Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Z1: Focus Session: Graphene - Multilayer and Interfacial Effect |
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Sponsoring Units: DMP Chair: Rui He, University of Northern Iowa Room: 001A |
Friday, March 6, 2015 11:15AM - 11:51AM |
Z1.00001: Layer breathing vibrations in AB-stacking few layer graphene and twisted bilayer graphene Invited Speaker: Rui He Interlayer interactions in few layer graphene can create a set of shear modes and layer breathing modes (LBMs) that involve lateral and vertical displacement of individual layers, respectively. LBMs are of importance because they can facilitate interlayer current conduction and are sensitive to external perturbations, such as the presence of substrate or surface adsorbates. We investigated low-frequency fundamental layer breathing vibrations in AB-stacking few layer graphene and twisted bilayer graphene using Raman spectroscopy. In AB-stacking few layer graphene we observed the Raman peaks from phonons at the Brillouin zone center for the lowest-frequency branch LBM vibration. The mode frequency depends strongly on the number of graphene layers. Notably, the LBM Raman response is unobservable at room temperature, and it is turned on at higher temperature ($>$600 K) with a steep increase of Raman intensity. The observation suggests that the LBM vibration is strongly suppressed by molecules adsorbed on the graphene surface but is activated as desorption occurs at high temperature. In twisted bilayer graphene, the fundamental LBM is observed in a small range of twisting angle at which the intensity of the G Raman peak is strongly enhanced. The dependence of this mode's frequency and linewidth on the rotational angle can be explained by the double resonance Raman process (LBM phonon with nonzero wavevector) mediated by the twisted bilayer graphene lattice which lacks long-range translational symmetry. The angle dependence also reveals the strong impact of electronic band overlaps of the two rotated graphene layers. [Preview Abstract] |
Friday, March 6, 2015 11:51AM - 12:03PM |
Z1.00002: Probing the Ni(111)-graphene interface using Raman spectroscopy Guangjun Cheng, Irene Calizo, Angela Hight Walker Theoretical simulations have shown that due to the hybridization of Ni d-electrons with the $\pi $-orbitals of graphene, graphene phonon dispersion is significantly altered (Nano Lett, 2010, 10, 4335-4340). There is no Raman signal from graphene on Ni(111) due to the suppression of the Kohn anomaly. In our work, we deposit a Ni thin film by thermal evaporation onto mechanically exfoliated graphene, few-layer graphene (FLG), and graphite, and probe the Ni-graphene interface using Raman spectroscopy. When the sample is annealed in forming gas, a Ni(111) thin film is produced on graphene, FLG, and graphite. We observe the disappearance of Raman signals from graphene underneath Ni(111) when using low laser power and the re-appearance of the Raman signals from the graphene with a higher power excitation laser. This work provides direct experimental evidence for the strong interaction between Ni(111) and graphene. [Preview Abstract] |
Friday, March 6, 2015 12:03PM - 12:15PM |
Z1.00003: Electronic properties of bilayer graphenes strongly coupled to interlayer stacking and an external electric field Changwon Park, Junga Ryu, Suklyun Hong, Bobby Sumpter, Gunn Kim, Mina Yoon In the design of bilayer graphene (BLG)-based switching devices, it is critical to understand the complex stacking structures observed experimentally and their impact on the overall electronic properties. Using a maximally localized Wannier function, a highly accurate tight-binding Hamiltonian based on density functional theory was constructed and the stacking-dependent evolution of BLGs electronic band structures and their response to an external electric field were systematically investigated. Although the crossing band structures remain at any stacking configurations (i.e., no energy gap opens), the wavefunction characteristics around the Fermi level can differ qualitatively for different stackings. This difference is conveyed to energy gap opening properties in the presence of an external electric field. We, for the first time, established a phase diagram summarizing the stacking-dependent electronic structures of BLG, separating metallic and semiconducting characteristics for a given external field. [Preview Abstract] |
Friday, March 6, 2015 12:15PM - 12:27PM |
