Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session L24: Focus Session: Quantum Transport Simulations and Computational Electronics -- GNRs and QDs |
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Sponsoring Units: DCOMP Chair: Zlatan Aksamija, University of Wisconsin-Madison Room: D167 |
Tuesday, March 22, 2011 2:30PM - 2:42PM |
L24.00001: Numerical simulation of time-dependent transport in graphene Dharmendar Reddy, Priyamvada Jadaun, Leonard F. Register, Sanjay K. Banerjee We present a numerical method for modeling time-dependent quantum transport in graphene. The time-dependent Schrodinger equation is solved with a pi-orbital-based atomistic tight-binding Hamiltonian. A novel variation of an alternating-direction semi-implicit scheme is employed on the hexagonal tight-binding lattice to maintain stability and conserve probability while achieving computational efficiency. Open boundaries including source terms to allow time-dependent non-equilibrium Green's function (NEGF) calculation of graphene devices will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 2:42PM - 2:54PM |
L24.00002: ABSTRACT WITHDRAWN |
Tuesday, March 22, 2011 2:54PM - 3:06PM |
L24.00003: Current-Voltage Characteristics of Graphane Nanoribbon Transistors Jun-Qiang Lu, Daniel Valencia Using first-principles transport calculations, we investigate current-voltage characteristics of transistors made by graphane nanoribbons (or hydrogenated graphene nanoribbons). Our results show that transistors made by graphane nanoribbons can achieve better performance than those made by graphene nanoribbons because of the intrinsic large band gap presented in graphane. [Preview Abstract] |
Tuesday, March 22, 2011 3:06PM - 3:18PM |
L24.00004: Switching Behavior of Carbon Chains Bridging Graphene Nanoribbons: Effects of Uniaxial Strain Brahim Akdim, Ruth Pachter Recently, several experiments [1,2] demonstrated the stability of chain-like carbon nanowires bridged between graphene nanoribbons, paving the way for potential applications in nano-devices. On the basis of density functional tight-binding calculations, we demonstrated switching for chains terminated with a five-membered ring under an applied strain, serving as a model for morphological changes in realistic materials. Electron transport calculations showed an increase of up to 100{\%} in the output current, achieved at a reverse bias-voltage of 2V and an applied strain of just 1.5{\%}. Structural analysis suggested that the switching is driven by conformational changes, in our case triggered by the formation and annihilation of a five-membered ring at the interface of the chain-graphene edge. In addition, we showed that a five-membered ring can easily be formed at the interface under a source-drain bias or through a gate voltage. This mechanism can serve as an explanation of experimentally observed conductance for the materials. \\[4pt] [1] Jin, C.; Lan, H.; Peng, L; Suenaga, K.; Iijima S. \textit{Phys. Rev. Lett.} \textbf{2009}, 102, 205501. \\[0pt] [2] Chuvilin, A.; Meyer, J. C.; Algara-Siller, G.; Kaiser, U. \textit{New J. Phys.} \textbf{2009}, 11, 083019 [Preview Abstract] |
Tuesday, March 22, 2011 3:18PM - 3:30PM |
L24.00005: Giant Mechanoelectrical Switching in Ferromagnetic Graphene Nanoribbons Hong Li, Rui Qin, Jing Zhou, Qihang Liu, Zhengxiang Gao, Jing Lu, Wai-Ning Mei, R.F. Sabirianov Giant mechanoelectrical effect is observed when twisting a ferromagnetic zigzag-edged graphene nanoribbon (ZGNR) with collinear spin configuration from ab initio quantum transport calculations. The resulting switch ratio is up to 10$^{10}${\%} when the ZGNRs are overturned once and can be even enhanced to over 10$^{14}${\%} via multiply overturnings. We find such a switch equivalent to a spin valve without resort to an external magnetic field. Furthermore, consideration under Noncollinear situation is aslo in progress. [Preview Abstract] |
Tuesday, March 22, 2011 3:30PM - 3:42PM |
L24.00006: Transport in Carbon Nanotubes: 2LSU(2) regime reveals subtle competition between Kondo and Intermediate Valence states G. Martins, C. Busser, E. Vernek, P. Orellana, G. Lara, E. Kim, A. Feiguin, E. Anda Three different numerical techniques are used to study the two-level SU(2) regime, obtained from an SU(4) Hamiltonian by orbital mixing via coupling to the leads. SU(4) Kondo physics has been experimentally observed, and studied in detail, in Carbon Nanotube Quantum Dots. Adopting a two molecular orbital basis, the Hamiltonian is rewritten, such that one of the molecular orbitals decouples from the charge reservoir, although still interacting capacitively with the other molecular orbital. This basis transformation explains in a clear way how the charge transport in this system turns from double- to single-channel when it transitions from the SU(4) to the 2LSU2 regime. The charge occupancy of these molecular orbitals displays gate-potential-dependent occupancy oscillations that arise from a competition between the Kondo and Intermediate Valence (IV) states. The determination of whether the Kondo or the IV state is more favorable, for a specific value of gate potential, is assessed by the definition of an energy scale $T_0$, which is calculated through DMRG. We speculate that the calculation of $T_0$ may provide experimentalists with a useful tool to analyze correlated charge transport in many other systems. For that, a current work is underway to improve the numerical accuracy of its DRMG calculation and explore different definitions. [Preview Abstract] |
Tuesday, March 22, 2011 3:42PM - 4:18PM |
L24.00007: Scattering matrix approaches for dissipative quantum transport Invited Speaker: The Usuki method, which is closely related to both the scattering matrix approach and recursive Green's functions provides a stable numberical method for the simulation of quantum transport in semiconductors. It has major advantage over the Green's function method for self-consistent simlations in that the electron density involves integrals in the contacts where the distribution is near equilibrium, rather than throughout the active area. Various applications of this approach have been studied, and we discuss primarily ballistic transport in quantum dots and dissipative transport in gated quantum wire transistors. Dissipation is introduced via a proper self-energy determined in the mode representation, which is then transformed to the site representation used in the recursive approach. [Preview Abstract] |
