Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A37: 2D Materials - TMDCs IFocus
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Sponsoring Units: DMP Chair: Sanfeng Wu, Massachusetts Institute of Technology-MIT Room: LACC 411 |
Monday, March 5, 2018 8:00AM - 8:12AM |
A37.00001: Coherent Light-matter Interaction in Monolayer Molybdenum Diselenide Chaw Keong Yong, Jason Horng, Feng Wang The recently discovered monolayer transition metal dichalcogenides (TMDs) provide a platform to explore new coupled spin-valley physics. Engineering the quantum states of matter using light has emerged as promising and effective routes to coherently control the valley and pseudospin in condensed matters. Here, we used ultrafast pump-probe spectroscopy to investigate the coherent light-matter interactions in monolayer Molybdenum diselenide (MoSe2). We demonstrate that optical excitation in such an atomically thin layer is strongly modified by the exciton-exciton interactions. The exciton-biexciton coupling in monolayer MoSe2 breaks down the valley selection rules based on the non-interacting exciton picture and provides an additional route to manipulate the Floquet states in MoSe2 that leads rich coherent phenomena. By systematically varying the driving frequency below the exciton transition, we observed the Floquet states exhibit energy redshift, splitting or blueshift. Our study reveals the crucial roles of many-body excitonic interactions in coherent light-matter interaction in low-dimensional systems. |
Monday, March 5, 2018 8:12AM - 8:24AM |
A37.00002: Ultrafast Electron-Phonon and Phonon-Phonon Interactions in Multilayer 2H-MoTe2 Ming-Fu Lin, Aravind Krishnamoorthy, Clemens Weninger, Xiang Zhang, Vidya Kochat, Amey Apte, Xiaozhe Shen, Rajiv Kalia, Pulickel Ajayan, Aiichiro Nakano, Priya Vashishta, Xijie Wang, David Fritz, Uwe Bergmann Transition metal dichalcogenides are emerging novel functional materials for many potential applications. However, its fundamental properties and physics at a short time scale is still unclear. Here, we used the ultrafast electron diffraction to investigate the nonradiative process such as electron-phonon and phonon-phonon interactions with a temporal resolution of ~200 fs. A multilayer 2H-MoTe2 was photoexcited at 400 nm and 800 nm, respectively, to create a high carrier density, leading to a temperature jump of tens of kelvin. A strong diffuse scattering appears in the diffraction pattern in a picosecond time scale, implying a fast and effecient electron-phonon interaction. We also observed an anisotropic population of phonon branches specifically located at M-point, corresponding to the Mo-Mo dimerization coordinate. The nonadiabatic quantum molecular dynamics (NAQMD) simulation was incorporated to disentangle the complex light-induced structure disorder dynamics, which also sheds new light on light-driven structure phase transition for 2D materials. |
Monday, March 5, 2018 8:24AM - 8:36AM |
A37.00003: Spin Valley Hall Effects in Transition-metal Dichalcogenides Tong Zhou, Katsuhisa Taguchi, Yuki Kawaguchi, Yukio Tanaka, Kam Tuen Law In gated transition-metal dichalcogenides (TMDs), electrons near the K-valleys experience both Ising spin-orbit coupling (SOC) due to the intrinsic noncentrosymmetric lattice symmetry and Rashba SOC due to gating. In this work, we show that the coexistence of Ising and Rashba SOCs leads to a new type of valley Hall effect, which we call spin valley Hall effect. Importantly, near the conduction band edge of TMDs, the valley-dependent Berry curvatures due to SOCs are highly tunable by external gates and dominate over the intrinsic Berry curvatures. We show that the spin valley Hall effect can be manifested in the gate dependence of the valley Hall conductivity, which can be detected by Kerr effect experiments. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A37.00004: Large, valley-exclusive Bloch-Siegert shift in monolayer WS2 Invited Speaker: Nuh Gedik Coherent light-matter interaction can be used to manipulate the energy levels of atoms, molecules and solids. When light with frequency ω is detuned away from a resonance ω0, repulsion between the photon-dressed (Floquet) states can lead to a shift of energy resonance. The dominant effect is the optical Stark shift, but there is an additional contribution from the so-called Bloch-Siegert shift. Although it is common in atoms and molecules, the observation of Bloch-Siegert shift in solids has so far been limited only to artificial atoms since the shifts were small (<1 µeV) and inseparable from the optical Stark shift. Here we observe an exceptionally large Bloch-Siegert shift (~10 meV) in monolayer WS2 under infrared optical driving by virtue of the strong light-matter interaction in this system. Moreover, we can disentangle the Bloch-Siegert shift entirely from the optical Stark shift, because the two effects are found to obey opposite selection rules at different valleys. By controlling the light helicity, we can confine the Bloch-Siegert shift to occur only at one valley, and the optical Stark shift at the other valley. Such a valley-exclusive Bloch-Siegert shift allows for enhanced control over the valleytronic properties in two-dimensional materials. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A37.00005: Phase engineering of monolayer MoS2 Zhichang Wang, Jianqi Zhu, Xiaoqiang Liu, Ji Feng, Guangyu Zhang, Ying Jiang Monolayer transition metal dichalcogenides (TMDs) can exist in different phases (2H, 1T, 1T′) with distinct properties. They provide a platform to investigate novel quantum states, such as superconductor, quantum spin Hall insulators (QSH) and Weyl semimetals and show great potentials in a great many fields ranging from electronic device to electrochemical catalysis. To date, phase engineering in TMDs have been realized through laser irradiation, alkali-metal intercalation, strain and electrostatic doping. Here we found Ar-plasma bombardment could locally induce 2H → 1T or 2H → 1T′ phase transition in monolayer MoS2 , resulting in mosaic structures depending on the intensity of Ar-plasma and exposure time. On the basis of a selected-area phase patterning process, we fabricated MoS2 FETs including 1T phase transition within the metal contact areas, which exhibit substantially improved device performances. In addition, scanning tunneling spectroscopy (STS) measurement revealed that the 1T′-MoS2 phase is insulating with a 60 mV bandgap, while the domain boundary between 1T′ and 2H phases is conducting. Density functional theory (DFT) calculations further confirm the topological nature of the metallic boundary states. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A37.00006: Tunable Γ−Κ Valley Populations in Hole-Doped Trilayer WSe2 Hema Movva, Timothy Lovorn, Babak Fallahazad, Stefano Larentis, Kyounghwan Kim, Takashi Taniguchi, Kenji Watanabe, Sanjay Banerjee, Allan MacDonald, Emanuel Tutuc Semiconducting transition metal dichalcogenides (TMDs) exhibit thickness dependent bandstructures with the monolayers possessing a direct gap at the K points of the Brillouin zone. Beyond the monolayer limit, the TMD bandstructure becomes more complicated with the band extrema locations in few-layer TMDs remaining an open question for the vast majority of TMDs. Here, we address the valence band maxima locations in trilayer WSe2 using magnetotransport measurements in high-mobility, h-BN encapsulated, dual-gated samples. Shubnikov-de Haas oscillations evince holes populating two distinct subbands with effective masses m* = 0.5me belonging to the K valley, and m* = 1.2me belonging to the Γ valley of trilayer WSe2; me is the bare electron mass. At a fixed total hole density, the K and Γ occupations can be tuned by an applied transverse electric field (E), with Γ being the lowest energy state at low E-field and K being the lowest energy state at high E-field. Ab-initio calculations support these findings and explain the shift of the valence band maxima, and the consequent transfer of holes from Γ to K with increasing E-field. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A37.00007: Time-resolved energy and fluence dependent carrier dynamics in MoS2 and WS2 Alexandra Brasington, Dheeraj Golla, Arpit Dave, Bin Chen, Sefaattin Tongay, John Schaibley, Arvinder Sandhu, Brian LeRoy Monolayer transition metal dichalcogenides (TMDs) exhibit interesting optical properties due to the change from indirect band gap to direct band gap when going from bulk to single layer. We investigate the carrier recombination dynamics in CVD grown MoS2 and WS2 on sapphire substrates using femtosecond pump-probe spectroscopy. Utilizing a 3.2 eV pump pulse, we excite electrons into the conduction band and tune the probe energy above, below, and on resonance with the exciton level in each sample. We measure differential reflection signals at each probe energy over a range of pump fluences. Depending on the probe energy compared to the exciton level, we observe different carrier dynamics. We observe evidence of band structure renormalization and its dependence on pump fluence. The time dynamics of the differential reflectivity signals allows us to monitor defect mediated relaxation pathways. |
Monday, March 5, 2018 9:48AM - 10:00AM |
A37.00008: One-dimensional states in transition metal dichalcogenide interfaces: an analytical and computational approach Oscar Avalos Ovando, Diego Mastrogiuseppe, Sergio Ulloa The growth and exfoliation of 2D materials have led to the creation of edges and novel interfacial states (IFS) at the juncture between different crystals. IFS can exist between different materials, such as at a lateral interface of MoS2/MoSe2, or between different crystalline phases of the same material. IFS inherit properties from both crystals, giving rise to robust states with unique non-parabolic dispersion and strong spin-orbit effects. Although several computational approaches have been used to describe IFS in different materials, it is important to have analytical insights into the nature of such states. We present here a general theory of lateral IFS for Dirac-like HHamiltonians with different characteristics across the interface. We focus here on results for IFS in MoS2/WS2, and study their behavior for different interfaces (zigzag and armchair). We also compare these findings to numerical results obtained from a suitable 3-orbital tight-binding model of the structure, which considers experimental and DFT parameters as input. We finally propose different scenarios where IFS could act as effective 1D-physics hosts. |
Monday, March 5, 2018 10:00AM - 10:12AM |
