Quantum transport in Graphene Moire Superlattice and p-n junction

Abstract

The discovery of graphene has revolutionized the field of mesoscopic condensed matter physics. It has started a new field of van-der Waals heterostructure in which different two-dimensional materials including graphene can be stacked on top of each other. In the last few years graphene based van-der Walls heterostructure has lead to many interesting physics like Hofstadter’s butterfly, Valley Hall effect, Mott insulator and superconductivity. In this thesis, two different kinds of graphene heterostructures, namely, graphene moiré superlattice (GMSL) and graphene p-n junction (GPNJ) have been studied extensively. The GMSL is realized by aligning and stacking graphene and hexagonal boron nitride with an accuracy of . This leads to additional set of Dirac cones known as cloned Dirac cones (CDC) which are placed symmetrically around the primary Dirac cone (PDC). As a part of the thesis, we have studied the magneto-conductance on GMSL devices as a function of carrier concentration and temperature, which reveals a transition from weak anti-localization (WAL) near the PDC to weak localization (WL) near the CDC. The transition is explained due to the shift of Berry phase, which is measured experimentally by probing the Shubnikov-de Haas oscillations. Furthermore, we study the low frequency 1/f noise at multiple Dirac cones in GMSL devices. Our results reveal that the low-frequency noise in GMSL devices can be tuned by more than two-orders of magnitude by changing carrier concentration as well as by modifying the band structure. We find that the noise is suppressed at the CDC compared to the PDC and understood in terms of screening. In the second part of the thesis, we study the equilibration of quantum Hall edges in GPNJ devices, which are realized in dual gated geometry. The equilibration of electron-like and hole-like edges in graphene p-n junction have been studied in single and bilayer graphene in both unipolar and bipolar regime, when the different symmetries likes valley, spin and orbital degrees of freedoms are broken. Our studies reveal the partial equilibration based on the spin polarization of the quantum Hall edges. Furthermore, we have carried out shot noise measurements to understand the dynamics and mixing of quantum Hall edges at the p-n junction, which could be used as an electronic beam splitter