Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T2: Invited Session: Valley Polarization Physics: Transition Metal Dichalcogenides and Other |
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Sponsoring Units: DCMP Chair: Tony Heinz, Columbia University Room: Ballroom II |
Thursday, March 21, 2013 8:00AM - 8:36AM |
T2.00001: Valley optoelectronics and spin-valley coupling: from graphene to monolayer group-VI transition metal dichalcogenides Invited Speaker: Wang Yao The Bloch bands in many crystals have a degenerate set of energy extrema in momentum space known as valleys. The band-edge carriers then have an extra valley index which may also be used to encode information for device applications provided that dynamic control of valley index is possible. In this talk, we show that, when inversion symmetry is broken, a pair of valleys which are equivalent by time-reversal are distinguishable by their magnetic moment and Berry curvature. These quantities give rise to valley Hall effect and circularly-polarized valley optical transition selection rule both in graphene (where inversion symmetry can be broken in a controlled way in gated bilayers), and in monolayer group-VI transition metal dichalcogenides (where the 2D crystal has inherent structural inversion asymmetry). Moreover, in monolayer dichalcogenides, we find the electrons and holes at the band edges are described by massive Dirac Fermions with strong spin-valley coupling, which further results in valley and spin dependent optical selection rule, and coexistence of valley Hall and spin Hall effects. These phenomena make possible dynamic control of valley and spin by electric and optical means for device applications in monolayer dichalcogenides. We will report photoluminescence studies on dichalcogenide thin films, which show the first evidence on valley optical selection rule and optical valley pumping, and signature of the spin-valley coupling. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 9:12AM |
T2.00002: Optical control of exciton valley polarization in MoS$_2$ Invited Speaker: Kin Fai Mak Atomic monolayers of transition metal dichalcogenides have emerged as an interesting class of 2-dimensional (2D) crystals beyond graphene. In particular, the isoelectronic family of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$ monolayers are direct band gap semiconductors.\footnote{Mak, K. F., Lee, C., Hone, J., Shan, J. {\&} Heinz, T. F. \textit{Phys Rev Lett} \textbf{105}, 136805 (2010); Splendiani, A.\textit{ et al.} \textit{Nano Lett} \textbf{10}, 1271-1275 (2010).}$^,$\footnote{Xiao, D., Liu, G.-B., Feng, W., Xu, X. {\&} Yao, W. \textit{Phys Rev Lett} \textbf{108}, 196802 (2012); Zhu, Z. Y., Cheng, Y. C. {\&} Schwingenschlogl, U. \textit{Phys Rev B} \textbf{84}, 153402 (2011).} Unlike graphene, because of the lack of inversion symmetry and the presence of strong spin-orbit interactions, the fundamental energy gaps of these compounds are located at two inequivalent high-symmetry valleys in the Brillouin zone (K and K') with coupled valley and spin degrees of freedom.\footnote{Ibid.} This electronic property makes them unique from conventional semiconductors. In this talk, we will discuss the properties of MoS$_{2}$ atomic layers as a prototype. Through characterization of the optical properties of the material as a function of thickness, we show that quantum confinement effects lead to a crossover in MoS$_{2}$ from a bulk indirect gap semiconductor to a direct gap semiconductor at monolayer thickness.\footnote{Mak, \textit{PRL} 105, 2010} With this basic property established, we show that complete valley polarization of the excitons in monolayer MoS$_{2}$ can be achieved by optical pumping with circularly polarized light.\footnote{Mak, K. F., He, K., Shan, J. {\&} Heinz, T. F. \textit{Nat Nano} \textbf{7}, 494-498 (2012); Zeng, H., Dai, J., Yao, W., Xiao, D., {\&} Cui, X. \textit{Nat Nano} \textbf{7}, 490-493 (2012); Cao, T. \textit{et al.} \textit{Nat Commun} \textbf{3}, 887 (2012); Sallen, G. et al. \textit{Phys Rev B} \textbf{86}, 081301(R) (2012).} Furthermore, this polarization can be retained for longer than 1ns. Our results thus highlight the great potential of this material family for studies of valley and spin Hall physics.\footnote{Xiao, D., Yao, W. {\&} Niu, Q. \textit{Phys Rev Lett} \textbf{99}, 236809 (2007); Yao, W., Xiao, D. {\&} Niu, Q. \textit{Phys Rev B} \textbf{77}, 235406 (2008); Xiao, D., Chang, M.-C. {\&} Niu, Q. \textit{Rev Mod Phys} \textbf{82}, 1959-2007 (2010).} [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:48AM |
