Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A16: Graphite and Intercalated Compounds |
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Sponsoring Units: DCMP DMP Chair: Arthur Hebard, University of Florida Room: B115 |
Monday, March 15, 2010 8:00AM - 8:12AM |
A16.00001: GW Investigation of Alkali Intercalated Graphite Bahadir Altintas , Resul Eryigit The amount of charge transferred from the alkali atoms to graphene layers and the character of electronic energy levels have been controversial for the alkali intercalated graphite. Experimental studies by different techniques indicate negligible to full charge transfer, while DFT level calculations show a low level of charge transfer. We will report first principles calculations on electronic properties, such as charge transfer, band structure and partial density of electronic states of Li, Na, K, Rb and Cs intercalated graphite at the GW level. The results show a systematic increase in charge transfer compared to the DFT results as well as a raising of alkali derived states around the Fermi level [Preview Abstract] |
Monday, March 15, 2010 8:12AM - 8:24AM |
A16.00002: Formation dynamics of graphite intercalation compounds: An {\em ab initio} study Bo Song , Haiping Fang , David Tom\'anek In response to the rising interest in functionalized graphitic nanostructures for energy applications, we study the intercalation of potassium in graphene bi-layers. Our {\em ab initio} molecular dynamics calculations, based on the density functional force field and simulating conditions at 900~K, provide microscopic insight into the dynamics of the intercalation process. Our model system consists of wide graphitic ribbons with hydrogenated edges and a varying number of K atoms in the unit cell. We find that following initial charge transfer from K to graphite upon adsorption, K$^+$ ions diffuse efficiently along the surface. After reaching the edge, K$^+$ ions experience further stabilization upon entering the region in-between graphene layers, accompanied by a substantial increase of the graphene inter-layer distance. We observe both intercalation and de-intercalation as competing processes in our canonical ensemble under steady-state conditions. [Preview Abstract] |
Monday, March 15, 2010 8:24AM - 8:36AM |
A16.00003: Why is KC$_8$ superconductor and LiC$_6$ is not? Z.-H Pan , J. Camacho , M.H. Upton , A.V. Fedorov , A.C. Walters , C.A. Howard , M. Ellerby , T. Valla Superconductivity in graphite intercalated compound(GIC) has been studied for decades. Discovery of CaC$_6$ with $T_c = 11.5 K$ has resurged the intense study of GICs.Many GICs have been found to be superconducting with $T_c$ ranges from milikelvins to more than 10 kelvins, however there is not a clear trend. We performed a systematic angle resovled photoemission spectroscopy(ARPES) study on both KC$_8$ and LiC$_6$, the former is superconductor while the latter is not. We found a trend that superconductivity is correlated to electron phonon coupling and doping. Our result gives a natural explanation of why the KC$_8$ is superconductor and LiC$_6$ is not. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A16.00004: The nature and strength of inter-layer binding in graphite Leonardo Spanu , Sandro Sorella , Giulia Galli We compute the interlayer bonding properties of graphite using an ab initio many-body theory. We carry out variational and diffusion quantum Monte Carlo calculations and find an equilibrium interlayer binding energy in good agreement with most recent experiments. We also analyze the behavior of the total energy as a function of interlayer separation at large distances comparing the results with the predictions of the random phase approximation. We then estimate the ratio between exfoliation and binding energy.\\[4pt] {\it Phys. Rev. Lett} {\bf 103}, 196401 (2009) [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A16.00005: Delocalization by Disorder in Layered Systems Vladimir Yudson , Dmitrii Maslov , Andres Somoza , Miguel Ortuno Motivated by anomalously large conductivity anisotropy in graphite and other layered materials, we propose a simple model of randomly spaced potential barriers (mimicking stacking faults) with isotropic impurities in between the barriers. We solve this model both numerically and analytically, by utilizing an exact solution for the conductivity of a one-dimensional (1D) disordered system. In the absence of bulk disorder, electron motion in the out-of-plane direction is localized. Bulk disorder destroys 1D localization. As a result, the out-of-plane conductivity is finite and scales linearly with the