Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L51: Optical Spectroscopy of 2D Materials |
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Sponsoring Units: DCMP Chair: Yunqiu (Kelly) Luo, Cornell University Room: Mile High Ballroom 1D |
Wednesday, March 4, 2020 8:00AM - 8:12AM |
L51.00001: Raman enhancement of PTCDA on monolayer WSe2 Christine Muccianti, Bekele Badada, Sara Zachritz, Calley Eads, Angel Garlant, Oliver Monti, John Schaibley Monolayer transition metal dichalcogenide (TMD) semiconductors have garnered interest due to their direct band gap, strong excitonic effects, and valley optical selection rules. Organic semiconductors also host strongly bound excitons and allow for broadly tunable emission energies. Together, heterointerfaces of a molecular semiconductor adsorbed on a monolayer TMD are characterized by interesting effects including enhanced Raman scattering. These Raman enhancements are poorly understood but are attributed to different charge transfer mechanisms [1], [2], [3]. Here, we study a system of monolayer 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on WSe2, chosen for its type-II band alignment. We performed low temperature Raman and photoluminescence spectroscopy and observe a large enhancement of the PTCDA Raman signal compared to monolayer PTCDA on SiO2. The behavior of this Raman scattering is explored through electrostatic doping of the TMD and excitation energy dependence. Control over the Raman enhancement would yield new understanding of the Raman enhancement phenomena in these systems. |
Wednesday, March 4, 2020 8:12AM - 8:24AM |
L51.00002: Dark Excitons in Monolayers of Molybdenum based Transition Metal Dichalcogenide Cedric ROBERT, Piotr Kapuscinski, Alex Delhomme, Bo HAN, Takashi Taniguchi, Kenji Watanabe, Bernhard Urbaszek, Xavier Marie, Clement Faugeras, Marek Potemski Excitons with binding energies of a few hundreds of meV drive the optical properties of Transition Metal Dichalcogenide (TMD) monolayers. One can expect that the optoelectronic properties will change drastically whether the spin-forbidden dark excitons lie below or above the bright excitons. This exciton fine structure splitting was accurately determined recently for WS2 and WSe2 monolayers using various experimental techniques [1-5]. In contrast the energy of the dark exciton in MoS2 ML has not been measured yet though this was the first member of the TMD family to be established as a direct gap semiconductor in the monolayer form. |
Wednesday, March 4, 2020 8:24AM - 8:36AM |
L51.00003: Effect of many-body interactions on the valley-selective optical Stark in WS2 Paul Cunningham, Aubrey T. Hanbicki, Thomas L Reinecke, Kathleen M McCreary, Berend Thomas Jonker Breaking the valley degeneracy in monolayer transition metal dichalcogenides through the valley-selective optical Stark effect can be exploited for quantum valleytronic operations such as coherent manipulation of valley superposition states. The strong light-matter interactions that give rise to the optical Stark effect have historically been described by a two-level dressed-atom model, which assumes non-interacting particles. While this model works well far from resonance where the Rabi frequency is larger than the exciton formation rate, here we show experimentally that it does not apply for excitation near resonance in monolayer WS2. Instead, our observations are well described by an excitonic model of the optical Stark effect that includes many-body Coulomb interactions. We observe a blue-shift due to the valley-selective optical Stark effect for excitation both below and above resonance, confirming the prediction from this theory that repulsion between virtual excitons dominates the light-matter interactions for photoexcitation detuned from resonance by less than the exciton binding energy. We expect our findings to be general to low-dimensional semiconductors that support bound excitons and other many-body Coulomb interactions. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L51.00004: Direct Observation of Gate-Tunable Dark Trions in Monolayer WSe2 Tianmeng Wang, Zhipeng Li, Zhengguang Lu, Mandeep Khatoniar, zhen lian, Yuze Meng, Mark Blei, Takashi Taniguchi, Kenji Watanabe, Stephen A McGill, Sefaattin Tongay, Vinod M Menon, Dmitry Smirnov, Sufei Shi Spin-forbidden intravalley dark excitons in tungsten-based transition-metal dichalcogenides (TMDCs) have attracted intense research interest. It has been discovered that tungsten-based TMDCs such as WSe2 and WS2 have a unique bandstructure, in which the spin-orbit coupling also induces the splitting of the conduction band. The resulted ground state of the exciton, however, is a spin-triplet state as the spin-forbidden dark exciton. We show that we can control the dark exciton electrostatically by dressing it with one free electron or free hole, forming the dark trions. The existence of the dark trions is suggested by the unique magneto-photoluminescence spectroscopy pattern of the monolayer WSe2 device. And the existence of the dark trions is unambiguously demonstrated by directly resolving the radiation angle of the dark trions through back focal plane imaging. The dark trions possess binding energy of ∼15 meV, and inherit the long lifetime and large g-factor from the dark exciton. The dark trions open the door to new possibilities of valleytronics and excitonic applications. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L51.00005: Spin-allowed quantum emission from strain confinement in monolayer semiconductors Leo Yu, Minda Deng, Jingyuan Linda Zhang, Sven Borghardt, Beata Kardynal, Jelena Vuckovic, Tony F Heinz Here we demonstrate quantum emitter behavior from strained monolayer MoSe2. Exciton confinement has been achieved using strain associated with causing the monolayer to conform to a nanoscale indentation prepared in the substrate. By sufficiently reducing the lateral scale of the confinement, we have achieved single quantum emitter behavior, with the A exciton feature showing a drop in linewidth to below 0.2 meV at cryogenic temperatures, in an emission 70-meV redshifted from the bulk A exciton feature of the unstrained monolayer. The quantum emitter characteristics of these confined excitons has been demonstrated directly by g(2) measurements. Unlike recent demonstrations of quantum emitters in monolayer WSe2, the exciton involved for our MoSe2 samples has a spin aligned configuration. This was demonstrated by Zeeman measurements indicating a g factor of 4, and was also reflected in the relatively short, 0.2 ns measured photoluminescence lifetimes. Our approach has the potential for creating spin-allowed quantum emitters that can be arbitrarily located laterally and tuned by the engineered strain properties. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L51.00006: Propagation of excitons in TMDC monolayers with suppressed disorder Koloman Wagner, Jonas Zipfel, Jonas D. Ziegler, Raul Perea Causin, Roberto Rosati, Samuel Brem, Marvin Kulig, Takashi Taniguchi, Kenji Watanabe, Ermin Malic, Mikhail Glazov, Alexey Chernikov Coulomb-bound electron-hole pairs, or excitons, dominate the electro-optical properties in single layers of semiconducting transition metal dichalcogenides (TMDCs) and their heterostructures. In these 2D systems, excitons can move freely across large distances1. We study exciton propagation by directly monitoring their spatial behavior in single layers of TMDCs by spatially- and time-resolved photoluminescence microscopy2. In high-quality boron nitride encapsulated samples with strongly suppressed disorder3 we find very efficient diffusion at low excitation densities and a strongly non-linear behavior at high densities. Single layers of encapsulated TMDCs provide a highly promising platform to explore exciton transport phenomena, including low-temperature conditions with suppressed phonon-scattering and manipulation of spin-valley polarized excitations. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L51.00007: Nonlinear responses in inversion-breaking 2D lattices Qiong Ma, Zhiren Zheng, Huitao Shen, Hiroki Isobe, Yang Zhang, Zhen Bi, Kenji Watanabe, Takashi Taniguchi, Liang Fu, Suyang Xu, Nuh Gedik, Pablo Jarillo-Herrero Sensing, detecting and manipulating electromagnetic waves are in general nonlinear processes based on optical or electrical rectification processes. Such processes are conventionally based on semiconductor p-n junctions, in which a strong built-in electric field provides the directionality to rectify signals. Such a mechanism has been challenging in the THz and infrared regimes, calling for new physics and materials. Intriguingly, the intrinsic crystal directionality, set by either the atomic lattice or spin orientation, can be translated into directional properties of electron quantum wavefunctions (Berry phase, Berry curvature, etc.), and rectify electron’s motion in a dramatic way. In this talk, I will show that a dipole moment of Berry curvature can lead to a strong nonlinear (AC excitation, DC output or double-frequency output) Hall effect in non-magnetic bilayer WTe2 without external magnetic field. I will also show our recent studies of nonlinear electrical and optical responses of bilayer graphene whose inversion symmetry can be broken by an out-of-plane electric field. |
