe2, including Hall bars, FET-like devices and Josephson junctions, have been fabricated and measured in the magnetic fields up to 12 T at low temperatures down to 20 mK. In order to improve the performance of the devices, the hexagonal boron nitride (hBN) flakes are used to build sandwiched structures for thin WTe2 flakes. The main results are presented as follows. First, thickness-dependent quantum transport measurements suggest that the novel ’turn-on’ behavior in WTe2 take the origin of the Kohler’s rule in Fermi liquid state. The ’turn-on’ behavior accompanied by the large magnetoresistance (MR) will be effectively suppressed by the loss of perfect carrier compensation. Strikingly, however, the trend of non-saturation is unaffected at all which indicates the possibility of other origins of the non-saturating MR. In addition, the angle-dependent MR measurements reveal that the electronic 3D nature of multi-layer WTe2 and the Fermi surface anisotropy depends on the sample thickness. Second, we observe an obvious crossover between weak anti-localization (WAL) and weak localization (WL) in an disordered ultrathin WTe2 flake. The mechanism of the crossover shows coexistence and competition among several characteristic lengths, including the dephasing length, the spin-flip length, and the mean free path. Furthermore, the interplay of quantum interference and electron-electron interaction is also observed. Third, an unconventional quasi-3D quantum Hall effect (QHE) is observed in a high quality flake with much lower carrier density and higher mobility than ordinary WTe2. The quasi-3D QHE act as a collection of several weakly-coupled 2D QHE layers, which might be resulted from a dimerization or tetramization effect. Fourth, in the Ta-WTe2-Ta Josephson junctions, supercurrent state is successfully induced into the multi-layer WTe2 by proximity effect. We observe the fast mode superconducting quantum interference pattern, which indicates the presence of edge supercurrent resulted from the intrinsic edge states of WTe2. In addition, the multiple frequencies observed in the interference pattern might be from the terrace structure along the sample edges. Finally, the presence of the multi-dips in differential resistance in the Josephson junctions with incomplete superconducting state marks the multiple Andreev reflections in WTe2, which might be due to the multiple channels formed along the Josephson junction length. Furthermore, the related experimental procedures, improvements and different data processing methods have also been presented in the main text and appendixes.



Quantum entanglement, Quantum Hall effect, Superconductors, Transition metal compounds, Tungsten compounds