Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization II |
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Sponsoring Units: DMP Chair: Aaron Jones, University of Washington Room: Mile High Ballroom 1E |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J51.00001: Horizontally aligned single walled carbon nanotube arrays on quartz for electrochemical biosensing Yuehai Yang, Xuewen Wang, Wenzhi Li, Jin He We have fabricated and characterized a simple and high performance electrochemical sensor using horizontally aligned single walled carbon nanotube arrays on transparent single crystal quartz substrates grown by chemical vapor deposition. The electrochemical activities of redox probes Fe(CN)$_{6}^{3-/4-}$, Ru(NH$_{3})_{6}^{3+}$ and protein cytochrome c on these pristine SWCNT thin films have been studied. Because the surface coverage of CNTs is extremely low and aligned, the shape of cyclic voltammograms of these molecules in stationary solution is strongly affected by the mass transport rate of molecules and the interactions between molecules and the SWCNT surface. We also studied the electrochemical flow sensing capability of the device for detecting neurotransmitter dopamine at physiological conditions with the presence of \textit{Bovine serum albumin}. Good sensitivity, fast response, high stability and anti-fouling capability are observed. Therefore, this device shows great potential for sensing applications in complex solution. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J51.00002: Transition from Coulomb Blockade to Resonant Transmission in a MoS$_{2}$ Nanoribbon Yanjing Li, Nadya Mason We have measured a side-gated nanoribbon of MoS$_{2}$ at low temperature, and observed the transition from Coulomb blockade to resonant transmission when the Fermi level is tuned with a gate. We show that near the crossover between these regimes, the entire nanoribbon acts as a single quantum dot. Our findings may shed light on quasi-ballistic transport in the material. We also discuss the quantum dot formation in terms of a substrate-induced disorder potential, and consider other possible origins of disorder. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J51.00003: Spin-orbit coupling, quantum dots, and qubits in transition metal dichalcogenides Andor Korm\'{a}nyos, Viktor Z\'{o}lyomi, Neil D. Drummond, Guido Burkard We derive an effective Hamiltonian describing the dynamics of electrons in the conduction band of transition metal dichalcogenides (TMDC) in the presence of perpendicular electric and magnetic fields [1]. We discuss both the intrinsic and Bychkov-Rashba spin-orbit coupling (SOC) induced by an external electric field. We identify a new term in the Hamiltonian of the Bychkov-Rashba SOC which does not exist in III-V semiconductors. Due to the strong intrinsic SOC is an effective out-of-plane $g$-factor for the electrons which differs from the free-electron $g$-factor $g\simeq 2$. Using first-principles calculations, we estimate the various parameters appearing in the theory. Finally, we consider quantum dots (QDs) in TMDC materials and derive an effective Hamiltonian allowing us to calculate the magnetic field dependence of the bound states in the QDs. We find that all states are both valley and spin split, which suggests that these QDs could be used as valley-spin filters. We explore the possibility of using spin and valley states in TMDCs as qubits, and conclude that, due to the relatively strong intrinsic SOC in the conduction band, the most realistic option appears to be a combined spin-valley (Kramers) qubit at low B fields.\\[4pt] [1] A. Korm\'{a}nyos et al., arXiv:1310.7720. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J51.00004: Valley relaxation dynamics in monolayer semiconductors studied by transient absorption and multidimensional spectroscopies Kenan Gundogdu, Cong Mai, Andrew Barrette, Yifei Yu, Linyou Cao, Yuriy Semenov, Ki Wook Kim Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley dependent career spin polarization, which is the basis for recently proposed valleytronic applications. These systems also exhibit unusually strong many body affects, such as strong exciton and trion binding, due to reduced dielectric screening of Coulomb interactions. Recently observed large photoluminescence helicity suggests beyond ns hole spin and valley lifetimes. But there is not much known about the impact of strong many particle correlations on spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation dynamics in single layer MoS$_{2}$. We found that excitonic many body interactions significantly contribute to the relaxation process.. Biexciton formation reveals hole valley/spin relaxation time. Our results suggest that initial fast intervalley electron scattering and electron spin relaxation leads to loss of valley polarization for holes through an electron-hole exchange mechanism. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J51.00005: Magnetoelectric effects and valley dependent spin resonances in transition metal dichalcogenide bilayers Zhirui Gong, Gui-Bin Liu, Hongyi Yu, Di Xiao, Xiaodong Cui, Xiaodong Xu, Wang Yao In addition to spin, valley is an internal degrees of freedom of carriers in monolayer group-VI transition metal dichalcogenides (TMDCs). In bilayer, carrier is also characterized by the layer pseudospin, which is associated with the electrical polarization. Here we show in TMDC bilayers, the spin, valley and layer pseudospins of carriers are strongly coupled to each other. Because of this coupling, most of the spin physics in TMDC monolayer such as the spin Hall effect and spin-dependent selection rule for optical transitions are inherited in TMDC bilayers. The strong coupling between spin and layer pseudospin also gives rise to a variety of magnetoelectric effects that make possible quantum manipulation of these electronic degrees of freedom. For example, electric polarization will oscillate in a magnetic field, while electric field can be used to tune the spin precessions. Moreover, the coupling between spin, valley and layer pseudospins makes possible valley dependent spin resonances such that spin rotations can be selectively addressed in the two valleys. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J51.00006: Exction-related electroluminescence from monolayer MoS$_{2}$ Yu Ye, Ziliang Ye, Majid Gharghi, Hanyu Zhu, Mervin Zhao, Xiaobo Yin, Xiang Zhang Excitons in MoS$_{2}$ dominate the absorption and emission properties of the two-dimensional system. Here, we study the microscopic origin of the electroluminescence from monolayer MoS$_{2}$ fabricated on a heavily $p$-type doped silicon substrate. By comparing the photoluminescence and electroluminescence of a MoS$_{2}$ diode, direct-exciton and bound-exciton related recombination processes can be identified. Auger recombination of the exciton-exciton annihilation of bound exciton emission is observed under a high electron-hole pair injection rate at room temperature. We expect the direct exciton-exciton annihilation lifetime to exceed the carrier lifetime, due to the absence of any noticeable direct exciton saturation. We believe that our method of electrical injection opens a new route to understand the microscopic nature of the exciton recombination and facilitate the control of valley and spin excitation in MoS$_{2}$. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 4:18PM |
J51.00007: Spin-valley coupling in atomically thin tungsten dichalcogenides Invited Speaker: Xiaodong Cui The monolayers of group VI transition metal dichalcogenides feature a valence band spin splitting with opposite sign in the two valleys located at corners of 1st Brillouin zone. This spin-valley coupling, particularly pronounced in tungsten dichalcogenides, can benefit potential spintronics and valleytronics with the important consequences of spin-valley interplay and the suppression of spin and valley relaxations. In this talk we discuss the optical studies of WS2 monolayers and multilayers. The PL spectra and first-principle calculations consistently reveal a spin-valley coupling of 0.4 eV which suppresses interlayer hopping and manifests as a thickness independent splitting pattern at valence band edge near K valleys. This giant spin-valley coupling, together with the valley dependent physical properties, may lead to rich possibilities for manipulating spin and valley degrees of freedom in these atomically thin 2D materials. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J51.00008: Electrically Tunable Excitonic Light Emitting Diodes based on Monolayer WSe2 p-n Junctions Jason Ross, Philip Klement, Aaron Jones, Nirmal Ghimire, Jiaqiang Yan, David Mandrus, Takashi Taniguchi, Kenji Watanabe, Kenji Kitamura, Wang Yao, David Cobden, Xiaodong Xu Light-emitting diodes (LEDs) are of vital importance for lighting, displays, optical interconnects, logic and sensors. The development of new systems that allow improvements in their efficiency, spectral properties, compactness and integrability could have dramatic ramifications. Monolayer transition metal dichalcogenides have recently emerged as interesting candidates for optoelectronic applications due to their unique optical properties. Electroluminescence (EL) has already been observed from monolayer MoS2 devices. However, the EL efficiency was low and the linewidth broad due both to the poor optical quality of MoS2 and ineffective contacts. In this talk, we present EL from lateral p-n junctions in monolayer WSe2 induced electrostatically using a thin boron nitride dielectric layer with multiple metal gates beneath. This structure allows effective injection of electrons and holes, and combined with the high optical quality of WSe2 it yields bright EL with 1000 times smaller injection current and 10 times smaller linewidth than in MoS2. Further, by increasing the injection bias we can tune the EL between regimes of impurity-bound, charged, and neutral excitons. This system has the required ingredients for new kinds of optoelectronic devices such as spin- and valley-polarized LEDs, on-chip lasers, and two-dimensional electro-optic modulators. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J51.00009: Enhanced valley polarization in 3R-MoS$_{2}$ Ryuji Suzuki, Yuijin Zhang, Yoshihiro Iwasa Transition-metal dichalcogenides (TMDs) are growing interest as graphene-like layered materials and have rich physical properties including spin/valley physics. Since the spin/valley degeneracy is resolved, inversion symmetry breaking is necessary, and then researchers mainly focused on monolayer TMDs, which has NO inversion symmetry. The size of monolayer, however, is 30um at maximum in the lateral size. This is not sufficient for deepening spin/valley physics. On the other hand, 2H bulk crystal, a famous bulk polytype, has inversion symmetry. Hence, noncentrosymmetric bulk crystals are desired. Here we introduce 3R bulk crystal of MoS$_{2}$ as a new promising material for spin/valley physics. 