Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M2: Focus session: Beyond Graphene - Optics in 2D semiconductors III |
Hide Abstracts |
Sponsoring Units: DMP Chair: Xavie Marie, INSA Toulouse Room: 001B |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M2.00001: Ultrafast generation of pseudo-magnetic field for valley excitons in WSe2 monolayers Jonghwan Kim, Xiaoping Hong, Chenhao Jin, Su-Fei Shi, Chih-Yuan S. Chang, Ming-Hui Chiu, Lain-Jong Li, Feng Wang The valley pseudospin emerges as a new degree of freedom in atomically thin two-dimensional transition metal dichalcogenides (MX2). In analogy to the control of spin in spintronics, the capability to manipulate the valley pseudospin can provide exciting opportunities in valleytronics. Here we present that femtosecond pulses with circular polarization can generate ultrafast and ultrahigh valley pseudomagnetic field in a monolayer MX2. Our polarization-resolved transient absorption measurement shows that the degeneracy of valley exciton transitions at K and K' valley in WSe2 monolayers can be lifted by optical Stark effect from the non-resonant pump. Energy splitting due to the optical Stark effect is linear with both the pump intensity and the inverse of pump detuning. We observe that valley-selective optical Stark effect can create an energy splitting more than 10 meV which corresponds to a pseudomagnetic field over 60 Tesla. Our study demonstrates efficient and ultrafast control of the valley excitons with optical light which can open up the possibility of coherent manipulation of the valley polarization in MX2. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M2.00002: Valley splitting and polarization by the Zeeman effect in monolayer MoSe2 Yilei Li, Jonathan Ludwig, Tony Low, Alexey Chernikov, Xu Cui, Ghidewon Arefe, Young Duck Kim, Arend van der Zande, Albert Rigosi, Heather Hill, Suk Hyun Kim, James Hone, Zhiqiang Li, Dmitry Smirnov, Tony Heinz We have measured circularly polarized photoluminescence in monolayer MoSe$_2$ under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of $\mp$ 0.12 meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to $\mp$ 0.18 meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M2.00003: Nonlinear Optical and Excitonic Effects in Two-Dimensional Transition Metal Dichalcogenides Wang-Kong Tse We present a theory for coherent optics in two-dimensional transition metal dichalcogenides. Using the density matrix formalism, we derive the kinetic equations for the interband polarization and band population distributions, and study the regime of strong optical fields where Coulomb interaction effects are small and the regime of weak optical fields where excitonic effects are important. In particular, the influence of the optical Stark effect on the excitonic properties is studied within our theory. We also address the excitonic effects of Coulomb interaction on the optical conductivity and compare our results with that of graphene. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:27PM |
M2.00004: Direct observation of spin-valley-layer locking in centrosymmetric bulk WSe$_2$ by spin- and angle-resolved photoemission Invited Speaker: Phil King Methods to generate spin-polarized electronic states in non-magnetic solids are strongly desired to enable all-electrical manipulation of electron spins for new quantum devices. This is generally accepted to require breaking global structural inversion symmetry. In contrast, I will report our observation from spin- and angle-resolved photoemission spectroscopy of spin-polarized bulk states in the centrosymmetric transition-metal dichalcogenide 2H-WSe$_2$ [1]. Mediated by a lack of inversion symmetry in constituent Se-W-Se monolayers of the bulk crystal where the electronic states are localized, we show how enormous spin splittings up to $\sim\!0.5$~eV result, with a spin texture that is strongly modulated in both real and momentum space. Through this, our study provides direct experimental evidence for a putative locking of the spin with the layer and valley pseudospins in transition-metal dichalcogenides, of key importance for using these compounds in proposed valleytronic devices. \\[4pt] [1] J.M.~Riley {\it et al.}, Nature Phys. {\bf 10} (2014) 835 [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M2.00005: Topological induced valley polarization in bilayer graphene/Boron Nitride Leonardo Basile, Juan C Idrobo Novel electronic devices relay in our ability to control internal quantum degrees of freedom of the electron e.g., its spin. The valley number degree of freedom is a pseudospin that labels degenerate eigenstates at local maximum/minimum on the valence/conduction band. Valley polarization, that is, selective electronic localization in a momentum valley and its manipulation can be achieved by means of circular polarized light (CPL) in a system with strong spin-orbit coupling (SOC). In this talk, we will show theoretically that despite the fact that neither graphene or BN have a strong SOC, a bilayer of graphene on BN oriented at a twist angle has different absorption for right- and left- CPL. This induced polarization occurs due to band folding of the electronic bands, i.e., it has a topological