Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B30: Focus Session: Graphene Devices: Fabrication, Characterization and Modeling: Trilayer and Bilayer Graphene: Transport and Stacking |
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Sponsoring Units: DMP Chair: Jeanie Lau, University of California, Riverside Room: 605 |
Monday, March 3, 2014 11:15AM - 11:51AM |
B30.00001: Transport in bilayer and trilayer graphene: band gap engineering and band structure tuning Invited Speaker: Jun Zhu Controlling the stacking order of atomically thin 2D materials offers a powerful tool to control their properties. Linearly dispersed bands become hyperbolic in Bernal (AB) stacked bilayer graphene (BLG). Both Bernal (ABA) and rhombohedral (ABC) stacking occur in trilayer graphene (TLG), producing distinct band structures and electronic properties. A symmetry-breaking electric field perpendicular to the sample plane can further modify the band structures of BLG and TLG. In this talk, I will describe our experimental effort in these directions using dual-gated devices. Using thin HfO$_2$ film deposited by ALD as gate dielectric, we are able to apply large displacement fields D $>$ 6 V/nm and observe the opening and saturation of the field-induced band gap E$_g$ in bilayer and ABC-stacked trilayer graphene, where the conduction in the mid gap changes by more than six decades. Its field and temperature dependence highlights the crucial role played by Coulomb disorder in facilitating hopping conduction and suppressing the effect of E$_g$ in the tens of meV regime. In contrast, mid-gap conduction decreases with increasing D much more rapidly in clean h-BN dual-gated devices. Our studies also show the evolution of the band structure in ABA-stacked TLG, in particular the splitting of the Dirac-like bands in large D field and the signatures of two-band transport at high carrier densities. Comparison to theory reveals the need for more sophisticated treatment of electronic screening beyond self-consistent Hartree calculations to accurately predict the band structures of trilayer graphene and graphenic materials in general. In collaboration with: Ke Zou, Jing Li, Fan Zhang, C Clapp, and Allan MacDonald. References: Zou and Zhu, PRB 82, 081407 (2010) Zou et al, Nano Letters, 13, 369 (2013) [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B30.00002: Electric Field Control of Stacking-Order Solitons in Trilayer Graphene Matthew Yankowitz, Joel I-Jan Wang, A. Glen Birdwell, K. Watanabe, T. Taniguchi, Pablo San-Jose, Philippe Jacquod, Pablo Jarillo-Herrero, Brian J. LeRoy Trilayer graphene exhibits two low-energy stacking configurations (Bernal and rhombohedral). In graphene flakes with both stacking configurations, the area separating them consists of a localized soliton-like region of strain where one graphene layer shifts by the carbon-carbon bond distance. Under a perpendicular electric field, Bernal-stacked trilayer graphene remains metallic whereas rhombohedrally-stacked trilayer graphene develops a band gap. Consequentially, the electric field modifies the relative energy cost of each stacking configuration, permitting rare control over the crystal structure of a material via only the application of an external electric field. We demonstrate the ability to control the stacking configuration in trilayer graphene via an electric field using scanning tunneling microscopy. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B30.00003: Broken Symmetry States in Dual Gated Rhombohedral Trilayer Graphen Yongjin Lee, David Tran, Kevin Myhro, Jairo Velasco Jr., Nathaniel Gillgren, Chung Ning Lau, Yafis Barlas, Jean-Marie Poumirol, Dmitry Smirnov, Francisco Guinea We perform low temperature transport measurements of dual-gated rhombohedral-stacked trilayer graphene device. At the charge neutral point, we observe a giant interaction-induced gap, $\sim$ 41mV that is suppressed by an interlayer potential or a critical temperature Tc $\sim$ 28K, suggesting a layer antiferromagnetic ground state with broken time reversal symmetry. In the quantum Hall regime, we observe~QH plateaus at filling factors $\nu =$0, 1, 2 and 3 in a high magnetic field. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B30.00004: Mechanism of photocurrent generated at a junction between ABA- and ABC-stacked tri-layer graphene Minjung Kim, Seon-Myeong Choi, Ho Ang Yoon, Sun Keun Choi, Jung Cheol Kim, Sang Wook Lee, Young-Woo Son, Hyeonsik Cheong Tri-layer graphene has two stacking orders, ABA and ABC stacking, which have different electronic band structures. We observed photocurrent generated at the ABA and ABC stacking junction in tri-layer graphene and investigated the mechanism of the photocurrent by measuring the back-gate voltage dependence of the photocurrent. In general, there are two mechanisms of photocurrent generated in graphene photodevices without bias; a density of states (DOS) mismatch and the Seebeck coefficient difference. The dominant mechanism of photocurrent at a junction between single- and bi-layer graphene has been suggested as being due to the difference in Seebeck coefficients [X. Xu et. al., Nano lett. 10, 562 (2010).]