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Research

Interacting electron systems in two dimensions

Confining electrons to two or fewer dimensions enhances the effect of interactions, leading to emergent quasiparticles and phenomena depending on the conditions. Graphene and related two-dimensional materials provide an optimal solid-state platform for exploring emergent physics, particularly when combined in engineered multi-layered systems. We start with raw materials like graphite and boron nitride, isolate atomically thin crystals from them, and recombine the layers to form specific structures with desired properties for electronic experiments at low temperatures. For instance, by assembling two layers of graphene with a small twist angle between the layers, a moiré pattern forms in real space that can lead to dramatic hybridization of the electronic structure. The electron kinetic energy can be strongly suppressed, leading to enhanced Coulomb interactions between electrons, correlated phenomena, symmetry breaking, and even superconductivity. We perform low-temperature electrical measurements of moiré systems to explore these electronic phases, discover new phenomena, and to understand their origin.

Topological phenomena

Two-dimensional systems are also excellent for investigating phenomena that arise from the geometry of electronic wavefunctions. Leading examples are topological insulators, Chern insulators, quantum Hall, quantum anomalous Hall, and quantum spin Hall effects, those many others are possible. Moiré systems, which provide direct control of lattice scales and symmetries, open up the possibility of engineering topological systems. Since moiré materials can also generate enhanced electronic interactions, we assemble multi-layered structures that combine both strong interactions and non-trivial topology to explore correlated topological phenomena. The fractional quantum Hall effect and fractional quantum anomalous Hall effect, or fractional Chern insulators, are examples that arise in graphene-based systems. We explore these effects by creating favorable conditions for them and probing their electrical properties at low temperatures and in large magnetic fields. While our interest in topological phases is primarily fundamental, there are potentially groundbreaking applications of interacting topological phenomena in quantum computing and other quantum technologies.

Novel electronic and scanning probe measurements

We investigate systems like the ones described above by designing and constructing microscopic electronic devices and measuring their response with electron transport. Though transport measurements are powerful and versatile, we are always thinking about new methods for more effectively exploring topological and quantum phenomena in quantum materials. To this end, we develop and employ techniques beyond conventional transport:

  • Quantum capacitance: precise capacitive measurements of quantum systems accesses thermodynamic quantities including the electronic compressibility, an equilibrium measure of the electronic density of states.
  • Tunneling spectroscopy: applying a potential bias across a thin tunneling barrier yields a tunneling current that depends on the spectroscopic density of states on both sides of the barrier. Since tunneling measurements are typically performed away from equilibrium, they can be used to measure the quasiparticle spectrum of superconductors and in many other useful situations.
  • Scanning probe techniques: in systems where behavior is not spatially uniform, like in moiré materials or in certain quantized transport regimes, local probes provide essential information about electronic systems. We start with conventional methods and seek to extend their capabilities or develop entirely new ones.