Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A40: Graphene Properties 
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Sponsoring Units: DCMP Chair: Qiong Ma, Massachusetts Institute of TechnologyMIT Room: LACC 501C 
Monday, March 5, 2018 8:00AM  8:12AM 
A40.00001: Microscopic theory for electron hydrodynamics in monolayer and bilayer graphene Derek Ho, Indra Yudhistira, Nilotpal Chakraborty, Shaffique Adam

Monday, March 5, 2018 8:12AM  8:24AM 
A40.00002: Hydrodynamic theory of electron transport in doublelayer graphene Hongyi Xie, Matthew Foster, Alex Levchenko We study the transport properties of doublelayer graphene within the hydrodynamic regime, taking into account the slow carrier population imbalance relaxation due to interlayer electron Coulomb interaction and electronopticalphonon scattering. Assuming fast intralayer equilibration we derive the hydrodynamic theory from the Boltzmann equation, incorporating weak quenched disorder, electronphonon scattering, and magnetic field. The system is described by three macroscopic currents carrying electron charge, energy, and carrier population, that are the response of electrochemical field, temperature gradient, and gradient of particlehole imbalance chemical potential. We apply our hydrodynamic theory to the Coulomb drag experiments in graphenehBN heterostructures and focus on the strong drag regime about charge neutrality where the drag resistance behaves anomalously. In the limit of infinite imbalance relaxation of various quantities, our theory reproduces existing drag theories such as those based on the momentum and energy mechanism. We also propose a diagnostics for different transport regimes base on nonlocal drag measurement. 
Monday, March 5, 2018 8:24AM  8:36AM 
A40.00003: Electron hydrodynamics at the boundaries Arthur Barnard, Aaron Sharpe, Simone Fasciati, Kenji Watanabe, Takashi Taniguchi, David GoldhaberGordon When electronelectron interactions dominate in mesoscopic solidstate devices, current flow can remarkably resemble that of hydrodynamic flow in a continuum fluid. Exciting glimpses of this behavior have emerged in recent experiments; in graphene, for example, negative local resistances arise due to viscous sheer forces (Geim, et. al. 2015). While a modified Hallbar geometry—as was used in those experiments—can give us some important insights, we need new device architectures and measurement schema that take control of the boundaries, which become critically important in the hydrodynamic regime. 
Monday, March 5, 2018 8:36AM  8:48AM 
A40.00004: Resonant electronlattice cooling in graphene Jian Feng Kong, Leonid Levitov, Dorri Halbertal, Eli Zeldov Controlling energy flows in solids through switchable electronlattice cooling can grant access to a range of interesting and potentially useful energy transport phenomena. Here we discuss a unique switchable electronlattice cooling mechanism arising in graphene due to phonon emission mediated by resonant scattering on defects in crystal lattice, which displays interesting analogy to the Purcell effect in optics. This mechanism strongly enhances the electronphonon cooling rate, since nonequilibrium carriers in the presence of momentum recoil due to disorder can access a larger phonon phase space and emit phonons more efficiently. Resonant energy dependence of phonon emission translates into gatetunable cooling rates, exhibiting giant enhancement of cooling occurring when the carrier energy is aligned with the electron resonance of the defect. 
Monday, March 5, 2018 8:48AM  9:00AM 
A40.00005: Gateinduced localized states along armchair chain in graphene Luyao Wang, CheYuan Chang, C.S. Chu In this presentation, we show that a gateinduced line potential in the armchairchain direction (along y) on a graphene sheet gives rise to localized states. The generation of these localized states is found once the gateinduced potential V_{0} becomes nonzero. This is demonstrated, for a nonzero k_{y }, by our two findings: (1) that the two valleyassociated evanescent states on one side of the linepotential have their pseudospins lying on the same plane, and their relative pseudospin orientation span the 2π angular space when the energy is scanned across the energy gap for the k_{y}, and (2) that the boundary condition for the localized states, formulated into an operator connecting the two evanescent states, has the operator being transformed into a rotation operator by a nonzero V_{0}, with the rotation axis normal to the plane of the pseudospins. Our results reveal that the formation of the localized states is topological. 
