Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U51: Graphene: Imaging and Spectroscopy |
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Sponsoring Units: DCP Chair: Jennifer DeMell, Laboratory for Physical Sciences Room: Mile High Ballroom 1D |
Thursday, March 5, 2020 2:30PM - 2:42PM |
U51.00001: Imaging magnon transport in a quantum Hall ferromagnet: Part 1 Seung Hwan Lee, Andrew Pierce, Yonglong Xie, Patrick R Forrester, Kenji Watanabe, Takashi Taniguchi, Amir Yacoby Graphene exhibits quantum Hall ferromagnetism at partial fillings in the N=0 Landau level where its SU(4) isospin symmetry is spontaneously broken due to strong electron-electron interactions. It has been demonstrated that it is possible to generate and detect magnons, collective excitations of spins, in the ν= ±1 quantum Hall ferromagnets by exploiting equilibration of spin-polarized edge states. However, the nature of magnon transport in the system is still under active investigation. In this two-part talk, we will discuss scanning gate microscopy (SGM) of magnon transport in the graphene quantum Hall ferromagnet. Using a metallic scanning tip, a small region of graphene can be gated away from integer filling, effectively acting as a barrier for magnon transport inside the system. In this talk, we present a general scheme of performing SGM of magnon transport, and report observation of localized response in transport as we place the local barrier near the magnon generation and absorption sites. |
Thursday, March 5, 2020 2:42PM - 2:54PM |
U51.00002: Imaging magnon transport in a quantum Hall ferromagnet: Part 2 Andrew Pierce, Seung Hwan Lee, Yonglong Xie, Patrick R Forrester, Kenji Watanabe, Takashi Taniguchi, Amir Yacoby The spin-polarized ν=1 quantum Hall state of monolayer graphene constitutes a unique laboratory for studying mesoscopic magnetic phenomena. Among many interesting properties, the system supports magnons which can be generated and detected electrically. Here we study the ν=1 state of graphene using a scanning single-electron transistor (SET) operated simultaneously as a local gate and electrometer. Processes involving localized states near device contacts are shown to strongly influence transport response, suggesting a microscopic mechanism for magnon generation and absorption. Moreover, operating the SET as an electrometer enables us to perform local compressibility measurements which further clarify the effects of magnon generation on the system. |
Thursday, March 5, 2020 2:54PM - 3:06PM |
U51.00003: Visualizing unexpected magnetic field-tuned electron flow through ballistic channels Joseph Sulpizio, Lior Ella, Asaf Rozen, John Birkbeck, David Perello, Debarghya Dutta, Moshe Ben-Shalom, Takashi Taniguchi, Kenji Watanabe, Andre Geim, Shahal Ilani The advent of ultra-clean materials has enabled the exploration of ballistic flow, where electrons traverse a sample unperturbed before colliding with the walls. We have developed a nanoscale probe based on a scanning nanotube single electron transitor that is sufficiently sensitive to image the electrostatic potential of ballistic electrons in such materials for the first time. We apply this probe to high-mobility graphene channel devices, and image how the potential of the flowing electrons drops sharply at the contacts while remaining constant along the bulk, which is the textbook picture of ballistic electron flow. We then image how the Hall voltage profiles of the flowing electrons develop a rich set of patterns as the cyclotron radius is tuned by varying the perpendicular magnetic field. Contrary to the expectation that the electron current primarily flows along the edges in magnetic field, our results reveal that the current varies considerably across the bulk of the channel, and can even concentrate near the center. These results open the path for visualizing other ballistic electron flow phenomena that previously could only be studied via transport, including electron optics. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U51.00004: Realization of Nanoscale Mapping of Noise Source Activities on Graphene Domains JEEHYE PARK, Hyungwoo Lee, Duckhyung Cho, Shashank Shekhar, Jeongsu Kim, Jaesung Park, Byung Hee Hong, Seunghun Hong We report a “noise spectral imaging” strategy to directly map the noise source activities in graphene domains. Here, the lateral current and noise maps were measured by scanning a conductive contact probe on the graphene surface. The measured data were analyzed by using a two-dimensional network model to obtain the sheet resistance and charge trap density maps inside graphene domains, on domain boundaries, and on the edge of graphene. The results showed high activities of noise sources and large sheet resistance values at the domain boundary and edge of graphene. Furthermore, the top layer of double-layer graphene was found to have lower noises than those of single-layer graphene, as predicted previously. Our method allows one to image the localized noise source distribution in two-dimensional electronic channels such as graphene, and thus it can be a powerful tool for the basic research and applications about two-dimensional transports in nanostructures. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U51.00005: Photothermal nanoimaging of dissipative surface polaritons Honghua U Yang, Liang-Chun Lin, tao jiang, Samuel Berweger, Fabian Menges, Markus B. Raschke Controlling the functionalities of 2D material structures via strong light-matter coupling requires understanding of the dissipation and thermalization dynamics of surface polaritons. However, the intricate details of polaritonic decay processes are rooted in a plethora of physical mechanism spanning widely separated length and time scales, even down to regimes where dissipation is no longer a readily measurable quantity. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U51.00006: THz spectroscopy of graphene and graphene nanoribbons using LaAlO3/SrTiO3 nanoscale junctions Erin Sheridan, Lu Chen, Qing Guo, Jianan Li, Jung-Woo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy
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Thursday, March 5, 2020 3:42PM - 3:54PM |
U51.00007: Lattice vibrations of single and multi-layer isotopologic graphene HWANSOO JEON, Tokuyuki Teraji, Kenji Watanabe, Takashi Taniguchi, Sunmin Ryu Isotope-substituted materials, isotopologues, have been crucial in understanding various scientific problems. In this work, we report full Raman spectroscopic investigation of 13C graphene, its multilayers, and isotopologic bilayers. All samples were prepared using 13C-enriched bulk crystals that were grown by the high-pressure and high-temperature method. Frequencies of G, 2D, and combination peaks showed a linear relation between two isotopologues of 12C and 13C, which led to a precise optical determination of isotopic purity in agreement with mass spectrometric results. Single to eight-layered graphene exhibited correspondence in spectral features between isotopologues: thickness-dependent progressions of intensities and line shapes, and excitation energy-dependent dispersion of 2D frequency. Line shape analyses of stacking-sensitive 2D revealed the spatial distribution of stacking domains and dominance of ABC-polytype unlike natural or kish graphite. Using isotopologic bilayers of 12C/13C, we quantified the change in Fermi velocity induced by vdW interlayer interactions separately from strain and charge density. This work also presents direct evidence that the puzzling thermally activated doping is induced by hole dopants located at the graphene-substrate interface. |
Thursday, March 5, 2020 3:54PM - 4:06PM |
U51.00008: Detecting Photo-Induced Topological Edge States in a Graphene Nanoribbon Using Pump-Probe Spectroscopies Yuan Chen, Yao Wang, Martin Claassen, Brian Moritz, Thomas Devereaux Recently, photo-induced topological edge states in low dimensional materials have attracted considerable attention due to the tunability of dispersion and topological properties using light. Here, we present a numerical study of various pump-probe spectroscopies for a graphene nanoribbon subject to circularly polarized pump light. In particular, we calculate time-resolved resonant inelastic x-ray scattering (tr-RIXS) for the graphene nanoribbon, which clearly resolves the pump-induced band gap and the edge states. These tr-RIXS features have a direct correspondence with Floquet states and the calculated time-resolved ARPES spectra. We also calculate the pump-probe optical conductivity and nonresonant Raman scattering, demonstrating that the signatures from the pump-induced gap and edge states also should be visible, especially at low energy. These pump-probe techniques provide powerful tools for detecting photo-induced topological states in low dimensional materials. |
Thursday, March 5, 2020 4:06PM - 4:18PM |
U51.00009: Graphene Conductivity and Dielectric Response at THz frequencies David Carey The calculations of the dynamical conductivity of graphene allows further determination of the high frequency, from dc to optical frequencies, response of the real and imaginary parts of the dielectric constant of graphene. In this study we explore how the dynamical conductivity, dielectric properties and refractive index of single layer graphene vary as a function of frequency for different carrier concentrations and scattering times. We play particular attention to the differing roles of the intra- and interband transitions and explore the THz response of graphene as a possible functional material to bridge the so-called THz gap between electronics and optics. We find in the 1-10 THz frequency region that for moderately doped graphene (i) ε1 is negative and approaches values in the mid -105 but does not vary strongly at lower frequencies and (ii) ε2 is positive for all THz frequencies explored but is one to two orders lower in magnitude; comparisons are made with noble metals. We also calculate the variation of real and imaginary components of the refractive index with wavelength. Potential applications in graphene based antennas and metamaterials are discussed. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U51.00010: Strong magnetophonon oscillations induced by Dirac fermion - transverse acoustic phonon scattering in graphene Piranavan Kumaravadivel, Mark Greenaway, David Perello, Alexey Berdyugin, John Birkbeck, Joshua Wengraf, Song Liu, James H. Edgar, Andre Geim, Laurence Eaves, Roshan Krishna Kumar Two dimensional electron gas systems exhibit a plethora of quantum phenomena. Graphene has not only provided a versatile platform for studying many of these phenomena but also revealed new effects. However one of the first discoveries in quantum transport, well known for fifty years has remained conspicuously absent in graphene : magnetophonon oscillations. Here we present our recent work on magnetotransport in boron nitride encapsulated graphene devices of different widths [1] and show that the magnetoresistance of wider graphene devices reveal this hitherto elusive quantum phenomenon. In devices of channel width greater than ten micrometres we observe pronounced magnetophonon oscillations caused by resonant scattering of Landau quantised Dirac quasiparticles with acoustic phonons in graphene. Using this we determine graphene’s low energy phonon dispersion and also find that transverse acoustic modes are the dominant source of phonon scattering. The results highlight the importance of device size in studying new quantum phenomena and demonstrates a spectroscopic technique for studying the nature of electron-phonon interactions in van der Waals heterostructures. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U51.00011: Magneto-infrared spectroscopy of non-polar epigraphene Tianhao Zhao, Yuxuan Jiang, Yiran Hu, Yue Hu, Grant H Nunn, Mykhaylo Ozerov, Dmitry Smirnov, lei ma, claire berger, walter deheer, Zhigang Jiang We report on the magneto-infrared spectroscopy study of the new generation epigraphene grown on a non-polar facet of SiC [arXiv:1910.03697]. Interband Landau level (LL) transitions are observed and fitted well with 1×106 m/s as Fermi velocity in the massless Dirac fermion model. The transitions remain visible as the magnetic field is down to 0.25 T, indicating that the carrier density is no greater than 3.6×1010 cm-2 as expected for charge-neutral non-polar epigraphene. When the samples are grown thicker, LL transition splittings are spotted at high magnetic fields. Failing to explain the splittings with electron-hole asymmetry, we suggest that it could arise from (twist) bilayer epigraphene components and we will discuss it with a numerical twist bilayer graphene model. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U51.00012: Direct Imaging of Electronic Symmetries in Twisted Double-Bilayer Graphene Simon Turkel, Carmen Rubio Verdú, Larry Song, Dante Kennes, Lede Xian, Hector Ochoa, Kenji Watanabe, Takashi Taniguchi, Angel Rubio, Abhay Pasupathy When two graphene bilayers are twisted relative to one another at about one degree, the resulting system has been shown to host correlated insulating and superconducting states. Unlike in the case of simple twisted bilayer, the insulating states in the magic double bilayer are enhanced under parallel magnetic field, suggesting that electronic correlations may be mediated by ferromagnetic order. We directly image the local density of states in twisted double bilayer graphene using scanning tunneling microscopy and spectroscopy and observe the evolution of electronic wave functions within the moiré unit cell as a function of carrier density and applied electric field. In addition to corroborating theoretical band structure calculations via point spectroscopy, our observations of symmetry-broken states across a range of dopings and long-range order persisting across hundreds of nanometers offer insights into the complex charge and spin order that exists in magic angle twisted double bilayer graphene. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U51.00013: Imaging double moiré pattern in twisted bilayer graphene grown on hBN with microwave impedance microscopy Xiong Huang, Lingxiu Chen, Shujie Tang, Chengxin Jiang, Chen Chen, Huishan Wang, Zhixun Shen, Haomin Wang, Yongtao Cui Stacking multiple atomic layers of similar crystal structure will create moiré patterns whose periodicity and orientation are determined by the stacked layers. In this study, we image the moiré patterns in CVD grown bilayer graphene/hBN heterostructures using microwave impedance microscopy and conductive atomic force microscopy. Two sets of moiré patterns are observed: one formed between the first graphene layer with hBN and a second one between the two twisted graphene layers. The graphene/hBN moiré pattern indicates a perfect alignment between the two layers. The graphene/graphene moiré pattern has a smaller period due to a larger twist angle. This moiré pattern experiences abrupt phase change across the boundaries of the graphene/hBN moiré pattern. We present our analysis of such double moiré pattern based on lattice reconstruction occurred in the graphene layers. |
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