Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B57: 2D Semiconductors: Optical SpectroscopyFocus
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Sponsoring Units: DMP DCOMP Chair: Hanan Dery Room: Mile High Ballroom 3A |
Monday, March 2, 2020 11:15AM - 11:51AM |
B57.00001: Manipulating the potential landscape of 2D materials through external dielectric screening Invited Speaker: Archana Raja Atomically thin, quasi two-dimensional (2D) materials like semimetallic graphene and semiconducting transition metal dichalcogenide (TMDC) monolayers have been shown to have extraordinary optoelectronic properties, along with the possibility to tailor physical phenomena by assembling individual layers into novel heterostructures. The interaction between charge carriers in these 2D materials is strongly influenced by the local environment. By tuning the external dielectric screening, the band gap and exciton energies in 2D TMDCs can be modified to create lateral heterojunctions without modifying the material itself [1]. Using a combination of optical and angle-resolved photoemission spectroscopies with microscopic spatial resolution, we show that the band structure rigidly shifts in response to the change in local dielectric screening [2]. Furthermore, this environmental sensitivity can lead to a new type of disorder that leads to spatially inhomogeneous band gap and exciton energies as a consequence of spatial variations in the external dielectric screening rather than any material imperfections [3]. |
Monday, March 2, 2020 11:51AM - 12:03PM |
B57.00002: Nano-optical cavity imaging and control of interlayer exciton emission in WSe2/MoSe2 heterostructures Molly A May, Chenfeng Du, Tao Jiang, Kyoung-Duck Park, Xiaodong Xu, Markus B. Raschke, Markus B. Raschke Long lived interlayer excitons in transition metal dichalcogenide heterobilayers hold promise for applications from high temperature exciton condensates to nano-lasers with extended spatial coherence and other 2D optoelectronic devices. However, fundamental properties of interlayer excitons, their relaxation processes, and the mechanism for their formation through interlayer charge tunneling are still poorly understood. Furthermore, new methods for control of interlayer exciton emission and the associated nonradiative decay pathways are desired to establish high temperature exciton condensation and coherent emission. Here, we use a configurable nano-optical cavity based on a plasmonic scanning probe tip to quantify the interlayer exciton lifetime of (39 +/- 8) ps and charge transfer rate of (80 +/- 20) fs at room temperature. Further, using tip-sample force perturbation with deep sub-nm precision, we actively control interlayer exciton formation and tune the competition between nonradiative charge transfer and Purcell enhancement. We discuss the extension of this tip nano-cavity approach to low and variable temperature modalities for nano-spectroscopy, imaging, and control from dark excitons to localized states. |
Monday, March 2, 2020 12:03PM - 12:15PM |
B57.00003: Exciton valley depolarization in two-dimensional MoSe2 and WSe2 Min Yang, Dinh Van Tuan, Hanan Dery The valley degree of freedom is a sought-after quantum number in monolayer transition-metal dichalcogenides. Similar to the spins of electrons and holes, the valley degree of freedom is not a conserved quantity and excitons lose their original helicity over time. Valley depolarization of excitons in monolayer transition-metal dichalcogenides due to long-range electron-hole exchange typically takes a few ps at low temperatures. Exceptions to this behavior are monolayers MoSe2 and MoTe2 wherein the depolarization is much faster. We elucidate the enigmatic anomaly of these materials, finding that it originates from Rashba-induced coupling of the dark and bright exciton branches next to their degeneracy point. When photoexcited excitons scatter during their energy relaxation between states next to the degeneracy region, they reach the light cone after losing the initial helicity. The valley depolarization is not as fast in monolayers WSe2, WS2 and likely MoS2 wherein the Rashba-induced coupling is negligible. |
Monday, March 2, 2020 12:15PM - 12:27PM |
B57.00004: Deciphering the rich photoluminescence spectrum of monolayer WSe2. Hanan Dery High-quality monolayer WSe2 samples when gated and encapsulated by hBN exhibit unprecedented rich photoluminescence spectrum. In this talk, I will summarize what we (think we) already understand, identifying the signatures of dark and indirect excitons as well as of dark trions, along with their zone-center and zone-edge phonon replicas. I will then focus on the signatures of many-body interactions, proposing novel electron-assisted optical transitions of dark excitons and the brightening of these excitons when they are coupled to collective charge or spin excitations (plasmons and magnons). I will explain why these many-body interactions have the right conditions to manifest in the photoluminescence spectra of electron-doped tungsten-based monolayers. |
Monday, March 2, 2020 12:27PM - 12:39PM |
