Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session R57: 2D Semiconductors: SpectroscopyFocus Live
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Sponsoring Units: DMP Chair: Aubrey Hanbicki, Laboratory for Physical Sciences |
Thursday, March 18, 2021 8:00AM - 8:36AM Live |
R57.00001: Tracking magnons, spin flips, and spin canting with magneto-Raman spectroscopy in atomically thin CrI3 Invited Speaker: Amber McCreary The discovery of 2-dimensional (2D) materials, such as CrI3, that retain magnetic ordering at monolayer thickness has resulted in a surge of both pure and applied research in 2D magnetism. Here, we report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two additional features in the spectra that appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Spin canting by applying the magnetic field parallel to the atomic layers is also investigated. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3. |
Thursday, March 18, 2021 8:36AM - 8:48AM Live |
R57.00002: Raman studies of a natural van der Waals heterostructure Viviane Z. Costa, Liangbo Liang, Addison Miller, Sam Vaziri, Shirin Jamil, Andrew Ichimura, Eric Pop, Akm Newaz Van der Waals heterostructures (vdWH) comprised of two-dimensional (2D) materials offer a platform to obtain materials by design with unique electronic properties. Franckeite (Fr) is a naturally occurring vdWH comprised of two distinct alternately stacked semiconducting layers. Because it is naturally heterostructured, it enables the study of a complex layered system without the presence of fabrication defects, and where the crystal orientation between layers has been preserved. Unlike other layered sulfide-based 2D materials, the exfoliation of thin flakes of Fr is especially challenging, leading to a knowledge gap of its fundamental properties. Raman spectroscopy is a powerful method to characterize and study the fundamental properties of 2D crystals. All the power this technique provides in characterizing 2D materials stems from well-established literature on the signature of these crystals, which is lacking for Fr. In this work, we performed an extensive micro-Raman spectroscopy study of Fr. We show resonance-enhanced Raman signal, the evolution of the Raman peaks for increasing temperature, and the presence of breathing and shearing modes. Moreover, we performed theoretical calculations to fully characterize the vibrational modes of Fr. |
Thursday, March 18, 2021 8:48AM - 9:00AM Live |
R57.00003: Probing valley population imbalance in TMDs with T-dependent SHG E. Stratakis, L. Mouchliadis, S. Psilodimitrakopoulos, G. Maragkakis, I. Demeridou, G. Kourmoulakis, A. Lemonis, George Kioseoglou Valleys in transition metal dichalcogenides (TMDs) provide an additional degree of freedom that can be used for information transport and storage. When TMDs interact with intense laser light, the SHG-field inherits characteristics that reflect not only the broken inversion symmetry in real space, but also the valley anisotropy in reciprocal space. The latter is present whenever there exists a valley population imbalance (VPI) between the two valleys and affects the polarization state of the detected SHG. In this work, it is shown that the temperature-induced change of the SHG intensity dependence on the excitation field polarization, is a unique fingerprint of VPI. In particular, VPI mapping based on polarization-resolved raster-scanning imaging microscopy was performed inside a cryostat to generate the SHG contrast in the presence of VPI, from every point of a TMD flake. The generated contrast is marked by rotation of the SHG intensity polar diagrams at low temperatures and is attributed to the VPI-induced SHG. |
Thursday, March 18, 2021 9:00AM - 9:12AM Live |
R57.00004: Controlling interlayer excitons in MoS2 layers grown by chemical vapor deposition Ioannis Paradisanos, Shivangi Shree, Antony George, Nadine Leisgang, Cedric ROBERT, Kenji Watanabe, Takashi Taniguchi, Richard J. Warburton, Andrey Turchanin, Xavier Marie, Iann C Gerber, Bernhard Urbaszek Combining MoS2 monolayers to form multilayers allows to access new functionalities. Deterministic assembly of large area van der Waals structures requires concrete indicators of successful interlayer coupling in bilayers grown by chemical vapor deposition. In this work, we examine the correlation between the stacking order and the interlayer coupling of valence states in both as-grown MoS2 homobilayer samples and in artificially stacked bilayers from monolayers, all grown by chemical vapor deposition. We show that hole delocalization over the bilayer is only allowed in 2H stacking and results in strong interlayer exciton absorption and also in a larger A-B exciton separation as compared to 3R bilayers. Comparing 2H and 3R reflectivity spectra allows to extract an interlayer coupling energy of about t⊥ = 49meV. Beyond DFT calculations including excitonic effects confirm signatures of efficient interlayer coupling for 2H stacking in agreement with our experiments.1 |
