Electronic transport and planar tunneling spectroscopy in twisted graphene structures
Creators: name: Zhang, Zhenyuan
Publisher: name: No Publisher Supplied
Year: 2024

description: Graphene, since its first discovery, has attracted countless attention from various fields due to its unique honeycomb structure and novel properties. Stacking two layers of graphene with a twist adds an additional degree of freedom to tune the electronic properties since it generates a new periodic pattern besides the atomic lattice. It was shown that the central moiré bands become extremely flat near the so-called “magic angle” ~1.1°. Strong correlation effects dominate the carrier behavior due to the suppression of kinetic energy and bandwidth, exhibiting unexpected phenomena like unconventional superconductivity, quantized anomalous Hall effect, Chern insulators, heavy fermion states, etc. In this dissertation, I will present studies of electronic properties in magic angle twisted bilayer graphene (TBG) using both electronic transport and planar tunneling spectroscopy. The transport results focus on the conducting behavior of charge carriers on the Fermi surface while the tunneling measurements study the evolution of flat bands from the perspective of energy, not limited to the Fermi energy.Magic angle TBG is an ideal platform for studying strong correlation systems owing to the suppressed kinetic energy and the narrow isolated central bands. TBG near magic angle tends to have much higher resistivity but weaker carrier density dependence at room temperature, which might be related to scattering from the moiré superlattice. From the magnetoresistance and Hall resistance at low temperatures and under perpendicular magnetic fields, we discovered quantized Hall conductivity following non-zero Chern numbers and phase transitions marked by the resetting of Hall carrier density. Our discoveries reveal the non-trivial topology of the degenerate quasiparticle flat bands and demonstrate possible symmetry-breaking ground states at each integer filling which can be facilitated by van Hove singularities.With the optimized design of tunneling device structure, flat bands with 26 meV total bandwidth reveal a cascade of phase transitions. An additional gap opening right at filling of 2 below 4 K agrees with the symmetry-breaking insulating state observed in transport measurements. Prominent heavy to light fermion transitions (depicted by the inverse compressibility and effective mass extracted from the tunneling current) at each non-zero filling and solid-like local moment states (shown as plateaus in the high-resolution entropy data) support the coexistence of localized f electrons and mobile c electrons discussed in the recently introduced topological heavy fermion models and Kondo lattice models.Unusual phenomena like carrier confinement and negative differential conductance were also observed in TBG tunneling devices. It is demonstrated that the band gap between flat bands and remote bands amplifies the confinement of carriers induced by potential variations. The negative differential conductance originates from the misalignment of moiré flat bands in different regions, in a similar way to flat Landau levels developed under magnetic fields.