Yanmeng Shi, UC Riverside

The study of two-dimensional (2D) materials began with the seminal work of experimental isolation of monolayer graphene in 2004, and has remained one of the frontiers of condensed matter physics ever since. Mono- and few-layer graphene, which host chiral charge carriers with competing symmetries (valley, spin and orbital), have proved to be fascinating platforms for investigating the quantum Hall (QH) physics. Research efforts were soon extended to other 2D materials. One such material is phosphorene (mono- or few-layer black phosphorous), which has attracted much attention due to its large direct band gap and high mobility. In this talk, I will present our comprehensive transport studies of bi- and tetra-layer graphene, as well as few-layer phosphorene (FLP). In bi-layer graphene (BLG), the interplay between symmetries and electric and magnetic felds gives rise to two distinct phases of the QH state at flling factor f = 1, with different pseudospin and real spin polarizations. Using transport spectroscopy, we investigate the energy gaps and transitions at flling factors f = 1 and 2/3. In another graphene system, we observe unusual transport behavior at B = 0 in hexagonal boron nitride (hBN)-encapsulated Beranal-stacked tetralayer graphene, which is attributed to the trigonal warping that induces Lifshitz transitions as a function of charge density and electric field. In the QH regime, we observe rich Landau level (LL) crossing patterns between the two BLG-like bands. These works provide us with the insight of the importance of the interplay betweencompeting symmetries and applied electric and magnetic felds. Finally, in hBN-encapsulated FLP devices, we show the observations of weak localization (WL), from which the dephasing lengths could be extracted to be ~ 30 - 100 nm, and exhibit power- law dependences on temperature and charge density. We conclude that the dominant source of phase-relaxation is the electron-electron interactions, shedding light onto the understanding of the scattering mechanisms in FLP devices at low temperatures.