Nanoscale Infrared and Microwave Imaging of Stacking Faults in Multilayer Graphene.

Graphite exhibits a range of metastable stacking orders, with the number of possible configurations increasing exponentially with the number of layers. Most experimental studies have focused on Bernal and rhombohedral stacking due to the difficulty of identifying and isolating intermediate stacking orders. Motivated by this challenge, we present two atomic force microscopy (AFM) techniques that unambiguously distinguish stacking orders and defects in graphite flakes.

Optical imaging of flavor order in flat band graphene.

Spin- and valley flavor polarization plays a central role in the many-body physics of flat band graphene, with Fermi surface reconstruction - often accompanied by quantized anomalous Hall and superconducting state - observed in a variety of experimental systems. Here we describe an optical technique that sensitively and selectively detects flavor textures via the exciton response of a proximal transition metal dichalcogenide layer. Through a systematic study of rhombohedral and rotationally faulted graphene bilayers and trilayers, we show that when the semiconducting dichalcogenide is in direct contact with the graphene, the exciton response is most sensitive to the large momentum rearrangement of the Fermi surface, providing information that is distinct from and complementary to electrical compressibility measurements.

Superconductivity and spin canting in spin-orbit-coupled trilayer graphene.

Graphene and transition metal dichalcogenide flat-band systems show similar phase diagrams, replete with magnetic and superconducting phases. An abiding question has been whether magnetic ordering competes with superconductivity or facilitates pairing. For example, recent studies of Bernal bilayer graphene in the presence of enhanced spin-orbit coupling show a substantial increase in the observed domain and critical temperature T of superconducting states; however, the mechanism for this enhancement remains unknown.

Fluctuating magnetism and Pomeranchuk effect in multilayer graphene.

Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wavefunctions in f- and d-atomic orbitals. By contrast, in rhombohedral multilayer graphene (RMG), magnetism-manifesting as spontaneous polarization into one or more spin and valley flavours-originates from itinerant electrons near a Van Hove singularity. Here we show experimentally that the electronic entropy in this system indicates signatures typically associated with disordered local magnetic moments, unexpected for electrons in a fully itinerant metal.

Superconductivity and quantized anomalous Hall effect in rhombohedral graphene.

Inducing superconducting correlations in chiral edge states is predicted to generate topologically protected zero energy modes with exotic quantum statistics. Experimental efforts so far have focused on engineering interfaces between superconducting materials-typically amorphous metals-and semiconducting quantum Hall or quantum anomalous Hall systems. However, the strong interfacial disorder inherent in this approach can prevent the formation of isolated topological modes.

Imaging inter-valley coherent order in magic-angle twisted trilayer graphene.

Magic-angle twisted trilayer graphene (MATTG) exhibits a range of strongly correlated electronic phases that spontaneously break its underlying symmetries. Here we investigate the correlated phases of MATTG using scanning tunnelling microscopy and identify marked signatures of interaction-driven spatial symmetry breaking. In low-strain samples, over a filling range of about two to three electrons or holes per moiré unit cell, we observe atomic-scale reconstruction of the graphene lattice that accompanies a correlated gap in the tunnelling spectrum.

Isospin magnetism and spin-polarized superconductivity in Bernal bilayer graphene.

In conventional superconductors, Cooper pairing occurs between electrons of opposite spin. We observe spin-polarized superconductivity in Bernal bilayer graphene when doped to a saddle-point van Hove singularity generated by large applied perpendicular electric field. We observe a cascade of electrostatic gate-tuned transitions between electronic phases distinguished by their polarization within the isospin space defined by the combination of the spin and momentum-space valley degrees of freedom.

Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt.

Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin phases.