Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S51: Graphene: Electron interactions |
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Sponsoring Units: DCMP Chair: George Kioseoglou, Univ of Crete Room: Mile High Ballroom 1D |
Thursday, March 5, 2020 11:15AM - 11:27AM |
S51.00001: Fabry–Pérot Quantum Hall Interferometry in Graphene Yuval Ronen, Thomas Werkmeister, Danial Haei Najafabadi, Andrew Pierce, Bobae Johnson, Kenji Watanabe, Takashi Taniguchi, Amir Yacoby, Philip Kim The fractional quantum Hall effect (FQHE) manifests when a large magnetic field is applied to a 2-dimensional electron gas with strong electron-electron interactions. The excitations of this highly correlated phase are expected to possess anyonic exchange statistics. Inspired by experiments in GaAs heterostructures that have attempted to demonstrate anyonic exchange statistics via quantum Hall interferometry, we explore the possibility of determining quasiparticle charge and statistics by measuring quantum Hall interferometers in graphene heterostructures. We utilize hBN-encapsulated monolayer graphene with top and bottom graphite gates to electrostatically define interferometers in the FQHE regime. We demonstrate that two quantum point contacts (QPC), which individually act as beam splitters for quantum Hall edge states, cascaded in series forms a Fabry–Pérot interferometer. We will discuss development toward realizing Aharanov-Bohm dominated interference. |
Thursday, March 5, 2020 11:27AM - 11:39AM |
S51.00002: Optical conductivity of graphene with second-nearest neighbors coupling Enrique Munoz, Horacio Falomir, Marcelo Loewe, Renato Zamora Optical transparency experiments show that optical conductivity depends on the fine structure constant, as it can be fully explained within the effective theory of two-dimensional relativistic Dirac fermions. In this work, we investigate how this property is affected by second-nearest-neighbors coupling t’. Within a field theoretical representation in the continuum approximation arising from an underlying tight- binding atomistic model, we obtain the dependence of the optical conductivity on frequency, temperature and finite chemical potential. We investigated the model in the Keldysh contour representation at zero temperature[1], as well as in Matsubara space for finite temperature and chemical potential[2]. Our results reproduce the universal and experimentally verified value at zero frequency and, more importantly, they reveal that a small but still measurable shift in the conductance minimum at finite temperatures arises as a function of the second-nearest neighbors hopping t’, thus providing the possibility to directly measure this parameter in transport experiments. |
Thursday, March 5, 2020 11:39AM - 11:51AM |
S51.00003: The equilibration dynamics of spin-polarized quantum Hall edges at graphene p-n junction Arup Kumar Paul, MANAS SAHU, Chandan Kumar, Kenji Watanabe, Takashi Taniguchi, Anindya Das The Understanding of equilibration dynamics of quantum Hall (QH) edges is essential to realize an electron interferometer, which has the potential to exhibit fractional statistics and quantum entanglement. Recently, graphene pn junction with spin-valley polarized QH edges showed an ideal platform for an electron interferometer. However, the interplay of different scattering mechanisms in equilibration of polarized QH edges at the graphene pn junction remain elusive. In this report, we have carried out the conductance together with shot noise measurements for symmetry broken QH edges co-propagating along a pn junction. The pn junction was realized in a dual |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S51.00004: Light-induced anomalous Hall effect in graphene James McIver, Benedikt Schulte, Falk-Ulrich Stein, Toru Matsuyama, Gregor Jotzu, Guido Meier, Andrea Cavalleri In this talk I will discuss our recent observation of an anomalous Hall effect in monolayer graphene driven by a femtosecond pulse of circularly polarized light [1]. This was achieved using an ultrafast device architecture based on photoconductive switches. The dependence of the Hall effect on a gate potential used to tune the Fermi level reveals multiple features that reflect a Floquet-engineered topological band structure [2], similar to the band structure originally proposed by Haldane [3]. This includes an approximately 60 meV wide conductance plateau centered at the charge neutrality point, where a gap of equal magnitude is predicted to open. We find that when the Fermi level lies within this plateau, the non-equilibrium anomalous Hall conductance saturates around 1.8±0.4 e^2/h. |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S51.00005: Landau Levels in Bilayer Graphene under Pressure Brett Green, Jorge Osvaldo Sofo Bilayer graphene in a magnetic field hosts various ordered phases near neutrality, built from 8 low-energy Landau levels labeled by orbital n=0,1, valley ξ=+,- and spin σ=↑,↓. Phase energies depend strongly on interactions and the variables magnetic field, interlayer bias, and pressure. We find the ground state as function of these variables using a Hartree-Fock treatment including the effects of metallic gates, layer separation, layer thickness. Gates screen long-range interactions, separation weakens intervalley interactions, and thickness weakens the Coulomb blockade between layers; these effects distort the phase diagram but do not change its topology. We find that pressure strengthens the noninteracting scale relative to the Coulomb energy, which drives phase transitions to occur at lower fields. This stabilizes two orbitally polarized states not yet predicted or observed, adding to previously-identified valley-, spin-, and partially polarized states. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S51.00006: Ballistic to hydrodynamic crossover in twisted bilayer graphene Isabelle Phinney, Denis Bandurin, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero At elevated temperatures, electron-electron (e-e) collisions dominate transport in high-mobility graphene, causing the flow of charge carriers to resemble that of classical fluids or gases and allowing for a hydrodynamic description [1][2]. However, experimental conditions are typically such that the onset of fluidity and truly hydrodynamic phenomena occur in a regime where electron-phonon scattering becomes significant. Twisted bilayer graphene (TBG), which has already seen success as a host for the study of strongly interacting electron systems [3], can provide an exceptional alternative platform to study hydrodynamic electron flow due to reduced Fermi velocity and thus a higher e-e scattering rate. Here we show that, at liquid helium temperatures, such devices exhibit micrometer-scale ballistic transport, probed using transverse magnetic focusing. At higher temperatures, we observe the development of a negative potential in the vicinity of the current injecting contact, which can be attributed to the onset of hydrodynamic electron behavior. |
Thursday, March 5, 2020 12:27PM - 12:39PM |
S51.00007: Hydrodynamic electron flow in a graphene-based device with engineered edge conditions Rupini Kamat, Arthur W Barnard, Joe Finney, Takashi Taniguchi, Kenji Watanabe, Marc Kastner, David Goldhaber-Gordon Hydrodynamic electron flow is expected to emerge in the regime where electron-electron interactions dominate over all other interactions. In this regime, electrons should flow like a viscous fluid. Evidence of such behavior in graphene has been found experimentally in the form of superballistic flow through constrictions [1] and negative local resistance measurements [2]. Hydrodynamic electron transport may strongly depend on boundary conditions, which can range from no-slip (all tangential momentum is dissipated at the boundary) to no-stress (the boundary exerts no force on the fluid). To isolate the impact of boundary conditions on hydrodynamic electron flow, we have fabricated a graphene device in a pipe geometry with the first half of the pipe having smooth edges and the second half having jagged edges. Through a combination of simulations and transport measurements with this device, we tune between the two extreme boundary conditions and identify the impact of edge geometry on the flow profile of a viscous electron fluid. |
Thursday, March 5, 2020 12:39PM - 12:51PM |
S51.00008: Hydrodynamic and Ballistic AC transport in two-dimensional Fermi Liquids Manichandra Morampudi, Gitansh Kataria, Deshdeep Sahdev, Ravishankar Sundararaman Electron transport in clean two-dimensional systems with weak electron-phonon (e-ph) coupling can transition from an Ohmic to a ballistic or a hydrodynamic regime. The ballistic regime occurs when electron-electron (e-e) scattering is weak whereas the hydrodynamic regime arises when this scattering is strong. Despite this difference, we find that vortices and a negative nonlocal resistance believed to be quintessentially hydrodynamic are equally characteristic of the ballistic regime. These non-Ohmic regimes cannot be distinguished in DC transport without changing experimental conditions. Further, as our kinetic calculations show, the hydrodynamic regime in DC transport is highly fragile and is wiped out by even sparse disorder and e-ph scattering. We show that microwave-frequency AC sources by contrast readily excite hydrodynamic modes with current vortices that are robust to disorder and e-ph scattering. Current reversals in the non-Ohmic regimes occur via repeated vortex generation and mergers through reconnections (movie : vimeo.com/261891439). Crucially, AC sources give rise to strong spatiotemporal correlations across the entire device that unambiguously distinguish all regimes. |
Thursday, March 5, 2020 12:51PM - 1:03PM |
