Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session G53: TMDCs: Excitons, magnetism and spin dependent effectsFocus
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Sponsoring Units: GMAG Chair: Matt Stone, Oak Ridge National Lab Room: Room 307 |
Tuesday, March 7, 2023 11:30AM - 12:06PM |
G53.00001: Many-body Hexcitons and Oxcitons in Monolayer Semiconductors Invited Speaker: Hanan Dery Hydrogen-like bound states of photoexcited electron-hole pairs in semiconductors -- that is, excitons -- have been a focus of considerable study for more than half a century. In undoped direct-gap semiconductors, neutral excitons comprise the photogenerated electron and hole in the conduction and valence bands (CB and VB), respectively, and typically manifest as discrete optical resonances below the free-particle band-gap energy. More interesting states arise when electron-hole (e-h) pairs are photoexcited into doped semiconductors containing a Fermi-sea of mobile carriers. Most well-known are the charged excitons (or trions) that emerge in lightly electron- or hole-doped semiconductors. In the simplest picture, a trion is a three-particle complex consisting of a carrier from the Fermi sea bound to the photoexcited e-h pair. Trions appear as an additional optical resonance below the neutral exciton, and the energy difference between the two provides a measure of the binding energy between the exciton and the resident carrier. In the archetypal monolayer semiconductor WSe2, the distinct ordering of spin-polarized valleys (low-energy pockets) in the CB allows for studies of not only simple neutral excitons and charged excitons (i.e., trions), but also more complex many-body states that are predicted at higher electron densities [1]. I will discuss magneto-optical measurements of electron-rich WSe2 monolayers [2], and interpret the spectral lines that emerge at high electron doping as optical transitions of 6-body exciton states (``hexcitons'') and 8-body exciton states (``oxcitons''). These many-body states emerge when a photoexcited electron-hole pair interacts simultaneously with multiple Fermi seas, each having distinguishable spin and valley quantum numbers. [1] Dinh Van Tuan and Hanan Dery, Composite excitonic states in doped semiconductors, Phys. Rev. B 106, L081301 (2022). [2] Dinh Van Tuan, Su-Fei Shi, Xiaodong Xu, Scott A. Crooker and Hanan Dery, Hexcitons and oxcitons in monolayer WSe2, Phys. Rev. Lett. 129, 076801 (2022). |
Tuesday, March 7, 2023 12:06PM - 12:18PM |
G53.00002: Optical Response of Transition-Metal Dichalcogenides with Trigonal Warping and Rashba Spin-Orbit Coupling David J Cao, Gaofeng Xu, Benedikt Scharf, Igor Zutic The optical response with spin-valley coupling in monolayer transition-metal dichalcogenides (TMDs), often dominated by tightly bound excitons, provides a valuable probe for their inherent properties and the properties of van der Waals heterostructures. For example, unlike in the widely-studied magnetic proximity effects that are typically limited to the single-particle picture in many materials, in TMDs the key fingerprint of magnetic proximity is contained in the modification of their excitons [1]. Such excitons reveal a removal of valley degeneracy in TMDs [2,3] and show the changes in the optical selection rules with noncolinear orientation between the magnetization of a substrate and the intrinsic spin-orbit field [1]. While the necessity of an accurate many-body description for TMDs is now recognized [2,3], the role of trigonal warping and Rashba spin-orbit coupling on optical properties remains unexplored. By carefully solving the Bethe-Salpeter equation, we study both their individual and combined effects in optical absorption and discuss how they can be observed experimentally. |
Tuesday, March 7, 2023 12:18PM - 12:30PM |
G53.00003: Asymmetric Magnetic Proximity Interactions in Semiconductor/Ferromagnet van der Waals Heterostructures Scott A Crooker, Junho Choi, Christopher A Lane, Jian-Xin Zhu Magnetic proximity interactions (MPIs) between atomically-thin semiconductors and two-dimensional magnets provide a means to manipulate spin and valley degrees of freedom in nonmagnetic monolayers, without the use of applied magnetic fields. In such van der Waals (vdW) heterostructures, MPIs originate in the nanometer-scale coupling between the spin-dependent electronic wavefunctions in the two materials, and typically their overall effect is regarded as an effective magnetic field acting on the semiconductor monolayer. Here we demonstrate that this picture, while appealing, is incomplete: The effects of MPIs in vdW heterostructures can be markedly asymmetric, in contrast to that from an applied magnetic field [1]. Valley-resolved optical reflection spectroscopy of MoSe2/CrBr3 vdW structures reveals strikingly different energy shifts in the K and K' valleys of the MoSe2, due to ferromagnetism in the CrBr3 layer. Strong asymmetry is observed at both the A- and B-exciton resonances. Density-functional calculations indicate that valley-asymmetric MPIs depend sensitively on the spin-dependent hybridization of overlapping bands, and as such are likely a general feature of such hybrid vdW structures. These studies suggest routes to selectively control specific spin and valley states in monolayer semiconductors. [1] J. Choi et al., arXiv:2206.09958, to appear in Nature Materials. |
