Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W67: Angle Resolved Photoemission Spectroscopy and Scanning Near-Field Optical Microscopy of 2D MaterialsFocus Recordings Available
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Sponsoring Units: DMP Chair: Christopher Gutiérrez, University of California Los Angeles Room: Hyatt Regency Hotel -Hyde Park |
Thursday, March 17, 2022 3:00PM - 3:36PM |
W67.00001: Investigating momentum and spatially resolved view of two-dimensional material-based systems utilizing focused angle-resolved photoemission spectroscopy Invited Speaker: Jyoti Katoch Two-dimensional (2D) materials offer the freedom to create novel condensed matter systems, with unique properties, by mechanically assembling different (or same) 2D materials layer-by-layer to form atomically sharp vertical or lateral heterostructures. The van der Waals (vdW) heterostructures with small lattice mismatch and a relatively small twist angle between the constituent layers, have shown to exhibit coexisting complex phases of matter including Mott insulating state, superconductivity, bound quasiparticles, and topological states. The advent of the state-of-the-art angle-resolved photoemission spectroscopy with high spatial resolution (micro- and nano-ARPES) and the ability to perform these measurements on fully functional devices, has made it possible to directly probe many exotic physical phenomena in 2D based material systems [1, 2]. In this talk, I will discuss the utilization of the in-operando nanoARPES to investigate the highly tunable many-body effects in graphene and twisted bilayer graphene devices [3, 4]. |
Thursday, March 17, 2022 3:36PM - 3:48PM |
W67.00002: Anisotropic Carrier Dynamics in Graphite/Graphene Probed with Time-and Angle-Resolved Photoemission Spectroscopy Jin Bakalis, Sergii Chernov, Alice Kunin, Christopher Corder, Peng Zhao, Ziling Li, Shuyu Cheng, Roland K Kawakami, Michael G White, Gerd Schönhense, Thomas K Allison Most HHG-based ARPES have suffered from severe restrictions imposed by space-charge limits and low collection efficiency of the photoelectrons from low repetition rates. These restrictions have largely precluded the study of the intrinsic dynamics of excited states in weakly excited systems. At Stony Brook, we have developed a tunable high rep-rate HHG source operating at 61 MHz in the range 8 to 40 eV. Combining with a time-of-flight momentum microscopy enables rapid time-resolved ARPES measurements with coverage of the full Brillouin zone in the perturbative excitation limit. In this work, we will discuss measurements of polarization dependent anisotropic distribution of excited electrons in HOPG graphite and monolayer graphene. We will discuss how different pump polarizations and probed photon energies affect this anisotropic angular distribution in all K/K’ in incident pump fluence ranging from 1 to 80 μJ/cm2. We compare our measurements to a theory[1] that includes both electron-electron and electron-phonon scattering. |
Thursday, March 17, 2022 3:48PM - 4:00PM |
W67.00003: Imaging the Electronic Structure of Strained Epitaxial Monolayer Graphene Falk Niefind, Henry Bell, Thuc T Mai, Angela R Hight Walker, Randolph E Elmquist, Sujitra Pookpanratana Epitaxial graphene (EG) has applications in quantum Hall resistance standards, chemical and biochemical sensing. Here, using a photoemission electron microscope (PEEM), we image EG topography in real space and measure the electronic structure of monolayer EG regions with micrometer-scale angle resolved photoemission (μ-ARPES). We detect characteristic electronic features of graphene such as the Dirac points and the π-band. On different regions of the sample which look topographically similar, we observe electronic structure including the electronic flat band. We performed Raman spectroscopy on the same regions that were analyzed by PEEM, and we estimated a significant amount of compressive strain (~1.2%) by comparing the 2D and G positions using a vector decomposition model [1]. It has been proposed by others that the compressive strain could modify the electronic band structure in monolayer graphene [2, 3]. |
Thursday, March 17, 2022 4:00PM - 4:12PM |
W67.00004: Quantum Lifetime Spectroscopy and Magnetotunneling in Double Bilayer Graphene Heterostructures Nitin Prasad, G W Burg, Kenji Watanabe, Takashi Taniguchi, Leonard F Register, Emanuel Tutuc Energy resolved lifetime measurements provide unique insight into the fundamental relaxation mechanisms of quantum states in a material. We describe a tunneling spectroscopy technique in a double bilayer graphene heterostructure where momentum-conserving tunneling between different energy bands serves as an energy filter for the tunneling carriers, and allows a measurement of the quasi-particle state broadening at well-defined energies and temperatures. The data reveal the quasi-particle state broadening increases linearly with the state energy separation from the Fermi level, and is relatively insensitive to temperature. These observations suggest the quasi-particle state broadening at the Fermi level is disorder limited, and at excited states is dominated by carrier-carrier interactions or phonon emission. We use magnetotunneling spectroscopy to confirm the high degree rotational alignment of the two bilayer graphenes and an absence of momentum randomizing processes. Using a combination of measurements and calculations, we determine the interlayer separation in the double bilayer graphene heterostructure. |
