Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W51: Graphene: Valleytronics, Plasmonics, and Excitonics |
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Sponsoring Units: DCMP Chair: Aubrey Hanbicki Room: Mile High Ballroom 1D |
Friday, March 6, 2020 8:00AM - 8:12AM |
W51.00001: Strain-induced valley polarization in monolayer graphene Duxing Hao, Chen-Chih Hsu, Marcus L Teague, Jiaqing Wang, Nai-Chang Yeh We use nearly strain-free PECVD-grown graphene [1] to induce controllable strain and pseudo-magnetic fields by nanoscale strain engineering [2]. By placing strain-free monolayer graphene and monolayer h-BN on architected silicon nanostructures fabricated by electron-beam lithography, we demonstrate broken global inversion symmetry and provide experimental evidences for strain-induced giant pseudo-magnetic fields and valley polarization by scanning tunneling spectroscopic studies at room temperature. Here we report transport properties of monolayer graphene devices using both DC and low-frequency lock-in techniques. Non-local resistance and non-local magnetoresistance are measured on both strained and unstrained graphene devices to confirm the strain-induced valley Hall effect. We further use different circularly-polarized light to enhance or suppress the valley Hall effect via selectively exciting electrons in the corresponding valley. Our approach thus paves a new way to realize scalable graphene-based valleytronics. |
Friday, March 6, 2020 8:12AM - 8:24AM |
W51.00002: Plasmonic Doppler Effect in Graphene Yinan Dong, Lin Xiong, Isabelle Phinney, Ran Jing, Zhiyuan Sun, Alexander McLeod, Shuai Zhang, Michael M Fogler, Pablo Jarillo-Herrero, Leonid Levitov, Denis Bandurin, Dmitri Basov High mobility two-dimensional electron gases reveal an intriguing phenomenon of the plasmonic Doppler shift. The plasmonic response is altered when direct current (DC) is applied provided the drift velocity of electrons reaches a substantial fraction of the Fermi velocity. When plasmons are coupled with light, surface plasmon polaritons (SPP) are predicted to acquire a quasi-relativistic Doppler effect [D.S. Borgnia and L.Levitov, arXiv: 1512.09044]. Here we utilize cryogenic nano-imaging technique to search for the current-induced Doppler effect in the SPP dynamics in graphene. Directional carrier flow breaks time-reversal symmetry and causes non-reciprocal plasmonic responses in infrared frequencies. Changes of SPP wavelength in real space are attributable to the Doppler effect. SPP imaging data inform us of the behavior of hybrid quasiparticles under current flow. |
Friday, March 6, 2020 8:24AM - 8:36AM |
W51.00003: Plasmonic Nonreciprocity and Doppler Effect: The Role of Electron-Electron Interactions Haoyang Gao, Zhiyu Dong, Egor Kiselev, Leonid Levitov Plasmonic modes propagating in the presence of an electrical DC current offer an appealing tool for achieving optical nonreciprocity at the nanoscale. This talk will discuss an approach relying on the DC-current-induced plasmonic Doppler effect, wherein downstream (upstream) propagation results in a blue (red) frequency shift of plasmon resonance, respectively. The carrier drift velocity in modern high-mobility two-dimensional electron systems can reach a substantial fraction of the Fermi velocity, leading to a strong Doppler effect. Since time reversal symmetry is broken in the presence of a flow, neither nonlinear coupling nor pumping, conventionally used to achieve nonreciprocity, are required in this case. We find that electron-electron interactions impact the magnitude of the effect, enhancing the Doppler shift substantially relative to the free-particle base value. Estimates of this effect based on Landau Fermi-liquid theory will be discussed and compared with the results of a microscopic approach. |
Friday, March 6, 2020 8:36AM - 8:48AM |
W51.00004: Valley polarization braiding in strained graphene Daiara Faria, Carlos León, Leandro R. F. Lima, Andrea Latgé, Nancy Sandler Previous works on deformed graphene predict the existence of valley-polarized states, however, optimal conditions for their detection remain challenging. We show that in the quantum Hall regime, edge-like states in strained regions can be isolated in energy within Landau gaps. We identify precise conditions for new conducting edges-like states to be valley polarized, with the flexibility of positioning them at chosen locations in the system. A map of local density of states as a function of energy and position reveals a unique braid pattern that serves as a fingerprint to identify valley polarization[1]. |
Friday, March 6, 2020 8:48AM - 9:00AM |
