Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G2: Focus session: Beyond Graphene - Valley properties |
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Sponsoring Units: DMP Chair: Di Xiao, Carnegie Mellon University Room: 001B |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G2.00001: $\mathbf{k}\cdotp\mathbf{p}$ theory for two-dimensional transition metal dichalcogenide semiconductors Andor Kormanyos, Guido Burkard We present $\mathbf{k}\cdotp\mathbf{p}$ Hamiltonians (for a review see [1]) parametrised by ab initio density functional theory calculations to describe the dispersion of the valence and conduction bands at their extrema (the $K$, $Q$, $\Gamma$, and $M$ points of the hexagonal Brillouin zone) in atomic crystals of semiconducting monolayer transition metal dichalcogenides. We review the parametrisation of the essential parts of the $\mathbf{k}\cdotp\mathbf{p}$ Hamiltonians for MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$, including the spin-splitting and spin-polarisation of the bands We use $\mathbf{k}\cdotp\mathbf{p}$ theory to analyse: i) optical transitions in two-dimensional transition metal dichalcogenides over a broad spectral range; ii) to discuss magnetotransport properties of the charge carriers in the $K$ and $-K$ valleys. \newline [1] A. Korm\'anyos, G. Burkard et al, arXiv:1410.6666 [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G2.00002: Magneto-optics in WSe2 and MoSe2 monolayers Bernhard Urbaszek, Gang Wang, Louis Bouet, Etienne Palleau, Mael Vidal, Xavier Marie, Thierry Amand We perform photoluminescence (PL) experiments at T=4K on MoSe$_2$ and WSe$_2$ in magnetic fields up to 9T applied perpendicular to the monolayer (ML) plane. In both systems the neutral exciton is spectrally well separated from the charged exciton (trion). For both exciton complexes in both systems we observe a clear Zeeman splitting of the order of -2meV at 9T between the $\sigma^+$ and $\sigma^-$ polarized PL components, from the K$^+$ and K$^-$ valley, respectively. The extracted g-factors for both exciton complexes in both materials are of the order of $g\approx -4$. This indicates a dominant contribution from the transition metal valence band $d$-orbitals to the exciton magnetic moment, contributions from the valley magnetic moments are discussed. In ML MoSe$_2$ the exciton valley polarization can be tuned with the magnetic field, independent of the excitation laser polarization. In the investigated ML WSe$_2$ sample the evolution of the valley polarization depends for the trion both on the applied magnetic field and the excitation laser helicity, for the neutral exciton only on the latter. In the absence of optical orientation, the trion polarization amplitude increases linearly with the applied magnetic field, albeit with opposite signs in MoSe$_2$ compared to WSe$_2$. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G2.00003: Experimental evidence for dark excitons in monolayer transition metal dichalcogenide crystals Xiaoxiao Zhang, Yumeng You, Frank Zhao, Tony Heinz Transition metal dichalcogenides in the family of MoS$_{\mathrm{2}}$, MoSe$_{\mathrm{2}}$, WS$_{\mathrm{2}}$, and WSe$_{\mathrm{2}}$ have been identified as direct-gap semiconductors in the limit of monolayer thickness. In addition to the optically bright states associated with dipole-allowed excitonic transitions between these bands, it is predicted that excitonic states will form that are optically dark either because of momentum or spin selection rules. In this paper, we report studies of the temperature dependence of the bright exciton population using photoluminescence and time-resolved photoluminescence spectroscopy. The experimental results indicate the presence of dark states lying below the optically bright states in some members of this family of materials. These states, unlike the usual bright states, are not be constrained by rapid radiative decay and offer new avenues for control of the valley and spin degrees of freedom. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G2.00004: Probing the valley Hall effect in MoS2 transistors Invited Speaker: Kin Fai Mak Two-dimensional (2D) atomic layers of molybdenum disulfide (MoS2) have attracted much recent attention due to their unique electronic properties. In addition to charge and spin, electrons in MoS2 monolayers possess a new valley degree of freedom (DOF) that has finite Berry curvatures. As a result, not only optical control of the valley DOF is allowed, but each valley is also predicted to exhibit an anomalous Hall effect whose sign depends on the valley index. In this talk, we will discuss our recent observation of this new valley Hall effect (VHE) in monolayer MoS2 transistors. This is manifested experimentally as a finite anomalous Hall effect when circularly polarized light is used to preferentially excite electrons into a specific valley. We will describe the dependence of the anomalous Hall conductivity on photon helicity, photon energy, doping levels and crystal symmetry, and will compare these observations with theoretical predictions. Possibilities of using the valley DOF as an information carrier in next-generation electronics and optoelectronics will also be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G2.00005: Magneto-optical Kerr effect in Transition Metal Dichalcogenides Ryuji Suzuki, Sandor Bordacs, Yoshinori Tokura, Yoshihiro Iwasa Transition-metal dichalcogenides (TMDs) are attracting a great deal of interest as beyond graphene materials because of their rich physical properties. The key of the monolayer TMDs in contrast to the bulk is the broken inversion symmetry, which results in novel valley properties, coupled with spins through their strong spin-orbit interaction. On the other hand, 3R-MoS2 is known to keep the broken inversion symmetry and thus strong valley polarization in PL spectra even in multilayers, providing new opportunities to investigate properties of monolayers with use of bulk materials and the stacking dependent properties between the 2H (centrosymmetric) and 3Rnon-centrosymmetric) [R. Suzuki \textit{et al., Nat. Nano.