Emergent Quantum Phases of Electrons in Multilayer Graphene Heterostructures

Abstract

The flexibility of two-dimensional materials allows for the creation of various structural configurations to engineer many-body electronic Hamiltonians and explore new phases of matter. In this thesis, I will discuss two different classes of graphene heterostructures that have enabled our observation of novel quantum phases of electrons. The first utilizes small-angle twisting of 2D materials to create a moiré pattern, introducing a new length scale for electrons to move and interact. By twisting two layers of Bernal bilayer graphene, we have observed emergent correlated metallic and insulating states, along with their spin-polarization nature. By twisting three graphene layers with alternating twist angles of ±1.56 degrees, we have discovered displacement field-tunable superconductivity, which occurs in close conjunction with correlation-driven flavor polarization. Similar phenomenology can also be found in twisted quadrilayer graphene, with more intricate interplay between the superconducting phases and the single particle dispersion. I will also discuss two experimental efforts in investigating the nature of the superconducting pairing symmetry: fabricating Josephson junctions between twisted trilayer graphene and an s-wave superconductor, and measuring superfluid stiffness using microwave reflectometry. The latter has indicated the nodal nature of the superconducting gap. The second heterostructure involves a graphene double-layer structure where two parallel graphene sheets are brought close to each other but remain separated. This enables Coulomb interactions to couple electrons across the two layers, leading to a variety of interlayer correlated states under strong magnetic fields, including exciton condensation, interlayer fractional quantum Hall states, and exciton condensation formed by composite fermions. The exploration of novel material manipulation methods presented in this thesis not only reveals new phenomena and underlying physical principles, but also holds significant promise for future material design and device applications.