origin. ~ [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M2.00006: Anomalous temperature dependence of charged exciton photoluminescence polarization in monolayer WS$_{2}$ A.T. Hanbicki, G. Kioseoglou, M. Currie, C.S. Hellberg, A.L. Friedman, K.M. McCreary, B.T. Jonker Monolayer WS$_{2}$ is a direct-gap transition metal dichalcogenide semiconductor. Its low-dimensional hexagonal structure leads to two inequivalent K-points in the Brillioun zone. The valley index and spin are intrinsically coupled with spin-dependent selection rules that enable populating and interrogating each valley using circularly polarized light. Here, we probe the degree of circular polarization of the emitted photoluminescence (PL) as function of the photo-excitation energy and temperature to elucidate spin-dependent inter- and intra-valley relaxation mechanisms. Monolayer WS$_{2}$ flakes have PL emission from the free and charged exciton near 2.0 eV. We reproducibly isolate these excitons via appropriate sample preparation. With excitation using positive helicity light, we analyze the PL for positive and negative helicities to determine polarization. Unlike MoS$_{2}$ \footnote{G. Kioseoglou, et al. \textit{Appl. Phys. Lett.} \textbf{101}, 221907 (2012).}, we measure significant polarization from the charged exciton for high excitation energies, even at room temperature. There is also an enhancement of polarization of the charged exciton at intermediate temperatures. We discuss the polarization behavior in terms of phonon assisted intervalley scattering processes. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M2.00007: First-Principles Calculations of LEEM Reflectivity Spectra of Molybdenum Disulfide John McClain, Karsten Pohl, Jian-Ming Tang We present calculations of the low-energy electron specular reflectivity spectra of systems of a few layers of molybdenum disulfide at general angles of incidence using a newly modified algorithm within our first-principles theoretical approach, which leverages the self-consistent scattering potentials produced by density-functional theory [1]. Our calculated normal-incidence spectra for MoS$_2$ reveal layer-dependent features around 7-8 eV and 15 eV, allowing for a characterization of the number of layers via LEEM reflectivity and thus an in-situ technique for growth monitoring. We have previously described the application of our approach to the off-normal spectra of few-layer graphene, but the lack of mirror symmetry in MoS$_2$ requires a new algorithm for finding degenerate pairs of solutions for the matching procedure. The computed off-normal spectra illustrates the complexity of the electronic structure of MoS$_2$. We also present the way in which our new off-normal algorithm leads naturally to an approach to higher-order diffraction intensity calculations with the wave-matching scheme, along with our results for higher-order diffraction in model systems and progress towards results for real systems. [1] McClain et al., arXiv.1311.2917. [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M2.00008: Spin-orbit coupling in the band structure WSe$_{2}$ monolayers Iori Tanabe, Alexei Barinov, Duy Le, Edwin Preciado, Miguel Isarraraz, Ludwig Bartels, Talat Rahman, Peter Dowben We have mapped the occupied band structure of monolayer WSe$_{2}$ by small spatial spot angle resolved photoemission and have found significant spin-orbit coupling in excess of 500 meV, far larger than for MoS$_{2}$. The experimental band mapping is consistent with theoretical expectations with the top of the valence band is seen at K, not $\Gamma $, thus distinct from the band structure for the bilayer and bulk single crystals. This shift of the top of the valence band in monolayer WSe$_{2}$, from $\Gamma $ to K, is also predicted in density functional theory. In general the wave vector dependent experimental band structure confirms the expectations of density functional theory. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M2.00009: Intrinsic circular polarization in centrosymmetric stacks of transition-metal dichalcogenide Qihang Liu, Xiuwen Zhang, Alex Zunger The circular polarization (CP) that the photoluminescence~inherits from~the excitation source in~n monolayers of transition-metal dichalcogenides (MX$_{\mathrm{2}})_{\mathrm{n}}$ has been previously explained as a special feature of odd values of n, where the inversion symmetry is absent. This valley polarization effect results from the fact that in the absence of inversion, charge carriers in different band valleys could be selectively excited by different circular polarized light. Such restriction to non-centrosymmetric systems poses a limitation on the material selection for achieving CP. Although several experiments observed CP in centrosymmetric MX$_{\mathrm{2}}$ systems e.g., for bilayer in MX$_{\mathrm{2}}$, they were dismissed as being due to some extrinsic sample irregularities. Here we show~that also for n $=$ even where inversion symmetry is present and valley polarization physics is strictly absent, such intrinsic selectivity in CP is to be expected on the basis of fundamental spin-orbit physics. First-principles calculations of CP predict significant polarization for n $=$ 2 bilayers: from 69{\%} in MoS$_{\mathrm{2}}$ to 93{\%} in WS$_{\mathrm{2}}$. This realization could broaden the range of materials to be considered as CP sources. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M2.00010: Electric field and spin-orbit coupling effects on the band structure of monolayer WSe$_{2}$ Ittipon Fongkaew, Walter R.L. Lambrecht Transition metal dichalcogenides are known to switch from indirect to direct gap between bulk and monolayer form. Here we show that in WSe$_{2}$, an electric field perpendicular to the layer of order a few 0.1 MV/cm has strong effects on the conduction band and can convert the material back to an indirect gap. The competing minima at different points in the Brillouin zone undergo different shifts with electric field because of their different orbital character. Using first-principles GGA calculations in the presence of an electric field with and without spin-orbit coupling, we determine the critical field at which the minimum between $\Gamma $ and $K$ (where the gap occurs in bulk) becomes back the lowest conduction band minimum. For even stronger electric fields we find the CBM to shift to the $\Gamma $-point. While the electric fields considered here are much larger than the fields obtained in gated structures, they may be possible using electric double layers using an electrolyte. Such measurements have already been done on bulk WSe$_{2}$, [Nature Physics 9, 563 (2013)] but focused on the valence band and Rashba effects instead of the conduction band. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M2.00011: Spin-dependent refraction at the atomic step of transition-metal dichalcogenides Mikito Koshino, Tetsuro Habe We theoretically propose a spin-dependent electronic transport mechanism in the transition metal dichalcogenide, in which the spin-unpolarized electron beam is split into different directions depending on spins at an atomic domain boundary. Specifically, we calculate the electronic transmission across a boundary between monolayer and bilayer of the transition metal dichalcogenide, and demonstrate that up-spin and down-spin electrons entering the boundary are refracted and collimated to opposite directions. The phenomenon is attributed to the strong spin-orbit interaction, the trigonally-warped Fermi surface, and the different crystal symmetries between the monolayer and bilayer systems. The spin-dependent refraction suggests a potential application for a spin splitter, which spatially separates up-spin and down-spin electrons simply by passing the electric current through the boundary. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M2.00012: Two dimensional valley electrons and excitons in the noncentrosymmetric 3R MoS2 Ryosuke Akashi, Masayuki Ochi, Sandor Bordacs, Ryuji Suzuki, Yoshinori Tokura, Yoshihiro Iwasa, Ryotaro Arita Possible control of the valley-dependent spin polarization in transition-metal dichalcogenides has been a hot topic as the valleytronics. Through the recent great progress based on the monolayer systems, people's interest is shifting to multilayered polytypes. The centrosymmetric 2H-stacked systems have been much studied for switching of the valley-dependent spin polarization. On the other hand, some of the authors [Suzuki et al., Nat. Nanotechnol. 9, 611 (2014)] have successfully fabricated the noncentrosymmetric 3R-stacked MoS2 multilayer and demonstrated the valley polarization independent of the number of layers. On the basis of this success, we further examined the valley electronic states in the 3R-MoS2 and found their novel two-dimensional properties utilizable for the valleytronics [Akashi et al., submitted.]. Namely, interlayer hopping of the valley electrons was proved to be zero as a consequence of a quantum-interference effect caused by the 3R-stacking geometry. In the talk, we report the results of the reflectivity measurement and analysis with an anisotropic hydrogen atomic model and show that the zero hopping causes 2D-hydrogen-like spectral series and confinement of the wave function within a single layer of the valley exciton. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M2.00013: Electrical Control of Exciton-Enhanced Second-Harmonic Generation in Monolayer WSe2 Kyle Seyler, John Schaibley, Pu Gong, Pasqual Rivera, Aaron Jones, Sanfeng Wu, Jiaqiang Yan, David Mandrus, Wang Yao, Xiaodong Xu Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies, is ubiquitous in modern photonic systems. However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material, and so far, dynamical control of optical nonlinearities remains confined to research labs. In this talk, we report a new mechanism to electrically control second-order optical nonlinearities in monolayer WSe2. We show that the intensity of second-harmonic generation (SHG) at its lowest exciton resonance is widely tunable through electrostatic doping in a field-effect transistor device. Such remarkable tunability arises from the strong exciton charging effects in monolayer semiconductors, which allow for exceptional control over the exciton and trion oscillator strengths. Our study paves the way for a new platform of chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700