Monday, March 5, 2018 9:00AM  9:12AM 
A40.00006: Validity of the Adiabatic BornOppenheimer Approximation in the TightBinding Model of Graphene Vaibhav Mohanty, Eric Heller The adiabatic BornOppenheimer (ABO) approximation is widely used in molecular and atomic physics to simplify quantum mechanical calculations. For a molecular system, an ABO electronic state can be computed by solving the timeindependent Schrödinger equation (TISE) at several time points for a Hamiltonian that is dependent on the nuclear configuration. We show that this approach breaks down when calculating the time evolution of the singleelectron wavefunction of a finite graphene sheet in the presence of thermal nuclear vibrations. By classically solving for the timedependence of the nuclear coordinates, we construct the electronic Hamiltonian explicitly as a matrix in the nearestneighbor tightbinding approximation. We then calculate an allowed ABO state by solving the TISE at multiple time points. Comparing the ABO state with the true state determined from the timedependent Schrödinger equation, we find that the ABO approximation breaks down in the timescale of the nuclear vibrational period when the ABO state’s timedependent energy approaches avoided crossings. 
Monday, March 5, 2018 9:12AM  9:24AM 
A40.00007: GrapheneBased Active Modulation of Near Field Thermal Radiation Nathan Thomas, Michelle Scherrot, Harry Atwater, Austin Minnich Modern heat switches for thermal regulation of electronics or refrigeration systems require mechanical components, either as pumps to transport a working fluid or as thermally conducting mechanical linkages. Such systems are difficult to apply at sufficiently small length scales and lack the reliability of nonmechanical systems. Here, we theoretically propose and experimentally demonstrate the modulation of heat flow in a solidstate system using nearfield radiative effects. Modulating the carrier concentration in graphene using electrostatic gating enables the radiative flux to another graphene sheet to be actively tuned. Our work highlights the capability for active control of radiative heat flow using electrical biasing of atomically thin films. 
Monday, March 5, 2018 9:24AM  9:36AM 
A40.00008: Layerdecoupling of multilayered graphene by electridebased electron injection Sera Kim, Jongho Park, Sung Wng Kim, Heejun Yang Graphene has been extensively explored due to its promising intrinsic characteristics such as high mobility and thermal conductivity, which inspired applications in nanoscale electrical devices. The Fermi level of graphene that affects its electrical performance can be largely modulated by (chemical) doping. To realize the doping of carriers in graphene, physical deposition of N, B or H molecules on graphene have been tried. However, these studies showed inevitable deformation of graphene lattices, which could be observed as D peak in Raman spectrum. Clean methods for an efficient doping without defects have been demanded. 
Monday, March 5, 2018 9:36AM  9:48AM 
A40.00009: Fieldeffectdriven halfmetallic multilayer graphene Jacopo Baima, Francesco Mauri, Matteo Calandra The realization of modern spintronic devices requires injection of spinpolarized currents. Materials of choice for such devices are halfmetals, namely compounds conducting in one spin channel and insulating in the other, like Heussler alloys and some transition metal oxides. Here, by using firstprinciples calculations, we show that fieldeffect doping of graphene multilayers with rhombohedral stacking induces a perfect halfmetallic behaviour with 100% of spin current polarization already at dopings attainable in conventional field effect transistors with solid state dielectrics. Our work defines a new kind of spintronic devices where the spin state is selected and driven by electric fields. 
Monday, March 5, 2018 9:48AM  10:00AM 
A40.00010: Giant Intrinsic Photoresponse in Pristine Graphene Qiong Ma, Chun Hung Lui, Justin Song, Jian Feng Kong, Yuan Cao, Nityan Nair, Wenjing Fang, Kenji Watanabe, Takashi Taniguchi, Suyang Xu, Jing Kong, Nuh Gedik, Leonid Levitov, Nathaniel Gabor, Pablo JarilloHerrero When the Fermi level matches the Dirac point in graphene, the reduced charge screening can dramatically enhance electronelectron (ee) scattering to produce a strongly interacting Dirac liquid. While the dominance of ee scattering already leads to novel responses such as the electron hydrodynamic flow, further exotic phenomena have been predicted to arise specifically from the unique kinematics of ee scattering in a strongly interacting massless Dirac system, although their experimental evidence remains lacking. Here, by using optoelectronic probes, which are highly sensitive to the kinematics of electron scattering, we uncover a large, previously uncharacterized photocurrent even in the absence of defined pn junctions, which arises uniquely from the strong, unusual electron interaction in chargeneutral graphene. This photocurrent emerges exclusively at the charge neutrality point and vanishes abruptly at nonzero charge densities. It is observed at places with broken reflection symmetry, and is selectively brightened at free graphene edges with sharp bends. Our findings reveal that the photocurrent relaxation is strongly suppressed by a drastic change of fast photocarrier kinematics in graphene when its Fermi level matches the Dirac point. 