. Here, we studied the dominant mechanism of the photocurrent in the ABA and ABC stacking junction in tri-layer graphene. If the DOS mismatch is the dominant mechanism, the direction of photocurrent is from ABC to ABA stacking in p-doped tri-layer graphene. On the other hand, if the Seebeck coefficient difference is dominant, the direction of the photocurrent is opposite. In our devices, it has been found that the DOS mismatch is dominant. In addition, we measured the photocurrent at between at a junction single- and bi-layer graphene and bi- and tri-layer graphene for comparison. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 1:03PM |
B30.00005: Quantum Hall phase diagram of ABC-trilayer graphene Invited Speaker: Yafis Barlas At low-energies, the massive Dirac electrons in ABC-stacked trilayer graphene exhibit a cubic dispersion with a Berry phase of $3 \pi$. Landau quantization of ABC-trilayer graphene leads to a quantum Hall (QH) plateau sequence $\sigma_{xy} = \pm 4(N +3/2) e^2/h$(where $N \geq 0 $ is the Landau level index). This results in a 12-fold degenerate zero-energy Landau level (LL) which supports a degenerate set of triplet ($n=0,1,2$) LL orbitals along with the spin and valley degeneracies. In this talk, I will show that interactions within the zeroth LL induce charge gaps which drive additional integer QH plateaus at intermediate filling factors $\nu$ ($-6 < \nu < 6$). The competition of remote hopping between the layers, interactions and pseudo-spin anisotropy leads to various ferromagnetically and anti-ferromagnetically pseudo-spin ordered states. Additionally, the unique LL orbital degeneracy influences the ground state at filling factors $\nu =-5,-2,1,4$. At these filling factors, a quantum phase transition from a quantum Hall liquid state to a triangular charge-density wave occurs when an electric potential difference $\Delta _{V}$ between the layers is reduced below a critical value $\Delta_{V}^{\left( c\right)}$ [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B30.00006: Electronic correlation in ABC graphene trilayers Xu Dou, Akbar Jaefari, Yafis Barlas, Bruno Uchoa At low energies, undoped ABC-stacked trilayer graphene can be described by an effective two-band model which features a cubic noninteracting energy spectrum. The divergence of the density of states at the neutral points provides a large phase space for electronic instabilities, which differentiates it from single-layer and bilayer graphene. In the large N limit, where N is the number of fermion species, we show that the self-energy is logarithmically renormalized by Coulomb interactions in leading order in 1/N. We show that the dynamical polarization bubble is also logarithmically divergent near the edge of the particle hole continuum. We investigate the renormalization of different physical observables accounting for dynamical polarization effects. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B30.00007: Magneto-optical spectra of ABA trilayer graphene: interplay between monolayer and bilayer signatures Yen-Hung Ho, Wu-Pei Su, Ming-Fa Lin We utilize a generalized tight-binding model to study the Landau level spectra of ABA trilayer graphene. The spatially resolved Landau wave functions enable the characterization of Landau levels and the calculation of magneto-optical properties. The spectra consist of monolayer-like and bilayer-like features, in which the inter-valley symmetry is lifted, especially the levels close to zero energy. Applying a bias voltage effectively increases the splitting, and furthermore, triggers the optical transitions between monolayer-like and bilayer-like Landau states. The calculated results can be further verified by optical measurements, and this numerical method can be applied to other layered materials, such as Molybdenum disulfide. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B30.00008: Chiral electron transport in CVD bilayer graphene Kyunghoon Lee, Yun Suk Eo, Cagliyan Kurdak, Zhaohui Zhong Charge carriers in bilayer graphene have a parabolic energy spectrum. Due to this band structure they are massive quasiparticles having a finite density of state at zero energy like other non-relativistic charge carriers in conventional two dimensional materials. However, they are massive Dirac fermions which have a chiral nature similar to the case of massless Dirac fermions in single layer graphene. Coupling of pseudospin and motion of charge carrier via chirality can result in dramatic consequence for transport in bipolar regime like Klein tunneling, Fabry-Perot interference, collimation of charge carrier, Veslago lens, etc. However, little