W51.00005: Interacting Valley Chern Insulator in Moiré Systems Xiaochuan Wu, Yichen Xu, Chao-Ming Jian, Cenke Xu One salient feature of systems with Moiré superlattice is that the Chern number of “minibands" originating from each valley of the original graphene Brillouin zone becomes a well-defined quantized number because the miniband from each valley can be isolated from the rest of the spectrum due to the Moiré potential. Then a Moiré system with a well-defined valley Chern number can become a nonchiral topological insulator with U(1) × Z_3 symmetry and a Z classification at the free fermion level. Here we demonstrate that the strongly interacting nature of the Moiré system reduces the classification of the valley Chern insulator from Z to Z_3, which is very different from the previously known examples of interaction reduced classification of topological insulators. We also show that an interacting valley Chern insulator is topologically equivalent to a bosonic symmetry protected topological state made of local boson operators. The effect of this interaction influenced classification is estimated in experimental systems. |
Friday, March 6, 2020 9:00AM - 9:12AM |
W51.00006: 30° Twisted Bilayer Graphene: Atomic Crystal Structure, Electronic Structure and Plasmonic Interactions Zhongwei Dai, Zhaoli Gao, Calley Eads, Samuel Tenney, Alan T Johnson, Jerzy T. Sadowski Recent discovery of unconventional superconductivity in twisted bilayer graphene (tBLG) has triggered intensive discussions about the importance of interlayer coupling effects in commensurate tBLG, which was overlooked previously. Much of its macroscale properties, such as conducting electron behavior and light matter interactions in the commensurate tBLG systems remain unknown. Here we present study of the interlayer coupling effects in the twisted 30° bilayer graphene system using surface sensitive low energy electron microscopy (LEEM), Raman scattering and infra-red scattering near-field optical microscopy (IR-sSNOM). Strong crystal structure coupling between two graphene sheets was revealed by a sharp 12-fold symmetrical LEED diffraction pattern. Enhancement of interlayer light scattering was also observed by Raman spectroscopy. Most surprisingly, the plasmonic interaction was observed to be sharply quenched on 30° twisted bilayer graphene in contrast with enhanced plasmonic interaction on non-twisted, bilayer graphene. |
Friday, March 6, 2020 9:12AM - 9:24AM |
W51.00007: Filling-factor-dependence of the magnetoplasmon spectrum in graphene Jordan Pack, Jordan Russell, Yashika Kapoor, Jesse Balgley, Jeff Ahlers, Takashi Taniguchi, Kenji Watanabe, Erik Henriksen We present cyclotron resonance measurements of a graphite-gated, hexagonal boron nitride-encapsulated monolayer graphene device. We observe a remarkable progression of the CR lineshape as a function of the Landau level filling factor, ν, from a single peak at ν=0 to four peaks at ν=1 to two peaks for ν>2, giving an unprecedented spectroscopic view of the evolution of the magnetoplasmon spectrum arising from the broken spin and valley symmetries in the N=0 Landau level. By comparing to many-particle theories of graphene CR, we extract precise values for the renormalized Fermi velocity and a Dirac mass due to sublattice-symmetry breaking by the hBN. A simple model of LL shifts due to interactions allows extraction of the effective g values for Zeeman gaps in the N=0 Landau level both at and below the Fermi energy. Additionally we find evidence for a small yet clear electron-hole asymmetry. |
Friday, March 6, 2020 9:24AM - 9:36AM |
W51.00008: Band Structures and Localized Graphene Surface Plasmons with Periodic Charge Doping Michael Sammon, Diego Rabelo da Costa, Tony Low We present recent band structure calculations of artificial plasmonic crystals created by periodic doping. We focus on the Lieb and Kagome lattice, two lattices known to exhibit topological features. We show that within the Drude approximation of the conductivity and for neutral doping with alternating charges, the Kagome lattice exhibits both a flat band and a band gap with a large local density of states. Additionally, for doping of alternating. both lattices exhibit strong localization of the plasmon electric field intensity near the region of vanishing charge density. We discuss the effect of interband transistions on these properties. |
Friday, March 6, 2020 9:36AM - 9:48AM |
W51.00009: Isotopic Effect of Carrier Relaxation in Graphene-hBN Heterostructures Alexandra Brasington, James H. Edgar, Takashi Taniguchi, Kenji Watanabe, Arvinder S Sandhu, Brian J LeRoy In atomically thin systems, the choice of substrate plays an important role in the relaxation of photo-excited carriers. In previous work, hexagonal boron nitride (hBN) substrates have been shown to improve the thermal relaxation rates of carriers in graphene as compared to silicon oxide substrates. Naturally occurring boron contains a mixture of two isotopes with atomic masses 10 and 11 with abundances of 20% and 80% respectively. Theoretical studies have predicted a higher thermal conductivity with higher isotopic purity of hBN, due to reduced phonon scattering from isotopic defects. We utilize femtosecond pump-probe spectroscopy to observe the time dynamics of photo-excited carriers in graphene-hBN heterostructures for both natural and isotopically pure hBN. |
Friday, March 6, 2020 9:48AM - 10:00AM |
