Wigner crystal and bubble phases in graphene in the quantum Hall regime

Graphene, a single free-standing sheet of graphite with honeycomb lattice structure, is a semimetal with carriers that have linear dispersion. A consequence of this dispersion is the absence of Wigner crystallization in graphene, since the kinetic and potential energies both scale identically with the density of carriers. We study the ground state of graphene in the presence of a strong magnetic field focusing on states with broken translational symmetry. Our mean-field calculations show that at integer fillings a uniform state is preferred, whereas at non-integer filling factors Wigner crystal states (with broken translational symmetry) have lower energy. We obtain the phase diagram of the system. We find that it is qualitatively similar to that of quantum Hall systems in semiconductor heterostructures. Our analysis predicts that non-uniform states, including Wigner crystal state, will occur in graphene in the presence of a magnetic field and will lead to anisotropic transport in high Landau levels.Comment: New references added; 9 pages, 9 figures, (paper with high-resolution images is available at http://www.physics.iupui.edu/yogesh/graphene.pdf

Irreversibility at macromolecular scales in the flake graphite of the lithium-ion battery anode.

Charging a commercial lithium-ion battery intercalates lithium into the graphite-based anode, creating various lithium carbide structures. Despite their economic importance, these structures and the dynamics of their charging-discharging transitions are not well-understood. We have videoed single microcrystals of high-quality, natural graphite undergoing multiple lithiation-delithiation cycles. Because the equilibrium lithium-carbide compounds corresponding to full, half, and one-third charge are gold, red, and blue respectively, video observations give direct insight into both the macromolecular structures and the kinematics of charging and discharging. We find that the transport during the first lithiation is slow and orderly, and follows the core-shell or shrinking annuli model with phase boundaries moving at constant velocities (i.e. non-diffusively). Subsequent lithiations are markedly different, showing transport that is both faster and disorderly, which indicates that the initially pristine graphite is irreversibly and considerably altered during the first cycle. In all cases deintercalation is not the time-reverse of intercalation. These findings both illustrate how lithium enters nearly defect-free host material, and highlight the differences between the idealized case and an actual, cycling graphite anode

Local analysis of oxygen reduction catalysis by scanning vibrating electrode technique : a new approach to the study of biocorrosion

The scanning vibrating electrode technique (SVET)was employed to investigate oxygen reduction catalysis by the presence of enzyme in an aerobic medium. Heme protoporphyrin (hemin) was chosen as a model of the enzymes that are able to catalyze oxygen reduction. A strict experimental protocol was defined for preparing the graphite surface by deposition of hemin with a simple configuration mimicking the presence of enzyme on the samples. The same configuration was adapted to a stainless steel electrode. Different geometric arrangementswere investigated by SVET to approach the local conditions. The results demonstrated that hemin deposited on the electrode surface led to an increase in the cathodic current, which indicated a catalytic effect. Based on the SVET analysis, itwas demonstrated that hemin caused the appearance of galvanic cells on the material surface. The SVET proved able to locate active catalytic centres and therefore to foresee the contribution of the enzyme to the creation of galvanic cells, thus leading to localized corrosion. The application of SVET to the study of the interaction between biological molecules and material provides a newapproach for visualizing and understanding microbially influenced corrosion (MIC) in an aerobic medium

A model of large volumetric capacitance in graphene supercapacitors based on ion clustering

Electric double layer supercapacitors are promising devices for high-power energy storage based on the reversible absorption of ions into porous, conducting electrodes. Graphene is a particularly good candidate for the electrode material in supercapacitors due to its high conductivity and large surface area. In this paper we consider supercapacitor electrodes made from a stack of graphene sheets with randomly-inserted "spacer" molecules. We show that the large volumetric capacitances C > 100 F/cm^3 observed experimentally can be understood as a result of collective intercalation of ions into the graphene stack and the accompanying nonlinear screening by graphene electrons that renormalizes the charge of the ion clusters.Comment: 13 pages, 5 figures; additional discussion and supporting calculations adde

Dependence of band structures on stacking and field in layered graphene

Novel systems of layered graphene are attracting interest for theories and applications. The stability, band structures of few-layer graphite films, and their dependence on electric field applied along the c-axis are examined within the density functional theory. We predict that those of Bernal type and also rhombohedral type tri- and tetra-layer graphite films exhibit stability. The rhombohedral-type systems including AB-bilayer, show variable band gap induced by perpendicular electric field, whereas the other systems such as the Bernal-type films stay semi-metallic.Comment: 5 pages, 5 figures, accepted for publication in Solid State Communication