S51.00009: Dynamic response functions of graphene with short-range interactions Megha Agarwal, Eugene Mishchenko, Oleg Starykh We consider a short-range interaction between electrons in graphene and determine their dynamic response functions, such as a longitudinal and a transverse electric conductivity and a polarization function. The calculations are performed to all orders in the interaction by taking into account the self-energy renormalization of the electron velocity and using a ladder approximation to account for the vertex corrections, ensuring that the Ward identity (charge conservation law) is satisfied. We discuss whether a collective mode can exist in this system corresponding to the propagation of electric polarization of correlated electron-hole pairs. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S51.00010: Detecting Hall viscosity in the absence of inversion or time reversal symmetries Muhammad Imran The Hall viscosity is experimentally observed in graphene. The Hall viscosity is characterized by the induced voltage with its polarity opposite to the Hall voltage. A formula is derived for the induced voltage entering from the Hall viscosity for the prototype models of bilayer graphene in the absence of time reversal symmetry and the semiconductor hetrostructures with strong spin orbit coupling. The fault tolerant Hall viscosity is predicted to be observed in the non-local resistance measurement. |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S51.00011: Reciprocal and non-reciprocal energy and momentum drag between 1D and 2D conductors Laurel Anderson, Austin K Cheng, Kenji Watanabe, Takashi Taniguchi, Philip Kim Double-layer drag experiments are an exceptionally sensitive tool for studying interactions in low-dimensional systems, since the drag response depends on long-range Coulomb interactions rather than particle exchange. Graphene in particular has exhibited unusual drag effects near charge neutrality that have been attributed to competing energy and momentum transfer between the layers. We have performed measurements of 1D-2D drag resistance between a single carbon nanotube (CNT) and monolayer graphene separated by a few-layer boron nitride flake. At high temperatures, the drag response is symmetric under interchange of the drive and drag layers, and resembles momentum transfer from drive to drag layer. This layer reciprocity is broken near the charge neutrality point of graphene at lower temperatures, which can be attributed to energy transfer or thermoelectric effects. Intriguingly, the high-temperature drag response decays with distance from the CNT slower than expected from the Ohmic diffusive current distribution. This suggests additional electron interactions coming into play near charge neutrality in the graphene layer. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S51.00012: Cavity quantum electrodynamics with cyclotron resonance transitions in graphene Yashika Kapoor, Jordan Russell, Ellie Irene Hunt, Rebecca Lim, Erik Henriksen Cyclotron resonance transitions between highly-degenerate Landau levels are good candidates to achieve the ultrastrong coupling regime in cavity QED, as shown recently for GaAs quantum wells [1]. Graphene provides a similarly ideal platform, with the additional advantage of a strongly anharmonic Landau level spectrum so that a two-level system may be readily explored. We have recently demonstrated graphene cyclotron transitions with quality factor Q~500 at a field of 8 T. Incorporated into cavities with either thin metal or dielectric mirrors, we estimate that cooperativities C~100 and reduced coupling strengths \eta~g/\omega~0.2 can be reached using graphene in an infrared cavity. We will present initial results of measurements on such graphene cavity QED devices and outline prospects for reaching higher coupling strengths. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S51.00013: Cyclotron Resonance in Dual-Gated Bilayer Graphene Jordan Russell, Jesse Balgley, Yashika Kapoor, Takashi Taniguchi, Kenji Watanabe, Erik Henriksen We report observations of cyclotron resonance in boron nitride-encapsulated, dual-gated bilayer graphene by way of transmission infrared magnetospectroscopy. The inter-Landau level transitions -2 → 1 and 1 → 2 are studied at a fixed magnetic field of 13 T as a function of the Landau level filling factor, ν, and the layer symmetry-breaking perpendicular electric displacement field, D. Varying D at ν = 4 yields a direct and high-resolution measurement of the optical bandgap in bilayer graphene at low energies. For increasing D at ν = 0 we observe a varying number of transitions as the K and K’ valleys diverge, while a sharp change in the CR energies signals the transition to a layer polarized state at finite D. A detailed description of the data requires consideration of band parameters beyond the lowest order inter- and intra-layer hopping terms. Measurements at D = 0 reveal a pronounced electron-hole asymmetry and many-body interaction effects as ν is varied. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S51.00014: Dynamic gauge field and quantum pumping in graphene nanoelectromechanical resonators Rui Pu, Du Xu Quantum pumping generates charge or valley current through periodic variation in its internal parameters. Graphene, with its large electron mobility and strong coupling between electronic properties and mechanical strain, is a promising candidate for the experimental realization of quantum pumping. Here we study the case of graphene nanoelectromechanical resonator suspended over a gate electrode as a quantum charge pump. The deformation of graphene sheet from electrostatic potential induces a cyclic modulation of carrier density as well as an effective dynamic gauge potential. By explicit breaking of spatial inversion symmetry, we observed a finite charge pumping current near the mechanical resonance frequency. We also explore the possibility of generating valley current in graphene nanoelectromechanical resonators. |
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