Princeton Graphene Workshop

The following wiki page provides my attempt at summarizing the talks given at the workshop on the Electronic Properties for Graphene hosted by the Princeton Center for Theoretical Science and the Princeton Center for Complex Materials. The event was organized by Dmitry Abanin, Joseph Checkelsky, and N. Phuan Ong on October 8-9, 2010. The level of the talks at this workshop was given at a very deep theoretical level and was most impressive. All the talks addressed the edge of our understanding of how electrons behave in graphene under various conditions.

Magnetism and Strong Correlations in Chemically Modified Graphene
Antonio Castro Neto, Boston University

• Graphene is not strongly correlated but adding adatoms to graphene can result in the adatoms becoming magnetic. The location of the adatoms on the graphene lattice has a role in determining the onset of the Kondo effect. The adatoms induce a sp³ hybridization of the electron levels and a uniform or regular placement of adatoms will produce ferromagnetism. Irregular or random adatom placement can produce antiferromagnetism. Ripples in the graphene will produce some variation in the magnetic states in the material.
• Website: Antonio Castro Neto

Charge Transport in Single- and Bi-layer Graphene
Michael Fuhrer, University of Maryland

• Studied the conductivity of graphene FETs by inducing defects and adding potassium to the graphene lattice. Disorder in graphene is still unknown but thinks point defects give rise to constant conductivity.
• Website: Michael Fuhrer

High Resolution Tunneling Spectroscopy of Graphene in Strong and Weak Disorder Potentials
Joseph A. Stroscio, Center for Nanoscale Science and Technology, NIST

• Made STM measurements on graphene grown on SiO₂ and SiC to understand the conductivity. Disorder appears to be related to the substrate roughness. Energy levels split into Landau levels at high magnetic fields and are stair step in shape. The landau levels have four peaks indicating a four fold degeneracy. When plotted in a certain way, the peaks form a diamond shape that he referred to as a coulomb diamond and said it was similar to a coulomb blockade. These areas look like quantum dots. Graphene grown on the C face of SiC are highly n-doped. Multiple graphene layers form and the layers can be rotated between 0.5 to 2 degrees from each other. Relaxation time is 0.4 ps and the valley splitting process is unknown.
• Website: Joseph A. Stroscio

Broken symmetry in suspended bilayer graphene
Amir Yacoby, Harvard University

• Bilayer graphene is stacked so that the lattice of each layer is alighted with each other such that a carbon atom appears in the center of the hexagon of the opposite layer when viewed from the top. This is known as Bernal stacking (in graphite alternate layers align with the middle layer being offset so that an atom aligns with the center of the hexagon in the above and below layers). Bilayer graphene produces 8 degenerate Landau levels in a magnetic field (compared with only 4 in single layer graphene). Quantum Hall Effect (QHE) was measured with magnetic fields as low as 50 mT. .
• Website: Amir Yacoby

Electron transport in Graphene on Boron Nitride
Cory Dean, Columbia University

• Graphene FETs were produced using BN as the gate insulator instead of SiO₂. BN has a bandgap between 3.2 and 7.6 eV, and produces an atomically smooth layer (smoother than SiO₂). The mobility for such devices ranges from 20,000 to 30,000 cm²/Vs. No effort was made to align the graphene lattice with the BN lattice.
• Website: Cory Dean

Fermi-line topology, Lifshitz transitions and symmetry breaking in bilayer graphene
Vladimir Fal'ko, Lancaster

• No useful notes.
• Website: Vladimir Falko

Tunable optical properties of graphene
Feng Wang, UC Berkeley

• The optical transmission through a graphene FET can be tuned in wavelength by controlling the gate. The absorption of graphene is constant. The photoluminescence of graphene is low because electron-hole pairs don't recombine symmetrically or are weekly coupled. Thus, electrons can relax to another unfilled site in band or to a site in the valance band that is at a different level than where the excited electron was first ejected. Scattering in CVD graphene tend to occur at grain boundaries (200 nm) which are chemically active or charged.
• Website: Feng Wang

Guiding Electrons in Graphene p-n Junctions
James Williams, Stanford University

• Graphene that is p-doped can produce a negative optical index of refraction. Enhanced conduction is seen in a p-n junction.
• Website: not found

Investigations of Scattering and Coulomb Interactionsin Graphene by ARPES Measurements
Eli Rotenberg, Lawrence Berkeley University

• Graphene is grown on SiC using a CVD process. Multiple layers form but only the top layer is considered active. Potassium is used to dope the graphene. Van hove singularities were observed (discontinuity in the Density of States (DOS)). Hydrogen p-dopes the graphene. Plasmarons are created when the SiC surface bonds below the graphene layer are terminated. Thinks that graphene can be superconductor and has observed plasmaronic quaisparticles.
• Website: not found