} \textbf{9}, 611 (2014)]. In this presentation, we report comparative studies of 3R MoS2 and 2H series of TMDs on magnetooptical properties, with a particular focus on magneto-optical Kerr effect (MOKE) spectroscopy. We found systematic evolusion of MOKE spectra in the 2H series of TMDs, and more interestingly, that 3R polytypes displayed significant difference not only in exciton dimensionality and but also in the $g$ values estimated from the excitons peak splitting in the MOKE spectra. Discussion will be given based on the electronic structures and the spin-orbit interactions [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G2.00006: Valley entanglement of carriers in monolayers of transition-metal dichalcogenides Alexey Belyanin, Mikhail Tokman The entanglement of two quantum systems or ensembles is usually generated as a result of coupling between them. This coupling can be mediated by classical electromagnetic fields. At the same time, one can also entangle \textit{non-interacting} quantum systems by a quantum field. Here we consider the optical excitation of electron-hole or exciton states near the band gap of a transition-metal dichalcogenide monolayer in two valleys K' and K with opposite valley indices. We show that a linearly polarized single-photon field in a cavity or a stationary stream of linearly polarized single photons gives rise to an efficient entanglement of non-interacting carriers in different valleys, i.e. the generation of electron states entangled with respect to the valley degree of freedom. An intuitive explanation of this effect is that the carriers ``view'' linearly polarized photons as entangled left- and right-circularly polarized photon states. Valley entanglement of carriers gives rise to peculiar properties of the reemitted optical field and photocurrent fluctuations. [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G2.00007: Electrical Valley Excitation by Spin Injection in Monolayer Transition Metal Dichalcogenide Heterojunction Yu Ye, Xiaobo Yin, Hailong Wang, Ziliang Ye, Hanyu Zhu, Yuan Wang, Jianhua Zhao, Xiang Zhang Embracing the spin degree of freedom of charge carriers enables nonvolatile electronics with increased operation speed and reduced power consumption. Recently discovered atomic materials of monolayer transition metal dichalcogenides (TMDs) possess unbalanced carrier distribution in the momentum space and introduce a new independent valley of freedom. Here we demonstrate experimentally the unique spin and valley locking relationship in TMDs and report a new scheme of electronic devices taking advantages of the both degrees of freedoms. A valley-polarized light-emitting device is achieved experimentally through spin injection using (Ga, Mn)As as a spin aligner. The electrical generation and the control of valley polarization in TMD semiconductors through spin manipulation opens the new dimension in utilizing both spin and valley degrees of freedom for next-generation electronics and computing. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G2.00008: Interplay of spin, valley and layer pseudospins in folded MoS$_{2}$ bilayers Tao Jiang, Hengrui Liu, Di Huang, Shuai Zhang, Yingguo Li, Xingao Gong, Yuen-Ron Shen, Wei-Tao Liu, Shiwei Wu Two dimensional material such as graphene and transition metal dichalcogenide is much like a piece of sheet paper. Different from sheet paper, each of two dimensional materials has its own crystal lattice. Folding of two dimensional materials can make artificial bilayer or even multilayer structures, whose structural symmetry depend on how the folding line is oriented relative to the crystal lattice. The folded artificial structures with different stacking orders are also expected to tune the interlayer coupling, thus leading to unusual behaviors and new phenomena. In this talk, I will present our recent study of folded MoS$_{2}$ bilayers, which were obtained by folding exfoliated MoS$_{2}$ monolayers. As characterized by second harmonic generation and photoluminescence, folded bilayers can exhibit broken inversion symmetry and reduced interlayer coupling, evoking strong valley and/or spin polarizations that were not achieved in natural MoS$_{2}$ bilayer of Bernal stacking. Our work provides an effective and versatile means to understand the interaction between spin, valley and layer pseudospin degrees of freedom in MoS2 bilayer and engineer transition metal dichalcogenide materials with desirable electronic and optical properties. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G2.00009: Hofstadter spectrum in MoS$_2$ Yen-Hung Ho, Wu-Pei Su, Ming-Fa Lin In studying the Hofstadter problem of monolayer molybdenum disulfide (MoS$_2$), we systematically demonstrate the magnetic energy spectra due to various hoppings between $d$-orbital electrons. The magnetoelectronic spectrum shows a mirror symmetry as a result of the particle-hole symmetry in the Bloch bands. At small field, specific Landau fan diagrams can be ascribed to certain Bloch-band singularities. In the spectrum of real MoS$_2$, we further illustrate a breaking of spectral symmetry, the spin and valley polarization, and a flux-dependent energy gap. Our numerical results can facilitate the qualitative understanding of topological nature of $d$-bands and provide a basis for exploring the Landau levels in transition-metal dichalcogenides. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G2.00010: Spatially-Resolved Modeling of Spin and Valley Hall Effects in Two-Dimensional Semiconductors E.J. Lenferink, Y. Jia, N.P. Stern In monolayers of transition metal dichalcogenides (1L-TMDs), a valley degree of freedom emerges for charge carriers due to the absence of spatial inversion symmetry. Strong spin-orbit interaction couples spin and valley, resulting in correlated spin, valley, and charge transport such as transverse Hall effects. Spatially-resolved measurements of these Hall effects have recently been achieved in monolayer MoS$_2$~\footnote{K. F. Mak, K. L. McGill, J. Park, and P. L. McEuen. The Valley Hall Effect in MoS$_2$ Transistors. \emph{Science}, 344(6191):1489--1492, 2014.}, necessitating a detailed picture for understanding transport and relaxation mechanisms in 1L-TMDs that considers carrier, valley, and spin motion and generation processes. Here, we study spin and valley Hall effects in 1L-TMD devices by simulating the transport of spin- and valley-polarized carriers with a generalized drift diffusion model incorporating circularly polarized optical excitation. Spin and valley accumulation and the transverse voltage are analyzed in different device geometries. We compare the electron and hole contributions to the transverse voltage and discuss the potential for a measurement of the valley relaxation times of free carriers in 1L-TMDs. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G2.00011: Valley selective optical Stark effect in monolayer WS2 Edbert J. Sie, James W. McIver, Yi-Hsien Lee, Liang Fu, Jing Kong, Nuh Gedik Monolayer semiconductors, such as WS2, have a pair of valleys that, by time-reversal symmetry, are energetically degenerate. Lifting the valley degeneracy in these materials is of great interest because it would allow for valley specific band engineering and offer additional control in valleytronic applications. Here we show that circularly polarized light, which breaks time-reversal symmetry, can be used to lift the valley degeneracy by means of the optical Stark effect. We demonstrate that this effect is capable of raising the exciton level in monolayer WS2 by as much as 18 meV in a controllable valley selective manner. The resulting energy shift is extremely large, comparable to the shift that would be obtained using a very high magnetic field (~200 Tesla). These results offer a novel way to control valley degree of freedom, and may provide a means to realize new valley-selective Floquet topological state of matter. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G2.00012: Valley-Exciton Locked Optical Selection Rule in Monolayer WS2 Jun Xiao, Ziliang Ye, Ying Wang, Yuan Wang, Xiang Zhang Layered transition metal dichalcogenide (TMDC) with hexagonal lattice structure has six valleys at corners of the Brillouin zone. The nontrivial Berry curvature distribution renders the adjacent valleys with distinguishable valley angular momentum, which enables itself as an ideal 2D valleytronic platform. Recent studies reported strong excitonic effect in monolayer WS2 and each excitonic state is identified with a well-defined orbital angular momentum, however the anticipated selection rules involve nonlinear optical processes are not clear. Here we show valley angular momentum (VAM) together with exciton angular momentum (EAM) impose different valley-exciton locked selection rules for second harmonic generation (SHG) and two photon luminescence (TPL) in monolayer WS2. Moreover, the two-photon induced valley populations yield net circular polarized photoluminescence after a sub-ps interexciton relaxation. The work demonstrates a new approach to control valley population at different excitonic states for next generation of optical circuits and quantum information computing. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G2.00013: Trion dynamics in Transition Metal Dichalcogenide Monolayers Akshay Singh, Kha Tran, Sanweng Wu, Jason Ross, Galan Moody, Xiaodong Xu, Elaine Li Transition Metal Dichalcogenides (TMD's) in the monolayer limit, exhibit interesting phenomena including increased photoluminescence, spin-valley coupling and many-body effects. Excitons (bound electron-hole pairs) and trions (charged excitons) in these materials have unusually large binding energy and dominate the optical response near the band gap. In particular, trions can drift under application of an electric field and have higher spin lifetimes increasing applications in quantum spin models. We study the temporal dynamics of excitons and trions including their formation and lifetimes using time resolved two-color pump-probe spectroscopy on a monolayer TMD (MoSe2). Trions are observed to have vastly different temporal dynamics with much slower decay than excitons. We also observe rapid formation of trions when resonantly pumped while a slow rise (in temporal response) is seen for other excitation energies. We suggest that localization of trions needs to be taken into account to explain these observations. [Preview Abstract] |
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