Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session T2: Focus Session: Beyond Graphene - Interaction & Correlation Effects |
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Sponsoring Units: DMP Chair: Ming-wei Wu, University of Science and Technology, China Room: 001B |
Thursday, March 5, 2015 11:15AM - 11:27AM |
T2.00001: ABSTRACT MOVED TO W1.00010 |
Thursday, March 5, 2015 11:27AM - 11:39AM |
T2.00002: Electronic and thermoelectric properties of Mexican hat bands in van-der-Waals materials Darshana Wickramaratne, Ferdows Zahid, Roger Lake Mexican hat dispersions are relatively common in few-layer two-dimensional materials. In one to four monolayers of the group-III chalcogenides (GaS, GaSe, InS, InSe) and Bi$_{2}$Se$_{3}$ the valence band undergoes a band inversion from a parabolic to an inverted Mexican hat dispersion as the film thickness is reduced from bulk to a single monolayer. The band inversion is robust against changes in stacking order, omission or inclusion of spin-orbit coupling and the choice of functional. The Mexican hat dispersion results in a 1/$\sqrt{E}$ singularity in the two-dimensional density of states and a step-function turn on in the density of modes. The largest radius of the ring of states occurs for a single monolayer of each material. The dispersion with the largest radius coincides with the maximum power factor and ZT for a material at room temperature. Ab-initio electronic structure calculations are used with a Landauer approach to calculate the thermoelectric transport coefficients. Analytical models of the Mexican hat and the parabolic dispersions are used for comparison and analysis. Vertically biased bilayer graphene could serve as an experimental test-bed for measuring this effect since the radius of the Mexican hat band edge increases linearly with vertical electric field. [Preview Abstract] |
Thursday, March 5, 2015 11:39AM - 11:51AM |
T2.00003: Ab-initio study of the temperature effects on the optical properties of transition metal dichalcogenides Alejandro Molina-Sanchez, Maurizia Palummo, Andrea Marini, Ludger Wirtz Research on ultra-thin two-dimensional materials has been booming since the discovery of graphene along with its interesting physical properties. The transition metal dichalcogenides as MoSare gaining considerable attention due to their potential application in photovoltaics and nanoscale transistors. The optical properties of these layered materials depend strongly on the number of layers. The paradigmatic example is the transition from indirect to direct bandgap when we change from multi-layer to single-layer MoS. In this work, we study the effects of the electron-phonon interaction on the optical properties of single-layer MoS. In the framework of the GW method we calculate the contribution of the electron-phonon coupling to the self-energy. This allows us to calculate the zero-point re-normalization of the quasi-particle energies and to include temperature effects. We discuss the bandgap dependence on the temperature, and the change in the linewidth of the quasi-particle states. The impact of temperature on the exciton states is also addressed. [Preview Abstract] |
Thursday, March 5, 2015 11:51AM - 12:27PM |
T2.00004: Interaction and Correlation Effects in Quasi Two-dimensional Materials Invited Speaker: Steven G. Louie Experimental and theoretical studies of atomically thin quasi two-dimensional materials (typically related to some parent van der Waals layered crystals) and their nanostructures have revealed that these systems can exhibit highly unusual behaviors. In this talk, we discuss some theoretical studies of the electronic, transport and optical properties of such systems. We present results on graphene and graphene nanostructures as well as other quasi-2D systems such as monolayer and few-layer transition metal dichalcogenides (e.g., MoS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, and WSe$_{\mathrm{2}})$ and metal monochalcogenides (such as GaSe and FeSe). Owing to their reduced dimensionality, these systems present opportunities for unusual manifestation of concepts and phenomena that may not be so prominent or have not been seen in bulk materials. Symmetry and many-body interaction effects often play a critical role in shaping qualitatively and quantitatively their properties. Several quantum phenomena are discussed, including novel and dominant exciton effects, tunable magnetism, electron supercollimation by disorder, unusual plasmon behaviors, and possible enhanced superconductivity in some of these systems. We investigate their physical origins and compare theoretical predictions with experimental data. [Preview Abstract] |
Thursday, March 5, 2015 12:27PM - 12:39PM |
