Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J57: 2D Semiconductors: opto-magnetic propertiesFocus
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Sponsoring Units: DMP DCOMP Chair: Jing Li, Los Alamos National Laboratory Room: Mile High Ballroom 3A |
Tuesday, March 3, 2020 2:30PM - 2:42PM |
J57.00001: Landau quantized excitonic absorption and photoluminescence in a monolayer valley semiconductor Jeremiah Van Baren, Erfu Liu, Takashi Taniguchi, Kenji Watanabe, Yia-Chung Chang, Chun Hung Lui We observe charge-density-dependent quantum oscillations in the excitonic absorption and luminescence of monolayer WSe2 under magnetic fields up to B = 17.5 T. Valley-selective quantum oscillations occur for both the exciton and trions (or exciton-polarons) and reveal distinct intravalley and intervalley coupling between excitons and Landau levels (LLs). We observe spin- and valley-polarized LLs with filling factors n = +0, +1 in the lower conduction band and n = –0 to –6 in the valence band, including the Berry-curvature-induced n = ±0 LLs of massive Dirac fermions. The LL filling produces periodic plateaus in the exciton energy shift accompanied with sharp oscillations in the exciton absorption width and magnitude. This peculiar exciton behavior can be simulated by semi-empirical calculations. In addition, the experimentally deduced g-factors of the conduction band (g ~ 2.5) and valence band (g ~ 15) are much larger than the g-factors predicted in a single-particle model. Such g-factor enhancement implies strong many-body interactions at high charge density in monolayer WSe2. The complex interplay between Landau quantization, excitonic effects, and many-body interactions, as demonstrated in our research, provides a new platform to explore novel correlated quantum phenomena. |
Tuesday, March 3, 2020 2:42PM - 2:54PM |
J57.00002: Spin relaxation in InSe probed by time-resolved Kerr rotation Jovan Nelson, Teodor Stanev, Trevor LaMountain, Haolin Chen, Nathaniel Patrick Stern Two dimensional materials have shown much promise as a platform for novel optical and spin based devices.Recently, van der Waals layers of group-III monochalcogenide Indium Selenide (InSe) have attracted much attention because of their high electron mobility, strong second harmonic generation, and layered-dependent direct band gap. While these optical and electrical properties have been experimentally demonstrated, spin properties of InSe are still poorly understood despite intriguing predictions of layer-dependent optical spin selection rules. Here, we present measurements of spin relaxation in InSe using time-resolved Kerr rotation in the near infrared spectrum. These results will contribute to evaluating spin properties of the high-mobility carries in InSe. |
Tuesday, March 3, 2020 2:54PM - 3:06PM |
J57.00003: Observation of strong valley magnetic response in monolayer transition metal dichalcogenide alloys of Mo0.5W0.5Se2 and Mo0.5W0.5Se2/WS2 heterostructure Ting Yu Monolayer (1L) transition metal dichalcogenide (TMD) alloys have emerged as a new material system towards future applications for electronic, optoelectronic and spintronic devices. Particularly, the unique valley physics associated with valley polarization and Zeeman effect opens new opportunities for valleytronic applications. However, valley magnetic response in 1L TMD alloys largely remains unexplored. Here, we report strong valley magnetic response of trions in Mo0.5W0.5Se2 and Mo0.5W0.5Se2/WS2 heterostructures investigated by cryogenic magneto-photoluminescence microspectroscopy. The large g-factors have been extracted for Mo0.5W0.5Se2 and Mo0.5W0.5Se2/WS2, respectively, which are attributed to the significant impact of strong Coulomb interactions on the trion emission under a perpendicular magnetic field. The reduction of the valley Zeeman splitting in the heterostructure of Mo0.5W0.5Se2/WS2 is explained in terms of the doping variation caused by the interlayer charge transfer between Mo0.5W0.5Se2 and WS2, which agrees well with our density functional theory calculations of the band alignment in the Mo0.5W0.5Se2/WS2 heterostructure. Our findings give new insights into the optical properties of 1L TMD alloys and the interlayer coupling and shed light on the valleytronics. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J57.00004: Revealing the exciton masses and dielectric properties of monolayer semiconductors with high magnetic fields Scott Crooker, Mateusz Goryca, Jing Li, Andreas V. Stier, Takashi Taniguchi, Kenji Watanabe, Emmanuel Courtade, Shivangi Shree, Cedric Robert, Bernhard Urbaszek, Xavier Marie In semiconductor physics, many essential optoelectronic material parameters can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For monolayer transition-metal dichalcogenide semiconductors, this field scale is substantial -- of order 100 tesla! -- due to heavy carrier masses and huge exciton binding energies. Here we report absorption spectroscopy of monolayer MoS2, MoSe2, MoTe2, and WS2 in very high magnetic fields to 91 T. We follow the diamagnetic shifts and valley Zeeman splittings of not only the exciton’s 1s ground state but also its excited 2s, 3s, … ns Rydberg states. This provides a direct experimental measure of the effective (reduced) exciton masses and dielectric properties. Exciton binding energies, exciton radii, and free-particle bandgaps are also determined. Unexpectedly, the measured exciton masses are significantly heavier than theoretically predicted, especially for the Mo-based monolayers. These results provide essential and quantitative parameters for the rational design of optoelectronic van der Waals heterostructures incorporating 2D semiconductors. [1] Goryca et al., Nature Comm. 10, 4172 (2019). |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J57.00005: Spectroscopic signatures of few- and single layer MPS3 (M= Mn, Ni, Fe) complexes Sabine Neal, Heung Sik Kim, Kenneth R O'Neal, Amanda Haglund, Kevin Arthur Smith, David Mandrus, Hans Bechtel, G Carr, Kristjan Haule, David Vanderbilt, Janice Lynn Musfeldt Layered magnetic chalcogenides have become increasingly important over the last decade because they offer a unique platform for combining the complexity of bulk materials with the tunability of few- and single layer systems. These van der Waals complexes have been well studied by Raman spectroscopy, however, infrared spectroscopy is vastly underexplored due to the inability to overcome the diffraction limit. It is these ungerade infrared active modes that are vital in understanding material functionality. Synchrotron infrared nanospectroscopy, a fusion of near-field optical microscopy with high brightness infrared synchrotron radiation, has overcome this fight for photons and enabled a better understanding of size-induced effects, including symmetry breaking, that are quite different from the single crystal. This approach will be illustrated with members of the MPS3 (M= Mn, Ni, Fe) family of complex chalcogenides. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J57.00006: Imaging Spin-Split Impurity States in Monolayer Semiconductors Caleb Zerger, Alexander Contryman, Hong Li, Tyler Layden, Xiaolin Zheng, Hari C. Manoharan A two dimensional dilute magnetic semiconductor (2D-DMS) is a state sought after for its important implications for spintronics. It is predicted that this state could be achieved by doping monolayer transition metal dichalcogenides with magnetic dopants. Using scanning tunneling microscopy, we investigate the origins of a 2D-DMS state by probing the local density of states in magnetically doped monolayer dichalcogenide semiconductors. We find sharp features in the spectral maps due to an alignment of impurity states with the Fermi level due to tip-induced band bending (TIBB). By modeling the TIBB and comparing to density functional theory, we find evidence for strongly spin-split states in individual dopants, a necessary precursor to the 2D-DMS state. We also use these signatures to characterize impurity types in these samples, finding evidence for both transition metal subsitutional dopants and dichalcogenide substitutions, which collectively alter the magnetic landscape. |
Tuesday, March 3, 2020 3:42PM - 4:18PM |
J57.00007: Spin-orbit coupling, exchange, and magnetism in exciton physics of 2D semiconductors Invited Speaker: Meng Wu Interactions (e.g., spin-orbit coupling (SOC), electron-hole (e-h), magnetic ordering, etc.) often give rise to dramatic new features in the photophysics of 2D materials. With newly developed full-spinor ab initio GW & Bethe-Salpeter equation (GW-BSE) methods, we investigate the interplay among these interactions in 2D semiconductors. We discover that e-h exchange interaction in monolayer transition metal dichalcogenides mixes the well-known A and B excitons that heretofore were believed to be completely independent of each other, since A and B excitons had been viewed as derived from inter-band transitions between different pairs of spin-polarized bands [1]. In another study, we elucidate the physical origin of giant excitonic and magneto-optical responses in monolayer CrI3, a prototypical 2D ferromagnetic semiconductor [2]. Our calculations demonstrate that the optical properties of ferromagnetic monolayer CrI3 are dominated by exciton states that extend over several unit cells. By simulating a realistic experimental setup, we further predict a strong dependence of magneto-optical Kerr effect signals on excitation frequency and substrate configuration. |
Tuesday, March 3, 2020 4:18PM - 4:54PM |