Wednesday, March 4, 2020 9:24AM - 9:36AM |
L51.00008: Polarizability of finite-bandwidth two-dimensional electron gas Kaveh Khaliji, Tobias Stauber, Tony Low The two-dimensional electron gas (2DEG), characterized by its parabolic electron energy dispersion, has been the subject of intense research for more than half a century. The main fuel to this interest has been its simplicity and physical relevance to describe the low energy behavior of most known 2D electronic systems, up to a point where any deviation from a 2DEG behavior (such as what was observed in graphene) has been pinpointed as anomalous and receives extra attention. Following this, here, we ask what would happen to the polarizability if we restrict the parabolic energy dispersion to a finite bandwidth (FBW), for which the 2DEG behavior would then be a natural limit (i.e. allowing for the bandwidth to be infinite). We discuss the polarizability of the FBW-2DEG, in dynamical and static limits, as the key physical quantity to obtain elementary excitation spectra, collective modes, charged impurity-limited transport properties, and spin-spin exchange interaction. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L51.00009: Quasiparticle and optical properties of monolayer C3N Zhao Tang, Weiyi Xia, Yabei Wu, Wenqing Zhang, Peihong Zhang Highly ordered honeycomb structure carbon nitride (C3N) is an emerging 2D material that has attracted much interest recently. The (indirect) band gap of this material has recently be predicted to be about 1.5 eV [1]. In this work, we present fully converged GW+BSE results for monolayer C3N. The calculated the GW band gap of monolayer C3N is about 1.5 eV, an ideal value for electronics applications. Detailed investigations of the band-edge electronic states suggest a strong band edge absorption. Our fully converged GW+BSE calculation predicts an optical gap about 1.9 eV, with an excitonic binding energy of 0.7 eV. |
Wednesday, March 4, 2020 9:48AM - 10:00AM |
L51.00010: Hyperbolic Plasmon Polaritons in the Delafossite PdCoO2 Francesco Ruta, Seunghyun Khim, Gaurab Rimal, Brian S Y Kim, Kevin Kam, Aaron Sternbach, Yinming Shao, Christopher Homes, Seongshik Oh, Andrew Mackenzie, Dmitri Basov Hyperbolic plasmon polaritons are uncommon light-matter modes in materials at the extreme of anisotropy - where metallic and dielectric characters coexist along different crystallographic directions. Using Kramers-Kronig analysis and finite-difference time-domain simulations, we have identified a new quasi-two-dimensional material, palladium cobaltate (PdCoO2), that is expected to support low-loss hyperbolic plasmon polaritons at infrared frequencies. Simulations are consistent with the polariton dispersion calculated from the complex reflection coefficient. The large optical anisotropy can be recognized by the order of magnitude difference between the in-plane and out-of-plane plasma frequencies. Scanning near-field optical microscopy can be used to study these light-matter modes. However, because hyperbolic polaritons travel inside the material and reflect off the surfaces, observing them requires a thin layer (50-150 nm) of PdCoO2 with smooth surfaces. This presents a challenge to an experimental demonstration of hyperbolic modes in PdCoO2 since the material does not exfoliate readily from flux-grown single crystals and forms twin domains in films grown by molecular beam epitaxy. Efforts to isolate flat, thin flakes of PdCoO2 for near-field measurements will be presented. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L51.00011: Directional shift current and dipole selection rules in the layered semiconductor BC2N Julen Ibañez-Azpiroz, Ivo Souza, Fernando de Juan We study the shift-current optical response in a noncentrosymmetric polytype of the quasi-2D layered semiconductor BC2N [1]. Employing a recently developed first-principles Wannier-interpolation technique [2] implemented in the latest release of Wannier90 [3], we find that the photoconductivity exhibits two distinctive features at the band edge. First, it ranks among the largest bulk nonlinear responses reported to date, and peaks in an energy range suitable for optical manipulation. Secondly, it is strongly anisotropic, due to the vanishing of particular tensor components not foretold by phenomenological symmetry arguments; this is a consequence of dipole selection rules imposed by mirror symmetry, which imply that the relative parities between valence and conduction bands are key for determining the directionality of the band-edge response. The implications of the dipole selection rules should apply to a broad class of nonlinear responses. