Graphene, a two-dimensional carbon honeycomb lattice, has generated immense interest within the condensed matter physics community due to its fascinating electrical, optical, and mechanical properties. The differences in band structure of mono-, bi- and tri-layer graphene give rise to drastically different electronic ground state configurations and competing symmetries (such as spin, valley, orbital, and layer). Recent efforts have significantly improved electronic its charge carrier mobility and enabled the observation of a number of exciting phenomena in monolayer and few layer graphene. In this thesis we present an experimental study of spin transport through monolayer graphene antiferromagnet insulator (AFMI), and quantum Hall (QH) phases in multiple Dirac band trilayer graphene, which provide further insight into both single-particle and many-body physics in these exciting two-dimensional (2D) systems.
These projects require samples of exceptional quality. To this end, I, together with Nathaniel Gillgren, developed a dry transfer technique (first pioneered by the Columbia group), in order to fabricate graphene devices encapsulated within hexagonal boron nitride (hBN) layers. Since hBN sheets are atomically flat and host very few trapped charges and defects, they are ideal substrates for graphene devices, which boost charge
carrier mobility as high as ˜ 10^5. In my research these ultraclean devices enabled the
resolution of symmetry-broken quantum Hall phases and fractional quantum Hall states, as well as the establishment of an antiferromagnetic insulator the affords long distance spin transport.
In the first part of thesis, we focus on the observation of tunable symmetries of the integer and fractional quantum Hall (QH) states in ABA-stacked trilayer graphene, which hosts multiple Dirac bands. At finite doping and in the quantum Hall regime, we use transport measurements to map the Landau levels of hBN-encapsulated ABA-stacked trilayer graphene as a function of charge carrier density n, magnetic field B, and interlayer displacement field D. We observe the transitions among states with different spin, valley, orbital, and parity polarizations. This extremely rich pattern arises from crossings between Landau levels from different sub-bands, which reflects the evolving symmetries that are tunable in situ. Notable, we observe fractional QH (FQH) states at filling factors 2/3 and -11/3 at ¿ = 0. Unlike those in bilayer graphene, these FQH states are destabilized by a small interlayer potential that hybridizes the different Dirac bands.
At the charge neutrality point (CNP), trilayer graphene displays a striking phase diagram. Here, we discover a new class of QH effect, the quantum parity Hall (QPH) effect, in which boundary channels are distinguished by even or odd parity under the system’s mirror reflection symmetry. Quantized conductance is first observed at low perpendicular magnetic field ¿2 at ¿xx = 4e2/h, confirming the presence of four edge channels that travel from source to drain. As ¿2 increases, the quantized conductance decreases, first to ¿xx = 2e2/h, and then to smaller values. This interesting sequence of
the plateaus can be explained by a crossing between even- and odd-parity bulk Landau levels (LLs). Exchange interactions with the Fermi sea cause these LLs to move toward each other, yielding a spin-polarized state at the intermediate ¿2 and an ordinary insulator ground state at strong ¿2. Interestingly, the transitions between spin-polarized and unpolarized states can be tuned by Zeeman energy. These observations demonstrate topological phases that are protected by a gate-controllable symmetry and sensitive to Coulomb interactions.
The last part of this thesis investigates long-distance spin transport through a graphene anti-ferromagnetic insulators (AFMI). Antiferromagnetic are promising candidates for spintronics application due to their robustness against stray fields, ultra- fast intrinsic dynamics, and possibly support near-dissipationless transport through their bulk. As monolayer graphene hosts AFMI state at the CNP and high magnetic fields, it has been proposed as a promising platform to realize a spin transport through its AFM bulk. Here we utilize gate-controlled QH edge states in graphene as spin-dependent injectors and detectors., and observe large, non-local electrical signals across a 5-µm-long region that is tuned into ¿ = 0 AFMI state. Among a number of possible transport mechanisms that may give rise to non-local signals, spin superfluidity transport is the most consistent with our control measurements at different magnetic fields, temperatures and filling factors. This work demonstrates that graphene in the QH regime is a powerful model playground for fundamental studies of ferromagnetic and antiferromagnetic spintronics.
Graphene and its few-layer analogous are powerful platforms to explore
interesting aspects of complicated physical phenomena. Despite apparent similarity and simplicity, they afford some very different electronic properties and many-body interactions effects. The development of van der Waals stacking techniques enables fabrication of ultra-clean samples and heterostructures. Given ever-growing number of 2D materials, there are almost infinite possibilities of novel device architectures and properties that await discovery.