Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Q21: Focus Session: Graphene: Bilayers I |
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Sponsoring Units: DMP Chair: Sankar Das Sarma, University of Maryland Room: Portland Ballroom 251 |
Wednesday, March 17, 2010 11:15AM - 11:27AM |
Q21.00001: Probing the scattering mechanism in bilayer graphene by changing dielectric constant Shudong Xiao, Jianhao Chen, Ellen Williams, Michael Fuhrer We have examined the charge carrier transport in bilayer graphene in ultra-high vacuum (UHV) at low temperatures while changing the background dielectric constant through deposition of ice overlayers. Bilayer graphene sheets are mechanically exfoliated on Si/SiO$_{2}$ substrates, and the number of layers is verified by micro-Raman spectroscopy. We controlled the deposition of ice from water vapor dosage down to the sub-monolayer level and observed the effect on the transport properties of pristine bilayer graphene as well as bilayer graphene with added charged impurities (potassium). Changing dielectric constant has a negligible effect on the charge carrier mobility in pristine bilayer graphene, but significantly increases the mobility in bilayer graphene with adsorbed potassium. The results are consistent with pristine bilayer graphene being dominated by short-range disorder rather than charged impurity scattering. [Preview Abstract] |
Wednesday, March 17, 2010 11:27AM - 11:39AM |
Q21.00002: Carrier transport in disordered graphene bilayers Shaffique Adam, Hongki Min, Mark Stiles Biased graphene bilayers are expected to have an excitation gap, and indeed recent optical experiments have demonstrated robust spectrum gaps as large as 200 meV [Zhang et al. Nature (2009); Mak et al. PRL (2009)]. Yet these same experiments showed no evidence of a transport gap i.e. the bilayer graphene remained conducting with a finite minimum d.c. conductivity. In this theoretical work, we study the conductivity of both biased and unbiased graphene bilayers close to the charge neutrality point. We employ a self-consistent Poisson equation to determine the band structure and calculate the conductivity using an effective medium theory that averages over the inhomogeneous carrier density in electron and hole puddles induced by the disorder potential. We discuss how transport through these electron and hole puddles could explain the unusual transport properties. [Preview Abstract] |
Wednesday, March 17, 2010 11:39AM - 11:51AM |
Q21.00003: Measurement of the electronic compressibility of bilayer graphene E. A. Henriksen, J. P. Eisenstein We report on recent measurements of the electronic compressibility in bilayer graphene. The devices consist of a mechanically exfoliated bilayer graphene flake in a dual-gated configuration, having a global back gate from the underlying Si substrate and a lithographically defined top gate. With suitable shielding, an oscillating voltage applied to the back gate will generate corresponding signals in the top gate only via electric fields which penetrate the graphene, thereby allowing a direct measurement of the compressibility of the bilayer [1]. In our experiments, we map this quantity as a function of the back and top gate bias voltages and compare it to similar maps of the graphene sheet resistivity and capacitance. We discuss our results in light of numerical calculations of the underlying band structure as well as recent theoretical predictions. [1] J. P. Eisenstein, L. N. Pfeiffer, K. W. West, Phys. Rev. B 50, 1760 (1994). [Preview Abstract] |
Wednesday, March 17, 2010 11:51AM - 12:27PM |
Q21.00004: Screening and transport of bilayer graphene Invited Speaker: We present the screening function of bilayer graphene (BLG) which is calculated within the random phase approximation, and compare our results with the corresponding single layer graphene. We use the screening function to calculate transport properties of bilayer graphene. Even though the charged Coulomb impurity invariably present in the graphene environment is the most dominant scattering source in single layer graphene, the scattering strength of charged Coulomb impurity in BLG is significantly reduced due to the enhancement of screening. Due to the reduction of Coulomb scattering other scattering sources (for example, neutral short-range impurity), which are negligible in single layer graphene, play more significant role in BLG transport. We find that the calculated density dependent conductivity with proper BLG screening agrees well with current experimental results. We also present the temperature dependent conductivity and thermopower of bilayer grapheme. The purely electronic temperature dependence of our theory arises from two independent mechanisms: the explicit temperature dependence of the finite temperature screening and the finite temperature energy averaging of the transport scattering time. We also present the transport of bilayer graphene in the presence of in-plane magnetic field. We take into account the magnetic field induced spin polarization and the change of the screening behaviour due to the polarization. [Preview Abstract] |
Wednesday, March 17, 2010 12:27PM - 12:39PM |
