Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L17: 2D Devices: Charge, Spin, and Valley ControlFocus
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Sponsoring Units: DMP Chair: Kin Fai Mak, Penn State University Room: 316 |
Wednesday, March 16, 2016 11:15AM - 11:27AM |
L17.00001: Properties of Edge States at the Graphene P-N Junction Interface Son Le, Nikolai Klimov, David Newell, Jun Yan, Ji Ung Lee, Curt Richter The Landau level edge states from the p- and the n-section of a graphene P/N junction (\textit{pn}J) interact with each other differently across the junction depending upon the properties of the junction and the graphene. Full equilibration was reported for a two terminal graphene \textit{pn}J device in Williams et al. [1]. In our four-terminal device, however, only the lowest Landau level edge state is equilibrated across the \textit{pn}J [2]. When the two devices are compared, the LL energy spacings, the length of the edge states along the \textit{pn}J interface, and the carrier mobility are similar. Electrostatic simulations for our device geometry and that of [1] contrast the rate of change of the electrostatic potential across the \textit{pn}Js. Edge states at an electrostatically smooth junction are spatially further apart than those at a relatively abrupt junction, which decreases the probability of edge states mixing. Thus, we attribute the difference in equilibration in our device and that of [1] to the dramatic difference in the shape of the electrostatic junction. [1] J. R. Williams, L. DiCarlo, and C. M. Marcus, Science 317, 638 (2007) [2] Nikolai N. Klimov, Son T. Le, \textit{et al.}, Phys. Rev. B: Rapid Comm. (2015 [Preview Abstract] |
Wednesday, March 16, 2016 11:27AM - 11:39AM |
L17.00002: Quantum oscillations as a probe of massive Dirac fermions Scott Dietrich, Carlos Forsythe, Jesse Balgley, Cory Dean Significant band structure engineering has been recently accomplished by stacking monolayer and bilayer graphene onto hexagonal Boron Nitride (hBN). The slightly mismatched lattice constants and varying alignment angle of the two materials creates a tunable moir\'{e} superlattice potential and breaks the naturally occurring valley symmetry of graphene. These effects open a gap and distort the band dispersion near the charge neutrality point as well as at location of the moir\'{e} minibands. In this study, effective band mass as a function of density is extracted from Shubnikov-de Haas oscillations in devices with varying superlattice wavelengths. Our results demonstrate quantitative control of band mass in graphene devices, and could help clarify the possible role of the many-body physics in these systems. [Preview Abstract] |
Wednesday, March 16, 2016 11:39AM - 11:51AM |
L17.00003: Nanoscale Tunable Strong Carrier Density Modulation of 2D Materials for Metamaterials and Other Tunable Optoelectronics Cheng Peng, Dmitri Efetov, Ren-Jye Shiue, Sebastien Nanot, Marek Hempel, Jing Kong, Frank Koppens, Dirk Englund Strong spatial tunability of the charge carrier density at nanoscale is essential to many 2D-material-based electronic and optoelectronic applications. As an example, plasmonic metamaterials with nanoscale dimensions would make graphene plasmonics at visible and near-infrared wavelengths possible. However, existing gating techniques based on conventional dielectric gating geometries limit the spatial resolution and achievable carrier concentration, strongly restricting the available wavelength, geometry, and quality of the devices. Here, we present a novel spatially selective electrolyte gating approach that allows for in-plane spatial Fermi energy modulation of 2D materials of more than 1 eV (carrier density of $n \quad =$ 10$^{\mathrm{14\thinspace }}$cm$^{\mathrm{-2}})$ across a length of 2 nm. We present electrostatic simulations as well as electronic transport, photocurrent, cyclic voltammetry and optical spectroscopy measurements to characterize the performance of the gating technique applied to graphene devices. The high spatial resolution, high doping capacity, full tunability and self-aligned device geometry of the presented technique opens a new venue for nanoscale metamaterial engineering of 2D materials for complete optical absorption, nonlinear optics and sensing, among other applications. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:27PM |
L17.00004: Optical study of low-dimensional materials Invited Speaker: Feng Wang Electrons in monolayer graphene are described by massless Dirac electrons, which exhibit unique quantum phenomena due to the pseudospin and Berry phase of the massless electrons. In this talk, I will discuss our effort in probing massive Dirac electrons in gapped bilayer graphene, which can be described by a quantum valley Hall insulator with non-trivial Chern number for individual valleys. A topologically protected 1D conducting channel was predicted to exist at the layer-stacking domain boundary of AB-BA bilayers. We show that near-field infrared imaging provides a versatile tool to visualize the layer stacking domain walls, and demonstrate conducting channels arising from the quantum valley Hall edge states through electrical transport in gapped bilayer graphene. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L17.00005: Pseudo Magnetic Faraday and Quantum Hall Effect In Oscillating Graphene Anita Bhagat, Kieran Mullen When a graphene layer is stressed, the strain changes the phase between sites in a tight binding model of the system. This phase can be viewed as a pseudo-magnetic vector potential. The corresponding pseudo-magnetic field has been experimentally verified in static cases.\footnote{ N. Levy et al. {\it Science} {\bf 329}, 544 (2010)} We examine the case of oscillating graphene ribbons and explore two new effects. The first is to investigate an oscillating pseudo-magnetic field that produces a quantum Hall effect: we calculate the I-V characteristic of an oscillating graphene nanoribbon as a function of frequency, and amplitude in both the oscillations and the applied driving voltage. Second, the time dependent pseudo-magnetic field should produce a pseudo-Faraday effect driving electrons in different valleys in opposite directions. In both cases, we make explicit calculations for experiment. [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L17.00006: Twisted bilayer graphene with interlayer potential asymmetry Pilkyung Moon, Young-Woo Son, Mikito Koshino A twisted stack of two graphene layers (twisted bilayer graphene) exhibits an extremely long potential period arising from the moir\'{e} interference between the layers \footnote{C. Berger et al., Science 312, 1191 (2006).} \footnote{P. Moon and M. Koshino, Phys. Rev. B 85, 195458 (2012).} \footnote{P. Moon and M. Koshino, Phys. Rev. B 87, 205404 (2013).}. We investigate the band structure and optical absorption spectrum of twisted bilayer graphenes with changing interlayer bias and Fermi energy simultaneously \footnote{P. Moon, Y.-W. Son, and M. Koshino, Phys. Rev. B 90, 155427 (2014).}. We show that the interlayer bias lifts the degeneracy of the superlattice Dirac point, while the amount of the Dirac point shift is significantly suppressed in small rotation angles, and even becomes opposite to the applied bias, by the interlayer interaction. In addition, we show that the spectroscopic features are highly sensitive to the interlayer bias and the Fermi energy, and widely tunable by the external field effect. [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L17.00007: Control of commensuration between graphene and boron nitride Matthew Yankowitz, K. Watanabe, T. Taniguchi, Pablo San-Jose, Brian J. LeRoy The electronic properties of van der Waals (vdW) heterostructures can be controlled through the choice and ordering of materials, as well as through the relative rotation between the atomic layers. However, little has been done to directly control the interactions between these layers, which may act as another tunable degree of freedom in these systems. Here, we demonstrate the ability to control the interlayer interaction strength between graphene and boron nitride using pressure resulting from the vdW interaction of a nearby STM tip. In particular, controlling the relative layer separation dynamically modifies the adhesion-induced strains in the graphene as it forms a partially commensurate structure with the boron nitride. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L17.00008: Negative Electronic Compressibility and Tuneable Spin Splitting in WSe$_{2}$ J.M. Riley, W. Meevasana, L. Bawden, M. Asakawa, T. Takayama, T. Eknapakul, T.K. Kim, M. Hoesch, S.-K. Mo, H. Takagi, T. Sasagawa, M.S. Bahramy, P.D.C. King Recently, semiconducting transition metal dichalcogenides have gained attention for their extraordinarily large exciton-binding energies [1,2] and locking of the spin with valley and layer pseudospins [3,4]. Through sub-monolayer deposition of alkali metals onto the surface of WSe$_{2}$, analogous to the gating in a field-effect transistor, we create a 2DEG at the sample surface with tuneable carrier concentration [5]. Counter-intuitively, we find that the addition of carriers induces a reduction of the chemical potential in the near-surface. We attribute this to negative electronic compressibility [6] where strong Coulomb effects lead to the lowering of the chemical potential with band filling, which we find persists to remarkably high electron densities. Simultaneously, we show this is accompanied by a giant tuneable spin-splitting of the valence band states and a reduction of the quasiparticle band gap. [1] Ugeda, \textit{et al., Nature Mat.} \textbf{13} (2014) 1091 [2] Ye \textit{et al}., \textit{Nature} \textbf{513} (2014) 214 [3] Xu \textit{et al., Nature Phys. }\textbf{10} (2014) 343 [4] Riley \textit{et al., Nature Phys.} \textbf{10} (2014) 835 [5] Riley \textit{et al.} doi:10.1038/nnano.2015.217 [6] Eisenstein \textit{et al. Phys. Rev. Lett. }\textbf{68} (1992) 674 [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L17.00009: Floquet-Engineered Valleytronics in Dirac Systems Babak Seradjeh, Arijit Kundu, Herbert Fertig Valley degrees of freedom offer a potential resource for quantum information processing if they can be effectively controlled. We discuss an optical approach to this problem in which intense light breaks electronic symmetries of a two-dimensional Dirac material. The resulting quasienergy structures may then differ for different valleys, so that the Floquet physics of the system can be exploited to produce highly polarized valley currents. This physics can be utilized to realize a valley valve whose behavior is determined optically. We propose a concrete way to achieve such valleytronics in graphene as well as in a simple model of an inversion-symmetry broken Dirac material, such as monolayer transition-metal dichalcogenides. Simulating the system numerically, we find that the effect is robustness against moderate disorder and small deviations in optical parameters. We also study designs for coherent manipulation of valley degrees of freedom suitable for quantum information processing. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L17.00010: Transition-metal dichalcogenide-based dipolariton optoelectronic devices German Kolmakov, Tim Byrnes, Andy He, Roman Ya. Kezerashvili Using computational modeling, we simulate the dynamics of dipolaritons in an optical microcavity, which encompasses the transition-metal dichalcogenide double-layer structure. We demonstrate that dipolaritons, a three-way superposition of photons, direct excitons and indirect excitons, are guided by a pattern deposited on the microcavity and can be driven by an external electric field or voltage applied to the structure. Focusing on a normal dipolariton gas in Y- and Psi-shaped patterns, we isolate conditions when the dipolariton flow can be switched between the channel branches of the pattern by the electric field. We also studied the superfluid dynamics of dipolariton Bose-Einstein condensates in patterned substrates at low temperatures, showing that the condensate in the channels can be accelerated and then directed by the electric field. We compare the obtained results with those for GaAs-based microcavities and demonstrate that dipolaritons in transition-metal dichalcogenide-based microcavities can be utilized for the design of optical switches and transistors for optoelectronic integrated circuits. [Preview Abstract] |
(Author Not Attending)
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L17.00011: Electrical and optical properties of SnS$_{\mathrm{2}}$/WSe$_{\mathrm{2}}$ van der Waals Heterojunction FETs Ahmad Zubair, Amirhasan Nourbakhsh, Mildred Dresselhaus, Tomas Palacios Two dimensional crystals based on atomically thin films of transition metal dichalcogenides offer an exciting platform for various optoelectronic applications. Their unique crystal properties make them particularly attractive for van der Waals heterostructures which open up an additional degree of freedom to tailor the material properties into new physics and device applications. In this work, we explore, for the first time, the optoelectronic properties of van der Waals SnS$_{\mathrm{2}}$/WSe$_{\mathrm{2}}$ heterojunction. WSe$_{\mathrm{2}}$ is an ambipolar semiconductor while SnS$_{\mathrm{2}}$ is an $n$-type wide bandgap semiconductor. We use the pickup and dry transfer methods to fabricate SnS$_{\mathrm{2}}$/WSe$_{\mathrm{2\thinspace }}$heterojunction transistors (hetero-FETs). We observe negative differential transconductance in the SnS$_{\mathrm{2}}$/WSe$_{\mathrm{2}}$ hetero-FET. Also, the heterostructure couples strongly to incident light and shows high photovoltaic responsivity which can find applications in nano-devices such as photo-detectors and solar cells. [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L17.00012: Mapping of the photo-induced metastable and hidden phases in 2D electronic materials Faran Zhou, Tianyin Sun, Tzong-Ru Han, Christos Malliakas, Phillip Duxbury, Subhendra Mahanti, Mercouri Kanatzidis, Chong-Yu Ruan Using the ultrafast electron imaging techniques, we studied the light-induced phase transitions in transition-metal dichalcogenide materials. A succession of different phases was introduced transiently using femtosecond mid-infrared pulses and the local atomic scale charge-density-wave dynamics and morphological evolution of the long-range textured domains were \textit{in situ} characterized using the ultrashort coherent electron pulses. The various metastable and hidden states emerging under the controlled nonthermal, nonadiabatic driving highlight the interaction-driven nature of these transitions with limited involvement of lattice entropy. The methodology introduced here can be generally applied to survey the complex energy landscape in strongly correlated electron systems, avoiding the difficulty of electrostatic gating or confounding effects due to defects and/or disorder. In particular, the observation of robust non-thermal switching at meso-scales and at ultrafast timescales, provides a platform for designing high-speed low-energy consumption nano-photonics and electronics devices. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L17.00013: Transport properties of heterostructures composed of Mo(S,Se)$_2$ on \emph{h}-BN Qiong Zhou, Nihar Pradhan, Shahriar Meraman, Daniel Rhodes, Luis Balicas The thickness-dependent tunable band gap of transition metal dichalcogenides in the visible region has generated a lot of interest on their optoelectronic properties. Our single crystals of molybdenum disulphide (MoS$_2$) and molybdenum diselenide (MoSe$_2$) were grown though a chemical vapor transport technique. Few-layered flakes of MoS$_2$ and MoSe$_2$ were mechanically exfoliated and transferred onto \emph{h}-BN flakes, with this stack subsequently transferred onto pre-evaporated molybdenum bottom gate(s). Here, we report the fabrication and temperature-dependent electrical transport properties of few-layered MoS$_2$ and MoSe$_2$ field-effect transistors on \emph{h}-BN. [Preview Abstract] |
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