A37.00009: Effects of Point Defects on the Optical Spectra of Monolayer Transition Metal Dichalcogenides Diana Qiu, Sivan Refaely-Abramson, Jeffrey Neaton, Steven Louie Point defects are known to play an important role in the optical properties of quasi-two-dimensional materials. For instance, the photoluminescence spectra of transition metal dichalcogenides (TMDs) show broad, low energy features that are attributed to defects, and researchers have proposed exploiting defects in TMDs as catalytic centers and single-photon emitters. We use the GW and GW-BSE approach to study the quasiparticle energy levels and optical spectra associated with point defects in monolayer TMDs. We find that defect-state excitons hybridize very strongly with normal excitons in the pristine monolayer, allowing a defect to act as an exciton trap or as a pathway for exciton dephasing. |
Monday, March 5, 2018 10:12AM - 10:24AM |
A37.00010: First-Principles Studies of Chromium Line-Ordered Alloys in a Molybdenum Disulfide Monolayer Noeliarinala Felana Andriambelaza, Edwin Mapasha, Nithaya Chetty Density functional theory calculations have been performed to study the thermodynamic stability, structural and electronic properties of various chromium (Cr) line-ordered alloy configurations in a molybdenum disulfide (MoS2) hexagonal monolayer for band gap engineering. Only the molybdenum (Mo) sites were substituted at each concentration in this study. For comparison purposes, different Cr line-ordered alloy and random alloy configurations were studied and the most thermodynamically stable ones at each concentration were identified. The configurations formed by the nearest neighbor pair of Cr atoms are energetically most favorable. The line-ordered alloys are constantly lower in formation energy than the random alloys at each concentration. From density of states analysis, we found that the MoS2 band gap is tunable by both the Cr line-ordered alloys and random alloys with the same magnitudes. The reduction of the band gap is mainly due to the hybridization of the Cr 3d and Mo 4d orbitals at the vicinity of the band edges. The band gap engineering and magnitudes (1.65 eV to 0.86 eV) suggest that the Cr alloys in a MoS2 monolayer are good candidates for nanotechnology devices. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A37.00011: Electron-Hole Liquid Formation in Monolayer Semiconductors at Room Temperature Alexander Bataller, Robert Younts, Kenan Gundogdu Analogous to gas-to-liquid condensation in real particle systems, electronic excitations in semiconductors may also undergo liquification at high density and low temperature. An example of this quasiparticle condensation is the photoexcited state known as electron-hole liquid (EHL); a condensed degenerate electron-hole plasma. In the nearly 50 years since its discovery, EHL formation has been limited to cryogenic temperatures, and therefore outside the range of most practical applications. In contrast, 2D semiconductors are ideal candidates for high-temperature EHL formation due to their quantum confinement, reduced material screening, and long charge lifetime. We have recently discovered EHL in monolayer transition metal dichalcogenides (TMDs) that can be created above room temperature using less excitation power than a laser pointer. A description of this discovery and the process of EHL formation in monolayer TMDs will be presented. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A37.00012: Crack Formation induced by the Post-Growth Alloying of Two-Dimensional Transition-Metal Dichalcogenides Hossein Taghinejad, Ali Eftekhar, Mohammad Taghinejad, Yao Zhou, Evan Reed, Ali Adibi Alloying is a prime method for customizing the opto-electronic properties of two-dimensional (2D) transition-metal dichalcogenides (TMDs). In this method, the tuning of the composition ratio (x) in MxM’1-xX2 or MX2xX’2(1-x) ternary alloys (M, M’: transition-metals and X, X’: chalcogens) allows for engineering the electronic bandgap and obtaining properties that are not intrinsically available in binary TMDs (i.e., MX2). Ternary compounds can be synthesized via the incorporation of foreign atoms (i.e., M’ or X’) into an already-grown MX2 binary lattice, that is the post-growth alloying of binary crystals. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A37.00013: All-dry fabrication of hBN-encapsulated devices with high-quality CVD-grown transition metal dichalcogenide flakes Takato Hotta, Akihiro Ueda, Yosuke Uchiyama, Kenji Watanabe, Takashi Taniguchi, Hisanori Shinohara, Ryo Kitaura Two-dimensional (2D) semiconductors, including transition metal dichalcogenides (TMDs), have provided a fascinating opportunity to explore novel optical and electric properties at the two-dimensional limit. For this purpose, high-quality devices are indispensable to address their intrinsic properties, and TMDs prepared with the mechanical exfoliation method have been used. Here, we report all-dry fabrication of hexagonal Boron Nitride (hBN) encapsulated devices with CVD-grown monolayer tungsten disulfide (WS2). Using pre-patterned hBN fabricated with reactive ion etching, we have successfully picked up the CVD-grown TMDs directly from the substrate. Fabricated field effect transistor (FET) devices have shown FET mobility of ca. 40 cm2/Vs at room temperature. Considering the advantages of CVD-grown TMDs, large-area monolayer flakes is possible, the present method should contribute to further exploration of basic properties of TMDs. In this presentation, the details of device fabrication and electric transport of hBN-encapsulated devices will be explained. |
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