B57.00005: Candidates for the origin of many-body spectral features in monolayer WSe2. Dinh Van Tuan, Hanan Dery The combination of strong Coulomb interaction, spin and valley degrees of freedom give rise to multiple optical peaks in the spectrum of WSe2. While the physics of many peaks has been identified, the origin of a strong low-energy peak in highly electron-doped WSe2 remains elusive since its first observation in 2013 [1]. In this talk, we show possible explanations for the origin of this optical transition including the interaction of excitons with intervalley plasmons [2,3,4], or magnons induced by resident electrons. We explain why the proposed mechanisms are suitable for the peak to emerge in the spectrum of electron-doped WSe2 while being absent in hole-doped regime or in the whole spectrum of MoSe2 monolayer [5]. |
Monday, March 2, 2020 12:39PM - 12:51PM |
B57.00006: Saddle-point Excitons and Their Extraordinary Light Absorption in Two-Dimensional beta-phase Group-IV Monochalcogenides NanNan Luo, Chong Wang, Zeyu Jiang, Yong Xu, Xiaolong Zou, Duan Wenhui In two-dimensional (2D) materials, saddle-points in the electronic structure give rise to diverging density of states, which leads to intriguing physical phenomena useful for applications, including magnetism, superconductivity, charge density wave, as well as enhanced optical absorption. Using first-principles calculations, we show monolayer beta-phase of group-IV monochalcogenides are a new class of 2D materials that possess saddle-points. Due to the existence of saddle-points, a remarkable absorption peak within the fundamental gap is found in these materials. The properties of saddle-point excitons can be effectively tuned by both the strain and thickness of these materials. Importantly, the strong optical absorbance induced by saddle-point exciton absorptions and the appropriate band gap give ideal power conversion efficiencies as large as 1.11% for monolayer beta-SnSe, significantly higher than reported high-performance ultrathin solar cells using transition metal dichalcogenides. These results not only open new avenues for exploring novel many-body physics, but also suggest beta-phase MXs could be promising candidates for future optoelectronic devices. |
Monday, March 2, 2020 12:51PM - 1:03PM |
B57.00007: Temperature dependent valley polarization in WS2 heterostructures George Kioseoglou, Ioannis Paradisanos, Kathleen M McCreary, Aubrey T. Hanbicki, Leonidas Mouchliadis, Davoud Adinehloo, Vasili Perebeinos, Berend Thomas Jonker, Emmanuel Stratakis The absence of degeneracy in the valley indices of monolayer-TMDs serves as an essential property for the development of valleytronic devices. One criterion for the realization of such a device is to attain high polarization at RT. We examine vertical WS2 heterostructures and show that the material type used, significantly influences the valley polarization of WS2. The interaction between WS2 and graphene has a strong effect on the T-dependent depolarization, with a polarization of 24% at RT under near-resonant excitation. This contrasts with hBN- encapsulated WS2, which exhibits a RT polarization of 11%. The low depolarization rate in WS2/Graphene is attributed to a) rapid charge and energy transfer processes of the scattered excitons, b) absence of thermal dissociation of trions and thermally assisted dark-to-bright transitions and c) partial suppression of the T-dependent bandgap renormalization. Significant variations in the polarization are also observed at 4K. We propose that intervalley hole scattering in the VB proximity between the K and Γ points of WS2 is sensitive to the immediate environment, leading to the observed variations. |
Monday, March 2, 2020 1:03PM - 1:15PM |
B57.00008: Tunable Spontaneous Valley Coherence in Atomically Thin Semiconductors Mandar Sohoni, Muralidhar Nalabothula, Pankaj Jha, Tony Low, ANSHUMAN KUMAR An interesting feature of monolayer transition metal dichalcogenides (TMDCs) is the existence of two degenerate, but disparate, band extrema valleys in k space with different optical selection rules. In order to manipulate this “valley degree of freedom”, we should be able to control, and manipulate the coherence between the two valleys. |
Monday, March 2, 2020 1:15PM - 1:27PM |
B57.00009: Valley-selective optical Stark shift of exciton-polaritons in a monolayer semiconductor Trevor LaMountain, Jovan Nelson, Erik J Lenferink, Akshay Murthy, Vinayak D. Dravid, Nathaniel Patrick Stern Light provides a high-speed coherent medium for measurement and manipulation of electronic quantum states. Exploiting the optical selection rules of transition metal dicalchogenide monolayers (TMDs), the optical Stark effect allows for valley-selective control of energy levels using sub-resonant optical pulses. Recent discoveries have revealed that microcavity exciton-polaritons in TMDs preserve valley features while also incorporating many of the favorable properties of light. Here, we use polarization-dependent transient reflectance to demonstrate that the optical Stark effect can also be used for valley-selective manipulation of energy levels in WS2 exciton-polaritons. In the reflectance spectra we observe a simultaneous shift of both polariton branches when pump and probe are co-polarized, and no appreciable shift when they are cross-polarized. We find excellent agreement between measured data and a Lorentz oscillator model over a wide range of experimental parameters. The extracted polariton shift confirms the expected linear power dependence of the optical Stark effect. The polarization-dependent Stark shift of TMD exciton-polaritons provides a new tool for state control in coherent valleytronics. |
