Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A15: 2D Materials (Semiconductors) -- Monolayers |
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Sponsoring Units: DMP DCOMP Chair: Vinod Sangwan, Northwestern University Room: BCEC 154 |
Monday, March 4, 2019 8:00AM - 8:12AM |
A15.00001: Shortwave Coulomb excitations and local-field effects in monolayer transition-metal dichalcogenides. Hanan Dery, Dinh Van Tuan, Benedikt Scharf, Igor Zutic Many-body interactions in monolayer transition-metal dichalcogenides are strongly affected by local-field effects and their spin-split band structure. The former is caused by a strong contribution of umklapp processes to Coulomb excitations between the time-reversed valleys, where the effect is stronger for conduction-band electrons because of the nature of their atomic orbital. As a result, the blueshift of the neutral exciton, X0, in electron-doped samples can be larger than 10 meV when the electron density increases from 0 to 5x1012 cm-2, while the blueshift in hole-doped samples is nearly absent. We develop an analytical theoretical model that elucidates the important role played by intervalley plasmons and local-field effects. We compute the energy shift of X0 as a function of charge density and show that similar to experiment, the blueshift is evident only in electron-doped conditions, and that it is stronger in MoSe2 than in WSe2 due to differences in their band ordering and direct vs indirect exciton energies. In addition, the theory elucidates the observed emergence of exciton-plasmon peak in electron-doped WSe2. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A15.00002: Impact of dielectric environment on exciton binding energy in monolayer WS2 and WSe2 Wei-Ting Hsu, Jiamin Quan, Chun Yuan Wang, Li-Shuan Lu, Wen-Hao Chang, Xiaoqin (Elaine) Li, Chih-Kang Shih The large exciton binding energy in monolayer transition metal dichalcogenides (TMDs) was determined recently. The robust excitons open a venue to explore the exciton physics such as Bose-Einstein condensation at room temperature. Recent reports further demonstrated the Coulomb engineering via dielectric environment based on a few-layer graphene. However, due to the conducting nature, quenching of optical transitions is often unavoidable. Thus, it is desirable to show the tunability using insulating dielectrics. Here we investigate the impact of dielectric environment on exciton binding energy and quasiparticle bandgap in monolayer WS2 and WSe2 by exciton Rydberg spectroscopy. The dielectric constant is systematically varied from κ = 1.49 to 3.82. We found that, with increasing κ, the exciton binding energy and quasiparticle bandgap exhibit significant reductions. We found the model using nonlocally-screened Keldysh potential captures the results very well. Our work validates the applicability of Keldysh model which can be used to design TMD-based optoelectronic devices in different dielectric media. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A15.00003: Band Symmetries in Two-Dimensional Materials Edward Aris Fajardo, Roland Winkler The symmetries of the energy bands are of fundamental importance for understanding many properties of a material. Here we develop a general scheme to determine the irreducible representations of Bloch functions for a given wave vector. Using a tight-binding picture and exploiting the fact that the atomic orbitals are localized in the vicinity of the atomic sites, we demonstrate that this problem can be factorized into one characterizing the atomic orbitals times one characterizing the crystal-periodic plane waves. Each of these subproblems permits a universal classification, independent of the details of a particular crystal structure. We apply this general scheme to two-dimensional materials including transition metal dichalcogenides (such as MoS2, WS2, MoSe2, and WSe2) and few-layer graphene. We demonstrate that the irreducible representations characterizing the energy bands are not always uniquely determined by the symmetry of a crystal structure. However, we also show that this ambiguity does not affect observable physics such as selection rules or the effective Hamiltonians for Bloch states that can be derived by means of the theory of invariants. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A15.00004: Direct observation and gate manipulation of dark trions in monolayer WSe2 Erfu Liu, Jeremiah van Baren, Zhengguang Lu, Takashi Taniguchi, Kenji Watanabe, Dmitry Smirnov, Chun Hung Lui Dark trions, the bound states between a dark exciton and an electron (hole), are intriguing entities with novel applications, because their long lifetime and finite net charge allow us to efficiently control the excitonic dynamics by electric field. Detection of dark trions is, however, exceedingly challenging due to their optical inactivity. Prior research required indirect detection techniques, such as altering their spins with in-plane magnetic field and coupling to surface plasmons. Here we report the direct observation and gate manipulation of intrinsic dark trions in monolayer WSe2. By using ultraclean WSe2 devices encapsulated by boron nitride, we can directly resolve the weak photoluminescence of spin-forbidden dark trions and continuously tune between negative and positive charged dark trions with electrostatic gating. We also reveal their spin triplet configuration and distinct valley emission by their characteristic Zeeman splitting under magnetic field. The dark trions exhibit large binding energy (14-16 meV) and narrow line width (2.5 meV), signifying their high stability and long lifetime. Such robust and directly detectable dark trions provide a crucial component to realize electrically controllable trion transport in two-dimensional materials. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A15.00005: Observation of trion Rydberg states in monolayer MoSe2 Chun Hung Lui, Erfu Liu, Jeremiah van Baren, Takashi Taniguchi, Kenji Watanabe, Hongyi Yu, Wang Yao, Zhengguang Lu, Dmitry Smirnov Trions in monolayer transition metal dichalcogenides (TMDs) attract much attention due to their large binding energy (20 ~ 50 meV). The high stability of trions makes them possible to form excited states. Here we report the observation of trion Rydberg states in monolayer TMD. By measuring the photoluminescence of ultraclean boron-nitride-encapsulated MoSe2, we reveal the first and second excited Rydberg states of trions. The excited-state trions exhibit much stronger interaction with the Fermi sea as well as higher valley polarization and coherence than the ground-state trions. The results reveal rich physics of few-body quantum states in two-dimensional systems. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A15.00006: The Born-Oppenheimer approximation in graphene: A time-dependent perspective Vaibhav Mohanty, Eric Johnson Heller In graphene, electron-phonon interactions are known to play an important role in the loss of electronic wavefunction character and relaxation processes following photoexcitation. We model electronic interactions with nuclear vibrations from a microscopic, time-dependent perspective. Utilizing a time-dependent tight-binding Hamiltonian for the electronic degrees of freedom, we numerically determine the time-evolved electronic wavefunction in the presence of classical nuclei vibrating along normal modes. We examine the solutions by comparing them to those predicted within the adiabatic Born-Oppenheimer (ABO) approximation. We find that, for electronic states on energetically isolated potential energy surfaces, the adiabatic Born-Oppenheimer (ABO) approximation offers an accurate picture of time-evolution. But, in the presence of avoided crossings, the ABO approximation quickly breaks down as the electronic wavefunction becomes a superposition of ABO basis states. Moreover, electronic character is preserved over several vibrational periods for a finite lifetime, indicating highly diabatic time-evolution. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A15.00007: Transport and photoluminescent characterization of high-quality single layer WSe2 devices Kateryna Pistunova, Luis Jauregui, Andrew Y Joe, Kristiaan De Greve, Andrey Sushko, Daniel A Rhodes, James Hone, Hongkun Park, Mikhail Lukin, Philip Kim Single layer semiconducting transition metal dichalcogenides (TMD) are direct band gap |
Monday, March 4, 2019 9:24AM - 9:36AM |
A15.00008: The stable trion states in 2D WSe2 monolayer Shalva Tsiklauri, Roman Kezerashvili We study the binding energies of negatively (two electron and one hole) and positively (one electron and two holes) charged trions in suspended two-dimensional monolayer of WSe2 in the framework of effective-mass model by employing the method of hyperspherical harmonics in configuration space [1,2]. The binding energies of trions are calculated using the Ritova-Keldysh potential. Trion fine structure based on formation of intravalley trions in spin singlet (S=1/2) state and intervalley trions in triplet (S=3/2) state is addressed. Our calculations show that those state are stables. To understand the importance of dielectric screening on the formation of trions, we perform calculations of the binding energy for WSe2 placed in three different dielectric environments: supported on an SiO2 substrate, supported on an h-BN substrate, and encapsulated by h-BN, and compare these results with the case of suspended WSe2 binding energy. The analysis and comparison of our results for the binding energies of trions with those calculated via different theoretical methods and experimental data are presented. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A15.00009: Revealing the Unusual Excitonic Complexes in BN Encapsulated Monolayer WSe2 Zhipeng Li, Tianmeng Wang, Zhenguang Lu, Chenhao Jin, Yanwen Chen, Yuze Meng, Zhen Lian, Ting Cao, Takashi Taniguchi, Kenji Watanabe, Shengbai Zhang, Dmitry Smirnov, Sufei Shi Strong Coulomb interactions in single-layer transition metal dichalcogenides (TMDs) result in the emergence of strongly bound excitons, trions and biexcitons. These excitonic complexes possess the valley degree of freedom, which can be exploited for quantum optoelectronics. However, in contrast to the good understanding of the exciton and trion properties, the binding energy of the biexciton remains elusive, with theoretical calculations and experimental studies reporting discrepant results. In this work, we present the high-quality low-temperature photoluminescence (PL) spectra of BN encapsulated monolayer WSe2, which directly reveals the biexciton state only exists in charge neutral WSe2 and one free electron binds to a biexciton and forms the trion-exciton complex, the binding energy is ~17 meV and ~49 meV for biexciton and trion-exciton complex, respectively. The magneto-PL also reveals unambiguous evidence of the dark exciton. The improved understanding of the biexciton, trion-exciton complexes and other excitonic complexes improve our understanding of the many-body interaction in TMDs, promising novel application in low-dimensional quantum optoelectronics. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A15.00010: Edge states of two-dimensional materials Georgios Kopidakis, Georgios Vailakis, daphne davelou, Ioannis N. Remediakis Single layers of atomic thickness such as graphene, molybdenum disulfide and other transition metal dichalgogenides (TMDs) display unique electronic properties which depend on composition, dimensionality, strain, defects, chemical modification and nanostructuring, so that they may be engineered for specific applications. Intensive efforts to use two-dimensional (2D) materials in a wide range of technologies highlight the importance of edges in nanoribbons and nanoflakes. We will present theoretical results based on density functional theory calculations combined with simple models which clarify the effect of strain on TMD electronic and dielectric properties [1], the role of the metallic edge states in these otherwise semiconducting materials, especially in quasi-1D and 0D systems, and nanostructure stability [2]. The common origin of the edge states in TMD and graphene nanoribbons, as well as their main differences, will also be discussed. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A15.00011: 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 of bulk h-BN is direct or indirect, and, to the best of our knowledge, the quasiparticle band gap of monolayer 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. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A15.00012: Origins of spectral doublets from 2D semiconductors embedded in optical microcavities Trevor LaMountain, Hongfei Zeng, Pufan Liu, Nathaniel Stern Recent progress embedding atomically-thin materials such as monolayer transition metal dichalcogenides (TMDs) in microcavities has enabled applications such as nanolaser devices as well as novel regimes of polarization-sensitive exciton-polaritons [1]. Compared to the highly optimized III-V semiconductors typically used in microcavities, TMDs exhibit significantly more spatial inhomogeneity as well as additional tears, strains, and chemical contaminants introduced by the mechanical layer transfer process. We show how the spatial inhomogeneity of WS2 and MoS2 can produce spectral doublets that mimic the upper and lower polariton branches. We identify common pitfalls for misidentifying these doublets, as well as more robust measurements that can be used to distinguish the anti-crossing feature of exciton-polaritons. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A15.00013: Quantum yield engineering of quantum emitters in WSe2 by deterministically coupling to plasmonic nanocavities Yue Luo, Gabriella D. Shepard, Jenny V. Ardelean, Daniel A Rhodes, Bumho Kim, Katayun Barmak, James Hone, Stefan Strauf Solid-state single-quantum emitters are important resources for on-chip photonic quantum technologies. Efficient cavity-emitter coupling is required to realize quantum networks application. Recent studies explored the scalability aspect via spatially defined stressors to create quantum emitters from monolayer transition metal dichalcogenide semiconductor. Yet the low quantum yield of those quantum emitters is a crucial challenge to any real applications. Here we pressent a deterministic approach to achieve Purcell-enhancement at lithographically defined locations using the sharp corners of a Au nanocube for both electric field enhancement and to deform a two-dimensional material. This nanoplasmonic platform allows for studying the same quantum emitter before and after coupling. We reached record high quantum yield to near-unity in combination with flux grown high quality material that has naturally low non-radiative defect centers1. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A15.00014: Electronic Dipole Spin Resonance of 2D Semiconductor Spin Qubits Matthew Brooks, Guido Burkard Monolayer transition metal dichalcogenides (TMDs) offer a novel two-dimensional platform for semiconductor devices. One such application, whereby the added low dimensional crystal physics (i.e. optical spin selection rules) may prove TMDs a competitive candidate, is quantum dots as qubits. The band structure of TMD monolayers offers a number of different degrees of freedom and combinations thereof as potential qubit basis, primarily electron spin, valley isospin and the combination of the two due to the strong spin orbit coulping known as a Kramers qubit. Pure spin qubits in monolayer MoX2 (where X = S or Se) have been shown to be achievable by energetically isolating a single valley and tuning to a spin degenerate regime within that valley by a combination of a sufficiently small quantum dot radius and large perpendicular magnetic field. Within such a TMD spin qubit, we theoretically induce and analyse single qubit rotations with an electric dipole spin resonance. We employ a rotating wave approximation within a time dependant Schrieffer-Wolf approximation to derive analytic expressions for the Rabi frequency of single qubit oscilations, and compare this result to more exact numerics, as to find optimal operational regimes. |
Monday, March 4, 2019 10:48AM - 11:00AM |
A15.00015: Magneto-spectroscopy probe of exciton-electron interactions in monolayer MoSe2 Zhengguang Lu, Zhipeng Li, Tianmeng Wang, Yuze Meng, Yuxuan Jiang, Sufei Shi, Dmitry Smirnov Coulomb interaction driven many-body effects are expected to modify significantly the quasiparticles properties in monolayer transition metal dichalcogenides at high carrier densities. Here we report on low temperature magneto- photoluminescence and broadband reflection contrast measurements on high quality monolayer MoSe2 over a wide doping range. A valley g-factor around 4 is measured in the low-density regime, which is consistent with the value deduced from the single particle picture. As the carrier density increases, one can access the strongly interacting regime and observe a pronounced change of the valley magnetic response. The observed strong modification of the valley g-factor in highly doped MoSe2 will be discussed and compared to the previously reported results on WSe2 [1]. |
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