Tuesday, March 7, 2023 12:30PM - 12:42PM |
G53.00004: Tuning spin splittings in 2D compounds using Bayesian optimization Gustavo M Dalpian, Elton Ogoshi de Melo, Carlos Mera Acosta, Gabriel de Miranda Nascimento, Adalberto Fazzio Bayesian analysis offers a data-centered way to search for compounds with targeted properties. We have used Bayesian inference to search for 2D compounds with large spin splittings (SS) together with large band gaps. Our starting point is a collection [Scientific Data 9, 195 (2022)] of calculated SS for all 2D compounds available in the C2DB database. This collection classifies these SS in four different prototypes: Rashba, Dresselhaus and Zeeman and High Order SS. Each prototype has been indicated using different design principles that include the symmetry of the crystal, the types of atoms, the symmetry of the k-point in reciprocal space and the shape of the dispersion relation in the calculated band structures. As we have a large number of SS available, we use the calculated data to infer what are the atoms and crystal structures that maximize the targeted property. We will show the power of this method by focusing on the Rashba SS, but it also works for other properties. The structural clusters that are more likely to have a large RSS are transition metal (TM) dichalcogenides of the Janus type and where two TM share the M sites in MX2. Compounds containing Mo, W, Sb or Bi cations are also more likely to have large RSS. With the obtained results we can elaborate on the prediction of new compounds with optimized properties, followed by ab initio calculations to confirm the results. |
Tuesday, March 7, 2023 12:42PM - 12:54PM |
G53.00005: Composite excitonic states in doped semiconductors Van Tuan Dinh, Hanan Dery We present a theoretical model of composite excitonic states in doped semiconductors [1]. Many-body interactions between a photoexcited electron-hole pair and the electron gas are integrated into a computationally tractable few-body problem, solved by the variational method [2,3]. We focus on electron-doped monolayer (ML)-MoSe2 and ML-WSe2 due to the contrasting character of their conduction bands [4]. In both cases, the core of the composite is a tightly bound trion (two electrons and a valence-band hole) surrounded by a region depleted of electrons. The composite in ML-WSe2 further includes a satellite electron with different quantum numbers [1,5]. The theory is general and can be applied to semiconductors with various energy-band properties, allowing one to calculate their excitonic states and to quantify the interaction with the Fermi sea. |
Tuesday, March 7, 2023 12:54PM - 1:06PM |
G53.00006: Optical investigation of magnetic order in monolayer semiconductors Kai Hao, Robert T Shreiner, Andrew H Kindseth, Alexander A High In low-doped regions, the interaction between electrons dominates the behavior of individual electrons, forming correlated phases. Here, we experimentally study the spin correlation in electron-doped WSe2. With circular dichroism measurement, we demonstrate that long-range magnetic order can be stabilized by a circularly polarized optical pump. All optical manipulation of magnetic order allows us to study the dynamics of the magnetic order with optical pump-probe experiments. Utilizing a spatial resolved pump-probe experiment, we study the build-up dynamics of the magnetic order at spots several micrometers away from the pump. We also demonstrate that the initial pump-induced spin polarization is amplified by more than an order of magnitude due to the magnetic interaction. The research establishes the optical pump-probe experiment as a versatile tool in the study of correlated phases in two-dimensional electron gases and opens new applications of monolayer semiconductors in nanophotonics and spintronics. |
Tuesday, March 7, 2023 1:06PM - 1:18PM |