T2.00005: Atomistic model for excitons: Capturing Strongly Bound Excitons in Monolayer Transition-Metal Dichalcogenides Frank Tseng, Ergun Simsek, Daniel Gunlycke Monolayer transition-metal dichalcogenides form a direct bandgap predicted in the visible regime making them attractive host materials for various electronic and optoelectronic applications. Due to a weak dielectric screening in these materials, strongly bound electron-hole pairs or excitons have binding energies up to at least several hundred meV's. While the conventional wisdom is to think of excitons as hydrogen-like quasi-particles, we show that the hydrogen model breaks down for these experimentally observed strongly bound, room-temperature excitons. To capture these non-hydrogen-like photo-excitations, we introduce an atomistic model for excitons that predicts both bright excitons and dark excitons, and their broken degeneracy in these two-dimensional materials. For strongly bound exciton states, the lattice potential significantly distorts the envelope wave functions, which affects predicted exciton peak energies. The combination of large binding energies and non-degeneracy of exciton states in monolayer transition metal dichalogendies may furthermore be exploited in room temperature applications where prolonged exciton lifetimes are necessary. [Preview Abstract] |
Thursday, March 5, 2015 12:39PM - 12:51PM |
T2.00006: Observation of Rapid Exciton--Exciton Annihilation in Monolayer Molybdenum Disulfide Dezheng Sun, Yi Rao, Georg Reider, Gugang Chen, Yumeng You, Louis Br\'ezin, Avetik Harutyunyan, Tony Heinz In this paper, we present ultraist pump-probe spectroscopy results for monolayer MoS2 crystals in which we explore exciton dynamics as a function of exciton density. After a femtosecond excitation pulse of near-resonant radiation to create A excitons, we have monitored the temporal evolution of the exciton density using a continuum probe pulse. We observe a decay rate as long as 100 ps for samples at room temperature and at relatively low exciton density. The decay rate increases strongly with increasing exciton density. We are able to fit the entire set of density-dependent exciton dynamics using a simple model in which the dominant decay channel is an exciton-exciton annihilation process. From these measurements, we infer an exciton-exciton annihilation rate of (4.3 $\pm$ 1.1) $\times$ 10$^2$ cm$^2$/s. We compare this rate with that observed in other nanostructured materials. [Preview Abstract] |
Thursday, March 5, 2015 12:51PM - 1:03PM |
T2.00007: GW-BSE calculations on two-dimensional MXene phases Zhenglu Li, Liang Hong, Felipe Jornada, Ting Cao, Serdar Ogut, Steven G. Louie MXene is a promising candidate for new useful two-dimensional (2D) crystals. Experimentally, few-layer samples have been made from the bulk, and they demonstrate many excellent properties for electric and thermal transport, as well as other novel physics. In this work, we have performed GW-BSE calculations based on first-principles calculations to study some of the MXene family. We find that monolayer Ti$_2$CO$_2$ possesses insulating properties. Furthermore, the 2D screening effect is very strong, resulting in a GW band gap correction of almost ~1 eV. Based on these initial results, we expect that optical properties of these materials will also have strong excitonic effects. [Preview Abstract] |
Thursday, March 5, 2015 1:03PM - 1:15PM |
T2.00008: Bright Interlayer Exciton Dynamics in MoSe$_{2}$-WSe$_{2}$ Heterostructures Pasqual Rivera, Kyle Seyler, Jason Ross, John Schaibley, Hongyi Yu, Jon Ell, Marie Scott, Jiaqiang Yan, David Mandrus, Wang Yao, Xiaodong Xu Monolayer transition metal dichalcogenide heterostructures have recently demonstrated type-II band alignment, prompting great interest in characterizing the properties of this new material system. In the monolayer MoSe$_{2}$-WSe$_{2}$ heterostructure, bright spatially indirect excitons with dramatically extended lifetimes have been demonstrated. Since the interlayer excitons are permanent electrical dipoles, they allow for electrical and optical control. Here, we report on the investigation of interlayer exciton emission energy, lifetime, and in-plane spatial diffusion, as a function of electric field and exciton density, in the MoSe$_{2}$-WSe$_{2}$ heterostructure. [Preview Abstract] |
Thursday, March 5, 2015 1:15PM - 1:27PM |
T2.00009: Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor Aaron Bradley, Miguel M. Ugeda, Su-Fei Shi, Felipe H. da Jornada, Yi Zhang, Diana Y. Qiu, Wei Ruan, Sung-Kwan Mo, Zahid Hussain, Zhi-Xun Shen, Feng Wang, Steven G. Louie, Michael F. Crommie Reduced screening in 2D has been predicted to result in dramatically enhanced Coulomb interactions that should cause giant bandgap renormalization and exotic excitonic effects in single-layer TMD semiconductors. Here we present a direct experimental observation of extraordinarily high exciton binding energy and bandgap renormalization in a single-layer of a semiconducting MoSe2, grown on bilayer graphene, using high-resolution scanning tunneling spectroscopy and photoluminescence spectroscopy. We have measured both the quasiparticle electronic bandgap and the optical transitions, obtaining an exciton binding energy of 0.55 eV -- a value orders of magnitude larger than in conventional 3D semiconductors. We have also studied the influence of external dielectric screening by repeating measurements on MoSe2/HOPG. These results are important for room-temperature optoelectronic devices involving 2D TMDs, as well as more complex layered heterostructures. [Preview Abstract] |