Q21.00005: Observation of the Kohn anomaly near the K point of bilayer graphene Daniela Mafra, Leandro Malard, Steve Doorn, Han Htoon, Johan Nilsson, Antonio Castro Neto, Marcos Pimenta Graphene systems exhibit a strong electron-phonon coupling at special points in the Brillouin zone, that softens the phonon energy and gives rise to kinks in the phonon dispersion which are called Kohn anomaly. The electron-phonon coupling is expected to be strong at the K point, but Raman experiments in graphene systems performed with visible light cannot probe phonons near the K point. This work presents a resonance Raman investigation of AB-stacked bilayer graphene using laser lines in the near infrared and visible range. The Kohn anomaly for both symmetric and anti-symmetric phonons was evidenced, and our results show the importance of considering higher renormalization terms such as electron-electron interactions to correctly describe the phonon dispersion near the K point, confirming the theoretical predictions by Lazzeri et al. [1]. [1] M. Lazzeri et al., Phys. Rev. B. 78, 081406(R) (2008). [Preview Abstract] |
Wednesday, March 17, 2010 12:39PM - 12:51PM |
Q21.00006: Fermi velocity renormalization in twisted graphene layers A. Luican, G. Li, E.Y. Andrei, J.M.B. Lopes dos Santos, A.H. Castro Neto, A. Reina, J. Kong, R.R. Nair, K.S. Novoselov, A.K. Geim A twist between stacked graphene layers produces a super-lattice which for certain rotation angles gives rise to Moir\'{e} patterns. These patterns are often seen in STM images, but their effect on the electronic properties is not fully understood. ~Using scanning tunneling microscopy and spectroscopy, we obtain direct evidence for the electronic structure of twisted graphene layers. The samples were suspended membranes of CVD grown graphene which contain areas with various rotation angles. We find that the density of states on twisted layers develops two Van Hove singularities that flank the Dirac point [1] at an energy that is proportional to the twist angle. In the presence of a magnetic field the density of states develops quantized Landau levels (LL) characteristic of massless Dirac fermions. From the energy and field dependence of the LL sequence we obtain the Fermi velocity and find that it is renormalized by an amount that depends on the angle of rotation. These results are compared with theoretical predictions. [1] G. Li et al, Nature Physics (2009)~doi:10.101038/NPHYS1463 [Preview Abstract] |
Wednesday, March 17, 2010 12:51PM - 1:03PM |
Q21.00007: Landau level crossing and charging instabilities in a double-gated graphene bilayer L.M. Zhang, M.M. Fogler, D.P. Arovas, F. Guinea We show that the ``Mexican-hat'' band structure of a graphene bilayer in zero magnetic field leads to multiple Landau level (LL) crossings in a finite field. We present an intuitive picture of this phenomena based on a semiclassical quantization and explain the role of the Berry phase therein. We propose that the LL crossings would produce distinct experimental signatures, such as bistability, hysteresis, and transport anisotropy due to quantum Hall Ising ferromagnetism and the domain formation. A two-gate experimental setup for independent control of the band structure and chemical potential is proposed. The corresponding electrostatic problem is solved self-consistently and the LL spectrum is computed numerically. In ultraclean samples the nonlinearity of the electrostatic equations may result in bistability even in the absence of quantum Hall ferromagnetism. It would show up every time a new LL is populated. [Preview Abstract] |
Wednesday, March 17, 2010 1:03PM - 1:15PM |
Q21.00008: Curvature-induced p-n junction and spin-orbit interaction effects in bilayer graphene Yogesh Joglekar, Avadh Saxena A non-relativistic quantum particle on a two-dimensional curved surface experiences a surface-geometry induced attractive potential and an additional spin-orbit interaction that are both characterized by the principle curvatures $(\kappa_1,\kappa_2)$ at a given point. With bilayer graphene sheets in mind, we obtain the geometric potential $V_G (\kappa_1,\kappa_2)$ and corrections to the spin-orbit interaction $H_{so}(\kappa_1,\kappa_2)$ for several surface shapes. The geometric potential suppresses the local Fermi energy. By estimating the value for this potential, we show that in zero-gap materials surface-curvature will provide a novel avenue to create p-n junctions and, in general, to control local electronic properties. A similar analysis is carried out for surface-curvature correction to the spin-orbit coupling and its consequences. [Preview Abstract] |
Wednesday, March 17, 2010 1:15PM - 1:27PM |
Q21.00009: Transport in dually-gated suspended bilayer graphene devices in electric and magnetic fields R. Thomas Weitz, Monica T. Allen, Ben E. Feldman, Jens Martin, Amir Yacoby The layer pseudospin of bilayer graphene can be controlled by applying an electric field E across the flake. We demonstrate control over this pseudospin in suspended bilayer graphene devices with suspended top gates. At zero magnetic field B, we observe a significantly larger increase in resistance at the charge neutrality point with growing E than had been reported before, indicative of the high sample quality. At finite B, the 8-fold degeneracy of the lowest Landau level is lifted due to electron-electron interactions [1]. The strength and nature of these symmetry broken filling factors are found to be depended on E. The nu=1 and 2 states can be enhanced with E. In the nu=0 state phase transitions between two insulating phases are observed. The position of this transition in the E-B plane is marked by an increased conductance and depends on the relative strengths of B and E. [1] B. E. Feldman et al. Nature Physics [Preview Abstract] |
