Session X44: Focus Session: Nanoscale Transport - Molecules III

 Friday, March 9, 2007 8:00AM - 8:12AM X44.00001: Landauer conductance and twisted boundary conditions for Dirac fermions Shinsei Ryu , Christopher Mudry , Akira Furusaki , Andreas Ludwig We apply the generating function technique developed by Nazarov to the computation of the density of transmission eigenvalues for a finite graphene sheet in which a two-dimensional freely propagating massless Dirac fermion is realized. By modeling ideal leads attached to the sample as a conformal invariant boundary condition, we relate the generating function for the density of transmission eigenvalues to the twisted chiral partition functions of fermionic (c=1) and bosonic (c=-1) conformal field theories. We also discuss the scaling behavior of the ac Kubo conductivity and compare its \textit{different} $dc$ limits with results obtained from the Landauer conductance. Finally, we show that the disorder averaged Einstein conductivity is an analytic function of the disorder strength, with vanishing first-order correction, for a tight-binding model on the honeycomb lattice with weak real-valued and nearest-neighbor random hopping. Friday, March 9, 2007 8:12AM - 8:24AM X44.00002: Electron turbulence in nanoscale junctions Neil Bushong , John Gamble , Massimiliano Di Ventra Electron transport through a nanostructure can be characterized in part using concepts from classical fluid dynamics. [1] It is then natural to ask how far the analogy can be taken, and whether the electron liquid can exhibit nonlinear dynamical effects such as turbulence. Here we present a first-principles study using time-dependent current density functional theory of electron transport in nanojunctions which reveals that the electron liquid indeed exhibits behavior quite similar to that of a classical fluid. For example, a transition away from symmetric flow occurs at higher current densities, just as in the classical Navier-Stokes case. We will also discuss the behavior of the velocity correlation tensor in both laminar and turbulent regimes, as well as spontaneous symmetry breaking. Work supported in part by NSF and DOE. \newline \newline [1] R. D'Agosta and M. Di Ventra, J. Phys. Cond. Matt. in press. Friday, March 9, 2007 8:24AM - 8:36AM X44.00003: Conductance switching and electronic states in polymer nanodevices Nikolai Zhitenev , Alexander Sidorenko , Donald Tennant , Raymond Cirelli Organic materials offer new electronic functionality not available in the inorganic devices. However, the integration of organics within nanoscale electronic circuitry poses new challenges for material physics and chemistry. To rationally control the conducting properties of small devices, the electronic states in organics have to be optimized relative to the Fermi level of metal contacts. We demonstrate a novel approach to create and chemically modify such electronic states in thin polyelectrolyte films. Nanoscale devices fabricated using integrated shadow masks and the polyelectolyte film grafted to electrodes display reversible switching between conducting and non-conducting states. The conductance is related to the creation and annihilation of the chain of the electronic levels in the polymer. The electronic properties and the switching dynamics are broadly tunable by the chemical composition of the polymers. The open design of our nanodevices allows us to perform the chemical conversion targeting primarily carboxyl groups inside the completed junctions. The conduction memory effect is observed in devices with lateral size down to 30 nm. Friday, March 9, 2007 8:36AM - 9:12AM X44.00004: Pair Tunneling through Single Molecules Invited Speaker: Mikhail Raikh Coupling to molecular vibrations induces a polaronic shift, and can lead to a negative charging energy, U. For negative U, the occupation of the ground state of the molecule is even. In this situation, virtual pair transitions between the molecule and the leads can dominate electron transport. At low temperature, T, these transitions give rise to the charge-Kondo effect [1]. We developed the electron transport theory through the negative-U molecule [2] at relatively high T, when the Kondo correlations are suppressed. Two physical ingredients distinguish our theory from the transport through a superconducting grain coupled to the normal leads [3]: (i) in parallel with sequential pair-tunneling processes, single-particle cotunneling processes take place; (ii) the electron pair on the molecule can be created (or annihilated) by two electrons tunneling in from (or out to) opposite leads. We found that, even within the rate-equation description, the behavior of differential conductance through the negative-U molecule as function of the gate voltage is quite peculiar: the height of the peak near the degeneracy point is independent of temperature, while its width is proportional to T. This is in contrast to the ordinary Coulomb-blockade conductance peak, whose integral strength is T-independent. At finite source-drain bias, V$>>$T, the width of the conductance peak is $\sim$V, whereas the conventional Coulomb-blockade peak at finite V splits into two sharp peaks at detunings V/2, and -V/2. Possible applications to the gate-controlled current rectification and switching will be discussed. \newline \newline [1] A. Taraphder and P. Coleman, Phys. Rev. Lett. 66, 2814 (1991). \newline [2] J. Koch, M. E. Raikh, and F. von Oppen, Phys. Rev. Lett. 96, 056803 (2006). \newline [3] F. W. J. Hekking, L. I. Glazman, K. A. Matveev, and R. I. Shekhter, Phys. Rev. Lett. 70, 4138 (1993). Friday, March 9, 2007 9:12AM - 9:24AM X44.00005: Tuning the Kondo effect with a mechanically controllable break junction J. J. Parks , A. R. Champagne , G. R. Hutchison , S. Flores-Torres , H. D. Abruna , D. C. Ralph We study electron transport through C$_{60}$ molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The normalized linear conductance as a function of $T/T_\textrm{K}$ agrees with the scaling function expected for the spin-1/2 Kondo problem. The same devices can also exhibit finite-bias inelastic Kondo features at an energy that corresponds to the $H_g(1)$ intracage vibrational mode of C$_{60}$. Changes in electrode spacing can tune the energy and amplitude of these signals. Friday, March 9, 2007 9:24AM - 9:36AM X44.00006: Bandgap enhancement of phonon occupation Lutfe Siddiqui , Avik Ghosh , Supriyo Datta We explore the effect of electron transport through a vibration-coupled quantum dot weakly