Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A46: Bilayer Graphene: Stacking, Transport and Magnetics |
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Sponsoring Units: DCMP Chair: Jun Zhu, Pennsylvania State University Room: Mile High Ballroom 4E |
Monday, March 3, 2014 8:00AM - 8:12AM |
A46.00001: Spontaneous Domain Walls in Bilayer Graphene Xiao Li, Fan Zhang, Qian Niu, Allan MacDonald Intrinsic bilayer graphene is susceptible to a family of gapped broken symmetry states in which each spin-valley flavor spontaneously polarizes between layers. These states are close in energy and can coexist above a critical temperature, separated by domain walls (DW), the collective and topological excitations. Our simulation further reveals three important facts. (i) The critical temperature of DW nucleation is smaller than the homogenous mean-field estimation and determines the phase transition. (ii) A Ginzburg-Landau theory can be built to characterize the DWs. (iii) Each DW leads to a surprising nonlinear transverse pseudospin response, which can be explained by the presence of zero modes propagating along the DW. We discuss the rich classification of these spontaneous DWs based on their distinct zero modes which may form unusual Luttinger liquids. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A46.00002: Critical Behavior of Four-Terminal Junctions of Bilayer Graphene Domain Walls Benjamin Wieder, Fan Zhang, Charles Kane Bilayer graphene in a perpendicular electric field can host domain walls between regions of reversed field direction or interlayer stacking. The gapless modes propagating along these domain walls, while not strictly topological, nevertheless have interesting physical properties, including valley-momentum locking. A junction where four domain walls meet forms the analogue of a quantum point contact. We study theoretically the critical behavior of this junction near the pinch-off transition, which is controlled by a non-trivial quantum critical point. At low temperatures, the transition sharpens and the conductance is described by a universal scaling function, which we compute. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A46.00003: Disorder-tuned selection of ordered state in bilayer graphene Junhua Zhang, Rahul Nandkishore, Enrico Rossi The nature of the symmetry-broken state driven by interaction in bilayer graphene (BLG) has attracted a lot of interest. Theoretical studies predict various possible ordered phases as the candidate for the ground state of BLG. To identify what instability is the most favorable in BLG, a number of experiments have been performed by several groups. However, there is no consensus: some experiments show evidence for a fully gapped state while others seem more consistent with a nematic state. By exploring the influence of disorder on a variety of competing ordered states, we find that the pair breaking effect due to disorder varies among the candidate phases, giving rise to different amount of suppression on the mean-field transition temperatures. This suggests a simple and natural scenario to resolve the discrepancy between experimental observations. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A46.00004: Electronic transmission through AB-BA domain boundary in bilayer graphene Mikito Koshino We study the electron transmission through the domain boundary on bilayer graphene separating AB and BA stacking regions, which was recently found in the experiment. We calculate the electron transmission probability as a function of the electron energy and the incident angle, for several specific boundary structures. The transmission strongly depends on the crystallographic direction of the boundary and also on the atomic configuration inside. At the low energy, the boundary is either insulating or highly transparent depending on the structure. In insulating cases, the transmission sharply rises when the Fermi energy is increased to a certain level, suggesting that the electric current through the boundary can be controlled by the field effect. The boundary parallel to the zigzag direction generally have different transmission properties between the two different valleys, and this enables to generate the valley polarized current in a certain configuration. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A46.00005: Electron-electron interaction induced effective mass suppression in bilayer graphene Jing Li, Ke Zou, Adam Stabile, Donald Seiwell, Jun Zhu The effective mass of carriers m* captures fundamental properties of a material. In a two-dimensional electron system, the electron-electron (e-e) interaction manifests in the renormalization of m*. Extending previous studies[1] to lower carrier densities, where the interaction effect is expected to be stronger, we present precision measurements of the electron and hole effective mass m$_e$* and m$_h$* in high-quality ($\mu\sim$30,000cm$^2$/Vs) hexagonal boron nitride supported bilayer graphene using temperature-dependent Shubnikov-de Hass oscillations. Our measurements probe carrier densities down to 2$\times$10$^{11}$/cm$^2$. Comparison to tight-binding bands and previous data shows excellent agreement at carrier densities above 5$\times$10$^{11}$/cm$^2$, where m$_e$* and m$_h$* can be well described by a renormalized Fermi velocity of v$_F$= 1.11$\times$10$^6$m/s. At lower carrier densities, m$_h$*continuously decreases from the tight-binding band value, reaching m$_h$*=0.0234m$_e$ at n= 2$\times$10$^{11}$/cm$^2$. This corresponds to a suppression of 30\% and an increased v$_F$=1.37$\times$10$^6$m/s. The deviation is much smaller for electrons. We compare our results with theory and discuss its implications. [1] K. Zou, X. Hong, and J. Zhu, Phys. Rev. B 84, 085408 (2011). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A46.00006: Electric-field-dependent electronic structure of graphene bilayer: from the Bernal stacking to the unconventional orthorhombic stacking Gunn Kim, Changwon Park, Mina Yoon In this presentation, we report the electronic properties of bilayer graphene structures with various stackings, which can be formed, for instance, during the structural transition from graphite-to-diamond at high pressure, or at boundaries of stacking domains or at diamond surfaces. We performed ab initio calculations and the Wannier interpolations for accurate two-dimensional band structure with extremely dense (1600$\times$1600) $k$-point grid. Using tight-binding parameters obtained from maximally localized Wanneir function analysis, we also constructed the effective Hamiltonian for the graphene bilayer with various stacking. The overall electronic structures can be described by the relative shift and the coupling of two Dirac cones, depending on their stacking geometry. Our results reveal that external electric field is another parameter to control the electronic properties of the bilayer-graphene. In particular, the external fields significantly enhance the coupling of two Dirac cones, which result in additional or new van Hove singularities near the Fermi level. We compared the electronic structure of the orthorhombic stacking with those of AA and AB stackings. Our study may provide a deeper understanding of sliding effects of multilayer graphene. This work was supported by the Priority Research Center Program (2011-0018395) and the Basic Science Research Program through MEST/NRF (2013R1A1A2009131). This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A46.00007: Electronic Transport in Twisted Bilayer Graphene Superlattices Jason Luo, Javier Sanchez-Yamagishi, Sang Hyun Choi, Kenji Watanabe, Pablo Jarillo-Herrero Twisted bilayer graphene is the ultimate limit of a bilayer 2DEG, where two graphene layers are stacked with an interlayer distance of only 0.34nm. The interlayer tunnel coupling can be continuously tuned by twisting the two layers, leading to different physics in the small and large twist angle limits. At small twist angles, the two layers form a large superlattice unit cell and hybridization of the layers leads to low-energy van Hove singularities in the electronic spectrum. We report transport measurements of high-quality twisted bilayer graphene in the low twist angle regime where interlayer interactions have drastic effects on electronic properties. We demonstrate that in this regime, where the magnetic length scale is comparable to the superlattice constant, we can observe Hofstadter's butterfly physics at low magnetic fields. At zero magnetic field, we also observe a strong departure from the typical monolayer graphene transport properties. We discuss the possible roles that interlayer tunneling and electron-electron interactions play in our observed phenomena as well as the effect of interlayer electric field. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A46.00008: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:36AM - 9:48AM |
A46.00009: Magnetic breakdown in twisted bilayer graphene Chi-Ken Lu, H.A. Fertig Recently, superlattice patterns on graphene sheets have become available in both twisted bilayer systems and graphene on BN substrates. These systems are interesting due to the recent demonstration that in magnetic fields they host an observable Hofstadter spectrum, and that (for TBG) they host low energy saddle points(SPs) in their spectra which can lead to interesting many-body instabilities. We consider the role that these SPs play in weak magnetic fields, where the crossover from Landau level behavior to Hofstadter behavior in the spectrum is controlled by them. This phenomenon is a realization of magnetic breakdown, in which semiclassical trajectories change their topology when the energy passes through such SPs. In the TBG, we find that a description fully incorporating the magnetic symmetries is only possible if one doubles the Brillouin zone. This leads to a multiplicity of semiclassical quantization conditions on orbits above the saddle points, beyond the usual interior flux quantization, allowing the richness of the Hofstadter spectrum to become apparent at these higher energies. Possible experimental implications, including cyclotron resonance, magnetic susceptibility, and creation of open orbits (with accompanying metal-insulator transition) will be discussed. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A46.00010: Tunneling of dressed electrons in bilayer graphene Dipendra Dahal, Godfrey Gumbs, Andrii Iurov, Danhong Huang Closed-form analytic expressions have been derived for electron dressed states, resulting from the interaction between Dirac electrons in bilayer graphene and circularly polarized light. The dressed states in bilayer graphene exhibit some novel properties, such as left-right polarization asymmetry, which are absent in monolayer-layer graphene. We have investigated the dressed state tunneling through a square electrostatic potential barrier and present a detailed analysis of the way in which the physics involving the special type of zero-transmission Klein paradox in bilayer graphene is modified in the presence of strong electron-photon coupling. Additionally, we have investigated and compared various electron-tunneling behaviors in the presence of a finite-width magnetic barrier for both monolayer and bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A46.00011: Bound state energy of a Coulomb impurity in gapped bilayer graphene: ``Hydrogen atom with a Mexican hat'' Brian Skinner, Boris Shklovskii, Mikhail Voloshin Application of a perpendicular electric field induces a band gap in bilayer graphene, and it also creates a ``Mexican hat'' structure in the dispersion relation. This structure has unusual implications for the hydrogen-like bound state of an electron to a Coulomb impurity. We calculate the ground state energy of this hydrogen-like state as a function of the applied interlayer voltage and the effective fine structure constant. Unlike in the normal hydrogen atom, the resulting wavefunction has many nodes of density even in the ground state. Further, the electron state undergoes ``atomic collapse'' into the Dirac continuum both at small and large voltage. Our results have important implications for the pursuit of a robust, tunable band gap in bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A46.00012: Effect of long-range disorder on competing orders in bilayer graphene Martin Rodriguez-Vega, Christopher Triola, Junhua Zhang, Enrico Rossi Two general classes of spontaneously broken symmetry phases have been proposed for bilayer graphene: a gapped phase and a nematic phase. Some experiments suggest the establishment of a nematic phase whereas others suggest the presence of a gapped phase. In this talk I will present the results of our theoretical study of the effect of long-range disorder on the conditions for the establishment of a nematic or a gapped phase in bilayer graphene. In particular I will discuss the effect of the disorder-induced carrier density inhomogeneities on the properties and robustness of each phase. I will then discuss the relevance of our results for the current experiments. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A46.00013: Possible deconfined critical transition in bilayer graphene Junhyun Lee, Subir Sachdev Deconfined criticality describes a quantum transition between two ordered states with unrelated symmetry, which is not allowed in the Landau-Ginzburg-Wilson framework [1]. Although many numerical studies have shown evidence for deconfined criticality, it has not yet been observed in experiments. We point out that the conductivity measurements in suspended bilayer graphene [2] could imply the presence of such a transition. The phase transition is between two insulating phases and occurs in a finite magnetic field when we tune the electric field, both fields perpendicular to the graphene plane. We argue that in the strong coupling limit, the effective spin Hamiltonian of bilayer graphene suggests a ``N\'{e}el to a kekul\'{e} valence bond solid'' transition. We also present the study of zero-energy states in the VBS vortex near the critical point.\\[4pt] [1] T. Senthil et al, Science 303, 1490 (2004).\\[0pt] [2] R. T. Weitz et al, Science 330, 812 (2010). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A46.00014: Magnetic and Kohn-Luttinger instabilities near a Van Hove singularity: monolayer versus twisted bilayer graphene Jose Gonzalez We report on the many-body instabilities of electrons interacting near Van Hove singularities arising in monolayer and twisted bilayer graphene. It is found that a pairing instability must be dominant over the tendency to magnetic order as the Fermi level is tuned to the Van Hove singularity in the conduction band of graphene. As a result of the extended character of the saddle points in the dispersion, the pairing of the electrons takes place preferentially in a channel of f-wave symmetry, with an order parameter vanishing at the position of the saddle points along the Fermi line. In the case of the twisted bilayers, the dispersion has instead its symmetry reduced down to the $C_3v$ group and, most importantly, it leads to susceptibilities that diverge at the saddle points but are integrable along the Fermi line. This implies that a ferromagnetic instability becomes dominant in the twisted graphene bilayers near the Van Hove singularity, with a strength which is amplified as the lowest subband of the electron system becomes flatter for decreasing twist angle. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A46.00015: Tunable magnetic order in bilayer graphene Jin-Hua Sun, Dong-Hui Xu, Yi Zhou, Fu-Chun Zhang Layered antiferromagnetic spin density wave (LAF) state is one of the plausible ground state of charge neutral Bernal stacked bilayer graphene. Additionally, bilayer system offers a freedom of inducing a shift in the electrochemical potential to two graphene layers, which is proposed to induce half-metallicity into the system. In this talk, we will report the theoretical results on the effect of the electric field on the magnetic order in bilayer grapheme by using mean-field theory and determinant quantum Monte Carlo method. In neutral bilayer graphene, the ground state has layered antiferromagnetism at~weak electric field and undergoes a quantum phase transition to a charge order or possibly different type magnetic ordered states at a high electric field. [Preview Abstract] |
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