G53.00007: Manipulating the valley degree of freedom of transition metal dichalcogenides via van der Waals magnetic tunnel junctions Jia-Xin Li, Wei-Qing Li, Sheng-Hsiung Hung, Po-Liang Chen, Tian-Yun Chang, Horng-Tay Jeng, Chang-Hua Liu The monolayer transition metal dichalcogenides (TMDs) are ideal candidates for future valleytronics applications owing to the unique spin valley locking properties and valley-dependent optical selection rule. Over the past few years, several works have shown the possibility of manipulating the valley degree of freedom of TMDs using circularly polarized light. But to electrically control the valley-dependent polarization remains a critical challenge. In this talk, we demonstrate that Fe3GeTe2 (FGT)-based tunneling contact can effectively inject spin-polarized holes into the specific valley of monolayer TMD. This importantly gives rise to valley-dependent polarization as confirmed by our helicity-dependent electroluminescence and reflective magnetic circular dichroism (RMCD) measurements. Moreover, our density functional theory calculations reveal that FGT owns the strong exchange splitting of spin bands at point K in its Brillouin zone, which enables the efficient injection of spin-polarized carriers. Our results not only provide further insight into FGT, but also address challenges of valleytronics. |
Tuesday, March 7, 2023 1:18PM - 1:30PM |
G53.00008: Magnetic Order Change and Proximity Coupling of Chiral Quantum Emitters in Nano-Indented van der Waals Heterostructures Xiangzhi Li, Andrew Jones, Junho Choi, Huan Zhao, Vigneshwaran Chandrasekaran, Michael T Pettes, Andrei Piryatinski, David A Broadway, Märta Tschudin, Patrick Reiser, Patrick Maletinsky, Nikolai Sinitsyn, Scott A Crooker, Han Htoon The development of robust, deterministically placeable, quantum light emitters (QEs) represents a critical need for the development of solid-state photonic quantum technologies. To build complex quantum networks, highly chiral QEs are necessary. Previously, we have reported that strain engineering can create chiral QEs in WSe2/NiPS3 heterostructures due to local strain-introduced proximity effects. The origin of this discovery, however, is not well understood. Here, we present recent experimental findings addressing the fundamental puzzles. We show the direct evidence of magnetic order change after nano-indentation on NiPS3 by employing scanning NV microscopy which is consistent with the temperature-dependent magneto-PL studies. In addition, we have also extended the nano-indentation technique to other family members of TMPX3 and strikingly we observe proximity-induced chiral QEs here as well. These new discoveries establish TMD/TMPX3 heterostructures as exciting material platforms for the further exploration of novel emergent phenomena and the realization of solid-state quantum transduction and sensing technologies. |
Tuesday, March 7, 2023 1:30PM - 1:42PM |
G53.00009: Exploration of magnetic phases of monolayer TMDs Anna Movsheva, Peter Littlewood Monolayer transition metal dichalcogenides (TMDs) have been shown experimentally to exhibit an electron spin imbalance when optically pumped and electrically doped by a voltage gate. One possible mechanism for the magnetic behavior is the formation of a Wigner crystal due to the doping level. We investigate the dependence of the phase diagram and magnetic properties of the material on the dependence on electron density. We use four species of electrons to model the material characterized by two possible spins (up and down) and two possible crystal momentum (determined by the two valleys of the conduction band that the electrons are restricted to at low temperatures). We investigate the magnetic phase diagram of such an electron gas to detect the Wigner crystal phase found for the free two-dimensional electron gas. |
Tuesday, March 7, 2023 1:42PM - 1:54PM |
G53.00010: Interface-enhanced Magnetism in Two-dimensional V-doped MoS2/Graphene Heterostructures Yen T Pham, Da Zhou, Mingzu Liu, Edgar Dimitrov, Valery Ortiz Jimenez, Chang-Ming Hung, Mauricio Terrones, Manh-Huong Phan While two-dimensional transitional metal dichalcogenides (2D-TMDs) semiconductors often possess no magnetic orderings in their pristine forms, magnetic doping or coupling them to magnetic substrates can induce room-temperature ferromagnetism in these monolayers. Stacking 2D-TMDs with other 2D van der Waals materials, such as graphene, may offer a fertile ground for exploring interface-mediated magnetic phenomena in van der Waals heterostructures. This study demonstrates the interface-enhanced magnetism in monolayers of pristine (MoS2) and V-doped MoS2 stacked with graphene. We grow MoS2 and V-MoS2 monolayers by the chemical vapor deposition method and then transfer them onto graphene layers on Si/SiO2 substrates. Magnetometry