Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B33: Valley and Spin Dependent PropertiesFocus
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Sponsoring Units: DMP Chair: Keun-Su Kim, POSTECH Room: 296 |
Monday, March 13, 2017 11:15AM - 11:51AM |
B33.00001: Optical manipulation of valley pseduospin in 2D semiconductors Invited Speaker: Ziliang Ye Valley polarization associated with the occupancy in the energy degenerate but quantum mechanically distinct valleys in the momentum space closely resembles spin polarization and has been proposed as a pseudospin carrier for future quantum information technologies. Monolayers of transition metal dichalcogenide (TMDC) crystals, with broken inversion symmetry and large spin-orbital coupling, support robust valley polarization and therefore provide an important platform for studying valley-dependent physics.(1) Besides optical excitation and photoluminescence detection, valley polarization has been electrically measured through the valley Hall effect(2) and created through spin injection from ferromagnetic semiconductor contacts.(3) Moreover, the energy degeneracy of the valley degree of freedom has been lifted by the optical Stark effect.(4, 5) Recently, we have demonstrated optical manipulation of valley coherence, i.e., of the valley pseudospin, by the optical Stark effect in monolayer WSe$_{\mathrm{2}}$.(6) Using below-bandgap circularly polarized light, we rotated the valley pseudospin on the femtosecond time scale. Both the direction and speed of the rotation can be optically controlled by tuning the dynamic phase of excitons in opposite valleys. The pseudospin rotation was identified by changes in the polarization of the photoluminescence. In addition, by varying the time delay between the excitation and control pulses, we directly probed the lifetime of the intervalley coherence. Similar rotation levels have also been observed in static magneto-optic experiments.(7, 8) Our work presents an important step towards the full control of the valley degree of freedom in 2D semiconductors. The work was done in collaboration with Dr. Dezheng Sun and Prof. Tony F. Heinz.\newline (1) X. Xu et al. Nature physics \textbf{10}, 343 (2014). \newline (2) K. F. Mak et al. Science \textbf{344}, 1489 (2014). \newline (3) Y. Ye et al. Nature Nanotechnology \textbf{11}, 598 (2016). \newline (4) J. Kim et al. Science \textbf{346}, 1205 (2014). \newline (5) E. J. Sie et al. Nature Materials \textbf{14}, 290 (2014). \newline (6) Z. Ye et al. Nature physics, in press (2016). DOI:10.1038/nphys3891 \newline (7) R. Schmidt et al. Phys. Rev. Lett. \textbf{117}, 077402 (2016). \newline (8) G. Wang, et al. Phys. Rev. Lett. \textbf{117}, 187401 (2016). [Preview Abstract] |
Monday, March 13, 2017 11:51AM - 12:03PM |
B33.00002: All-optical, contactless real-space mapping of valley polarization in 2D materials F. Hipolito, Vitor M. Pereira Valley Polarization (VP), an induced imbalance in the population of a multi-valley electronic system, allows for Second Harmonic Generation (SHG) even in centrosymmetric crystals such as graphene. We study the quadratic response of a generic two-band electronic system on a honeycomb lattice to electromagnetic radiation and analyze the interplay between the intensity of SHG by valley polarization and that due to an intrinsic absence of inversion symmetry. Whereas in pristine monolayer graphene SHG could be a direct indication of VP, in a multilayer heterostructure, or a system such as MoS$_2$ or BN, second-harmonic will be present either intrinsically or spuriously from the interfaces. By characterizing the SHG response as a function of polarization state, temperature, electron density, and degree of VP, we demonstrate the possibility of identifying (and hence disentangling) the intrinsic and valley contributions to the SHG independently. A specific experimental setup is proposed to obtain direct quantitative information about the degree of VP and allow its remote mapping. This approach could prove useful for direct, contactless, real-space monitoring of valley injection and other applications of valley transport and valleytronics. [Preview Abstract] |
Monday, March 13, 2017 12:03PM - 12:15PM |