Thursday, March 17, 2022 4:12PM - 4:24PM |
W67.00005: Visualizing the conduction band and moiré effects in WS2/WSe2 heterobilayer devices by micro-ARPES Paul V Nguyen, Abigail J Graham, Heonjoon Park, James Nunn, Viktor Kandyba, Mattia Cattelan, Alessio Giampietri, Alexei V Barinov, Xiaodong Xu, David H Cobden, Neil R Wilson We investigate heterobilayers of WS2 and WSe2 aligned close to zero degrees, in which moiré superlattice effects are anticipated, using micron-scale-angle-resolved photoemission spectroscopy (micro-ARPES). The bilayer is partially capped by graphene and supported by a hexagonal boron nitride on graphite back gate in a field-effect transistor geometry. By applying a voltage to the back gate, we can electrostatically populate the heterobilayer conduction band minimum in the uncapped regions, finding that the conduction band edge is at the K-point in the WS2 as anticipated. We also see other spectral features resembling the conduction band edge but displaced in momentum by reciprocal lattice vectors corresponding to the expected heterobilayer moiré lattice. In the graphene-capped regions, we observe replicas of the graphene bands displaced by similar vectors. The particular symmetry in photoemission intensity of these replicas, combined with the absence of replicas displaced by graphene/WS2 or graphene/WSe2 moiré vectors, provide indications that the replicas are band structure alterations due to the moiré modulation rather than copies of the unperturbed bands produced by final-state diffraction. Differentiating between these two possible origins of the additional spectral features however remains challenging. |
Thursday, March 17, 2022 4:24PM - 5:00PM |
W67.00006: Observation of Pines' Demon in Sr2RuO4 with Momentum-Resolved EELS Invited Speaker: Peter Abbamonte The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1965, David Pines predicted that a distinct type of plasmon, dubbed a "demon," could exist in multiband metals containing more than one species of charge carrier. A demon corresponds to an out-of-phase oscillation of electrons in different bands, i.e., a modulation in the band occupancy. Demons have proven difficult to detect because they are neutral, meaning they do not couple to light, and are gapless so their excitation energy vanishes in the long-wavelength limit. In this talk I will present evidence for a demon in the multiband metal Sr2RuO4 from momentum-resolved electron energy-loss spectroscopy (M-EELS). The excitation is formed from electrons in the beta and gamma bands, is gapless with a velocity v = 0.5 eV*angstrom, and exhibits a critical energy of 60 meV. I will discuss how this excitation violates low-energy sum rules in Sr2RuO4, which is a defining property of demons, as well as its Landau damping into the strongly interacting continuum in this material. |
Thursday, March 17, 2022 5:00PM - 5:12PM |
W67.00007: Near-field Imaging of Transition Metal Dichalcogenides Anna Roche, Rachel L Nieken, Fateme Mahdikhanysarvejahany, Takashi Taniguchi, Kenji Watanabe, Michael Koehler, David G Mandrus, John Schaibley, Brian J LeRoy Monolayer transition metal dichalcogenide (TMD) (e.g., MoSe2, WSe2, MoS2, WS2) semiconductors have received massive attention after the discovery of strongly bound excitons at the +K and -K valleys. When TMDs are stacked in a heterostructure, the lattice mismatch and/or twist angle causes a periodic modulation of the bands resulting in trapped moiré excitons at the potential minima. To visualize these trapped moiré excitons requires an imaging technique with spatial resolution on the order of the moiré wavelength. Previous measurements of excitons in these systems have primarily relied on far-field optical spectroscopy techniques which are diffraction-limited to several hundred nanometers. Here, we present a study of the exciton spectra of TMD heterostructures using a cryogenic scattering-type scanning near-field optical microscope (s-SNOM). Using a tunable visible source, we map the exciton resonances in the TMD materials with sub 100 nm spatial resolution. |
Thursday, March 17, 2022 5:12PM - 5:24PM |