Thursday, March 18, 2021 9:12AM - 9:24AM Live |
R57.00005: Multipath optical recombination of intervalley dark excitons and trions in monolayer WSe2 Erfu Liu, Jeremiah van Baren, Ching-Tarng Liang, Takashi Taniguchi, Kenji Watanabe, Nathaniel Monroe Gabor, Yia-Chung Chang, Chun Hung Lui Excitons and trions in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violate the conservation of momentum. Here we reveal the intervalley recombination of dark excitons and trions through more than one path in monolayer WSe2. We observe the intervalley dark excitons, which can recombine by the assistance of defect scattering or chiral-phonon emission. We also reveal that a trion can decay in two distinct paths – through intravalley or intervalley electron-hole recombination – into two different final valley states. Although these two paths are energy degenerate, we can distinguish them by lifting the valley degeneracy under a magnetic field. The intra- and inter-valley trion transitions are coupled to zone-center and zone-corner chiral phonons, respectively, to produce distinct phonon replicas. The observed multipath optical decays of dark excitons and trions provide much insight into the internal quantum structure of trions and the complex excitonic interactions with defects and chiral phonons in monolayer semiconductors. |
Thursday, March 18, 2021 9:24AM - 9:36AM Live |
R57.00006: First-principles study of exchange-induced valley splitting in transition metal dichalcogenide monolayers Elizabeth Peterson, Jeffrey Neaton Monolayer transition metal dichalcogenides (TMDs) with spin-valley coupling are an emerging class of materials for valleytronics applications. Magnetic substrates have been shown to produce larger valley splitting than magnetic fields. In this work, we develop intuition for the physical mechanism driving valley splitting via magnetic substrates by performing first-principles density functional theory calculations for a series of Fe-decorated WSe2 and MoS2 monolayers. The valley splitting is computed as a function of different Fe heights above the TMD surface, different Fe alignment, and different Fe coverages. The non-Zeeman-like behavior of the valence band eigenvalues in the presence of variable magnetic atom position are rationalized using a magnetic impurity Hamiltonian, where the DFT trends in valley splitting can be recovered to second-order in the magnetic exchange coupling, a term that is strongly sensitive to TMD 4d/5d-Fe 3d orbital overlap. These computed trends are used to rationalize prior experiments involving magnetic exchange coupling-induced valley splitting, and are used to suggest new substrates to achieve large valley splitting. |
Thursday, March 18, 2021 9:36AM - 9:48AM Live |
R57.00007: Excitonic dispersion in monolayer C3N Miki Bonacci, Elisa Molinari, Alice Ruini, Marilia J Caldas, Andrea Ferretti, Daniele Varsano Monolayer C3N [1] is a 2D indirect band gap semiconductor with appealing mechanical, thermal, and electronic properties. In this work we present a full characterization of C3N optical properties, focusing on the so-called momentum-resolved exciton band structure. The study is performed using GW+BSE approach for zero and finite momentum transfer, as implemented in the Yambo code [2]. |
Thursday, March 18, 2021 9:48AM - 10:00AM Live |
R57.00008: Quasiparticle and Optical Properties of Two Dimensional C3N and C3B Zhao Tang, Fanhao Jia, Weiyi Xia, Yabei Wu, Peihong Zhang Highly ordered honeycomb structures carbon nitride (C3N) and boron carbide (C3B) are emerging 2D materials that have attracted much interest. In this work, we present fully converged GW results for monolayers C3N and C3B, and the C3N/C3B bi-layer heterostructure. The converged GW bandgap of monolayer C3N (C3B) is about 1.5 (2.6) eV; these results call for future experimental verifications. We have also carried out highly converged GW+BSE calculations of these systems. The nearly parallel valence and conduction states give rise to extremely strong optical absorption, suggesting potentials for future 2D optoelectronics applications. We will also report details of the excitonic structures of these materials. |
Thursday, March 18, 2021 10:00AM - 10:12AM Live |