3R polytype is composed of a trilayer stacking in such a way that the inversion symmetry is kept broken in the bulk form. Using 3R-phase, we observed the enhanced valley polarization through circular polarized photoluminescence measurements in the bilayer form. The degree of polarization $P$ of 3R bilayer reached 68{\%} at 4K, whereas the 2H form showed P $=$ 33 {\%} at most. The temperature dependency also indicated $P$ of 3R is about twice as large as 2H of all temperature range (4K $\sim$ 300K). The noncentrosymmetric 3R MoS$_{2}$ is promising for pushing forward valley/spin-tronics based on 2D crystals. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J51.00010: Observation of out-of-plane spin-polarization in bulk 3R-MoS$_2$ Masato Sakano, Ryuji Suzuki, Yijin Zhang, Ryosuke Akashi, Ayumi Harasawa, Koichiro Yaji, Kenta Kuroda, Koji Miyamoto, Taichi Okuda, Kyoko Ishizaka, Ryotaro Arita, Yoshihiro Iwasa Transition metal dichalcogenide of noncentrosymmetric 3R-type structure, 3R-MoS$_2$, is investigated by spin- and angle-resolved photoemission spectroscopy. The top of valence bands of 3R-MoS$_2$ at the Brillouin zone corners ($K$- and $K'$-points) are found to show huge spin splitting with $z$-oriented (Zeeman-type) spin-polarization, in contrast to the nearly spin-degenerate centrosymmetric 2H-MoS$_2$. The observed spin-polarizations reach $P_z \sim\pm 1$ at $\overline{K}$- and $\overline{K'}$-points, corresponding well with the relativistic first-principles band calculations. It provides the direct evidence of spin-valley coupling realized through the broken inversion symmetry and the strong spin-orbit interaction of Mo $4d$-orbitals, leading to the spin-valley polarized state in MoS$_2$. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J51.00011: Carrier and polarization dynamics in monolayer MoS$_2$: temperature and power dependence Bernhard Urbaszek, D. Lagarde, L. Bouet, T. Amand, X. Marie, C.R. Zhu, B.L. Liu, P.H. Tan In monolayer (ML) MoS$_2$ optical transitions across the direct bandgap are governed by chiral selection rules, allowing optical k-valley initialization [1,2,3]. Here we present the first time resolved photoluminescence (PL) polarization measurements in MoS$_2$ MLs [4], providing vital information on the electron valley dynamics. Using quasi-resonant excitation of the A-exciton transitions, we can infer that the PL decays within $\tau \simeq$4ps. The PL polarization of P$_c \approx 60\%$ remains nearly constant in time for experiments from 4K - 300K, a necessary condition for the success of future Valley Hall experiments [1]. $\tau$ does not vary significantly over this temperature range. This is surprising when considering the decrease of P$_c$ in continuous wave experiments when going from 4K to 300K reported in the literature [2,3]. By tuning the laser following the shift of the A-exciton resonance with temperature we are able to recover at 300K $\sim80\%$ of the polarization observed at 4K. For pulsed laser excitation, we observe a decrease of P$_c$ with increasing laser power at all temperatures.\\[4pt] [1] Xiao et al, PRL 108, 196802 (2012).\\[0pt] [2] Mak, et al Nat. Nanotech. 7, 494 (2012).\\[0pt] [3] Sallen et al, PRB 86, 081301 (2012).\\[0pt] [4] Lagarde et al, arXiv:1308.0696. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J51.00012: Linearly dispersing plasmons in monolayer transition metal dichalcogenides David Abergel, Kostyantyn Kechedzhi We describe the collective excitations of the electronic liquid in monolayer transition metal dichalcogenides such as MoS$_2$ in a strong Zeeman field. The combination of the Dresselhaus type spin-orbit coupling and the Zeeman field lifts the valley degeneracy, and this manifests in the appearance of an additional plasmon mode that has linear dispersion. There is a well-defined quasiparticle peak in the spectral function which corresponds to this second mode. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J51.00013: Curvature-controlled valley polarization and band-gap tuning in few-layer MoS$_{2}$ Pekka Koskinen, Ioanna Fampiou, Ashwin Ramasubramaniam Monolayer transition-metal dichalcogenides (TMDCs) display valley-selective circular dichroism due to time-reversal symmetry and lack of inversion symmetry, making them promising candidates for valleytronics. In contrast, few-layer TMDCs possess both time-reversal and inversion symmetry and hence, lose these desirable valley-selective properties. Here, by using density-functional tight-binding electronic structure simulation and revised periodic boundary conditions, we show that bending of multilayer MoS$_{2}$ sheets breaks band degeneracies and localizes states on separate layers due to bending-induced strain-gradients across the sheets. We propose a strategy for employing bending deformations as a simple yet effective means of dynamically and reversibly tuning band gaps while simultaneously tuning coupling between spin, valley, and layer pseudospin of charge carriers in few-layer TMDCs. [Preview Abstract] |
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