Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P33: Transport in Graphene |
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Sponsoring Units: DCMP Chair: Jinglei Ping, University of Pennsylvania Room: 296 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P33.00001: Unsaturated drift velocity of monolayer graphene seong chu Lim, Hee Jun Shin, Jaesu Kim, Sung Ho Kim, Homin Choi, Sahnghyub Lee, Young Hee Lee, Joo-Hiuk Son Despite terahertz (THz) high electric field E$_{\mathrm{THz}}$ \textgreater 70 kV/cm, at which optical phonons can be emitted by hot electrons, graphene heating by energetic carriers can be controlled by adjusting the electrostatic doping concentration or E$_{\mathrm{F}}$. For E$_{\mathrm{F}}$ \textgreater \textgreater k$_{\mathrm{BT}}$, where k$_{\mathrm{B}}$ is the Boltzmann constant and T is the temperature, the electron--phonon scattering is enhanced because of large available phase space, resulting in a significant increase in the optical phonon temperature. However, for E$_{\mathrm{F}}$ \textless \textless k$_{\mathrm{BT}}$, electron--phonon scattering is suppressed because of the diminishing density of states at the Dirac point. Therefore, the carriers are kept accelerating by E$_{\mathrm{THz}}$ without losing their energy. This contributes to the drift velocity of the carriers at the concentration n $=$ 7.3 × 10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$ comparable to the Fermi velocity without heating the graphene lattice on Si substrate at 300 K. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P33.00002: Scanning SQUID Microscopy of Electronic Transport in Graphene George M. Ferguson, Brian T. Schaefer, Lei Wang, Colin Clement, David H. Low, Long Ju, Paul L. McEuen, James P. Sethna, Katja C. Nowack Graphene is a playground for exploring electronic transport phenomena in two-dimensional electron systems. Using scanning superconducting quantum interference device (SQUID) microscopy, we can map out the magnetic field generated by electron flow and reconstruct the two-dimensional current density in graphene devices. Imaging current may allow us to explore unconventional transport phenomena including electronic focusing in graphene p-n junctions and viscous electron flow. Furthermore, magnetization from valley accumulation may be directly imaged in bilayer graphene devices. We will report on our progress imaging these phenomena as well as new methods we are implementing to perform the current reconstruction with optimal spatial resolution. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P33.00003: Devices for investigating low temperature electronic transport in graphene under surface acoustic wave irradiation Adrian Nosek, Alicia Lopez Paniagua, Jose Flores, Marc Bockrath Surface acoustic waves can be generated by an interdigital transducer on a piezoelectric material driven by an ac voltage. When exposed to surface acoustic waves, a tunable acoustic current has recently been realized by changing the charge carrier density on large scale CVD graphene devices using an electrolytic gate in aqueous solution [1]. Here we present the fabrication of devices to investigate the effects of such waves on the low-temperature transport properties of graphene using a Hall bar structured graphene-hBN heterostructure on a quartz substrate. Our latest results will be discussed. [1] Okuda S. et al., Acoustic carrier transportation induced by surface acoustic waves in graphene in solution, Appl. Phys. Express 9, 045104 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P33.00004: Non-Monotonic Temperature Dependence of Coulomb Drag Peaks in Graphene Derek Ho, Indra Yudhistira, Ben Yu-Kuang Hu, Shaffique Adam Coulomb drag is a direct measurement of the electron-electron interactions between two electronic layers. Graphene is a versatile electronic material with a high-degree of tunability opening up regimes that were not previously accessible. All previous theoretical studies of graphene Coulomb drag away from charge neutrality assume a spatially homogeneous carrier density which gives a peak in the Coulomb drag that decreases with temperature in contradiction to available experimental results. In this work [1], we develop an effective medium theory for Coulomb drag and show that including spatial inhomogeneity in the carrier density gives rise to a non-monotonic temperature dependence of the drag peaks that is in quantitative agreement with experimental data. Our results also show that at double-charge neutrality, there is a large negative momentum drag for correlated density fluctuations that competes with energy drag and is also non-monotonic with temperature. Lastly, we show that correlations between the density fluctuations in the two layers give rise to a violation of Onsager reciprocity between the active and passive layers. [1] D.