T2.00003: Single-layer MoS$_{2}$ - electrical transport properties, devices and circuits Invited Speaker: Andras Kis After quantum dots, nanotubes and nanowires, two-dimensional materials in the shape of sheets with atomic-scale thickness represent the newest addition to the diverse family of nanoscale materials. Single-layer molybdenum disulphide (MoS$_{2})$, a direct-gap semiconductor is a typical example of these new graphene-like materials that can be produced using the adhesive-tape based cleavage technique originally developed for graphene. The presence of a band gap in MoS$_{2}$ allowed us to fabricate transistors that can be turned off and operate with negligible leakage currents. Furthermore, our transistors can be used to build simple integrated circuits capable of performing logic operations and amplifying small signals. I will report here on our latest 2D MoS$_{2}$ transistors with improved performance due to enhanced electrostatic control, showing improved currents and transconductance as well as current saturation. We also record electrical breakdown of our devices and find that MoS$_{2}$ can support very high current densities, exceeding the current carrying capacity of copper by a factor of fifty. Furthermore, I will show optoelectronic devices incorporating MoS$_{2}$ with sensitivity that surpasses similar graphene devices by several orders of magnitude. Finally, I will present temperature-dependent electrical transport and mobility measurements that show clear mobility enhancement due to the suppression of the influence of charge impurities with the deposition of an HfO$_{2}$ capping layer. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:24AM |
T2.00004: Novel electronic degrees of freedom emerging from symmetry breaking of honeycomb lattices Invited Speaker: Ji Feng Electrons are central to the society-transforming information technologies. The intrinsic degrees of freedom of an electron, namely, its charge and spin, have been extensively explored in electronic and spintronic devices. As we are approaching the limit of device miniaturization, the exploration of novel electronic degrees of freedom, in terms of theoretical development and materials discovery, is of current interest. In this talk, we will focus on two strategies to break the symmetry of a Fermionic honeycomb lattice that lead to novel degrees of freedom of Bloch electrons. The essential idea in these approaches is to lift the isospin degeneracy a honeycomb lattice by introducing contrasting identities (chemical or magnetic) to the two sublattices. The new indices of Bloch electrons will then arise, corresponding to contrasting responses to external fields, such as in optical selectivity and anomalous electronic transport. Using combined computational, theoretical and experimental approaches, we go on to demonstrate that the proposed physics can be realized in real material systems. In particular, our results indicate that monolayer transition metal chalcogenides, such as non-magnetic MoX$_{2}$ and antiferromagnetic MnPX$_{3}$ (X = S, Se), can indeed exhibit selective circular dichroism. The associated Berry curvature-supported quantum transport will also be discussed. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 11:00AM |
T2.00005: Valley polarization in bismuth Invited Speaker: Benoit Fauque The electronic structure of certain crystal lattices can contain multiple degenerate \textit{valleys} for their charge carriers to occupy. The principal challenge in the development of \textit{valleytronics} is to lift the valley degeneracy of charge carriers in a controlled way. In bulk semi-metallic bismuth, the Fermi surface includes three cigar-shaped electron valleys lying almost perpendicular to the high symmetry axis known as the trigonal axis. The in-plane mass anisotropy of each valley exceeds 200 as a consequence of Dirac dispersion, which drastically reduces the effective mass along two out of the three orientations. According to our recent study of angle-dependent magnetoresistance in bismuth [1], a flow of Dirac electrons along the trigonal axis is extremely sensitive to the orientation of in-plane magnetic field. Thus, a rotatable magnetic field can be used as a valley valve to tune the contribution of each valley to the total conductivity. As a consequence of a unique combination of high mobility and extreme mass anisotropy in bismuth, the effect is visible even at room temperature in a magnetic field of 1 T. Thus, a modest magnetic field can be used as a valley valve in bismuth. The results of our recent investigation of angle-dependent magnetoresistance in other semi-metals and doped semiconductors suggest that a rotating magnetic field can behave as a valley valve in a multi-valley system with sizeable mass anisotropy.\\[4pt] [1] Zengwei Zhu, Aur\'elie Collaudin, Beno\^it Fauqu\'e, Woun Kang and Kamran Behnia Nature Physics 8, 89-94 (2011) [Preview Abstract] |
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