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L51.00012: First-principles calculations for high-harmonic generation in two-dimensional materials: from single atomic layer to thin films Shunsuke Yamada, Kazuhiro Yabana We present first-principles studies on high-harmonic generation (HHG) in thin materials using the time-dependent density functional theory (TDDFT). HHG in reflected and transmitted waves from a thin film exposed to intense light pulses is modulated from the bulk optical response by light propagation effect and surface effect. We have developed a real-time simulation method combining the time-dependent Kohn-Sham and the Maxwell equations for describing the microscopic dynamics of electrons and electromagnetic field in a thin film. This method can take into account both the propagation and surface effects. Using our method, we investigate the dependencies of HHG on the light intensity, polarization, and film thickness for semiconductor films. We discuss the mechanism of HHG signal growth in reflected and transmitted waves. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L51.00013: Quasiparticle and optical properties of hexagonal boron nitride: from monolayer to bulk Weiyi Xia, Weiwei Gao, Peihong Zhang Hexagonal boron nitride (h-BN), an emerging building block for van der Waals heterostructures, has become a research focus in recent years. Interesting, some of the most fundamental aspects of this material are still not fully understood. For example, there are still debates on whether the fundamental band gap is direct or indirect, for both monolayer and bulk h-BN. To the best of our knowledge, the quasiparticle band gap of few-layer h-BN has not been accurately determined. In this talk, we will present fully converged GW+BSE results for monolayer, bilayer and bulk h-BN, aiming to resolve some of the controversies and illustrate the effects of dielectric screening and interlayer interaction on the quasiparticle and optical properties of this material. |
Wednesday, March 4, 2020 10:36AM - 10:48AM |
L51.00014: Optical Absorption and Emission of Two-Dimensional Tetracene Crystals Seonghyun Koo, Sunmin Ryu Organic crystals have received much attention because of their unique properties different from inorganic crystals such as mechanical flexibility and high quantum yield of luminescence. Although various organic solids are used in many industrial applications, the electronic structures of low-dimensional forms are far from being understood not only because their van der Waals interactions between molecules are intractable but also because it has been a challenge to prepare such crystalline systems with sufficient stability. In this work, we present absorption and emission spectroscopy on single and few-layer tetracene (Tc) layers sandwiched between two-dimensional (2D) inorganic crystals including graphene and hexagonal BN. Tc deposited by thermal evaporation formed flat films with thickness of multiples of its minimum (~1.3 nm) and long-range order confirmed by polarized spectroscopy. Vibrational progressions and Davydov splitting observed in the absorption and emission spectra revealed significant deviation from their bulk counterparts, which suggests distinctive crystalline structures and dielectric environments of 2D Tc. We will also discuss structural details and excitonic dynamics probed by time-resolved spectroscopy. |
Wednesday, March 4, 2020 10:48AM - 11:00AM |
L51.00015: Low efficiency of the exciton-phonon scattering involving large momentum transfer in 2D-TMDC Hanz Yecid Ramírez Atomically thin transition metal dichalcogenide sheets are among the most promising systems for efficiently controlled emission of quantum light. However, like in all solid-state based emitters, temperature is a critical parameter to have into account, because of the ever present decoherence induced by phonons. |
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