W51.00010: Evidence for excitonic solid states in double layer graphene Yihang Zeng, Qianhui Shi, Anna Okounkova, Kenji Watanabe, Takashi Taniguchi, Jia Li, Cory Dean Spatially indirect excitons can form when electron-doped and hole-doped 2D quantum wells are brought into close proximity. In the condition that interlayer separation is sufficiently large to suppress recombination and small enough that interlayer coulomb interactions are strong, the system can spontaneously condense into a superfluid-like macroscopically coherent state, termed an exciton condensate. In a double layer graphene system in the presence of magnetic field, upon decreasing the exciton density, we observe a sharp transition from an exciton superfluid to an exciton insulator, which shows insulating behavior in both charge transport and exciton transport measurement. At elevated temperature, the excitonic insulating state exhibits perfect coulomb drag, which is the signature of an exciton superfluid. Our data suggests a phase diagram consisting of a pinned excitonic Wigner crystal at lower exciton density, which transitions to a superfluid-like state at elevated temperature. |
Friday, March 6, 2020 10:00AM - 10:12AM |
W51.00011: Tunable Graphene Split-Ring Resonators Qiaoxia Xing Graphene plasmonics, allowing strong light-matter interactions, subwavelength light confinement and in situ tunability, has gained much attention due to the potential to develop new optoelectronic and photonic devices. Here, we experimentally demonstrate graphene split ring resonators with deep subwavelength confined magnetic dipole, quadrupole and electric dipole responses in terahertz regime. All modes can be tuned via chemical doping or stacking multiple graphene layers. Finite-element frequency domain simulations nicely reproduce experimental results. Our study demonstrates an example of tunable multiple resonances based on graphene, and shed new light on its application in tunable high-frequency magnetic metasurfaces. |
Friday, March 6, 2020 10:12AM - 10:24AM |
W51.00012: Light emission from vertically-grown quasi one-dimensional (1D) graphene nanostripes (GNSPs): Temperature and thickness independent ultra-long carrier lifetimes Deepan Kishore Kumar, Nai-Chang Yeh We have previously shown that our PECVD-grown, quasi-1D GNSPs exhibited perfect purity, high mobility [1] and strong broadband absorption [2]. Here we report broadband photoluminescence (PL) and ultra-long carrier lifetimes in GNSPs from time-resolved PL (TRPL) studies. GNSPs were deposited on quartz substrates to form uniform films with thicknesses of 5, 10, 20, 40 and 50 μm for the TRPL measurements. We used Nd:YAG laser at 355 nm with 10 ps pulse width and 0.1 mJ pulse energy under a repetition rate of 10Hz to carry out TRPL studies at temperatures from 15 K to 290 K. Detailed analysis of the TRPL data revealed two carrier lifetimes of τ1 (~ 1 ns) and τ2 (~ 10 ns), both were independent of either temperature or thickness, and the values were ~ 103 times longer than any lifetimes reported to date for graphene-based materials. We attribute the ultra-long lifetimes to the nanoscale quasi-1D nature of GNSPs, which breaks the global inversion symmetry so that photo-excited hot electrons and holes are rapidly separated due to their large differences in mobility, a.k.a. the photo Dember effect. |
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W51.00013: Valley density wave in graphite in the quantum limit Zhiming Pan, Ryuichi Shindou Graphite in the quantum limit shows unconventional in-plane and out-of-plane transport behavior. To uncover its microscopic origin, we construct an interacting electron model with a pair of electron band and hole band with the valley degree of freedom. We use a fermionic renormalization group (RG) method and study a parquet RG equation of the model with screened Coulomb interaction. We found that, for strong screening region, the one-loop RG indicates a dominant instability of valley charge density wave (VCDW) with a finite pairing between the two valleys. We further construct an effective mean-field theory and clarify the in-plane transport behavior of the VCDW phase. |
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W51.00014: Generation and Detection of Valley Current in Single and Double Layered Graphene through the Electron-Phonon Interaction Ankang Liu, Alexander Finkelstein We report a possible method to generate and detect the valley current carried by quasi-particles in both single and double layered graphene. We observe that the term in the collision integral originated from mixing the scalar and vector gauge-field-like vertices in the electron-phonon interaction turns current carriers of two different valleys in opposite directions. As a result, the electron-phonon collision produces a valley current. Finally, we show that the valley current carried by quasi-particles could be not only generated but also detected in a non-local resistance measurement through the inverse version of this mechanism. Our study proposes a new approach to manipulate the valley degrees of freedom in a pristine graphene or bilayer graphene sample without breaking the spatial inversion symmetry. The effect increases with temperature owing to a higher rate of collisions with phonons at higher temperatures. |
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