B33.00003: Valley- and spin-polarized Landau levels in monolayer WSe2 Zefang Wang, Jie Shan, Kin Fai Mak Electrons in monolayer transition metal dichalcogenides (TMDs) are characterized by valley and spin quantum degrees of freedom, making it possible to explore new physical phenomena and applications in electronics and optoelectronics. Under a perpendicular magnetic field, theoretical studies have predicted the formation of discrete Landau Levels (LLs) in monolayer TMDs that are distinct from the case of two-dimensional (2D) electrons both in conventional semiconductor quantum wells and in graphene. Because of the broken sublattice symmetry and of the valley-contrasting Berry curvature effect, the zero-energy LLs at the K and K' valleys in monolayer TMDs are split by the material's bandgap. The strong spin-orbit interactions further spin-polarize the LLs at each valley. However, this unique LL structure has not been observed experimentally. In this talk we report the observation of fully valley- and spin-polarized LLs in high-quality WSe2 monolayers achieved by exploiting a van der Waals heterostructure device platform. We applied handedness-resolved optical reflection spectroscopy to probe the inter-LL transitions at individual valleys and derived the LL structure. Our results open up possibilities for studies of unconventional LL physics and the quantum Hall effect in a 2D semiconductor. [Preview Abstract] |
Monday, March 13, 2017 12:15PM - 12:27PM |
B33.00004: Electrical Tuning of Valley-Polarized Circular Photogalvanic Current in a Monolayer Transition Metal Dichalcogenide Lei Liu, Erik J. Lenferink, Teodor K. Stanev, Nathaniel P. Stern, Guohua Wei In a monolayer transition metal dichalcogenide that lacks structural inversion symmetry, the valley contrasting properties, particularly the magnetic moment and Berry curvature, offer the possibility to create a population imbalance between the two valleys simply with an external optical field\footnote{~D. Xiao, \textit{et al}. \textit{Phys. Rev. Lett.} \textbf{99}, 236809 (2007)}. With the circular photogalvanic effect, the generation of the spin-valley-coupled photocurrent has been demonstrated in chalcogenides\footnote{~H. Yuan, \textit{et al}. \textit{Nat. Nanotechnol.} \textbf{9}, 851 (2014)}. Continuously tuning the valley-polarized current so far has remained largely unexplored in monolayer devices. Here we show the voltage-tunable photocurrent polarization can be achieved in monolayer MoS$_2$ where electric field facilitates the disassociation of excitons and the carrier drift. Gating that modulates the contact barrier and carrier density can switch the monolayer photocurrent polarization on and off with a large valley-polarized current on-off ratio greater than 10$^3$. The efficient electrical tuning of valley-polarized photocurrent opens new possibilities for exploiting polarized currents in monolayer semiconductor devices. [Preview Abstract] |
Monday, March 13, 2017 12:27PM - 12:39PM |
B33.00005: Valley Polarization of Dark Excitons in Tungsten Diselenide Chaw Keong Yong, Jason Horng, Iqbal Bakti Utama, Feng Wang The recently discovered monolayer transition metal dichalcogenides (TMDs) provide a platform to explore new coupled spin-valley physics. Here, we used femtosecond transient absorption (TA) spectroscopy to directly probe the ultrafast recombination dynamics of electrons and holes in both K and K' valley in monolayer tungsten diselenide (WSe$_{\mathrm{2}})$. Following circularly polarized excitation with femtosecond pulse, we observed the A excitons and B excitons exhibit opposite valley polarization and persists to few 10s-picosecond timescales. The conduction band of B-exciton is lying below that of A-exciton in monolayer WSe$_{\mathrm{2}}$, allows the initially photoexcited electrons in the K valley of A-exciton relax to the conduction band of B-exciton in the K' valley in sub-100 femtosecond timescales to give opposite valley polarization for electron and hole. For TMDs with B-exciton conduction band lying above that of A-exciton, such opposite valley polarization of electrons and holes diminishes. Our results shed light for the importance of energetics in the control of valley polarization in atomically thin TMDs. [Preview Abstract] |
Monday, March 13, 2017 12:39PM - 12:51PM |