Z1.00004: Electromagnetic coupling of spins and pseudospins in bilayer graphene R. Winkler, U. Z\"ulicke We present a theoretical study of bilayer-graphene's electronic properties in the presence of electric and magnetic fields. In contrast to known materials, including single-layer graphene, any possible coupling of physical quantities to components of the electric field has a counterpart where the analogous component of the magnetic field couples to exactly the same quantities. For example, a purely electric spin splitting appears as the magneto-electric analogue of the magnetic Zeeman spin splitting. The measurable thermodynamic response induced by magnetic and electric fields is thus completely symmetric. The Pauli magnetization induced by a magnetic field takes exactly the same functional form as the polarization induced by an electric field. Although they seem counterintuitive, our findings are consistent with fundamental principles such as time reversal symmetry. For example, only a magnetic field can give rise to a macroscopic spin polarization, whereas only a perpendicular electric field can induce a macroscopic polarization of the sublattice-related pseudospin in bilayer graphene. These rules enforced by symmetry for the matter-field interactions clarify the nature of spins versus pseudospins. We have obtained numerical values of prefactors for relevant terms. [Preview Abstract] |
Friday, March 6, 2015 12:27PM - 12:39PM |
Z1.00005: Effective-mass theory of flattened carbon nanotubes as bilayer graphene with closed edges Takeshi Nakanishi, Tsuneya Ando We theoretically study effects of inter-wall interaction in collapsed carbon nanotubes, first directly calculating effective inter-wall interaction within an effective-mass scheme and second regarding collapsed tubes as ribbons of bilayer graphene with closed edges described by boundary conditions explicitly derived. Within the effective-mass scheme, effects of inter-wall interactions are shown to be important in non-chiral nanotubes such as zigzag and armchair. In fact, with the increase in the width of the flattened region, the band structure approaches that of a bilayer ribbon in which the electron motion in the ribbon-width direction is discritized. In chiral nanotubes, inter-wall interaction can essentially be neglected except in the vicinity of non-chiral tubes. Inter-wall interactions diminish rapidly when chiral angle deviates from zigzag or armchair, although the decay is slower in the vicinity of the armchair tube. When the flattened region has the structure of AA and AB stacked bilayer graphene, the same results can be derived by calculating boundary conditions corresponding to the closed-edge structure in which the top and bottom layers are smoothly connected through a monolayer graphene. [Preview Abstract] |
Friday, March 6, 2015 12:39PM - 12:51PM |
Z1.00006: Ordered states in spatially separated Coulomb-coupled double graphene AB bilayers Jung-Jung Su, Allan H. MacDonald Electron-electron interaction effects are strong in graphene bilayer systems because of the approximately quadratic crossing between conduction and valence bands at the Brillouin-zone corners. In isolated bilayers this circumstance leads to an unusual spin-density-wave ground state and to a first-order phase transition between states with and without broken time reversal symmetry as a function of interlayer displacement field strength. In this talk I will address possible ordered states in systems containing two AB bilayers separated by a tunnel barrier but coupled by inter-layer Coulomb interactions. I focus on the case in which displacement fields induce charge transfer between layers, showing that under appropriate conditions it is possible to induce states with spontaneous coherence between bilayers. The transition to this coherent exciton condensate is first-order and accompanied by a substantial rearrangement of charge within each bilayer. [Preview Abstract] |
Friday, March 6, 2015 12:51PM - 1:03PM |
Z1.00007: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 1:03PM - 1:15PM |