J57.00008: Discovery of tunable excitons, giant valley orbital magnetic moment, and unconventional optical selection rules in bilayer graphene Invited Speaker: Long Ju Multilayer graphene in the rhombohedral stacking order is a unique semiconductor where the bandgap can be continuously controlled by an external electric field. It provides an exciting platform to study conventional exciton physics in the context of valley pseudospin and quantum geometry in an in situ tunable semiconductor bandgap. We employed the photocurrent spectroscopy technique to study AB-stacked bilayer graphene, and observed tunable exciton states with unusual optical selection rules, strong optical resonances and extremely narrow line width. Upon the application of a perpendicular magnetic field, a large valley-dependent magnetic moment was observed which can be traced back to the fundamental Berry curvature effect. We also observed inter Landau level transitions with unconventional optical selection rules, and established the continuous evolution from Coulomb-interaction-dominated to Landau-quantization-dominated optical response. We further used photoluminescence and Raman spectroscopy to study the exciton evolution with tunable carrier density. I will also discuss exciton physics in rhombohedral stacked graphene with more than 3 layers and the effect of Moire superlattice from hBN substrate. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J57.00009: Investigation of the magnetic interactions in WSe2/WS2 moiré superlattice Yanhao Tang, Lizhong Li, Tingxin Li, Yang Xu, Song Liu, Katayun Barmak, Kenji Watanabe, Takashi Taniguchi, Allan MacDonald, Jie Shan, Kin Fai Mak Moiré superlattices formed in two-dimensional (2D) atomic crystals present a powerful platform to study interacting quantum particles in a lattice. Here, we present optical reflection spectroscopy studies on angle-aligned WSe2/WS2 bilayers, which form moiré superlattices because of the difference in lattice constant between the two materials. We measure the dependence of the lowest-energy moiré exciton on magnetic field and doping density. At half filling of the first hole moiré band, we observe a Mott insulating state with antiferromagnetic Curie-Weiss behavior. Past half filling, our experiment suggests an antiferromagnetic to weak ferromagnetic quantum phase transition near 0.6 filling. These results can be understood based on a triangular lattice Hubbard model in the strong interaction regime. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J57.00010: Probing Electronic Structure and Carrier Dynamics in the Ferromagnetic Semiconductor CrSiTe3 Giriraj Jnawali, Howard E Jackson, Leigh M Smith, Stephen Wilson The layered ferromagnetic semiconductor CrSiTe3 (CST) has attracted great scientific attention due to its potential as a 2-dimensional ferromagnet with higher Tc than in its bulk form. In order to understand the nature of magnetic correlations and possible magneto-optoelectronic applications, it is crucial to understand the electronic structure and photoinduced response near the band-edge in nanoscale samples. Here we present transient optical spectroscopy on exfoliated CST nanosheet samples at 300 K as well as at 10 K over an extended photon energy: 0.3 – 1.2 eV. We find clear signatures of optical transitions around 0.5 and 1.15 eV, which agree well with theoretical calculations of the indirect and direct conduction band edges. Photoexcited carriers are thermalized within a ps to the lattice through strong coupling to optical phonons and the subsequent decay persists over a nanosecond before undergoing recombination. Our work not only demonstrates direct measurements of the band structure but also sheds light on scattering behavior of photoexcited carriers in CST, which are critical for its future applications. |
Tuesday, March 3, 2020 5:18PM - 5:30PM |
J57.00011: First principles studies of valley splitting in monolayer transition metal dichalcogenides on BiFeO3 Elizabeth A Peterson, Jeffrey B Neaton Control of the spin and valley degrees of freedom of monolayer transition metal dichalcogenides (TMDs) via time reversal symmetry breaking at K and K’ has been successfully demonstrated via magnetic substrates [1], which can generate significantly larger valley splitting than applied magnetic fields. Multiferroic substrates offer an avenue to develop valleytronics devices with switchable in-situ polarization. Using first principles density functional theory calculations, we predict that the ferromagnetically-ordered surface of Fe-terminated (111)-BiFeO3 substrates can produce valley splittings in WSe2 an order of magnitude larger than previously proposed magnetic substrates. We discuss the details of the orbital exchange interactions driving the splitting; and we also develop design principles for tuning the valley splitting through lattice alignment of the TMD monolayer with the substrate, as well as the identity and spin state of the magnetic substrate ions. |
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