2025 Theses Doctoral

Transport Measurements of Correlated States in Multilayer Twisted Graphene

Swann, Joshua R.

In electronic flat bands, electron-electron interactions become dominant as the kinetic energy is quenched leading to the emergence of a variety of correlated electronic states. This thesis investigates two classes of strongly correlated flat band systems realized in graphene: highly degenerate Landau levels formed in an applied magnetic field and moiré flat bands in multilayer twisted graphene heterostructures.

For the first class, we examine interlayer coherence in graphene quantum Hall bilayers with an ultra-thin tunnel barrier. By separating two graphene layers with a single atomic layer of hexagonal boron nitride, the interlayer Coulomb coupling is significantly enhanced, leading to the observation of odd-integer quantum Hall gaps consistent with the emergence of interlayer coherent states. The remainder of the thesis focuses on moiré flat bands in magic angle twisted graphene systems. A central challenge in early studies of magic angle twisted graphene systems was achieving sufficient twist angle homogeneity to reliably access superconductivity and correlated insulating states.

To address this, a novel atomic force microscopy based fabrication technique is developed that enables in situ rotation of graphene layers, to reduce twist angle disorder and strain. Building on these developments, the structural and electronic properties of magic angle twisted trilayer graphene are investigated using piezoresponse force microscopy and scanning tunneling microscopy. These measurements reveal a relaxation unique to twisted trilayer graphene that stabilizes large regions of uniform twist angle while preserving mirror symmetry even in the presence of small rotational mismatches between layers.

Next, transport measurements of twisted trilayer graphene aligned to hexagonal boron nitride demonstrate how commensurate and incommensurate alignment profoundly alter disorder, band gaps, and superconductivity. While incommensurate alignment suppresses superconductivity at the magic angle, commensurate alignment stabilizes robust superconducting phases and opens a gap at charge neutrality, far from the nominal magic angle.

Finally, the effect of mirror symmetry breaking due to inequivalent twist angles and hBN alignment is investigated with transport measurements. The broken mirror symmetry leads to a system whose transport properties are well described by a set of twisted bilayer graphene-like flat bands plus a gapped Dirac band. Together, these results establish symmetry, relaxation, and alignment as key tuning parameters for correlated phases in twisted graphene systems and inform future efforts in flat band engineering across van der Waals materials.