Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T53: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures II |
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Sponsoring Units: DMP Chair: Blanka Magyari-Kope, Stanford University Room: Mile High Ballroom 2C |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T53.00001: Stannanane as a Topoligcal Insulator: a Study of Conducitivity and Mobility William Vandenberghe, Massimo Fischetti Recently, it was shown that monolayer tin (lat: stannum) which we refer to as stannanane, is a 2D topological insulator with a band gap exceeding 300 meV upon functionalization. We investigate the band structure of functionalized stannanane ribbons using ab-initio calculations and determine the Fermi-velocity of the edge states. We calculate the wavefunctions of the edge states closing the band gap in stannanane ribbons and demonstrate their spin-polarization. We compute the matrix element with a deformation-potential Hamiltonian to study back-scattering between opposite-edge states. The overlap of the edge states reduces with increasing ribbon width and depends on the energy. Finally, we calculate the stannanane conductivity and mobility as a function of Fermi level for different ribbon widths using the Kubo-Greenwood formalism and show that mobilities exceeding $10{}^{7}$ cm$^{2}$/(Vs) can be expected in stannanane ribbons. [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T53.00002: Quantum mechanical solver for confined heterostructure tunnel field-effect transistors Devin Verreck, Maarten Van de Put, Bart Soree, Anne Verhulst, Wim Magnus, William Vandenberghe, Guido Groeseneken Although the tunnel field-effect transistor (TFET) is a promising candidate to replace the MOSFET in low-power applications because of its sub-60mV/dec subthreshold swing (SS), on-currents are typically too low. Introducing a heterostructure of III-V materials at the tunnel junction enables higher on-currents, but the influence of quantum effects like size confinement is poorly understood. We therefore present a ballistic quantum transport formalism, combining for the first time a novel heterostructure envelope function formalism with the multiband quantum transmitting boundary method, extended to 2D potentials. First, the subband modes are obtained in the contacts, where the potential is assumed constant in the transport direction. Next, the modes are injected one by one into the device. Finally, the resulting transmission probabilities are integrated, weighted with a Fermi-Dirac distribution, to obtain the current. This multiband formalism has been implemented for the 2-band case. First, heterostructure diodes were simulated, showing a decrease in transmission probabilities for thin devices. Next, p-n-i-n heterostructure TFETs were studied. It was found that the improved gate control in thin devices counteracts the size confinement. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T53.00003: Full band quantum transport using mixed supercell and envelop function method Jingtian Fang, William Vandenberghe, Massimo Fischetti We study one-dimensional quantum transport in field-effect transistors with different channel materials, such as silicon nanowires, graphene nanoribbons, and carbon nanotubes. The normal (real) band structure and the complex band structure are calculated using the local empirical pseudopotential method. We employ the supercell approach to treat the two-dimensional quantum confinement and the envelop wavefunction approximation to deal with the open-boundary-condition transport problem. The proper open boundary conditions for atomically homogeneous systems along the transport direction are derived using the complex band structure of the contacts. The computational cost for solving the real-space quantum transport equation strongly increases when large cutoff energies are used and more realistic devices are simulated. We use a parallel computation technique to model devices with a length of 10 nm or larger. A sparse matrix solver enables the efficient solution of the transport equations as well. We present the electron density and potential profile in the device along the transport direction. The current-voltage characteristics of the device show that the current is almost linearly increasing when low biases are applied. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T53.00004: Phonon assisted carrier motion on the Wannier-Stark ladder Alfred Cheung, Mona Berciu It is well known that at zero temperature and in the absence of electron-phonon coupling, the presence of an electric field leads to localization of carriers residing in a single band of finite bandwidth. In this talk, we will present an implementation of the self-consistent Born approximation (SCBA) to study the effect of weak electron-phonon coupling on the motion of a carrier in a biased system. At moderate and strong electron-phonon coupling, we supplement the SCBA, describing the string of phonons left behind by the carrier, with the momentum average approximation to describe the phonon cloud that accompanies the resulting polaron. We find that coupling to the lattice delocalizes the carrier, as expected, although long-lived resonances resulting from the Wannier-Stark states of the polaron may appear in certain regions of the parameter space. We end with a discussion of how our method can be improved to model disorder, other types of electron-phonon coupling, and electron-hole pair dissociation in a biased system. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T53.00005: Phase Effects of Plasmon Polaritons in Hyperbolic Metamaterials Cyrus Vandrevala, Yuli Lyanda-Geller, Sabre Kais Metamaterials are artificial materials engineered to have properties that are generally not found in nature. They get their qualities from their structure rather than their chemical composition. Hyperbolic metamaterials are a subclass of metamaterials that have a hyperboloid-shaped dispersion curve. Due to this unique dispersion relation, light travels only in specific directions within the material for certain values of the wave vector. Although the exact mechanism that allows light to propagate through a hyperbolic metamaterial is still not exactly known, it is thought that surface plasmon polaritons at the interfaces between each metal and dielectric layer support the transmission of light from interface to interface. Additionally, recent experiments have shown that surface plasmon polaritons can demonstrate quantum effects like self interference and entanglement. We model the coupling of surface plasmon polaritons in a hyperbolic metamaterial using the Kronig-Penny model. From this, we analyze the phase of the plasmons as they propagate through the material. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T53.00006: Quantum heat transport in a spin-boson nanojunction: Coherent and incoherent mechanisms Yue Yang, Chang-Qin Wu Quantum heat transport in a spin-boson system is investigated by the nonequilibrium Green's function (NEGF) method. Spin-spin correlators are calculated via the Majorana fermion representation of spin operators, which allows us to make use of the Wick's theorem by standard diagrammatic techniques. A formula of heat current is obtained and numerical results are presented in comparison with other methods. Two kinds of transport mechanisms are identified in high and low temperatures, respectively, which indicate there exists a transition from incoherent to coherent transport with the temperature decreasing. Additionally, a saturation of heat current is confirmed by increasing the coupling strength between the baths and the intermediate system, which is possibly a sign of the quantum Zeno effect in the transport process. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T53.00007: Effects of dislocations on charge transport in a GaAs thin-film solar cell Andrey Semichaevsky, Chance Barrett Dislocations are known to form during the epitaxial growth of GaAs thin films [1]. These extended defects affect the mobility of charge carriers due to scattering. Dislocation scattering affects the open-circuit voltage of and the photocurrent density in a thin-film GaAs solar cell. The mobility degradation due to dislocation scattering in GaAs have been studied both experimentally [2] and theoretically [1]. In this paper we apply a Multiphysics approach [3] to model the transport of charges, including information about dislocation density, morphology, and size. We solve the Schrodinger-Poisson equation to find the scattering potential of an array of dislocations and the Boltzmann transport equation that uses this potential. The photogeneration and recombination terms are explicitly included into the equations. Our model can be of use to applied scientists and engineers in the thin film PV field. [1] J.H. You, H.T. Johnson, Solid State Physics, \textbf{61}, 143--261, 2009. [2] T. Wosinski, \textit{Journal of Applied Physics , }\textbf{65}, 1566 -- 1570, 1989. [3] A.V. Semichaevsky, H.T. Johnson, \textit{Solar Energy Materials and Solar Cells}, \textbf{108}, 189-199, 2013. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T53.00008: Optical phonon lasing and its detection in transport through semiconduc- tor double quantum dots Rin Okuyama, Mikio Eto, Tobias Brandes We theoretically propose optical phonon lasing for a double quantum dot (DQD) fabricated in a semiconductor substrate. No additional cavity or resonator is required. We show that the DQD couples to only two phonon modes that act as a natural cavity. The pumping to the upper level is realized by an electric current through the DQD under a finite bias. Using the rate equation in the Born-Markov-Secular approximation, we analyze the enhanced phonon emission when the level spacing in the DQD is tuned to the phonon energy. We find the phonon lasing when the pumping rate is much larger than the phonon decay rate, whereas anti-bunching of phonon emission is observed when the pumping rate is smaller.\footnote{R. Okuyama {\it et al.}, J.\ Phys.\ Soc.\ Jpn.\ {\bf 82}, 013704 (2013); New J.\ Phys.\ {\bf 15}, 083032 (2013).} Our theory can be also applicable to DQDs embedded in nanomechanical resonators to control the vibrating modes. We discuss detection of amplified modes using the electric current and its noise through the DQD, and another DQD fabricated nearby. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T53.00009: Length dependence of conductance and thermopower of hybrid alkyl-thiophene single molecule junctions Michele Kotiuga, William B. Chang, Cheng-Kang Mai, Fabian Pauly, Guillermo C. Bazan, Rachel A. Segalman, Jeffrey B. Neaton Single-molecule junctions are novel, controllable testbeds for understanding mixed electronic and thermal transport at interfaces. Here, we study a set of newly-synthesized molecules containing alkyl and thiophene units of increasing length in order to control junction level alignment and electronic coupling with~a combination of theory and experiment. Using a first-principles scattering-state approach, based on self-energy corrected density functional theory, we calculate the conductance and thermopower of thiol-terminated alkyl-thiophene-Au junctions, elucidating the relationship between length and thermopower. We compare our work to statistical measurements with a scanning tunneling microscope-based break junction technique, and discuss the impact of junction geometry on our results. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T53.00010: Temperature dependence of electron transport in GaAs nanowires Zhuting Sun, Andrei Kogan, Tim Burgess, Chenupati Jagadish We have measured nonlinear differential conductance through several (n=3) GaAs nanowire samples contacted by lithografically patterned gold-titanium films. The nanowires, 50 nm in diameter, are grown by metalorganic chemical vapour deposition (MOCVD) method in the same growth run and are doped with Silicon during the growth. We compare the measurements to a simple one-dimentional phenomenological model and show that it enables determination of the resistance of the wire and the saturation current and the ideality factor for each contact. Both the saturation current and the ideality factor vary strongly with temperature, as expected. We show that the temperature dependence of the saturation current can be used to determine the doping density and the effective barrier height for each metal-semiconductor contact. We find satisfactory consistency in the doping density obtained in all contacts and discuss variations in the barrier heights determined by this procedure, which we attribute to an inhomogeneous passivation of the surface states of the nanowire at the contact sites. Surprisingly, we find only a weak sensitivity of the nanowire resistance to temperature between 6K and 300 K and discuss a possible effect of the surface states on transport across the wire. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T53.00011: Effect of Disorder on Spectral Diffusion in GaAs Quantum Wells Studied Using Two-Dimensional Coherent Spectroscopy Rohan Singh, Galan Moody, Mark E. Siemens, Hebin Li, Steven T. Cundiff Disorder exists in even the highest quality semiconductor quantum wells (QWs) due to well-width fluctuations. A consequence of this disorder is inhomogeneity in the energies of excitonic resonances. Once an ensemble of excitons is excited, spatial migration of the excitons results in redistribution of exciton energies, known as spectral diffusion. Spectral diffusion in QWs is typically modeled in the strong-redistribution approximation, which means that the exciton energy redistribution is assumed to be independent of the initial exciton energy. In the present work, we study spectral diffusion in GaAs QWs using two-dimensional coherent spectroscopy (2DCS). 2DCS is an extension of the three-pulse transient four-wave mixing technique where the signal is unfolded onto two (emission and absorption) energy axes. The redistribution of exciton energies can be directly measured using 2DCS. We find that the disorder localized and delocalized excitons exhibit different spectral diffusion characteristics, and the distinction is more prominent at low sample temperatures ($<25$ K). Our results show that the strong-redistribution approximation is not sufficient to explain spectral diffusion of excitons in QWs, especially at low temperatures. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T53.00012: Magneto-transport Properties Using Top-Gated Hall Bars of Epitaxial Heterostructures on Single-Crystal SiGe Nanomembranes R.B. Jacobson, Yize Li, Ryan Foote, Xiaorui Cui, Donald Savage, Pornsatit Sookchoo, Mark Eriksson, Max Lagally A high-quality 2-dimensional electron gas (2DEG) is crucial for quantum electronics and spintronics. Grown heterostructures on SiGe nanomembranes (NMs) show promise to create these 2DEG structures because they have reduced strain inhomogeneities and mosaic tilt. We investigate charge transport properties of these SiGe NMs/heterostructures over a range of temperatures and compare them with results from heterostructures grown on compositionally graded SiGe substrates. Measurements are done by creating Hall bars with top gates on the samples. From the magneto-transport data, low-carrier-density mobility values are calculated. Initial results on the grown heterostructures give a typical curve for mobility versus carrier density, but extraction of the zero-carrier-density mobility is dependent on the curve-fitting technique. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T53.00013: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T53.00014: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T53.00015: Variation of the shot noise within an ensemble of atomic-scale metal junctions Ruoyu Chen, Douglas Natelson Shot noise originates from the discreteness of charge carriers. In nanoscale systems the noise carries additional information about transmittances of quantum channels beyond the conductance. In previous experiments with mechanical break junctions, we demonstrated that shot noise and its quantum suppression are still robust even at room temperature. In addition to studying the ensemble average of the noise over all the conductance traces involving many junction configurations, we can consider the whole ensemble of measurements. With STM-style gold junctions at room temperature, we present density maps of the noise as a function of conductance. The noise suppression when the conductance is near 1 G0 is still observed in such a map as usual. Furthermore, at that same conductance we observe a pronounced minimum of the noise's variation across the ensemble. We interpret this as experimental evidence that the number of atomic configurations in the ensemble with G near 1 G0 is comparatively reduced. [Preview Abstract] |
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