Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session S17: 2D Semiconductor Physics IIFocus

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Sponsoring Units: DMP Chair: Jeanie Lau, University of California, Riverside Room: 316 
Thursday, March 17, 2016 11:15AM  11:51AM 
S17.00001: Novel exciton systems in 2D TMD monolayers and heterobilayers Invited Speaker: Hongyi Yu In this talk, two exciton systems in transition metal dichalcogenides (TMDs) monolayer and heterobilayer will be discussed. In TMD monolayers, the strong eh Coulomb exchange interaction splits the exciton and trion dispersions into two branches with zero and finite gap, respectively \footnote{H. Yu, G.B. Liu, P. Gong, X. Xu, and W. Yao, Nat. Commun. \textbf{5}, 3876 (2014).} \footnote{H. Yu, X. Cui, X. Xu, and W. Yao, Natl Sci Rev \textbf{2}, 57 (2015).}. Each branch is a centerofmass wave vector dependent coherent superposition of the two valleys, which leads to a valleyorbit coupling and possibly a trion valley Hall effect. The exchange interaction also eliminates the linear polarization of the negative trion PL emission \footnote{A. Jones, H. Yu, N. Ghimire, S. Wu, G. Aivazian, J. Ross, B. Zhao, J. Yan, D. Mandrus, D. Xiao, W. Yao, and X. Xu, Nature Nanotech. \textbf{8}, 634 (2013).}. In TMD heterobilayers with a typeII band alignment, the low energy exciton has an interlayer configuration with the e and h localized in opposite layers. Because of the inevitable twist or/and lattice mismatch between the two layers, the bright interlayer excitons are located at finite centerofmass velocities with a sixfold degeneracy \footnote{H. Yu, Y. Wang, Q. Tong, X. Xu, and W. Yao, Phys. Rev. Lett. \textbf{115}, 187002 (2015).}. The corresponding photon emission is elliptically polarized, with the major axis locked to the direction of exciton velocity, and helicity determined by the valley indices of the e and h. Some experimental results on the interlayer excitons in the WSe$_2$MoSe$_2$ heterobilayers will also be presented. The interlayer exciton exhibits a long lifetime as well as a long depolarization time, which facilitate the observation of a PL polarization ring pattern due to the valley dependent excitonexciton interaction induced expansion \footnote{P. Rivera, K. L. Seyler, H. Yu, J. R. Schaibley, J. Yan, D. G. Mandrus, W. Yao, and X. Xu, to be published.}. [Preview Abstract] 
Thursday, March 17, 2016 11:51AM  12:03PM 
S17.00002: Engineering topological band with superlattice. Xiaoou Zhang, Wenyu Shan, Di Xiao Since the discovery of the quantum Hall effect, the search for topological states has been a major subject of interest in condensed matter physics.~Here we propose a general scheme to create nontrivial Chern band by fabricating superlattice structure on a system with nonzero Berry curvature.~We analyze the topological band structure by deriving an effective Hamiltonian that incorporates the Berry curvature effect.~The Chern number is tunable by the superlattice configurations that are realizable with existing experimental technology. [Preview Abstract] 
Thursday, March 17, 2016 12:03PM  12:15PM 
S17.00003: Optical properties of quantum dots in buckled graphenelike materials Thakshila Herath, Vadym Apalkov The band gap of buckled graphenelike materials, such as silicene and germanene, depends on external perpendicular electric field. A specially design profile of electric field produces a quantum dot, i.e., trapping potential for electrons in such materials. We theoretically study the optical properties of such silicene/germanene quantum dots. There are two types of absorption spectra in the quantum dot: interband (optical transitions between the states of the valence and conduction bands) and intraband (transitions between the states of conduction/valence band). The interband absorption spectra have triplepeak structure with peak separation around 10 meV, while intraband absorption spectra, which depend on the number of electrons in the dot, have doublepeak structure with separation between the peaks around 15meV. The interband optical spectra as a whole are redshifted with increasing electric field in the internal region of the quantum dot, while the energy separation between the peaks depends weakly on the electric field. With increasing the size of the quantum dot, the interband and intraband absorption spectra become red shifted as well. [Preview Abstract] 
Thursday, March 17, 2016 12:15PM  12:27PM 
S17.00004: Charge density wave transition in singlelayer titanium diselenide Peng Chen, Yanghao Chan, Xinyue Fang, Yi Zhang, MeiYin Chou, Sungkwan Mo, Zahid Hussain, Alexei Fedorov, TaiChang Chiang A single molecular layer of titanium diselenide (TiSe$_{\mathrm{2}})$ is a promising material for advanced electronics beyond graphenea strong focus of current exploration. Such molecular layers are at the quantum limit of device miniaturization and can show enhanced electronic effects not realizable in thick films. We show that singlelayer TiSe$_{\mathrm{2}}$ exhibits a charge density wave (CDW) transition at critical temperature T$_{\mathrm{C}}=$232 K, which is higher than the bulk T$_{\mathrm{C}}=$200 K. Angleresolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below T$_{\mathrm{C}}$ in conjunction with the emergence of (2x2) ordering. The results are rationalized in terms of firstprinciples calculations, symmetry breaking and phonon entropy effects. The observed BCS behaviour of the gap implies a meanfield CDW order in the single layer and an anisotropic CDW order in the bulk. [Preview Abstract] 