scattering rate by bulk impurities until planar and bulk disorder become comparable. The \emph{ac} out-of-plane conductivity is of a manifestly non-Drude form: the real part starts from finite value at zero frequency and has a maximum at the frequency corresponding to the scattering rate by potential barriers. [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A16.00006: Hot electron effect and weak localization in suspended multilayer graphene Carlos Diaz Pinto , Sungbae Lee , Nelka Wijesinghe , Haibing Peng We study the differential conductance (\textit{dI}/\textit{dV}) of suspended multilayer graphene as a function of source drain bias $V_{d}$ and temperature $T$. A dip of \textit{dI}/\textit{dV} has been observed near $V_{d }$= 0, along with phonon-induced anomalies at higher $V_{d}$. We find a logarithmic dependence of \textit{dI}/\textit{dV }on both $V_{d}$ and $T$. The logarithmic $V_{d}$ dependence can be explained with the hot electron effect and the logarithmic $T$ dependence is attributed to the weak-localization in two-dimensions. The magnetoconductance data for suspended multilayer graphene agree remarkably well with the weak localization theory considering both the inelastic and elastic scattering processes related to the chiral nature of graphene carriers. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A16.00007: Transport study of Berry's phase in Highly Oriented Pyrolytic Graphite Aruna N. Ramanayaka , Ramesh G. Mani Observation of a non-zero Berry's phase provides evidence for the existence of massless relativistic charge carriers in Graphene. Since stacked sheets of single layers of graphene with weak interlayer interactions is equivalent to Highly Oriented Pyrolytic Graphite (HOPG), it is of interest to look for a Berry's phase anomaly in HOPG. In this study, we report an experimental investigation of Berry's phase in graphite based on the analysis of Shubnikov-de Haas (SdH) oscillations of the diagonal magnetoresistance. Further, we compare the Berry's phase of graphite with the same for the other material systems such the GaAs/AlGaAs 2D electron system, n-GaAs epilayer, and bulk semiconducting Hg0.8Cd0.2Te. We conclude by reviewing the prospects for a Berry's phase anomaly in graphite. [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A16.00008: Room-Temperature Voltage Rectification in Graphite Yakov Kopelevich , J.C. Medina Pantoja , Robson R. da Silva , Alex M. Bratkovsky Four-probe current-voltage (I-V) characteristics were measured on 80-250 nm thick graphite samples, obtained by cleaving and placed on Si substrate, in the temperature interval 2 K $\le $ T $\le $ 300 K and applied magnetic field up to 9 T. Nonlinear and/or hysteretic I-V curves with a pronounced asymmetry with respect to the current polarity were obtained even at T = 300 K. We demonstrate that voltage rectification and related effects in graphite resemble very much that known for inhomogeneous superconducting systems [1], suggesting the occurrence of superconducting correlations in graphite at room temperature. We also discuss the role of the ferromagnetic ordering at graphitic edges in the observed phenomena. \\[4pt] [1] G. Carapella and G. Costabile, Phys. Rev. Lett. 87, 077002 (2001). [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A16.00009: Engineering of Ferromagnetic Graphite and Graphene Robson R. da Silva , Yakov Kopelevich , Ivan N. Naumov , Alexander M. Bratkovsky In the present work, we demonstrate both experimentally and by ab-initio simulations that room-temperature ferromagnetism can be induced in graphitic materials by means of adsorbed oxygen or sulfur. The results show that oxygen/sulfur-induced edges of graphitic fragments (via unzipping effect) play an essential role in this phenomenon, and that the finite magnetic moment takes place if zig-zag edges in a graphitic ribbon are terminated asymmetrically, i.e. with a different oxygen/sulfur occupation at opposite edges. In particular, our ab-initio calculations performed within the local spin density approximation showed that in the case of pure graphene ribbon, its edge carbon atoms carry large magnetic moment ($\sim $0.72 $\mu _{B}$/C). In an oxidized graphene, however, the magnetic moment at the edge with absorbed oxygen atoms vanishes, leading to effective ferro(ferri)-magnetic behavior of the sample. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A16.00010: Landau-level characterization of AB-stacked graphite Yen-Hung Ho , De-Hone Lin , Wu-Pei Su , Yu-Huang Chiu , Jei Wang , Ming-Fa Lin Magneto-electronic structures of AB-stacked bulk graphite are investigated by the Peierls tight-binding model, which takes account of all interlayer interactions and field-induced Peierls phases simultaneously. This model is feasible for all $\pi$ electronic states and capable of drawing the analytic solution for state wave functions. The external magnetic field $B_0\hat{z}$ condenses the planar motion of electrons into Landau levels, and the coupling between layers causes these levels to oscillate along $k_z$. The band edge states are mainly located at the Brillouin zone boundaries, where their frequencies as a function of field are closely related to the atomic hopping integrals. Landau levels are divided into two groups based on the wave function distribution. Besides, wave functions undergo a particular transition between atomic sites in response to $k_z$. From the behavior of energies and wave functions, we can infer the monolayer-like plus bilayer-like features in bulk graphite. The explicit characterization of Landau levels in this work could be applied to clarify and elucidate the experimental measurements on graphene layers. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A16.00011: Landau levels of graphene multilayers in a parallel magnetic field Sergey Pershoguba , Victor Yakovenko Much attention was paid recently to the Landau levels in graphene mono- and multilayers for a magnetic field perpendicular to the layers. In contrast to the previous investigations, we study the Landau levels of graphene multilayers (graphite) in a parallel magnetic field. We use the tight-binding approximation with the nearest-neighbor intralayer and interlayer hoping amplitudes $\gamma_0$ and $\gamma_1$ for the Bernal stacking. Electron dispersion is highly anisotropic in the plane perpendicular to the graphene layers and to the magnetic field. Employing numerical diagonalization and semiclassical approximations, we find the energy spectrum of the problem. The parabolic dispersion, well-known for graphene bilayers and multilayers without magnetic field, splits into an infinite series of Dirac cones due to the parallel magnetic field. These cones intersect and form a dispersive band around zero energy. The low energy levels exhibit an unusual Landau quantization $E_n\propto(nB)^2$, where $n$ is a quantum number, and $B$ is the magnetic field. We interpret these results in terms of closed semiclassical orbits on the two branches of the equienergetic surface. For the energies above $2\gamma_1$, the discrete energy spectrum transforms into a continuous dispersion $E(k)$, which corresponds to open semiclassical trajectories in the $k$-space. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A16.00012: Dissolution of Graphite into Graphene and Formation of Liquid Crystals Natnael Behabtu , Jay Lomeda , Micah Green , Alexander Sinitskii , Amanda Higginbotham , A. Nicholas G. Parra-Vasquez , Dmitry V. Kosynkin , Judith Schmidt , Ellina Kesselman , Yachin Cohen , Yeshayahu Talmon , James M. Tour , Matteo Pasquali Graphene is a promising new material with a wide number of potential applications, including electronics and nanocomposites, which often require that the graphene be dispersed and processed in a fluid phase. Here we show that in chlorosulfonic acid, graphene is spontaneously exfoliated from graphite into graphene, and dissolved at isotropic concentrations as high as 2 mg/ml, without covalent functionalization, surfactant stabilization, or sonication. STEM and cryo-TEM show that graphene in chlorosulfonic acid acts as rigid platelets. Thus, at higher concentrations, a liquid-crystalline phase forms spontaneously. The dissolution mechanism in superacids is protonation forming charge transfer complexes facilitating electrostatic repulsion, similar to nanotubes in superacids. Novel forms of graphene such as carbon nanoribbons can be dispersed as well. The isotropic phase is processed into conducting and transparent films. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A16.00013: Synthesis of RE$_{9}$ Mg$_{35}$ Zn$_{57}$ (RE = Gd, Tb, Dy, Ho, Er, and Y) from a metallic flux in 9 tesla magnetic fields Thomas J. Ott , Sally J. Tracy , Heather M. Volz , Jason C. Lashley , Jason C. Cooley We have precipitated RE$_{9}$ Mg$_{35}$ Zn$_{57}$ (RE = Gd, Tb, Dy, Ho, Er, Y) quasicrystals from molten metal fluxes in 9 tesla magnetic fields. We have measured the magnetic susceptibility and performed x-ray diffraction measurements on the materials grown in 9 tesla and 0 tesla. We will discuss the effect of growth in field on the magnetic susceptibility and on the crystal structure. [Preview Abstract] |
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