Monday, March 2, 2020 1:27PM - 1:39PM |
B57.00010: Anisotropic Optical Properties and Vibrational Characteristics of 2D Silicon Telluride Nanoplates Jiyang Chen, Romakanta Bhattarai, Xiao Shen, Jingbiao Cui, Thang Hoang The p-type semiconductor silicon telluride (Si2Te3) has a unique layered crystal structure with hexagonal closed-packed Te sublattices and Si-Si dimers occupying octahedral intercalation sites. The orientation of the silicon dimers leads to unique optical and electronic properties. Here, we report a combined experimental and computational study of the optical properties of individual Si2Te3 nanoplates (NPs). 2D Si2Te3 NPs with thickness of tens to hundreds of nanometers and diameter of tens of micrometers are synthesized by the chemical vapor deposition technique. Polarized reflection measurements at different temperatures show a 90-degree periodic change in the absorption coefficient, which indicates an anisotropic orientation of the Si-Si dimers and is in agreement with the theoretical calculation of the dielectric constants. Polarized and spatial Raman mapping measurements of single Si2Te3 NPs at different temperatures reveal various vibrational modes, which are in agreement with the Density Functional Theory calculation. |
Monday, March 2, 2020 1:39PM - 1:51PM |
B57.00011: Anomalous photovoltaic effect in van der Waals heterointerface Toshiya Ideue, Takatoshi Akamatsu, Ling Zhou, Sota Kitamura, Masaru Onga, Yuji Nakagawa, Joseph Laurienzo, Junwei Huang, Takahiro Morimoto, Hongtao Yuan, Yoshihiro Iwasa Van der Waals interfaces are unique platforms for novel properties and functionalities. In addition to being building blocks for functionalized devices such as p-n junctions or tunneling junctions, symmetry of the van der Waal interface has recently been attracting much attention, represented by Moiré physics in twisted interfaces [1-3] and pseudo landau level formation in graphene/black phosphorus heterostructures [4]. |
Monday, March 2, 2020 1:51PM - 2:03PM |
B57.00012: Deep-Learning-Based Image Segmentation Integrated with Optical Microscopy for Automatically Searching for Two-Dimensional Materials Satoru Masubuchi, Eisuke Watanabe, Yuta Seo, Shota Okazaki, Takao Sasagawa, Kenji Watanabe, Takashi Taniguchi, Tomoki Machida Deep-learning algorithms enable precise image recognition based on high-dimensional hierarchical image features. Here, we report the development and implementation of a deep-learning-based image segmentation algorithm in an autonomous robotic system to search for two-dimensional (2D) materials. We trained the neural network based on Mask-RCNN on annotated optical microscope images of 2D materials (graphene, hBN, MoS2, and WTe2). The inference algorithm is run on a 1024 × 1024 px2 optical microscope images for 200 ms, enabling the real-time detection of 2D materials. The detection process is robust against changes in the microscopy conditions, such as illumination and color balance, which obviates the parameter-tuning process required for conventional rule-based detection algorithms. Integrating the algorithm with a motorized optical microscope enables the automated searching and cataloging of 2D materials. This development will allow researchers to utilize unlimited amounts of 2D materials simply by exfoliating and running the automated searching process. |
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B57.00013: Chiral phonon replicas of dark excitonic states in monolayer WSe2 Erfu Liu, Jeremiah Van Baren, Takashi Taniguchi, Kenji Watanabe, Yia-Chung Chang, Chun Hung Lui We observe a set of replica luminescent peaks below the dark excitonic states (including excitons and trions) in monolayer WSe2. The replica redshift energies match the energies of the zone-center and zone-corner optical phonons. The phonon replicas exhibit parallel gate dependence and same g-factors as the dark excitonic states, but follow the valley selection rules of the bright excitonic states. Our results and symmetry analysis show that a K-valley dark excitonic state can decay into a left-handed chiral phonon and a right-handed photon, whereas a K’-valley dark excitonic state can decay into a right-handed chiral phonon and a left-handed photon. Such valley selection rules of chiral phonon replicas can be utilized to identify the valleys of the dark excitonic states and explore their chiral interactions with phonons. |
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