Tuesday, March 22, 2011 4:18PM - 4:30PM |
L24.00008: Electron hopping between Wurtzite CdSe Quantum Dots Linked by Molecules Iek-Heng Chu, Marina Radulaski, Nenad Vukmirovic, Hai-Ping Cheng, Lin-Wang Wang Recent experimental results show that the transport properties of quantum dot (QD) arrays will be tremendously improved after attached by cross-linking molecules. Here, we present an \textit{ab initio} study on the electron hopping rates between wurtzite CdSe QDs connected by Sn$_{2}$S$_{6}$ molecules. The conduction band minima (CBM) transports among connected QDs are calculated. The charge patching method (CPM) is used to construct the charge density of the QDs and the connected systems. The folded spectrum method (FSM) was applied to find the band edge states and the electronic coupling between the neighboring QDs. Electron-phonon couplings are calculated to yield the reorganization energy. The electron hopping rate is then calculated by Marcus theory and its corresponding quantum treatments. Hopping rates for three different sizes of QDs, and two different types of molecular attachments are also presented here for comparison. [Preview Abstract] |
Tuesday, March 22, 2011 4:30PM - 4:42PM |
L24.00009: Coulomb Drag in Open Quantum Dots Canran Xu, Maxim Vavilov We investigate the Coulomb drag effect in a system of two open quantum dots, in the presence of interdot and intradot Coulomb interactions. We present an analytical expression for the drag current at the low temperature limit obtained in the random-phase approximation. We show that the non-zero current arises from the asymmetry of electronic states with respect to the Fermi level. This asymmetry originates due to fluctuations of the transmission amplitudes in the chaotic quantum dots described by a random-matrix theory, and therefore the drag current exhibits interesting sample-to-sample mesoscopic fluctuations. [Preview Abstract] |
Tuesday, March 22, 2011 4:42PM - 4:54PM |
L24.00010: Nonequilibrium quantum many-body transport in multiple lead quantum dot devices Jong Han Recently proposed imaginary-time formalism of steady-state nonequilibrium is extended to three reservoir systems and discuss their interference effects. We first consider a quantum dot coupled to three non-interacting leads in the context of the Anderson impurity model driven by source-drain bias. We discuss the difference between the two and three reservoir systems. We then consider the system of interacting leads, used as a prototype for two-channel Kondo model in quantum dot device.\footnote{R. M. Potok et al, Nature {\bf 446},167 (2007)} We rewrite the charging interaction on the large dot via a gauge transformation to a correlated tunneling and perform quantum Monte Carlo simulation for equilibrium and nonequilibrium using the Matsubara-voltage formalism. We discuss the cross-over from local Fermi liquid to non-Fermi liquid as a function of the Coulomb parameter in the large dot in the electron self-energy and the magnetic susceptibility. We discuss the nonequilibrium spectral evolution of local Fermi liquid. [Preview Abstract] |
Tuesday, March 22, 2011 4:54PM - 5:06PM |
L24.00011: Continuous measurements of electron tunneling through a quantum dot by a quantum point contact Hsi-Sheng Goan The time-resolved charge detection through a quantum dot (QD) by a nearby quantum point contact (QPC) detector, each coupled to its own independent electrodes and gates, has been demonstrated. The conditional counting statistics of electron transport in this QD-OPC system has also been measured [1]. The conditional counting statistics that is the statistical current fluctuations of one system given the observation of a particular current in the other system could be substantially different from their unconditional counterparts. We provide a thorough analysis on the QD-QPC system. We use the stochastic master equation (or quantum trajectory) approach to describe the conditional dynamics of the QD under continuous measurements by a QPC. We simulate in each single experimental realization the observed QPC current which reveals the real time information of single-electron tunneling events through the QD. We then use the $n$-resolved master equation approach to calculate the conditional counting statistics through the QD (QPC) conditioned on the observed current in QPC (QD). Our investigation goes beyond the analysis presented in Ref.[1] in which they neglected, in the noise power(second cumulant) of the QPC, the QPC shot noise as compared to the telegraph noise contribution induced by the single-electron tunneling events through the QD. \\[0pt] [1] E.V.Sukhorukov et. al, Nature Physics, {\bf 3}, 243 (2007). [Preview Abstract] |
Tuesday, March 22, 2011 5:06PM - 5:18PM |
L24.00012: Designer switches: Effect of contact geometry on the transient current of a strongly correlated quantum dot Ali Ihsan Goker, Zhiyong Zhu, Udo Schwingenschlogl, Aurelien Manchon The time-dependent non-crossing approximation is utilized to investigate the influence of the geometry of contacts made of gold on time dependent current through a quantum dot suddenly shifted into the Kondo regime via a gate voltage. For an asymmetrically coupled system, instantaneous conductance exhibits complex fluctuations. We identify the frequencies participating in these fluctuations and they turn out to be proportional to the separation between the sharp features in the density of states and the Fermi level. Increasing ambient temperature or bias quenches the amplitude of these fluctuations. This suggests that the interference between the emerging Kondo resonance and the van Hove singularities in the density of states is the underlying microscopic mechanism for these fluctuations. Based on these observations, we predict that using different electrode geometries would give rise to drastically different transient currents which can be accessed with state-of-the-art ultrafast pump-probe techniques. [Preview Abstract] |
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