Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V16: Optical Studies of Transition Metal Dichalcogenides |
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Sponsoring Units: DCMP DMP Room: 315 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V16.00001: All-Optical Materials Design of Dissipationless Chiral Edge Modes in Transition-Metal Dichalcogenides Martin Claassen, Chunjing Jia, Brian Moritz, Thomas Devereaux Spurred by the recent progress in transient melting, enhancement and induction of electronic order, a particularly tantalizing prospect concerns the possibility to instead access dynamical steady states with distinct non-equilibrium phase transitions, to affect electronic transport. Here, we show that the interplay of crystal symmetry and optical pumping of monolayer transition-metal dichalcogenides (TMDCs) provides a novel avenue to engineer topologically-protected chiral edge modes, facilitating optically-switchable conduction channels that are insensitive to disorder. Intriguingly, while TMDCs are canonically described as condensed-matter realizations of massive relativistic fermions, here we predict from first principles that circularly-polarized pumping instead accesses the intrinsic three-band nature near the band edges to selectively photo-induce topological band inversions at low pump intensities, while simultaneously limiting absorption for sub-gap pump frequencies. The results presented provide a new strategy to predict and design topological materials out of equilibrium, and should be readily applicable to other classes of semiconductors. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V16.00002: Magnetic brightening of dark excitons in transitional metal dichalcogenides Xiao-Xiao Zhang, Zhengguang Lu, Ting Cao, Fan Zhang, James Hone, Steven G. Louie, Zhiqiang Li, Dmitry Smirnov, Tony Heinz Transitional metal dichalcogenides (TMDC) in the MX$_{\mathrm{2}}$ (M $=$ Mo, W, X $=$ S, Se) family represent an excellent platform to study of excitonic effects. At monolayer thickness, these materials exhibit both direct band-gap character and enhanced excitonic interactions. Theoretical studies suggest that both the valence and conduction bands are split and exhibit spin polarized character at the K/K' valleys. The lowest energy band-edge excitons are predicted to have different spin configurations for different materials in this family. When the lowest lying exciton has parallel electron and hole spin, radiative decay is forbidden and the state is dark. Here we demonstrate that by applying an in-plane magnetic field we can perturb the exciton spin configuration and brighten this state, allowing it to undergo radiative decay. We identify such a brightened dark state by the emergence of a new emission peak lying below the absorption peak, with a strength growing with applied in-plane magnetic field. On the other hand, for monolayer MoSe$_{\mathrm{2}}$, where no low-lying dark state is expected, we do not see the growth of a new emission feature under application of an in-plane magnetic field. Our experimental findings are in agreement with the calculated properties of dark excitons based on GW plus Bethe-Salpeter equation approach [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V16.00003: Studies of low temperature photoluminescence spectra and excitonic valley polarization in monolayer MoTe2 Sandhaya Koirala, Shinichiro Mouri, Yuhei Miyauchi, Kazunari Matsuda Recently, atomically thin layered transition-metal dichalcogenide (TMDs) in the form MX$_{2}$ (M $=$ Mo, W, X $=$ S, Se, Te) have attracted much interest from the viewpoints of their fundamental physics and potential applications [1, 2]. The characteristic optical features of semiconducting TMDs arise from excitons confined in their atomically thin layers. Molybdenum ditelluride MoTe$_{2}$ has attracted emerging research interest because of optical gap energy (lowest exciton transition) of 1.09 eV, and large spin-orbit coupling of 250 meV. Temperature-dependent photoluminescence (PL) and polarization-resolved PL measurement were performed for mechanically exfoliated monolayer MoTe$_{2}$ from 4.4 to 300 K. At a low temperature, the PL spectra from MoTe$_{2}$ show two sharp peaks for excitons and charged excitons (trions). The systematic temperature-dependent PL measurements revel that the homogeneous linewidth of the exciton peak broadens linearly as the temperature increased due to exciton--acoustic-phonon interactions [3]. From polarization-resolved PL measurements, the valley polarization of above 40 {\%} in the exciton state has been observed at low temperatures. In this meeting, we will discuss about exciton dephasing and valley polarization in monolayer MoTe2. [1] D. Kozawa, K. Matsuda, G. Eda et al., Nat. Commun. 5, 4543 (2014). [2] S. Mouri, Y. Miyauchi and K. Matsuda, Nano Lett. 15, 2336 (2015). [3] S. Koirala, K. Matsuda et al., submitted for publication. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V16.00004: Observation and Characterization of Biexciton States in high-quality Monolayer WS$_{\mathrm{2}}$ Mitsuhiro Okada, Yuhei Miyauchi, Kenji Watanabe, Takashi Taniguchi, Kazunari Matsuda, Hisanori Shinohara, Ryo Kitaura Group-VI transition metal dichalcogenides (TMDCs) have attracted a great deal of attention due to the optical properties dominated by excitonic effects, where emissions in PL arise from excitons and trions are seen even in room temperature (RT). In recent studies on PL emissions from WS$_{\mathrm{2}}$, one of TMDCs has shown that even biexcitons can be observed at 4 K with high-power excitation of 50,000 W/cm$^{\mathrm{2}}$. In this work, we report the observation of biexciton states at temperature higher than 80 K with excitation power lower than 25 W/cm$^{\mathrm{2}}$, by using a high-quality monolayer WS$_{\mathrm{2}}$ grown directly onto the hBN. PL spectra of the WS$_{\mathrm{2}}$/hBN measured at RT show a very sharp excitonic PL emission with a FWHM of 21.5 meV, and at 82.7 K the PL spectra show three additional peaks at the lower energy site. The excitation power dependence and the lifetime measurement of PL peak at 2.00 eV clearly show that this PL peak can be attributed to biexcitons, which has been observed with a low excitation power down to 24 W/cm$^{\mathrm{2}}$. We think that the minimal amount of trapping sites in high-quality WS$_{\mathrm{2}}$/hBN used is a key factor in the observation of biexcitons at over 80 K and a low excitation power. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V16.00005: High field magneto-spectroscopy of excitons in monolayer WSe$_{\mathrm{2}}$ Zhengguang Lu, Xiaoxiao Zhang, Jonathan Ludwig, Fan Zhang, Komalavalli Thirunavukkuarasu, Seongphill Moon, James Hone, Tony Heinz, Dmitry Smirnov We have performed circularly polarized photoluminescence (PL) experiments on monolayer WSe$_{\mathrm{2}}$ in magnetic fields up to \textpm 31T and at temperatures between 2K and 45K, focusing on the emission from the neutral (X$^{\mathrm{0}})$ and negatively charged (X$^{\mathrm{-}})$ excitons. A parallel magnetic field does not affect the exciton energy. At 45K, a perpendicular magnetic field (Faraday geometry) induces linear shift of about 0.12 meV/T$\approx $2$\mu $B for both X$^{\mathrm{0}}$ and X$^{\mathrm{-}}$ peaks indicating lifting of the valley degeneracy. The magnitude of this valley Zeeman shift agrees with the valence band edge lifting due to atomic orbital contribution. The change of the X$^{\mathrm{-\thinspace }}$PL intensity with the magnetic field suggests that the intravalley configuration is the lower energy state of the trion in WSe$_{\mathrm{2}}$. At lower temperatures, the X$^{\mathrm{0}}$ exhibits the same shift with the magnetic field as at 45K, while the X$^{\mathrm{-}}$ shows a more pronounced and non-linear shift with respect to magnetic field. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V16.00006: Strong Circularly Polarized Photoluminescence From Multilayer MoS$_{\mathrm{2}}$ Through Plasma Driven Direct-Gap Transition Rohan Dhall, Kyle Seyler, Zhen Li, Darshana Wickramaratne, Mahesh Neupane, Ioannis Chatzakis, Ewa Kosmowska, Roger Lake, Xiaodong Xu, Stephen Cronin We report circularly polarized photoluminescence spectra taken from few layer MoS$_{\mathrm{2}}$ after treatment with a remotely generated oxygen plasma. Here, the oxygen plasma decouples the individual layers in MoS$_{\mathrm{2}}$ by perturbing the weak interlayer van der Waals forces without damaging the lattice structure. This decoupling causes a transition from an indirect to a direct band gap material, which causes a strong enhancement of the photoluminescence intensity. Furthermore, up to 80{\%} circularly polarized photoluminescence is observed after plasma treatment of few layer MoS$_{\mathrm{2}}$ flakes, consistent with high spin polarization of the optically excited carriers. A strong degree of polarization continues up to room temperature, further indicating that the quality of the crystal does not suffer degradation due to the oxygen plasma exposure. Our results show that the oxygen plasma treatment not only engineers the van der Waals separation in these TMDCs multilayer for enhanced PL quantum yields, but also produces high quality multilayer samples for strong circularly polarized emission, which offers the benefit of layer index as an additional degree of freedom, absent in monolayer MoS$_{\mathrm{2}}$. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V16.00007: Nanosecond Valley Polarization in Suspended Monolayer Tungsten Sulfide Andy Barrette, Chao Xu, Yifei Yu, Yiling Yu, Linyou Cao, Kenan Gundogdu Monolayer transition metal dichalcogenides (TMD) have a vast range of interesting electronic and optical characteristics due to symmetry properties and selection rules. For carriers, these properties result in coupled spin and valley degrees of freedom and coupling between valley polarization circular polarization of excitation source. Because of these unique properties, TMDs are thought to have potential valleytronic applications, however to the detriment of these potential applications, recent optical studies have shown that carriers undergo valley relaxation very quickly, within tens of picoseconds. Using circularly polarized ultrafast transient absorption spectroscopy, we find that valley relaxation in suspended tungsten sulfide (WS$_{2})$ decays on the order of a nanosecond - two orders of magnitude slower than in supported samples. We discuss our results in the context of recent theoretical work which suggests that the predominant valley relaxation mechanism in monolayer TMDs is the electron-hole exchange interaction. Finally, we use valley relaxation measurements at several temperatures to conclude that the remaining nanosecond valley relaxation results from the flexural phonon mechanism. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V16.00008: Imaging Spin Dynamics in Monolayer WS$_{\mathrm{2}}$ by Time-Resolved Kerr Rotation Microscopy Elizabeth Bushong, Kelly (Yunqiu) Luo, Kathleen McCreary, Michael Newburger, Simranjeet Singh, Berend Jonker, Roland Kawakami Monolayer transition metal dichalcogenides (TMDs) such as WS$_{\mathrm{2}}$ offer a unique platform to probe spin and valley degrees of freedom in two-dimensional condensed matter systems. TMDs are of great interest because they have a direct band gap and optical selection rules that permit the excitation of both valley and spin-polarized electrons. Strong spin-orbit coupling leads to valley-dependent spin-splitting in both the conduction and valence bands, which may suppress spin relaxation and inhibit intervalley scattering, thereby increasing both the spin and valley lifetimes. To measure spin and valley dynamics of CVD grown WS$_{\mathrm{2}}$, we developed time-resolved Kerr rotation microscopy with spatial resolution of \textasciitilde 1 micron and temporal resolution of 150 fs. We observe a long spin lifetime of 5.1 ns in WS$_{\mathrm{2}}$ at T $=$ 6.5 K. We spatially map the spin populations at a given time delay, and observe a complex spatial dependence of the spin lifetimes with regions of spin lifetime less than 100 ps and regions of spin lifetime greater than 5 ns separated by only a few microns. To understand the origin of the long-lived spin states, we investigate the relationship between spin lifetime and the photoluminescence intensity. Application of in-plane magnetic fields shows an oscillatory and non-oscillatory component, indicating two spin populations that experience different effective fields. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V16.00009: Infrared and visible magneto-optical studies of centimeter-scale monolayer MoS$_{\mathrm{2}}$ Mumtaz Murat Arik, Alok Mukherjee, Jungryeol Seo, Chuan Zhao, Payam Taheri, Brett Blizzard, Hao Zeng, John Cerne We report extensive magneto-optical measurements on monolayer MoS2 at temperatures down to 10K and magnetic fields up to 7T. The centimeter-scale monolayer MoS2 films are grown by the vapor transport method, where a 5 {\AA}-thick pre-deposited MoO3 film is sulfurized. We measure polarization-sensitive transmission, reflection, photoluminescence, and Kerr response in the infrared and visible range (0.100 -- 2.75 eV). We explore the dependence of the optical and electronic properties on the substrate. This work is supported by NSF-DMR1410599 and NSF CBET-1510121. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V16.00010: Exciton and Trion Valley dynamics in WSe2 measured by two-color pump-probe Akshay Singh, Kha Tran, Joe Seifert, yiping wang, Marie Scott, Dennis Pleskot, Nathaniel Gabor, Jiaqiang Yan, David Mandrus, Xiaodong Xu, Xiaoqin Li Monolayer transition metal dichalcogenides are semiconducting materials demonstrating spin-valley coupling as well as quasiparticles with large binding energies. These quasiparticles, excitons and trions (charged excitons), have quite different spin polarization properties, with the trion having larger spin lifetimes than excitons. Photoluminescence and time resolved Kerr rotation techniques have been used earlier to measure spin lifetimes. However, most of these early optical measurements have relied on non-resonant excitation conditions which tend to mask the intrinsic valley (spin) scattering properties. Here, we use circularly polarized two-color pump probe spectroscopy to measure valley (spin) polarization in monolayer WSe$_{2}$ at low temperatures. We utilize quasi-resonant excitation with pump 1 meV (0.5 nm) spectrally separated from the probe, thus resulting in very efficient valley initialization. We present polarization resolved measurements on resonantly excited excitons and trions, which suggest that trions have larger spin lifetimes. Further, we probe spin polarization of trions when pumping at exciton energies, and vice-versa. We discuss the relative importance of different scattering mechanism at play. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V16.00011: Band edge identification and carrier dynamics of CVD MoS2 monolayer measured by broadband Femtosecond Transient Absorption Spectroscopy Shrouq Aleithan, Maksim Livshits, Jeffrey Rack, Martin Kordesch, Eric Stinaff Two-dimensional atomic crystals of transition metal dichalcogenides are considered promising candidates for optoelectronics, valleytronics, and energy harvesting devices. These materials exhibit excitonic features with high binding energy as a result of confinement effect and reduced screening when the material is thinned to monolayer. However, previous theoretical and experimental studies report different binding energy results. This work further examines the electronic structure and binding energy in this material using broadband Femtosecond Transient Absorption Spectroscopy. Samples of MoS2 were grown by chemical vapor deposition, pumped with femtosecond laser, and probed by femtosecond white light resulting in broadband differential absorption spectra with three distinct features related to the three dominant absorption peaks in the material: A, B, and C. The dependence of the transient absorption spectra on excitation wavelength and layer number provides evidence of a band gap located at C (2.9 eV) and therefore an excitonic binding energy of 1 eV. Additional features in the spectra identified as a broadening of the absorption features caused by carrier scattering, surface defects and trap states. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V16.00012: Ultrafast spectroscopy of exciton and exciton dynamics in mono and few layers of WS2 Sudiksha Khadka, Shrouq Aleithan, Max Livshits, Jeffrey J. Rack, Martin Kordesch, Eric Stinaff Single layer of Transitional metal dichalcogenides (MX2) are 2D semiconductors that have a direct band gap in visible spectrum and fill the gap in between 2D metallic and insulating materials. They have possible application in optoelectronic devices, photovoltaics and photodetection, molecular sensing, 'valleytronics', and flexible transparent electronics. Tungsten Disulphide (WS2), one of the member of MX2 family, has a direct band gap of 2.2 eV and a large valley splitting of about 0.4 eV. Here, we present a detailed study of exciton states and their decay mechanisms in mono and few layer WS2 using femto-second transient absorption spectroscopy. We report a new peak at 3.01±0.1 eV whose origin in k space is believed to be at or around K point and further investigation is underway. The exponential fitting of decay curve of the exciton A reveals three time components as 1.7±0.3 ps, 33.5±10 ps and 670±15 ps, most likely corresponding to carrier-carrier scattering, carrier-phonon scattering, and radiative relaxation respectively. [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V16.00013: Tunable second harmonic generation of monolayer MoS$_{2}$ by Se doping C. T. Le, D. J. Clark, V. Senthilkumar, J. I. Jang, H.-Y. Cho, Y. S. Kim As a transition metal dichalcogenides whose bandgap becomes direct with inversion symmetry breaking in the monolayer limit, MoS$_{2}$ has been getting ample attention as next-generation nonlinear optic material for its strong optical nonlinear properties. In this study, we demonstrate the wavelength second harmonic generation tunability of monolayer Mo(S, Se)$_{2}$. Employing the two-zone furnaces system, we selenized as-grown monolayer MoS$_{2\, }$at different temperature. X-ray photoluminescence spectroscopy was used to confirm the chemical composition of selenized film. Photoluminescence spectra shows the red shift in optical bandgap from 1.83 to 1.53 eV as a function of concentration Se replacing S. Second harmonic generation characteristics were measured in reflection geometry using $p$s pulse from Nd:YAG laser. Applying the previous bulk model, we calculated that the maximum value of $\chi^{(2)\, }$varied from \textasciitilde 40 pm/V for pure MoS$_{2}$ to \textasciitilde 100 pm/V for pure MoSe$_{2.\, }$We believe that our findings along with the ability to stack different 2D materials will create stacked 2D heterostructure with high $\chi^{(2)\, }$over a wide range of wavelength from visible to NIR. [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V16.00014: Optical coherence in atomic monolayer transition metal dichalcogenides limited by electron-phonon interactions Prasenjit Dey, Jagannath Paul, Zefang Wang, Christopher Stevens, Cunming Liu, Aldo Romero, Jie Shan, David Hilton, Denis Karaiskaj We systematically investigate the excitonic dephasing of three representative transition metal dichalcogenides, namely MoS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$ and WSe$_{\mathrm{2}}$ atomic monolayer thick and bulk crystals, in order to gain proper understanding of the factors that determine the optical coherence in these materials. Coherent nonlinear optical spectroscopy, temperature dependent absorption combined with `ab initio' theoretical calculations of the phonon spectra, indicate electron-phonon interactions to be the limiting factor. [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V16.00015: The Eigenstate Thermalization Hypothesis in 1D Anyon Chains Fiona Burnell, Anushya Chandran, Marc Schulz For ergodic systems with Hilbert spaces satisfying a local product structure, the eigenstate thermalization hypothesis (ETH) is relatively well-established. Using exact diagonalization studies, we investigate whether quantum spin chains based on SU(2)\_k anyon theories, which do not admit a Hilbert space with an exactly local product structure, also satisfy ETH, and which observables exhibit this behaviour. [Preview Abstract] |
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