measurements reveal a significant enhancement in saturation magnetization (MS) of the MoS2/graphene and V-MoS2/graphene heterostructures compared to the MoS2 and V-MoS2 monolayers on Si/SiO2 substrates. This difference in MS became more significant after annealing the heterostructure at 300oC to enhance stacking adherence and strengthen the bonding between MoS2 and graphene. It implies a vital role of interfacial couplings in facilitating charge transfer across their semiconductor/metal interface and manipulating the magnetic properties of the heterostructures. These findings pave a new pathway for developing novel van der Waals magnetic heterostructures for the next generation of spintronic, valleytronics, and opto-spincaloritronic devices. |
Tuesday, March 7, 2023 1:54PM - 2:06PM |
G53.00011: Magnetic interactions in intercalated transition metal dichalcogenides: a study based on ab initio model construction Tatsuto Hatanaka, Takuya Nomoto, Ryotaro Arita Transition metal dichalcogenides (TMDs), TX2, where T is a transition metal atom and X is a chalcogen atom, are known to have a wide variety of magnetic structures by hosting other transition metal atoms (M) in the van der Waals gaps. To understand the chemical trend of the magnetic properties of the intercalated TMDs, MxTX2, we carried out a systematic first-principles study for 48 compounds with different hosts (TX2), guests (M), and composition ratios (x). Starting with calculations based on spin density functional theory, we derive classical spin models by applying the Liechtenstein method to the ab initio Wannier-based tight-binding model. We show that the calculated exchange couplings are overall consistent with the experiments. In particular, when the composition rate x is 1/3, the chemical trend can be understood in terms of the occupation of the 3d-orbital in the intercalated transition metal. The present results give us a useful guiding principle to predict the magnetic structure of compounds that are yet to be synthesized. |
Tuesday, March 7, 2023 2:06PM - 2:18PM |
G53.00012: Itinerant spin polaron and magnetic states in semiconductor moiré superlattices Margarita Davydova, Yang Zhang, Liang Fu We study the instability of the ferromagnetic ground state of moiré TMD heterobilayers in magnetic field, which we model by the Hubbard model on triangular lattice. In strong interaction regime, we find a drastic difference between the charge e and -e excitations of the spin-polarized Mott insulator occurring in a wide range of magnetic fields below the saturation field hs~t>>J. In the limit U = inf, we find an exact solution for a hole spin polaron which leads to a magnetic instability at h~t. The hole spin polaron is an itinerant bound state of a spin flip and a hole in a fully polarized background, which has spin-3/2 and binding energy εb~t and forms due to a purely kinetic origin. The charge-e excitation is simply a doublon. The saturation field hs~ t for finite hole doping is much larger than that for electron doping, where hs~J<at small electron doping and decreases further due to the emergence metallic magnetism. We propose experimental signatures which will allow to unambiguously reveal the presence of the hole spin polaron physics and address the existing observations. |
Tuesday, March 7, 2023 2:18PM - 2:30PM |
G53.00013: Excitonic Chern insulator and heavy fermion liquid in MoTe2/WSe2 moire bilayer Zhihuan Dong, Yahui Zhang Since the observation of the quantum anomalous Hall (QAH) effect in the AB-stacked MoTe2/WSe2 system, various explanations have been proposed. However, these all point to a valley-aligned order, which contradicts the latest experiment using the technique of magnetic circular dichroism. In this talk, I will review the experiments and discuss a new mechanism for the QAH effect, which gives a valley order consistent with the experimental observations. In particular, based on the observation that the inter-layer tunneling is suppressed in the AB stacking, we consider a model with two layers coupled through the Coulomb interaction. Through mean field theory, a p±ip exciton condensation is found, leading to a Chern insulator with Chern number C = ±1. The valleys are polarized due to the kinetic energy instead of interaction. But the polarization in the two layers can be either the same or opposite depending on small perturbations away from a symmetric point. As a result, both valley polarized and inter-valley coherent (IVC) Chern insulator phases are possible. The latter has the same spin Sz in the two layers. If time allows, I will briefly discuss the heavy fermion physics in this setup. |
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