Interacting fermions on the honeycomb bilayer: from weak to strong coupling
Oskar Vafek, Florida State University

• Electrons will hop between the layers of bilayer graphene. An extra term is added to the Hamiltonian to account for this hopping.
• Website: Oskar Vafek

Layer and Orbital ordering in Bilayer graphene
Yafis Barlas, University of Florida

• Ferromagnetism in graphene is composed of electron spin, layer pseudo-spin, and Landau Layer pseudo-spin.
• Website: Yafis Barlas

Electronic Transport in Dual-Gated Bilayer Graphene at Large Displacement Fields
Pablo Jarillo-Herrero, MIT

• Applying a transverse magnetic field to bilayer graphene produces a bandgap. Conductivity of a dual-gated bilayer graphene was measured down to 300 mK. The FET had a bottom and top gates. A superconductor Josephson junction can be constructed and a robust Josephson Effect was measured. A Graphene FET was constructed using BN and was found to produce mobilities greater then 50,000 cm²/Vs. The Quantum Hall Effect was seen in the BN graphene FET with very low magnetic fields down to 100 mT.
• Website: Pablo Jarillo-Herrero
• Ref: Electronic Transport in Dual-gated Bilayer Graphene at Large Displacement Fields

Interactions in graphene
Philip Kim, Columbia University

• Electrons provide one spin component in graphene with up spin producing a triple and the down spin producing the singlet. The A and B lattice carbon atoms also provide a pseudo-spin with the A atom producing an up spin and the B atom producing a down spin. Insulating state can be produce with strong magnetic fields. Graphene FETs were made using an electrolyte for the gate dielectric and studied for conductivity. The conductivity was linear for temperatures under 100 K but then grew at T⁴ for temperatures above 100 K.
• Website: Philip Kim

Probing the electronic properties of graphene: from Landau level quantization to fractional quantum Hall effect
Eva Andrei, Rutgers University

• Described the history of graphene research today. Graphene FETs can be made using an exfoliation technique that uses scotch tape to separate the layers of graphite into a few number of layers. These layers can be transferred onto a Silicon wafer with a 300 nm thick layer of SiO₂. The single layer graphene turns out to be visible under a microscope in this configuration. Graphene can be also made using a CVD process with a gas like methane and a hot catalyst like In, Ru, Ni, or Cu. Graphene is impenetrable even to He atoms. The optical transmission of graphene is constant with T=1-απ where α=1/137. The transparency can be controlled with the gate voltage over a range of about 50 V. Graphene has a linear density of states, anomalous QHE, chirality, and berry phase. The QHE is considered anomalous because it is shifted by 1/2 compared to the normal QHE in other materials and the berry phase shift is due to the zero effective mass of the fermions at the Dirac point (charge neutral point). Small charge puddles occur in graphene due to the roughness of the substrate but this does not impair the QHE due to the DOS at Ef=0 (Dirac point) and the edge states. Landau levels appear under a magnetic field and are well defined. Fractional QHE can be seen on suspended graphene. The rotation angle or twist between bilayers graphene will affect the Density Of States (DOS).
• Website: Eva Andrei

Batch fabrication of single layer graphene devices and membranes
Jiwoong Park, Cornell University

• Used a Raleigh spectroscopic technique to measure the alignment and chirality of CNT over a large area. Single peaks in the spectrum identified the metallic CNT and the double peaks identified the semiconducting CNTs. Developed a process to create graphene films on Cu using a CVD process at 1000 °C. Grain boundaries of the resulting graphene film are chemically active but do not seem to interfere with the electron transport. The boundaries show a small potential drop. Grains measure 250 nm with a 1000 °C process temperature and the nucleation concentration or density has an effect on the grain size. The grains are not correlated to the copper lattice or copper grains. Low methane concentration produced the best graphene films.
• Website: Jiwoong Park

Ordered states of adatoms on graphene
Boris Altshuler, Columbia University

• Adatoms were attached to the graphene in certain locations to control the energy levels of the resultant materials. Three classes of materials were created: c-type, involving alkali and noble metals; s-type, involving H and Halogens; and e-type involving O, B, N, C, and H₂.
• Website: Boris Altshuler
• Ref: Partial Kekule Ordering of Adatoms on Graphene

Giant Flavor-Hall Effect and Nonlocal Transport in Graphene
Dmitry Abanin, Princeton University

• Described measurements and a model for QHE and transport in graphene. Electrons and holes drift in opposite directions but have opposite spins producing spin dependent currents that can be measured. At low magnetic fields, spin splitting is due to spin hall effect. At high magnetic fields, spin hall effect should become quantized with transport at the edge, but spin filter edge states become localized by disorder and transport is suppressed. Alternately, the splitting type changes form spin to valley as the field increases.
• Website: Dmitry Abanin