Z1.00008: Quantum multicriticality in bilayer graphene with a tunable energy gap Robert Throckmorton We extend previous renormalization group (RG) analyses of electron-electron interactions in gapless bilayer graphene at finite temperature to include the effect of an electric field applied perpendicular to the sample. We determine the possible outcomes of the resulting RG equations, represented by ``fixed rays'' along which ratios of the coupling constants remain constant and map out the leading instabilities of the system for an interaction of the form of a Coulomb interaction that is screened by two parallel conducting plates placed equidistant from the electron. We then construct maps of the leading instability (or instabilities) of the system for the screened Coulomb-like interaction as a function of the overall interaction strength and interaction range for four values of the applied electric field. We find that the pattern of leading instabilities is the same as that found in the zero-field case, namely that the system is unstable to a layer antiferromagnetic state for short-ranged interactions, to a nematic state for long-ranged interactions, and to both for intermediate-ranged interactions. However, if the interaction becomes too long-ranged or too weak, then the system will exhibit no instabilities. [Preview Abstract] |
Friday, March 6, 2015 1:15PM - 1:27PM |
Z1.00009: Quasiparticle renormalization in ABC graphene trilayers Xu Dou, Akbar Jaefari, Yafis Barlas, Bruno Uchoa We investigate the effect of electron-electron interactions in ABC stacked graphene trilayers. In the gapless regime, we show that the self-energy corrections lead to the renormalization of the dynamical exponent $z=3+\alpha_{1}/N$, with $\alpha_{1}\approx 0.52$ and $N$ is the number of fermionic species. Although the quasiparticle residue is suppressed near the neutrality point, the lifetime has a sublinear scaling with the energy and the quasiparticles are well defined even at zero energy. We calculate the renormalization of a variety of physical observables, which can be directly measured in experiments. [Preview Abstract] |
Friday, March 6, 2015 1:27PM - 1:39PM |
Z1.00010: Density of states and tunneling conductance of ABC-stacked trilayer graphene Jongbae Hong, David Abergel Experimental studies on ABC-stacked trilayer graphene suggest that the trilayer graphene may have a strongly correlated ground state which drives the opening of a band gap at the Fermi level. We propose a theoretical model for the tunneling conductance of strongly correlated trilayer graphene based on a Kondo-like mechanism of coherent transport where electrons tunnel into the correlated sample via an entangled singlet state. Our theory fits current experimental data extremely well, including peculiar features such as two pairs of kinks at low bias and a peak and shoulder at a relatively high bias. No other theoretical study has previously explained these features of the line shape. We also suggest a phenomenological estimate of the density of states in the strongly correlated regime. This has not previously been studied. [Preview Abstract] |
Friday, March 6, 2015 1:39PM - 1:51PM |
Z1.00011: Rich Magneto-electronic spectra of AAB-stacked trilayer graphene Thi Nga Do, Min-Fa Lin We develop the generalized tight-binding model to study the magneto-electronic properties of AAB-stacked trilayer graphene. Three groups of Landau levels (LLs) are characterized by the dominating sub-envelope function on distinct sub-lattices. Each LL group could be further divided into two sub-groups in which the wave-functions are, respectively, localized at 2/6 and 4/6 of the total length of the enlarged unit cell. The unoccupied conduction and the occupied valence LLs in each sub-group behave similarly. For the first group, there exists certain important differences between two sub-groups, including the LL energy spacings, quantum numbers, spatial distributions, and the field-dependent spectra. The LL crossings and anti-crossings occur frequently in each sub-group during the variation of field strength, which thus leads to the very complicated energy pectra and the seriously distorted wave-functions. [Preview Abstract] |
Friday, March 6, 2015 1:51PM - 2:03PM |
Z1.00012: Magnetic and electric field effects in trilayer graphene Jie Yuan, Carsten Honerkamp, Fu-Chun Zhang A 40meV gap has been observed in trilayer graphene in a recent experiment[1]. We study theoretcally a model for graphene trilayer with layer-antiferromagnetic state as the ground state due to on-site Hubbard interaction, employinmg mean-field and functional renormalization group techniques. The in-plane magnetic field causes a canting of the spins, but the gap decrease turn out to be insufficient to explain the experiment data, even with orbital effects included. The electric field makes the magnetic subbands non-degenerate. Upon doping, the system will develop a non-zero magnetization and become a half-metal. Notably, in the half-metallic state of trilayer graphene, the spin-polarization can be very large. [1].arXiv:1402.6413 [Preview Abstract] |
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