W67.00008: Nano-optical imaging of exciton-plasmon polaritons in WSe2/gold heterostructures Raghunandan B. Iyer, Yilong Luan, Ruth Shinar, Joseph Shinar, Zhe Fei Exciton-plasmon polaritons (EPPs) are hybrid modes formed due to the coupling of surface plasmons in metals with excitons in semiconductors. Here we present a nano-optical imaging study of EPPs in heterostructures formed by stacking tungsten diselenide (WSe2) thin flakes on gold films. By employing the scattering-type scanning near-field optical microscopy, we mapped the interference fringes of EPPs at various excitation energies, based on which we constructed the dispersion relations of these polaritonic modes. We found that the polariton dispersion demonstrates sensitive dependence on the thickness of WSe2 flakes. When the thickness of WSe2 is below 40 nm, EPPs tend to be plasmon-like surface modes. As the thickness surpasses 40 nm, EPPs behave as waveguide polaritons. The finite-element polaritonic simulations further confirm these observations. Our work uncovers in real-space the transport properties of EPPs and paves the way for future applications of these modes in nanophotonic circuits and devices. |
Thursday, March 17, 2022 5:24PM - 5:36PM |
W67.00009: Nano-optical studies of anisotropic waveguide exciton polaritons in Tin Sulfide Yilong Luan, Hamidreza Zobeiri, Xinwei Wang, Eli Sutter, Peter Sutter, Zhe Fei We report a nano-optical imaging study of waveguide exciton polaritons (EPs) in Tin Sulfide (SnS) in the near-infrared (IR) region by using the scattering-type scanning near-field optical microscope (s-SNOM). The samples that we studied are SnS microcrystals coated with a thin shell of Tin Disulfide. With s-SNOM, we mapped in real space the propagative EPs in SnS, which are sensitively dependent on the excitation energy and sample thickness. Moreover, we found that both the polariton wavelength and propagation length have shown strong in-plane anisotropy between the zigzag (a axis) and armchair (b axis) crystal directions. In particular, at a narrow spectral range from 1.32 to 1.44 eV, the EPs have shown a relatively long propagation length along the b axis but barely propagate along the a axis. The observed quasi-one-dimensional polaritons, which is originated from the different bandgap and exciton energies along the two axes of SnS, are promising for future applications related to directional control of the flow of nanophotonic energy and information. |
Thursday, March 17, 2022 5:36PM - 5:48PM |
W67.00010: Nano-spectroscopy of excitons in atomically-thin transition metal dichalcogenides Shuai Zhang, Baichang Li, Xinzhong Chen, Francesco L Ruta, Yinming Shao, Aaron Sternbach, Alexander S McLeod, Zhiyuan Sun, Lin Xiong, Samuel Moore, Sara Shabani, Lin Zhou, Fabian Mooshammer, P J Schuck, Cory R Dean, Abhay N Pasupathy, Michal Lipson, Andrew J Millis, James C Hone, Xiaodong Xu, Xiaoyang Zhu, Mengkun Liu, Dmitri N Basov Excitons play a dominant role in optoelectronic properties of atomically thin van der Waals(vdW) semiconductors, such as transition metal dichalcogenides (TMDs). These excitons are amenable to on-demand engineering with diverse controls, including dielectric screening, interlayer hybridization, and moiré potential. However, external stimuli frequently yield heterogeneous excitonic responses at the nano- and meso-scales, which are beyond the spatial resolution of conventional diffraction-limited optics. We use a scattering-type scanning near-field optical microscope (s-SNOM) to acquire exciton spectra in atomically thin transition metal dichalcogenide microcrystals with previously unattainable 20 nm resolution. Our nano-optical data reveal material- and stacking-dependent exciton spectra of MoSe2, WSe2 monolayers, and their heterostructures. Furthermore, the complex dielectric function of these vdW semiconductors can be accessed at the nanoscale. In addition, s-SNOM hyperspectral images uncover how the dielectric screening modifies excitons at length scales as short as few nanometers. This work paves the way towards understanding and manipulation of excitons in atomically thin layers at the nanoscale. |
Thursday, March 17, 2022 5:48PM - 6:00PM |
W67.00011: Study of momentum-resolved exciton dynamics in Monolayer Tungsten Disulfide David R Bacon, Xing Zhu, Vivek Pareek, Joel P Urquizo, Nicholas S Chan, Fabio Bussolotti, Kenji Watanabe, Takashi Taniguchi, Michael K Man, Julien Madeo, Kuan Eng Johnson Goh, Keshav M Dani Monolayer transition metal dichalcogenides (TMDs) exhibit unique characteristics among semiconductors. Desirable features such as direct bandgaps at K-points and highly efficient light-matter coupling have led to many potential applications in electronics, optoelectronics, and quantum devices. Recently, time and angle-resolved photoemission spectroscopy (TR-ARPES) techniques have succeeded in accessing the momentum-resolved dynamics of excitons in monolayer TMDC materials, such as WSe2 [1], and WS2 [2]. Here, we further probe the exciton dynamics in CVD grown monolayer WS2 samples using TR-ARPES by applying a variety of pump photoexcitation conditions, such as intensity and pump wavelength. Our results provide valuable insight into the ultrafast dynamics between free carriers, bright excitons and dark excitons. |
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