R57.00009: Measurements of cyclotron resonance of the interfacial states in strong-spin orbit coupled 2D electron gases proximitized with aluminum Prashant Chauhan, Candice Thomas, Tyler Lindemann, Geoff C Gardner, Jan Gukelberger, Roman Lutchyn, Michael Manfra, Peter Armitage The two-dimensional electron gas (2DEG) in InAs proximitized by aluminum (Al) is a promising platform for topological qubits based on Majorana zero modes. However, there are still substantial uncertainties associated with the nature of the electronic states at the interfaces of these systems. In this work, we have investigated InAs quantum well heterostructures with In0.8Ga0.2As or In.85Al.15As barrier and Al overlayers using high precision time-domain THz spectroscopy. In magnetic field, a prominent cyclotron resonance is observed that can be associated with the response of the interfacial states. Measurements of the THz range complex Faraday rotation allow the extraction of the sign and magnitude of the effective mass, the density of charge carriers, and scattering times. We compare the results of measurements with numerical calculations |
Thursday, March 18, 2021 10:12AM - 10:24AM Live |
R57.00010: Optically probing tunable band topology in atomic monolayers Gaofeng Xu, Tong Zhou, Benedikt Scharf, Igor Zutic Experiments on Bi monolayers on a SiC substrate reveal an interplay between a huge topologically nontrivial gap ~0.8 eV and strong spin-orbit coupling (SOC), leading to striking transport properties such as a robust quantum spin Hall effect (QSHE) [1]. With a suitable choice of substrates it is also possible to remove valley degeneracy and realize multiple Hall effects in a single materials system [2]. In contrast to transport properties, much less is known about how an optical response could yield topological signatures in these group V monolayers. By combining first-principles calculations and a careful inclusion of strong SOC in effective models as well as the Coulomb interaction in these monolayers, we show that the changes in optical response reveal topological properties inherent to these systems [3]. We explain how these findings offer new opportunities for proximitized materials [4]. |
Thursday, March 18, 2021 10:24AM - 10:36AM Live |
R57.00011: Unveiling anisotropic exciton splitting and momentum-dark states in ReS2 Avijit Dhara, Devarshi Chakrabarty, Pritam Das, Aswini Kumar Pattanayak, Shreya Paul, Shreyashi Mukherjee, Sajal Dhara ReS2 has a distorted 1T crystal structure, exhibiting non-equilibrium excitonic properties in electrically decoupled van der Waals layers. Our study finds four excitonic emissions which are highly polarized with dipole moments oriented along different directions with respect to the crystal axis. Dipole orientations of two newly discovered excitons X3 and X4 are aligned along two strong previously known excitons X1 and X2 respectively. In addition, we observe an anomalous variation in the photoluminescence intensity near 30K which is the signature of momentum-forbidden dark states below the excitonic resonances, as indicated by our rate equation model. Our work shed light on the quasi-direct nature of the electronic band structure for ReS2 multilayer system. |
Thursday, March 18, 2021 10:36AM - 10:48AM Live |
R57.00012: Directional shift current and dipole selection rules in the layered semiconductor BC2N Julen Ibañez, Ivo Souza, Fernando De Juan
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Thursday, March 18, 2021 10:48AM - 11:00AM Live |
R57.00013: Room-temperature diffusion of excitons in WS2 monolayers with suppressed disorder Koloman Wagner, Jonas Zipfel, Marvin Kulig, Raül Perea-Causín, Samuel Brem, Jonas-David Ziegler, Roberto Rosati, Takashi Taniguchi, Kenji Watanabe, Mikhail Glazov, Ermin Malic, Alexey Chernikov Excitons are found to dominate the electro-optical properties of single layers of semiconducting transition metal dichalcogenides (TMDCs) and their heterostructures. In addition they are also able to move freely across the two-dimensional plane, motivating detailed studies of the exciton transport in these materials. However, their intrinsic properties can be often obscured by inhomogeneities such as the fluctuations of the dielectric environment [1]. In our study, we address the topic of exciton propagation by using WS2 monolayers encapsulated in high-quality boron nitride, where long-range disorder effects are strongly suppressed [2]. Monitoring the spatial behavior of excitons through spatially- and time-resolved photoluminescence microscopy we find strongly enhanced efficient diffusion at low excitation densities in contrast to pristine, as-exfoliated flakes. In addition, we find a pronounced non-monotonous density-dependence of the effective diffusivity at elevated densities. We discuss our findings in view of the complex multi-valley structure and propose a mechanism for diffusion facilitated by free-electron hole plasma from entropy-ionized excitons. |
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