Thursday, March 17, 2016 12:27PM  12:39PM 
S17.00005: Spatially Indirect Exciton Condensates in Double Bilayer Graphene JungJung Su, Allan H. MacDonald Manybody interaction effects have a strong influence on the lowenergy electronic properties of graphene bilayers because of the nearly quadratic dispersion at the K/K’ bandcrossing points. In the single graphene bilayer systems, the ground state has an energy gap thought to be a consequence of spindensity wave order and other competing ordered states are believed to be nearby in energy. In systems with two closely spacing bilayer, spatially indirect exciton states are expected in neutral systems with interbilayer charge transfer. This transfer can be achieved by applying either a vertical electrical displacement fields or an interbilayer potential bias. Here we report that the different combinations of displacement field and potential bias can give rise to different types of indirect exciton condensate states that are distinguished by the twodimensional momentum dependence of the spontaneous interbilayer coherence. In general a displacement field prefers an excitonic condensate in which the phase coherence between the inner two layers of the four layer system, while the potential bias prefers momentumindependent coherence between remote layers. The complete phase diagram reported exhibits excitonic coherence states mentioned above, and more interestingly, their mixtures. [Preview Abstract] 
Thursday, March 17, 2016 12:39PM  12:51PM 
S17.00006: Excitonpolariton condensation in transition metal dichalcogenide bilayer heterostructure Ki Hoon Lee, JaeSeung Jeong, Hongki Min, Suk Bum Chung For the bilayer heterostructure system in an optical microcavity, the interplay of the Coulomb interaction and the electronphoton coupling can lead to the emergence of quasiparticles consisting of the spatially indirect exciton and cavity photons known as \emph{dipolariton}, which can form the BoseEinstein condensate above a threshold density. Additional physics comes into play when each layer of the bilayer system consists of the transition metal dichalcogenide (TMD) monolayer. The TMD monolayer band structure in the low energy spectrum has two valley components with nontrivial Berry phase, which gives rise to a selection rule in the excitonpolariton coupling, e.g. the exciton from one (the other) valley can couple only to the clockwise (counterclockwise) polarized photon. We investigate possible condensate phases of excitonpolariton in the bilayer TMD microcavity changing relevant parameters such as detuning, excitation density and interlayer distance. [Preview Abstract] 
Thursday, March 17, 2016 12:51PM  1:03PM 
S17.00007: Berry Phase Modification to the Energy Spectrum of Excitons Di Xiao, Jianhui Zhou, Wenyu Shan, Wang Yao By quantizing the semiclassical motion of excitons, we show that the Berry curvature can cause an energy splitting between exciton states with opposite angular momentum. This splitting is determined by the Berry curvature flux through the kspace area spanned by the relative motion of the electronhole pair in the exciton wave function. Using the gapped twodimensional Dirac equation as a model, we show that this splitting can be understood as an effective spinorbit coupling effect. In addition, there is also an energy shift caused by other “relativistic” terms. Our result reveals the limitation of the venerable hydrogenic model of excitons, and it highlights the importance of the Berry curvature in the effective mass approximation. [Preview Abstract] 
Thursday, March 17, 2016 1:03PM  1:15PM 
S17.00008: Optical and Electronic Properties of dopedMoS2: Joint Theoretical/Experimental Study Miller Eaton, Hansika Sirikumara, Hassana Samassekou, Dipanjan Mazumdar, Thushari Jayasekera, Laalitha Liyanage, Marco Buongiorno Nardelli Substitutional doping of transition metal dichalcogenides (TMDs) is an attractive way of engineering their electronic properties. The dependence of optoelectronic properties of TMDs on the dopant is largely underexplored. In this work, we will discuss how different species affect the optical properties of MoS2. The electronic structure calculations of doped TMDs are carried out using Density Functional Theory with the recently developed ACBN0 functional, a pseudohybrid Hubbard density functional that is a fast, accurate and parameterfree alternative to traditional DFT+U and hybrid exact exchange methods [L.A. Agapito, S. Curtarolo, and M. Buongiorno Nardelli, Phys. Rev. X 5, 011006 (2015)]. We compare our ACBN0 predictions with measurement of the electronic and optical properties of pristine and niobium doped MoS2 films synthesized via physical vapor deposition and characterized using spectroscopic ellipsometry and optical spectroscopy. [Preview Abstract] 
