EAGER: Layer Resolved Capacitance in Graphene Bilayers

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Non-Technical Abstract Two dimensional materials are composed of layers of atoms that are only a few atoms thick. Recent advances in the fabrication of these materials have led to the development of new types of electronic devices. For these extremely thin layers the quantum nature of the system dominates the properties in new and unusual ways. In these materials spontaneous ordering, similar to ferromagnetism, can occur and the topology of the system can affect the properties. However, these materials offer the advantage that electric fields can be used to modify the properties. This leads to new possibilities for enhancing the functionality of quantum electronic devices. This project aims to develop a new, ultrasensitive capacitive technique to study these materials. The novel measurement toolkit, moreover, will aid other scientists in the field studying devices relevant to electronics and energy harvesting. Undergraduate and graduate students will be trained in experimental design, device fabrication, and cryogenic measurement, advancing a new generation of condensed matter physicists ready to take on challenging problems across materials and measurement science. Technical abstract: The project will develop and provide a proof of principle for a capacitive measurement technique that directly senses electronic compressibility, layer polarization, and layer polarizability in atomic bilayers. The technique relies on the small difference in geometric capacitance between a bilayer and two proximal gates caused by interlayer motion of electrons, detected using a multiplexed high current gain cryogenic amplifier. The proposed research will enhance understanding of the physics of graphene bilayers by exhaustively cataloging integer and fractional quantum Hall effects and symmetry protected edge states, using a combination of thermodynamic and charge transport techniques. In Bernal-stacked graphene bilayers, the Principal Investigator will directly resolve valley- and orbital-order in the zero energy Landau level, with additional work to detect thermodynamic anomalies associated with the still poorly understood zero magnetic field correlated phase. Layer polarization and polarizability measurements effected by measuring diverse capacitance observables in a dual gated device will allow unambiguous observation of interlayer coherent states and disambiguation of the charge-valley-orbital symmetry breaking mechanisms. In large twist angle graphene bilayers, bulk and edge probes will be used to search for quantum spin Hall effects of both normal and fractionalized electronic degrees of freedom. In small angle bilayers, the measurements will reveal the basic parameters of physical and electronic structure, specifically the role of elastic strain reconstructions when the layers are nearly aligned. At high magnetic fields bulk and edge measurements will permit disentanglement of intertwined ferromagnetic, fractional, and fractal quantum Hall physics.