Monday, March 5, 2018 10:00AM  10:12AM 
A40.00011: Dynamically Tunable Extraordinary Light Absorption in Monolayer Graphene Alireza Safaei, Sayan Chandra, Michael N. Leuenberger, Debashis Chanda Graphene due to the absence of bandgap and monolayer thickness exhibits very weak light absorption in the visible to IR frequencies, which prevents being an efficient optical material. Here, we show theoretically as well as experimentally that the excitation of cavity coupled surface plasmons directly on graphene enhances absorbance up to 60% with low damping rate in both high and low mobility graphene. This result is based on the principle that the cavity and the surface plasmon concentrate the energy of the incident electromagnetic wave at the location of the graphene sheet. The advantages of graphene is the ease of electrical tunability of plasmon resonance frequency, the high degree of electric field confinement, and the low plasmon damping rate due to the high carrier mobility in graphene. Constructive interference between incident and scattered electric fields strengthens total electric field on the graphene nanomesh by coupling the perforated graphene to an optical cavity. In this work, the effects of carrier mobility and electronphonon coupling have been studied experimentally and theoretically. It is shown that the plasmonphonon coupling leads to the renormalization of the plasmonic dispersion relation. 
Monday, March 5, 2018 10:12AM  10:24AM 
A40.00012: Anderson Localization and Delocalization of Dirac Electrons in Monolayer Graphene in Onedimensional Random Scalar and Vector Potentials Seulong Kim, Kihong Kim We study Anderson localization and delocalization of Dirac electrons in monolayer graphene in onedimensional random scalar and vector potentials theoretically for two different cases. In the first case, the random parts of the scalar and vector potentials are uncorrelated while, in the second case, they are proportional to each other. We calculate the localization length for all values of the disorder strength in a numerically exact manner using the invariant imbedding method and derive analytical expressions for it, which are accurate in the weak disorder region. We also derive the incident angle at which obliquely incident electron waves are completely delocalized. We find that this condition, which includes the ordinary Klein tunneling as a special case, is equivalent to the condition that the effective wave impedance is uniform. In the presence of a vector potential, the delocalization angle can be tuned to nonzero values in contrast to Klein tunneling. We investigate the dependence of the localization length on the disorder strength and find that, in certain cases, the localization length increases monotonically to infinity as the disorder strength increases from zero. We discuss the implications of these results on electron supercollimation and other observable phenomena. 
Monday, March 5, 2018 10:24AM  10:36AM 
A40.00013: Probing interactions in graphene through global and local current noise measurements Trond Andersen, Javier SanchezYamagishi, Bo Dwyer, Kenji Watanabe, Takashi Taniguchi, Hongkun Park, Mikhail Lukin Electronelectron scattering plays an important role in graphene transport, and can even be the dominant scattering mechanism. This gives rise to a wealth of interesting collective phenomena, such as electron hydrodynamic behavior. Since ee interactions inevitably affect currentcurrent correlations, we can probe interactions by measuring current noise. I will present our recent observation of anomalous current fluctuations in biased graphene, using both global and local measurement techniques. Our electronic (global) noise measurements show not only an unexpected dependence on doping and bath temperature, but also a distinct noise spectrum. While this measurement provides a global spatial average of the noise, we also use nitrogen vacancy centers in diamond to probe the noise locally. The results from both measurements are incompatible with typical noise sources, and point to an unusual origin. 
Monday, March 5, 2018 10:36AM  10:48AM 
A40.00014: Capacitancebased tunneling spectroscopy of monolayer graphene Spencer Tomarken, Ahmet Demir, Kenji Watanabe, Takashi Taniguchi, Raymond Ashoori Traditional tunneling measurements do not function on insulating samples as there is no means to extract a tunneling current from them. We instead utilize a pulsed tunneling technique that exploits capacitive detection in a vertical sample geometry. Using this approach, we can tunnel into fully insulating materials such as the quantum Hall states of graphene. Moreover, the energy resolution of the method is limited only by the sample temperature. We measure the tunneling conductance between a graphite tunneling electrode and monolayer graphene through an atomically thin layer of hexagonal boron nitride in high magnetic fields and over a large range of carrier densities in the quantum Hall regime. We observe unexpected energy splittings in the Landau levels for filling factors that place the Fermi level in the cyclotron gap. 
Monday, March 5, 2018 10:48AM  11:00AM 
A40.00015: Spatially resovled photoemission studies on aligned square graphene sheets grown on copper substrate Haifeng Yang, Cheng Chen, Huan Wang, Zhongkai Liu, Yulin Chen

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