Wednesday, March 17, 2010 1:27PM - 1:39PM |
Q21.00010: Quantum Hall Effect in Dual-Gated Graphene Bilayers with Tunable Layer Density Imbalance Seyoung Kim, Emanuel Tutuc Recent quantum Hall effect studies on graphene bilayers, consisting of two graphene monolayers with Bernal stacking, revealed the existence of unique chiral quasi particles with 2$\pi $ Berry's phase. Here, we study the magnetotransport properties of high mobility dual-gated graphene bilayers in the quantum Hall regime. The dual-gated device geometry employed here enables \textit{independent} control of the total carrier density and density imbalance between two layers. At finite carrier layer density imbalance, we observe the emergence of a quantum Hall state (QHS) at filling factor \textit{$\nu $ = 0} evinced by a very large longitudinal resistance ($\rho _{xx})$, a finding consistent with the opening of an energy band-gap between the electron and hole bands at finite transverse electric fields. Interestingly, the $\rho _{xx}$ measured at \textit{$\nu $ = 0} decreases at high magnetic fields, indicating a suppression of the \textit{$\nu $ = 0} QHS. This observation can be explained by a loss of charge screening of the individual layers, and a reduction of the transverse electric field induced band-gap in high magnetic fields. [Preview Abstract] |
Wednesday, March 17, 2010 1:39PM - 1:51PM |
Q21.00011: Mapping the Dirac point in gated bilayer graphene Aparna Deshpande, Wenzhong Bao, Zeng Zhao, Chun Ning Lau, Brian LeRoy Bilayer graphene is a unique zero band gap semiconductor where the band gap can be tuned by applying an electric field or by chemical doping. We have carried out scanning tunneling microscopy and spectroscopy measurements on exfoliated bilayer graphene on SiO$_{2}$ at 4.5 K. Imaging shows the characteristic triangular lattice for the bilayer and modulations due to the SiO$_{2}$ substrate. Using a back gate, an electric field is applied perpendicular to the plane of the bilayer. Tunneling spectroscopy measurements reveal the band gap changing due to the electric field. Also, we observe a linear shift of the Dirac point with gate voltage as expected for a bilayer with a quadratic dispersion relation.~ Based on the variation of the band gap and shift of the Dirac point, we have estimated the effective mass of the charge carriers in the bilayer to be 0.023m$_{e}$. Our investigation demonstrates the ease of band gap tunability which gives bilayer graphene an edge over monolayer graphene for device applications. [Preview Abstract] |
Wednesday, March 17, 2010 1:51PM - 2:03PM |
Q21.00012: Ab initio study of hot carrier lifetime in graphene and bilayer graphene Cheol-Hwan Park, Feliciano Giustino, Catalin Spataru, Marvin Cohen, Steven Louie The lifetime of charge carriers through inelastic scattering processes determines transport properties of electronic devices operating at a high source-drain bias voltage at which the inelastic carrier mean free path is much shorter than the elastic one. Therefore, knowing the carrier lifetime arising from inelastic scattering processes is an important step towards the electronic device applications. We present a first-principles calculation of the carrier lifetime in graphene and bilayer graphene considering both electron-electron and electron-phonon interactions. We also compare our calculated results with recent ultrafast pump-probe optical and angle-resolved photoemission spectroscopy measurements on graphene. The results from these two kinds of experiments appear to contradict with each other. [Preview Abstract] |
Wednesday, March 17, 2010 2:03PM - 2:15PM |
Q21.00013: Competition between Long-Range and Short-Range Scattering in Bilayer Graphene Lain-Jong Li, Wenjing Zhang We introduce positively charged impurities to a BLG transistor with dimethylformamide vapor soaking, where it becomes stably n-doped after electrical annealing. Subsequent exposure to moisture results in positive shift of Dirac point and the increase in electron and hole mobility, suggestive of weakened long-range Coulomb scattering. Once these charged impurities are screened to a low concentration, the electron mobility starts to decrease but hole mobility increases, indicating that short-range scatters are positively charged in nature and therefore first affect the n-channel. Further exposure to moisture leads to the decrease in hole but increase in electron mobility, suggesting that negatively charged short-range scatters govern the p-channel. The asymmetric behaviors for p- and n-conductance is likely related to the competition between long- and short-range scatters, where the short-range scatters are suggested to be resulted from the interaction between BLG and moisture (H+ and OH- adsorbates). [Preview Abstract] |
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