Thursday, March 17, 2016 1:15PM  1:27PM 
S17.00009: Storing excitons in transitionmetal dichalcogenides using dark states Daniel Gunlycke, Frank Tseng, Ergun Simsek Monolayer transitionmetal dichalcogenides exhibit strongly bound excitons confined to two dimensions. One challenge in exploiting these excitons is that they have a finite life time and collapse through electronhole recombination. We propose that the exciton life time could be extended by transitioning the exciton population into dark states. The symmetry of these dark states require the electron and hole to be spatially separated, which not only causes these states to be optically inactive but also inhibits electronhole recombination. Based on an atomistic model we call the Triangular Lattice Exciton (3ALE) model, we derive transition matrix elements and approximate selection rules showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. [Preview Abstract] 
Thursday, March 17, 2016 1:27PM  1:39PM 
S17.00010: Excitonic effects in 2D semiconductors: Path Integral Monte Carlo approach Kirill Velizhanin, Avadh Saxena One of the most striking features of novel 2D semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states (e.g., excitons, trions and biexcitons), which could remain stable at nearroom temperatures and contribute significantly to optical properties of such materials. In my talk, I will report on our recent progress in using the Path Integral Monte Carlo methodology to numerically study properties of multicarrier bound states in 2D semiconductors. Incorporating the effect of the dielectric confinement (via Keldysh potential), we have investigated and tabulated the dependence of single exciton, trion and biexciton binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. The implications of the obtained results and the possible limitations of the used model will be discussed. The results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models. [Preview Abstract] 
Thursday, March 17, 2016 1:39PM  1:51PM 
S17.00011: Theoretical ultrafast spectroscopy in transition metal dichalcogenides Alejandro MolinaSanchez, Davide Sangalli, Andrea Marini, Ludger Wirtz Semiconducting 2Dmaterials like the transition metal dichalcogenides (TMDs) MoS$_2$, MoSe$_2$, WS$_2$, WSe$_2$ are promising alternatives to graphene for designing novel optoelectronic devices. The strong spinorbit interaction along with the breaking of inversion symmetry in singlelayer TMDs allow using the valleyindex as a new quantum number [1]. The practical use of valley physics depends on the lifetimes of valleypolarized excitons which are affected by scattering at phonons, impurities and by carriercarrier interactions. The carrier dynamics can be monitored using ultrafast spectroscopies such as pumpprobe experiments. The carrier dynamics is simulated using nonequilibrium Green’s function theory in an abinitio framework. We include carrier relaxation through electronphonon interaction. We obtain the transient absorption spectra of singlelayer TMD and compare our simulations with recent pumpprobe experiments [2]. [1] D. Xiao et. al., Phys. Rev. Lett. \textbf{108}, 196802 (2012). X. Xu et. al. , Nature Physics \textbf{10}, 343 (2014). [2] Y. T. Wang, et. al. , Scientific Reports \textbf{5}, 8289 (2015). [Preview Abstract] 
Thursday, March 17, 2016 1:51PM  2:03PM 
S17.00012: Interaction Induced Quantum Valley Hall Effect in Graphene Cristiane Morais Smith, Eduardo C. Marino, Leandro O. Nascimento, A Van Sergio We use Pseudo Quantum Electrodynamics (PQED) in order to describe the full electromagnetic interaction of the electrons in graphene in a consistent 2D formulation. We first consider the effect of this interaction in the vacuum polarization tensor or, equivalently, in the current correlator to evaluate the Kubo's formula. Thereby, we obtain the usual expression for the minimal conductivity plus corrections due to the interaction. We then predict the onset of an interactiondriven spontaneous Quantum Valley Hall effect by solving the SchwingerDyson equation. The obtained ValleyHall conductivity is exact and universal [1]. [1] E. C. Marino, Leandro O. Nascimento, V. S. Alves, and C. Morais Smith, Phys. Rev. X {\bf 5}, 011040 (2015). [Preview Abstract] 
Thursday, March 17, 2016 2:03PM  2:15PM 
S17.00013: Lateral Heterostructures of Monolayer Transition Metal Dichalcogenides: a Firstprinciples Study Meng Wu, Ting Cao, Steven G. Louie Using firstprinciples calculations, we investigate the electronic structure and optical properties of lateral heterostructures consisting of different monolayer transition metal dichalcogenides (TMDs). We find that the spinorbital coupling effect plays an important role in modifying the groundstate electronic structure and excitedstate properties such as optical responses. The anisotropy of optical absorption is investigated including localfield effects. [Preview Abstract] 
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