B33.00006: Tuning the valley polarization of localized excitons in atomically thin materials Chitraleema Chakraborty, Liangyu Qiu, Kenneth Goodfellow, Sajal Dhara, Nick Vamivakas Single photon emitters localized in atomically thin materials have begun to receive much attention from the solid-state quantum optics and quantum photonics communities. In this work we study the quantum emitters present in single layer tungsten diselinde and perform polarization resolved photoluminescence measuements in order to understand the electronic structure of the confined excitons. The emitters are embedded in a van der Waals heterostructure-based diode where quantum confined Stark shift from the excitons is observed. Moreover, we have also demonstrated tunable valley polarization of the confined excitons as a function of the applied electric field. [Preview Abstract] |
Monday, March 13, 2017 12:51PM - 1:03PM |
B33.00007: Ultrafast valley depolarization dynamics in monolayer transition metal dichalcogenides Stefano Dal Conte, Federico Bottegoni, Eva Pogna, Domenico De Fazio, Franco Ciccacci, Andrea Ferrari, Giulio Cerullo, Marco Finazzi The ability to control the valley degrees of freedom is the foundation of the emerging field of valleytronics. Atomically thin transition metal dichalcogenides, thanks to the interplay between the spin and the momentum of the carriers, are a promising platform for the implementation of new devices exploiting the valley and spin degrees of freedom. Here, we measure the exciton valley relaxation dynamics in monolayer MoS$_{2}$ by time-resolved Faraday rotation. We find that the temporal evolution of the Faraday angle has a double exponential decay, showing that the intervalley scattering of the photogenerated excitons is extremely quick (\textasciitilde 200 fs). On a slower time scale, a residual component of the valley polarization, lasting few ps, is detected. This physical scenario is confirmed by time-resolved circular dichroism experiments where the transient variation of the transmission is measured by co- and counter-circularly polarized broadband pulses. The intervalley relaxation processes of other two dimensional semiconductors (i.e. WS$_{2})$ have been studied with the same techniques at different probe energies, close to the excitons and the trion resonances. We also investigate how the valley relaxation dynamics depends on the density of the photoexcited carriers and the energy of the excitation pulses. [Preview Abstract] |
Monday, March 13, 2017 1:03PM - 1:15PM |
B33.00008: Doping-Dependent Study of Valley Relaxation Dynamics in Monolayer Transition Metal Dichalcogenides Yi-Hsin Chiu, Zefang Wang, Kin Fai Mak, Jie Shan Due to inherent broken inversion symmetry and the resulting spin-valley coupling, monolayer transition metal dichalcogenides (TMDs) hold great promise for exploiting valley-dependent physics and applications in electronics and optoelectronics. Here, we report an investigation of the valley relaxation dynamics of excitons in monolayer WSe2. Both the electron- and hole-doped regime can be readily accessed with a systematic variation of doping density in dual gated field-effect transistors of monolayer WSe2. Exciton valley polarization is probed by time-resolved photoluminescence measurements at varying temperatures, revealing intriguing dynamics with distinct depolarization behaviors in the two doping regimes. These observations highlight the importance of the electronic structure in the valley relaxation dynamics and shed light on the effect of valley-dependent electron-hole exchange and many-body interactions in atomically thin TMDs. [Preview Abstract] |
Monday, March 13, 2017 1:15PM - 1:27PM |
B33.00009: Long valley lifetime of free carriers in monolayer WSe2 Yan Tengfei, Xiaodong Cui Monolayer transition metal dichalcogenids (TMDs) feature valley degree of freedom, giant spin-orbit coupling and spin-valley locking. These exotic natures stimulate efforts of exploring the potential applications in conceptual spintronics, valleytronics and quantum computing. Among all the exotic directions, a long lifetime of spin and/or valley polarization is critical. The present valley dynamics studies concentrate on the band edge excitons which predominates the optical response due to the enhanced Coulomb interaction in two dimensions. The valley lifetime of free carriers remains in ambiguity. In this work, we use time-resolved Kerr rotation spectroscopy to probe the valley dynamics of excitons and free carriers in monolayer tungsten diselinide. The valley lifetime of free carriers is found around 2 ns at 70 K, about 3 orders of magnitude longer than the excitons of about 2 ps. The extended valley lifetime of free carriers evidences that exchange interaction dominates the valley relaxation in optical excitation. The pump-probe spectroscopy also reveals the exciton binding energy of 0.60 eV in monolayer WSe$ _2 $. [Preview Abstract] |