Thursday, March 5, 2015 1:27PM - 1:39PM |
T2.00010: Electron excitations in two-dimensional buckled honeycomb lattices Po-Hsin Shih, Yu-Huang Chiu, Min-Fa Lin The two-dimensional buckled honeycomb lattices system exhibits the rich Coulomb excitation spectra, being dominated by the free carrier density, band structure, and transferred momentum (q). There are two kinds of plasmon peaks in the energy loss spectra, calculated from the random phase approximation. They are, respectively, revealed at low and middle frequencies. The former, which arises from the free carriers, belongs to acoustic mode. It's frequency depends on $\sqrt{q}$ at long wavelength limit. On the other hand, the latter is due to all the $\pi$-electronic collective excitations is an optical mode. Whether such plasmon can service is mainly determined by q. The frequencies and intensities of plasmon modes are very different among graphene, silicene, germanene, and Tin. [Preview Abstract] |
Thursday, March 5, 2015 1:39PM - 1:51PM |
T2.00011: SPELEEM Studies on the Electronic Structure of MoS$_{2}$/Graphene Heterostructure Wencan Jin, Po-chun Yeh, Nader Zaki, Daniel Chenet, Ghidewon Arefe, Yufeng Hao, Alessandro Sala, Tevfik Mentes, Andrea Locatelli, James Hone, Richard Osgood Two-dimensional layered materials have been realized through the use of van der Waals heterostructures composed of weakly interacting layers. Among them, MoS2/graphene heterostructures can combine the advantages of high carrier mobility in graphene with the direct band gap of MoS2, which leads to potential applications in nanoelectronic devices with various functionalities. In this work, we study the influence of interlayer twist angle on the electronic structure of a MoS2/graphene heterostructure using Spectroscopic Photoemission and Low Energy Electron Microscopy (SPELEEM) system. MoS2/graphene heterostructures are prepared by transferring chemical-vapor-deposition (CVD)-grown monolayer MoS2 on top of CVD-grown graphene. Twist angles are characterized using the micro-LEED and the electronic structures are directly measured using micro-ARPES. [Preview Abstract] |
Thursday, March 5, 2015 1:51PM - 2:03PM |
T2.00012: Environment-Dependent Quasiparticle Bandgap of Monolayer MoS$_{2}$ Yong-Sung Kim, Ji-Young Noh, Hanchul Kim, Minkyu Park, K.C. Santosh, K.J. Cho 2D semiconductors are manifested by strong Coulomb interaction inside. The strong Coulomb interaction gives remarkable effects on various properties of the 2D semiconductors, including (i) large exciton binding energy (electron-hole), (ii) large quasi-particle self-energy (electron-electron), (iii) large scattering cross section in carrier transports by charged defects (electron-charged defects), (iv) deep defect transition level (bound electron-charged defects), and (v) strong interaction between charged defects (charged defects-charged defects). The ground state, optical, and transport properties are then largely affected by the dielectric environments surrounding the 2D semiconductors, because the Coulomb interaction is effectively screened by the dielectrics. We investigate the electronic band structures of a single-layer MoS2, as a prototype 2D semiconductor, with a variety of dielectric environments by using density-functional-theory (DFT) and GW calculations. [Preview Abstract] |
Thursday, March 5, 2015 2:03PM - 2:15PM |
T2.00013: The effects of surface polarity and dangling bonds on the electronic properties of MoS2 on SiO2 Ha-Jun Sung, Duk-Hyun Choe, Kee Joo Chang MoS2 has recently attracted much attention due to its intriguing physical phenomena and possible applications for the next generation electronic devices. In pristine monolayer MoS2, strong spin-orbit coupling and inversion symmetry breaking allow for an effective coupling between the spin and valley degrees of freedom, inducing valley polarization at the K valleys. However, the spin-valley coupling disappears in bilayer MoS2 because the inversion symmetry is restored. In this work, we investigate the effects of surface polarity and dangling bonds on the electronic properties of MoS2 on $\alpha $-quartz SiO2 through first-principles calculations. In monolayer MoS2, a transition can take place from the direct-gap to indirect-gap semiconductor in the presence of O dangling bonds. In bilayer MoS2, O dangling bonds induce dipole fields across the interface and thus break the inversion symmetry, resulting in the valley polarization, similar to that of pristine monolayer MoS2. Based on the results, we discuss the origin of the valley polarization observed in MoS2 deposited on SiO2 [Preview Abstract] |
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