Graphene, the recently isolated two dimensional crystal of carbon, possesses extraordinary electronic properties arising from a low energy excitation spectrum which mimics ultra relativistic particles. This makes it possible to achieve high quality field-effect transistors and to observe the quantum Hall effect at room-temperature. The object of this work is to access the intrinsic properties of graphene by isolating it from the environment and to explore the emergence of new physical phenomena in response to the controlled introduction of chemical or mechanical perturbations. We will seek to understand and minimize environmental effects on the electronic properties by exploring the influence of various substrates as well as that of removing the substrate altogether. We will explore the role of interactions, boundaries, magnetic field and dopants on the quasiparticle spectrum in search of emergent phases and new physical properties. We will investigate the feasibility of fabricating graphene nano-devices based on these properties. The primary experimental probes in this project include scanning tunneling microscopy and spectroscopy atomic force microscopy and magneto-transport. Work on this project will be carried out with the active participation of students at the undergraduate and graduate level and with post doctoral fellows.
Non-technical abstract
The recent discovery of graphene, a two-dimensional crystal consisting of a single atomic plane of graphite, gives access to extraordinary electronic properties arising from charge carriers that mimic ultra-relativistic particles. Because of the low dimensionality and the possibility of attaining charge transport with exceptionally high carrier mobility, graphene holds the promise of becoming the building block for a new generation of nanoelectronics devices. The goal of this project is to access the intrinsic properties of graphene by isolating it from the environment and to explore the emergence of new physical phenomena and new phases of matter in response to the controlled introduction of chemical or mechanical perturbations. We will explore the feasibility of fabricating graphene nano-devices based on these properties. The primary experimental probes will include scanning tunneling microscopy and spectroscopy, atomic force microscopy and magneto-transport. Work on this project will be carried out with the active participation of students at the undergraduate and graduate level. Work on this project will be carried out with the active participation of students at the undergraduate, graduate and post doctoral levels. They will receive training in cutting edge research and in the use of state of the art equipment that will prepare them for filling demanding technical and research positions.
During this funding period our work focused on the electronic properties of graphene and on understanding how they are modified by external perturbations dimensionality boundaries and interfaces. In order to effectively address these questions we developed and employed several sample preparation and fabrication techniques together with a variety of characterization tools that allowed us to access and probe the electronic properties of low dimensional systems and to follow their evolution with distance from an edge, with magnetic field, doping and gating. The primary characterization techniques include scanning tunneling microscopy, scanning tunneling spectroscopy, Landau level spectroscopy, atomic force microscopy, Raman spectroscopy. Below is a list of the significant outcomes from this research 1. Evolution of Landau Levels into edge states by scanning tunneling spectroscopy and microscopy. Two dimensional electron systems (2DES) support topologically ordered states in which the coexistence of an insulating bulk with conducting one dimensional chiral edge-states gives rise to the quantum Hall (QH) effect. In 2DES confined by sharp boundaries theory predicts a unique edge-bulk correspondence which is central to proposals of QH based topological qubits. However, in conventional semiconductor based 2DES, these elegant concepts were difficult to realize because in these systems edge-state reconstruction caused by insufficient screening destroys the edge-bulk correspondence. Our work demonstrated that edge-state reconstruction can be avoided in graphene. Using scanning tunneling microscopy and spectroscopy, we followed the spatial evolution of Landau levels towards an edge of graphene supported above a graphite substrate. To within one magnetic length of the edge no reconstruction was observed, in agreement with calculations based on an atomically sharp confining potential. Our results single out graphene as a system where the edge structure can be controlled and the edge-bulk correspondence preserved. 2. Direct observation and imaging of localized and extended electronic states and the transition between them in the quantum Hall effect regime in graphene. 3. Using landau level spectroscopy we observed the transition between compressible and incompressible electronic states as the Fermi energy was swept by gating from one landau level to the next. We showed that the Fermi energy remains pinned within partially filled Landau levels and scans through the all localized impurity states until reaching the next landau level. 4. Screening of charge impurities in graphene. Atomic orbitals become unstable when the nuclear charge exceeds a critical value, comparable to the inverse fine structure constant, when relativistic effects cause electrons to fall into the nucleus. Accessing this critical regime where new physics comes into play requires the creation of ultra-heavy nuclei which do not exist in nature. We demonstrate that in graphene it is possible to explore this regime by using an isolated charged impurity as the nucleus of an artificial atom together with a magnetic field and a gate voltage which define the electronic orbitals. Using scanning tunneling microscopy and spectroscopy in an ultra-clean graphene sample we showed that the effective charge of the impurity can be tuned from the subcritical to the supercritical regime by controlling the occupancy of the Landau levels with the gate voltage. We found that at low occupancy strong screening by the conduction electrons cloaks a positively charged impurity rendering it essentially invisible. As the impurity strengthens with increased occupancy the orbital degeneracy of nearby electronic states is lifted leading to fine splitting of the energy levels. At full occupancy where the impurity is unscreened it enters the supercritical regime which is identified by the appearance of a series of localized states in the negative energy sector. Our findings show that the magnetic field makes it possible to tune the strength of impurities in graphene from subcritical to supercritical providing unprecedented access to the regime of Coulomb criticality. The new knowledge created by this research was published in major scientific journals including Nature Communications and Reports on progress in physics. Results were reported through invited talks at 45 international conferences. 10 undergraduate students 1 graduate student and 1 post doctoral fellow were trained by participating in this research. .