Monday, March 13, 2017 1:27PM - 1:39PM |
B33.00010: Tuning of spin-orbit coupling in heterostructures formed by transition metal dichalcogenides and graphene Satrio Gani, Eric Walter, Enrico Rossi Graphene and bilayer graphene have extremely high mobilities but negligible spin orbit coupling (SOC). The ability to induce significant SOC in graphene without reducing its mobility would make it an ideal system to study transport properties that rely on the presence of SOC. Recent experimental results suggest that this could be achieved in van der Waals heterostructures in which graphene is in close proximity to materials with significant SOC. Using ab initio methods we systematically study the electronic structure of heterostructures formed by monolayers of transition metal dichalcogenides (TMDs) and graphene, or bilayer graphene. We consider heterostructures with different number of layers, different TMDs, and different stacking configurations to identify the optimal configurations that enhance the spin-orbit coupling in the graphenic layer and the key parameters of the structures that control its strength. [Preview Abstract] |
Monday, March 13, 2017 1:39PM - 1:51PM |
B33.00011: Spin-Orbit Coupling Effects on Excitonic States in Transition Metal Dichalcogenides Gaofeng Xu, Benedikt Scharf, Alex Matos-Abiague, Igor Zutic Strong Coulomb interactions in two-dimensional materials made of transition metal dichalcogenides (TMDs) have pronounced correlation effects in their optical properties. In particular, excitonic states have significant signatures in the absorption spectrum of TMD layer structures. These signatures can be affected by the sizable spin-orbit coupling (SOC) resulting from the lack of inversion symmetry of a single-layer TMD deposited on a substrate. Starting from a single-particle effective model, we use Bethe-Salpeter equation to calculate the effects of the SOC on both the excitonic states and corresponding optical absorption of the single-layer TMD. [Preview Abstract] |
Monday, March 13, 2017 1:51PM - 2:03PM |
B33.00012: Faraday Rotation in Single Layer Semiconductors with Anisotropic Carrier Effective Mass John Cavin Faraday rotation is a magneto-optical effect wherein the polarization of light is rotated upon transmission through some medium. Media where this effect is known to occur include plasmas, semiconductors, and some organic material. Typically, the angle of rotation is proportional to the distance the light travels through the medium. For this reason, it was a surprise when giant Faraday rotation was discovered in quintessentially thin graphene half a decade ago. Whereas symmetry clearly causes carriers in graphene to have isotropic effective masses, our research explored the nature of Faraday rotation in a single-layer materials with anisotropic carrier effective mass. One possible example of such a material would be black phosphorus. We reevaluated and rederived the Drude model expression of Faraday rotation in the framework of general effective mass. The result was non-trivial polarization-dependence: different rotation angles for different initial polarization states. Additionally, the Faraday rotation matrix is not norm-conserving, indicating energy exchange between carriers and the optical field. We believe these properties could prove useful in sensors, polarization rotators, and polarization measurement devices. [Preview Abstract] |
Monday, March 13, 2017 2:03PM - 2:15PM |
B33.00013: Electrical control of intervalley scattering via the charge state of defects Baoming Yan We study the intervalley scattering in defected graphene by low-temperature transport measurements. The scattering rate is strongly suppressed when defects are charged. The finding highlights `screening' of the short-range part of a potential by the long-range part. Experiments on calcium adsorbed graphene confirm the role of a long-range Coulomb potential. This effect is applicable to other multivalley systems, provided that the charge state of a defect can be electrically tuned. Our result provides a new means to electrically control valley relaxation and has important implications in valley dynamics in valleytronic materials. [Preview Abstract] |
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