~Y.~H.~Ho, I.~Yudhistira, B.~Y.-K.~Hu, and S.~Adam, arXiv: 1611.03089 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P33.00005: Stability of Dirac Liquids with Strong Coulomb Interaction Igor Tupitsyn, Nikolay Prokof'ev We develop and apply the Diagrammatic Monte Carlo technique to address the problem of stability of the Dirac liquid state (in a graphene type system) against strong Coulomb interaction. So far, all attempts to deal with this problem in the field-theoretical framework were limited either to perturbative or RPA treatments. Our technique allows to deal with long-range interactions in a fully self-consistent, approximations free, manner and obtain final results with controlled accuracy by computing vertex corrections from higher-order skeleton diagrams. We establish the renormalization group flow of the effective Coulomb coupling constant and unambiguously show that with increasing the system size L (up to ln(L)\textasciitilde 40), the coupling constant always flows towards zero; i.e. the two dimensional Dirac liquid is an asymptotically free T$=$0 state. Our approach is general and can be applied to any graphene-type system with arbitrary dispersion relation featuring Dirac cones, both doped and undoped, and with arbitrary shape of the interaction potential. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P33.00006: Unusual renormalization group (RG) flow in strongly-disordered monolayer epitaxial graphene Chieh-Wen Liu, Lung-I Huang, Yanfei Yang, Randolph E. Elmquist, Shun-Tsung Lo, Fan-Hung Liu, Chi-Te Liang We present a magneto-transport study on highly disordered, large- area monolayer epitaxial graphene grown on SiC. Quantum Hall-like characteristics are observed even when the sample is in the strongly insulating regime in the sense that the longitudinal resistivity decreases with increasing temperature. Interestingly, the temperature ($T)$-driven (renormalization group (RG)) flow diagram shows unusual features- a cusp-like structure close to ($\sigma_{xy} =\sigma_{xx} ={e^{2}} \mathord{\left/ {\vphantom {{e^{2}} h}} \right. \kern-\nulldelimiterspace} h)$ where the unstable point in the context of modular group symmetry is predicted. Instead of a quantum phase transition characterized by a $T$-independent point, a magnetic-field-independent crossing is observed at diagonal conductivity $\sigma_{xx} \sim {e^{2}} \mathord{\left/ {\vphantom {{e^{2}} h}} \right. \kern-\nulldelimiterspace} h$. Our new experimental results cannot be explained by conventional modular group symmetry and thus suggests further theoretical studies are required. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P33.00007: Quantum transport modeling of magnetic focusing in graphene p-n junctions Samuel LaGasse, Ji Ung Lee We demonstrate a new model for studying transverse magnetic focusing experiments in graphene \textit{p-n} junctions, using quantum transport methods. By including a combination of dephasing edge contacts and Landauer-B{\"u}ttiker multi-terminal analysis, we observe an exceptional degree of agreement with recent experimental data from Chen et al (\textit{Science}, 2016), without fitting parameters. Our model captures both the resonance and off-resonance non-local resistances from experiment. Our calculated quantum transmission functions indicate the origin of the sign of the measured resistance. Spatially resolved flow maps of local particle current density are used to explain our results and rapidly convey the mechanisms of device operation. Mode-by-mode analysis of transport shows the complex interplay between semi-classical skipping orbits and quantum effects. Quantum interference, \textit{p-n} filtering, and edge scattering are clearly seen. Additionally, we are able to explain subtle features from experiment, such as the $p$-$p^-$ to $p$-$p^+$ transition and the second \textit{p-n} focusing resonance. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P33.00008: Wave Packets Dynamics in Graphene with Gaussian Bump. Ramon Carrillo-Bastos, Marysol Ochoa, Saul Zavala, Francisco Mireles In monolayer graphene, out-of-plane strain-induced centro-symmetric deformations can be described as a six-folded pseudo-magnetic field in the low energy approximation [1]. It has been shown that such pseudo-magnetic field profiles can cause the formation of bound states [2], valley splitting, valley polarized, and valley filtering [3-5] of great interest in the realm of valleytronics and strain engineering. In this work, we study the dynamics of wave packets in graphene sheets that experiences local out-of-plane strain deformations in the form of Gaussian bumps. The study is carried out within the continuum Dirac Hamiltonian for graphene employing the time splitting spectral method for the time evolution operator. We present numerical results for different initial conditions (angle and energy of incidence) that shows wave packet focusing and beam splitting effects that can be exploited in the implementation of valleytronic devices. [1] Yang et al., J. Appl. Phys. 112, 073710; [2] Carrillo-Bastos et al. Phys. Rev. B 90, 041411R; [3] Settnes, et al., arXiv:1608.04569; [4] Milovanovic and Peeters, arXiv:1610.09916; [5] Carrillo-Bastos et al., Phys. Rev. B 94, 125422. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P33.00009: Nonlinear optical conductivity and subharmonic instabilities of graphene in a strong electromagnetic field Zhiyuan Sun, Dimitri Basov, Michael Fogler We study theoretically the second-order nonlinear optical conductivity $\sigma^{(2)}$ of graphene as a function of frequency and momentum. We distinguish two regimes. At frequencies $\omega$ higher than the temperature-dependent electron-electron collision rate $\gamma^{-1}_{ee}$, the conductivity $\sigma^{(2)}$ can be derived from the semiclassical kinetic equation. The calculation requires taking into account the photon drag (Lorentz force) due to the ac magnetic field. In the low-frequency hydrodynamic regime $\omega \ll \gamma^{-1}_{ee}$, the nonlinear conductivity has a different form and the photon drag effect is suppressed. As a consequence of the nonlinearity, a strong enough photoexcitation can cause spontaneous generation of collective modes in a graphene strip: plasmons in the high-frequency regime and energy waves (demons) in the hydrodynamic one. The dominant instability occurs at frequency $\omega / 2$. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P33.00010: Hyperbolic cooling of graphene Zener-Klein transistors Wei Yang, Simon Berthou, Xiaobo Lu, Emmanuel Baudin, Quentin Wilmart, Anne Denis, Michael Rosticher, Takashi Taniguchi, Kenji Watanabe, Gwendal Feve, Jean-Marc Berroir, Guangyu Zhang, Christophe Voisin, Bernard Placais Engineering of cooling mechanisms is a bottleneck in nanoelectroniscs. In graphene/hBN transistors, Wiedemann-Frantz cooling and supercollision-cooling prevails, and the latter is suppressed in high mobility graphene/hBN samples and substituted by the super-Planckian radiation of hyperbolic phonon-polaritons (HPPs) in the hBN substrate. Using electrical Joule heating and sensitive noise thermometry in several GHz range we report on prevailing HPP cooling in the upper Reststrahlen-band of hBN at high bias. We predict and observe its activation threshold, along with interband Zener-Klein tunneling. HPP cooling is able to evacuate at least several GW/m2 to the bottom gate, resulting in an unusual clipping of electronic temperature. As a scattering counterpart, HPPs of the lower Reststrahlen-band control current saturation at high doping. The combination of both mechanisms promotes graphene/hBN as a valuable nanotechnology for applications in the high power devices and radio frequency electronics. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P33.00011: Quantum wires and waveguides formed in graphene by strain Dawei Zhai, Yong Wu, Marc Bockrath, Nancy Sandler The effects of strain on the electronic and transport properties of graphene have triggered intensive theoretical investigations, which showed that graphene valley filters might be achieved by strain engineering. However, little experimental progress has been made in implementing these ideas. Here we report transport studies of graphene on top of hexagonal boron nitride with out-of-plane strained folds. Differential conductance measurements across the linear strain region reveal distinct transport regimes as the gate voltage is changed. For some samples, Coulomb blockade-characteristic of quantum dot behavior- is observed, while others show Fabry-Perot type resonances with higher transmission. The data is consistent with results from a Dirac model including pseudo-scalar and pseudo-magnetic fields produced by a finite length strained fold. Theoretical results for the dependence of energy level spacing on the geometric factors of the fold, as well as on the incident energy and angle of carriers for both types of samples are presented. Because these devices constitute the first step towards a practical realization of valley filters with graphene, implications for valley polarization properties of transmitted currents and appropriate modifications for device setups are discussed. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P33.00012: Electrons at the monkey saddle: a multicritical Lifshitz point Alex Shtyk, Garry Goldstein, Claudio Chamon We consider 2D interacting electrons at a monkey saddle with dispersion $\propto p_x^3-3p_xp_y^2$. Such a dispersion naturally arises at the multicritical Lifshitz point when three van Hove saddles merge in an elliptical umbilic elementary catastrophe, which we show can be realized in biased bilayer graphene. A multicritical Lifshitz point of this kind can be identified by its signature Landau level behavior $E_m\propto (Bm)^{3/2}$ and related oscillations in thermodynamic and transport properties, such as de Haas-van Alphen and Shubnikov-de Haas oscillations, whose period triples as the system crosses the singularity. We show, in the case of a single monkey saddle, that the non-interacting electron fixed point is unstable to interactions under the renormalization group flow, developing either a superconducting instability or non-Fermi liquid features. Biased bilayer graphene, where there are two non-nested monkey saddles at the $K$ and $K^\prime$ points, exhibits an interplay of competing many-body instabilities, namely $s$-wave superconductivity, ferromagnetism, and spin- and charge-density wave. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P33.00013: Electron transport in reduced graphene oxides in high electric field. Wen-Bin Jian, Jian-Jhong Lai, Sheng-Tsung Wang, Rui-Wen Tsao, Min-Chia Su, Wei-Yu Tsai, Baruch Rosenstein, Xufeng Zhou, Zhaoping Liu Due to a honeycomb structure, charge carriers in graphene exhibit quasiparticles of linear energy-momentum dispersion and phenomena of Schwinger pair creation may be explored. Because graphene is easily broken in high electric fields, single-layer reduced graphene oxides (rGO) are used instead. The rGO shows a small band gap while it reveals a graphene like behavior in high electric fields. Electron transport in rGO exhibits two-dimensional Mott's variable range hopping. The temperature behavior of resistance in low electric fields and the electric field behavior of resistance at low temperatures are all well explained by the Mott model. At temperatures higher than \textasciitilde 200 K, the electric field behavior does not agree with the model while it shows a power law behavior with an exponent of 3/2, being in agreement with the Schwinger model. Comparing with graphene, the rGO is more sustainable to high electric field thus presenting a complete high-electric field behavior. When the rGO is gated away from the charge neutral point, the turn-on electric field of Schwinger phenomena is increased. A summary figure is given to present electric field behaviors and power law variations of resistances of single-layer rGO, graphene, and MoS$_{\mathrm{2}}$. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P33.00014: Spatially resolving density-dependent screening around a single charged atom in graphene Dillon Wong, Fabiano Corsetti, Yang Wang, Victor Brar, Hsin-Zon Tsai, Qiong Wu, Roland Kawakami, Alex Zettl, Arash Mostofi, Johannes Lischner, Michael Crommie Due to the relativistic nature of its charge carriers, graphene has very unique screening properties. We have explored the screening of an individual Ca ion as a function of Dirac quasiparticle density in graphene by combining scanning tunneling microscopy (STM) with a gate-tunable graphene device. We find that the screening length in graphene is very gate-tunable, decreasing with increasing charge carrier density. Comparing our experimental results to tight-binding calculations provides new insight into electron-electron interactions in graphene, as well as the fundamental behavior of relativistic fermions in the presence of charged impurities. [Preview Abstract] |
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