Planar tunneling in twisted moire heterostructures

Abstract

Quantum tunneling is a phenomenon that has excited physicists for many decades. With the ability to exfoliate two-dimensional (2D) materials from the bulk, including graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDCs) such as tungsten diselenide (WSe2), experiments showcasing this exceptional quantum mechani- cal phenomenon is now feasible in low-dimensional layered systems. Some earlier works demonstrated electron tunneling in metal/graphene-insulator-graphene/graphite devices where a few atomic layers of hBN or WSe2 acted as the insulating tunnel barrier. In recent years, theoretical predictions have revealed that graphene layers are strongly cou- pled near the so-called ‘magic’ angles, where the low energy bands become extremely flat. Experiments have reported signatures of strong correlations, such as correlated insulators, superconductivity, and orbital ferromagnetism. In this thesis, we use planar tunneling as a novel tool to study twisted moir´e heterostruc- tures. We have fabricated vertical tunnel transistors of twisted bilayer graphene (TBG) and metal electrodes with WSe2 as the tunnel barrier. To make a TBG sample, it was essential to have precise control over the twist angle. The dry pick-up transfer method, including the tear-and-stack and cut-and-stack was employed to get the TBG samples. Moreover, the elaborate process of a Metal-WSe2-TBG device fabrication was optimized in this work with steps ranging from 2D material exfoliation to the assembly of van der Waals heterostructures. To access the exotic phases in TBG systems, it was essential to make sizeable, clean, and good-quality devices. However, during the entire process of device fabrication, there can be various possibilities that can induce defects in the system, such as the formation of air pockets between 2D layers during the assembly process, twist angle inhomogeneity, folding, and wrinkling. Therefore, every step of the fabrication pro- cess was optimized with the intention of getting the best quality devices possible. To study planar tunneling in low-dimensional systems, we begin with planar tunneling measurements on Metal-WSe2-Bilayer graphene (BLG) junctions. For planar tunneling between metal and BLG, it was essential to conserve the in-plane momentum for elastic tunneling, which was expected to be suppressed for these junctions. We observed sig- natures of in-elastic tunneling in these junctions, as expected, where electron tunneling was aided by phonons in the heterostructure. Further, we investigated tunneling in pla- nar Metal-WSe2-TBG junctions, where the twist angle in the TBG system was around 2°. For these junctions, we observed signatures of phonon-assisted tunneling where we anticipate a crucial role of electron-phonon coupling and the density of states of twisted bilayer graphene. The novelty of these results is expected to open avenues where planar tunneling can be a potent tool for studying various twisted moir´e systems