CAREER:Visualization and Control of Chiral Bound States and Ballistic Chiral Electrons in Two-Dimensional Material Heterostructures

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

Nontechnical abstract: This CAREER project is composed of a research and outreach plan that addresses fundamental questions regarding the control of very fast (Dirac) electrons in materials at the nanoscale. These exotic particles provide an exciting new testbed for electron motion in materials because their behavior across barriers is distinct from normal electrons. There are many theoretical predictions for the behavior of Dirac electrons at these interfaces, but it has not been possible to directly test them - until now. The PI recently developed techniques to create ideal nanoscale barriers with arbitrary shapes that enable the first imaging of confined Dirac electrons. Such control and visualization of Dirac electron motion will generate deep fundamental insights, paving the way toward entirely new electronic devices, such as novel sensors and tunable nanoscale electronic lenses. The outreach component of this CAREER project will increase Hispanic representation among STEM majors by using this research project with an existing NSF Research Experience for Undergraduates at the University of California Santa Cruz (UCSC). Hispanic high school students and high achieving community college students from Hispanic Serving Institutions from the local area will work and be mentored at three phases in their educational development: pre-college, early-college, and late-college. These stages will provide science career awareness to the pre-college group, provide opportunities for research experiences within the PI's research effort to the early-college group, and provide career-based mentoring and networking opportunities to the late-college group. These outreach efforts will benefit not only the participating students, but also students at UCSC, and the broader community. Technical abstract: This CAREER project addresses fundamental questions regarding chiral bound states and trajectories in graphene and bilayer graphene. The exotic quasiparticles-Dirac fermions, hosted by these two-dimensional materials provide a new testbed for electron motion because of their chiral nature and negative energy states. The PI recently developed a versatile technique to create pristine nanoscale PN junctions with arbitrary shapes, which enabled the first imaging of confined Dirac fermions. This project investigates the role of the potential shape and the differences in behavior between massless and massive Dirac fermions within these confinement potentials. Because Dirac fermions exhibit Klein tunneling (100% barrier transmission) and anti-Klein tunneling (100% barrier reflection), the research carried out by this project reveals novel bound states not found with Schrödinger fermions. Moreover, steered Dirac fermion trajectories across and in response to PN and graphene/superconductor junctions are mapped and compared directly to microscopic models for barrier transmission and superconductivity. Theoretical predictions for the behavior of Dirac fermions at these interfaces are abundant, but it has not been possible to directly test them - until now. Specifically, to reveal chiral bound states and trajectories this CAREER project contains three main research efforts: (1) Visualize and control chiral bound states in two-dimensional material heterostructures composed of graphene and bilayer graphene. This study creates pristine nanoscale PN junctions in two-dimensional material heterostructures and use scanning tunneling microscopy to map quantum interference and confinement within these PN junctions. Different junction shapes (circular vs. non-circular) and charge carrier types (massless vs. massive Dirac fermions) are investigated; (2) Visualize and control ballistic chiral electron trajectories in two-dimensional material heterostructures. This study uses scanning gate microscopy in conjunction with nanoscale PN junctions and magnetic fields to map and steer ballistic trajectories of massless and massive Dirac fermions; (3) visualize and control chiral trajectories at a graphene/superconductor interface. This study uses scanning gate microscopy to map Dirac fermion reflections off a superconductor's interface at the nanoscale. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.