attention has been paid to chiral dependent electron transport in bilayer graphene. Here we study these properties by probing phase coherent transport behavior in CVD bilayer graphene devices with sub-200nm channel length. Complex Fabry-Perot interference patterns are observed in resonant cavities defined by local gating. By applying Fourier analysis technique, we successfully analyze and identify the origin of each individual interference pattern in bipolar and monopolar regime. Our initial results also hint at the observation of cloaking of electronic states against chiral electrons in bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B30.00009: Electrochemical characterization of chemical vapor deposition grown few-layer graphene Rajaram Narayan, Mehmet Karakaya, Ramakrishna Podila, Prabhakar Bandaru, Apparao Rao The intrinsic double-layer capacitance (C$_{\mathrm{dl}})$ of graphene is an important fundamental parameter that has important implications in nano-carbon based energy storage devices. We used cyclic voltammetry to measure the C$_{\mathrm{dl}}$ of few-layer graphene (FLG) samples. Considering the fact that the specific C$_{\mathrm{dl}}$ of graphitic edge planes exceeds that of basal planes by an order of magnitude, the measured specific C$_{\mathrm{dl}}$ may be used to evaluate the relative area fraction of edge planes to that of basal planes. In our case, the specific C$_{\mathrm{dl}}$ of FLG grown on Ni foils was found to be $\sim$2-4 $\mu $F/cm$^{2}$, which is typical of basal plane capacitance, and indicating predominant basal plane coverage in our CVD process. Such samples are amenable to further physical/chemical modifications to create controlled defects which are expected to further enhance C$_{\mathrm{dl}}$. Electrochemical characterization of such ideal geometry in tandem with defects engineering can provide insights into the contribution of graphitic edge planes to charge storage in high surface area carbon electrodes. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B30.00010: Large area single and bilayer graphene with controlled orientation for each layer Lola Brown, Edward Lochocki, Christopher Guti\'errez, Abhay Pasupathy, Kyle Shen, Jiwoong Park The creation and exploration of artificial graphene structures has recently become the focus of great interest. In particular, controlling the interlayer twist angles in multilayer graphene stacks allows modulation of the overall band structure. However, producing such a structure remains difficult due to the random distribution of twist angles in as-grown samples. Here we report a novel way for creating large area graphene stacks with a pre-determined twist angle. We first grow single layer graphene whose orientation is aligned over a few cm length scale on copper foil. The overall angle alignment of the graphene is confirmed using low energy electron microscopy (LEED) and transmission electron microscopy techniques. Since the graphene is well aligned over a few centimeters, we can create large area graphene stacks with known twist angle by transferring these graphene layers while controlling the orientation of each layer during transfer. We confirm that the layers are coupled by probing the resulting band structure using angle resolved photoemission spectroscopy (ARPES), and examining their interlayer optical resonance features using spatially resolved hyperspectral (DUV-Vis-NIR wavelengths). This new method is scalable, and controllable and thus paves the way to explore and exploit the novel properties of two-dimensional crystals in artificial stacks with controlled interlayer structures. [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B30.00011: Stacking Faults and Topological Kink States in Bilayer Graphene Adam Tsen, Dennis Wang, Jiwoong Park, Abhay Pasupathy, Philip Kim For bilayer graphene, the lowest energy configuration consists of two mirror-symmetric stacking orders (AB and BA), which are connected by a lattice translation. In large-area bilayer systems grown by chemical vapor deposition, domains of both stacking configurations have been observed with transmission electron microscopy (TEM), and the boundaries were found to form by the continuous strain of one layer with respect to the other. Here, we perform similar TEM measurements on bilayer graphene prepared by mechanical exfoliation and observe identical stacking faults. These structures may present important ramifications for the electronic properties of such systems. In particular, they are predicted to support topologically protected, gapless kink states, and so their presence may explain the difficulty in opening a substantial transport gap in bilayer graphene even under large electric fields. We also present preliminary transport measurements on individual stacking faults resolved by TEM. [Preview Abstract] |
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