coupled with semiconducting leads and the thermal environment by writing master equations in the electron-phonon fock space. We show that the presence of bandgap in the semiconducting leads results in an enhancement of phonon occupation compared to the case of metallic contacts under certain bias as different phonon absorption processes get shut off by the bandgap. We also show that the presence of bandgap can lead to more than one negative differential resistance (NDR) peaks in the conductance spectrum of the dot. Friday, March 9, 2007 9:36AM - 9:48AM X44.00007: Interference Effects in Nanoscale Electron-Phonon Transport Andrei Sergeev , Michael Reizer , Vladimir Mititn Using the quantum transport equation we investigate interplay of the electron-phonon interaction and disorder in low-dimensional conductors. Interference of electron scattering mechanisms is beyond the Landau Fermi-liquid picture and result in nontrivial corrections, which violate the Mathiessen rule. Interfer-ence effects have been known for some time, however, the research in this field was mainly limited to disordered bulk materials and thin metallic films. In low dimensions, we found strong enhancement of the interference effects. As in the electron-phonon kinetics [1], this enhancement is due to a smaller electron momentum transferred in electron-phonon scattering processes in low dimensions. \newline [1] A. Sergeev et al., Phys. Rev. Lett. 94, 136602 (2005). Friday, March 9, 2007 9:48AM - 10:00AM X44.00008: Tuning interacting electron and phonon quantum states in molecular nanostructures via atomic manipulation Gabriel Zeltzer , Hari C. Manoharan We investigate atomically precise nanostructures assembled from CO molecules on Cu(111) using a custom-built low-temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM). The atomic manipulation capability of this instrument allows single molecule placement to desired locations, thus enabling the construction of nanostructures with \AA ngstrom level precision. We demonstrate the control of electronic and vibrational states within several quantum corral geometries, and investigate possible interactions between these two quantum degrees of freedom. Dependencies on corral shape and size are presented. $dI/dV$ spectroscopy reveals the ability to engineer the corral eigenstates with sub-meV accuracy. We also briefly discuss the design and performance specifications of our atomic manipulation STM which enable this class of experiments. Friday, March 9, 2007 10:00AM - 10:12AM X44.00009: Coulomb gas scaling of the non-equilibrium spin-boson model Aditi Mitra , Andrew Millis The nonequilibrium spin-boson model,'' a localized electronic level coupled to a fluctuating two-state system and to two electronic reservoirs, is solved via an Anderson-Yuval-Hamann mapping onto a plasma of alternating positive and negative charges time-ordered along the two Keldysh'' contours needed to describe nonequilibrium physics. The interaction between charges depend on whether their time separation is small or large compared to a dephasing scale defined in terms of the chemical potential difference between the electronic reservoirs, and a decoherence scale defined in terms of the current flowing from one reservoir to another. A nonequilibrium scaling transformation is introduced. An important feature is the presence in the model of a new coupling, essentially the decoherence rate, which acquires an additive renormalization similar to that of the energy in equilibrium problems. The resulting flow equations are used to study the competition between the dephasing-induced formation of independent resonances tied to the two chemical potentials, and the decoherence which cuts off the scaling and leads to effectively classical long-time behavior. Friday, March 9, 2007 10:12AM - 10:24AM X44.00010: Numerical Estimation of Keldysh-Countour Path Integrals for Nonequilibrium Problems Andrew Millis , Philipp Werner We propose an idea for simulating the dynamics of open (coupled to reservoirs) systems in a nonequilibrium steady state and present preliminary numerical results for the nonequilibrium spin boson and Anderson models. The method builds on the observation (Phys. Rev. Lett. ${\bf 94}$ 076404) that out of equilibrium the Keldysh time evolution operator exhibits an exponential time decay, and uses the stochastic hybridization expansion techniques of Phys. Rev. Lett. ${\bf 97}$, 07640. Friday, March 9, 2007 10:24AM - 10:36AM X44.00011: Full Counting Statistics for a Single-Electron Transistor: Nonequilibrium Effects at Intermediate Conductance Yasuhiro Utsumi , Dimitri Golubev , Gerd Sch\"on We calculate the probability distribution of current for a single-electron transistor (SET) with intermediate strength conductance where quantum fluctuations of the charge play a dominant role. The calculations are based on the multichannel anisotropic Kondo model in the Majorana representation and the fermionic Keldysh generating functional. The effects of quantum fluctuations are taken into account by the summation of a certain subclass of diagrams, which corresponds to the leading logarithmic approximation in the sense that the result is consistent with the RG analysis. We have shown that in non-equilibrium situations quantum fluctuations of the charge induce lifetime broadening for the charge states of the central island. Consequences which can be detected in experiments include a suppression of the probability of currents larger than the average value. Y. Utsumi, D. Golubev, G. Schoen; PRL. 96, 086803 (2006) Friday, March 9, 2007 10:36AM - 10:48AM X44.00012: Vibronic coupling effect on the electron transport through molecules Masaru Tsukada , Kunihiro Mitsutake Electron transport through molecular bridges or molecular layers connected to nano-electrodes is determined by the combination of coherent and dissipative processes, controlled by the electron-vibron coupling, transfer integrals between the molecular orbitals, applied electric field and temperature. We propose a novel theoretical approach, which combines \textit{ab initio} molecular orbital method with analytical many-boson model. As a case study, the long chain model of the thiophene oligomer is solved by a variation approach. Mixed states of moderately extended molecular orbital states mediated and localised by dress of vibron cloud are found as eigen-states. All the excited states accompanied by multiple quanta of vibration can be solved, and the overall carrier transport properties including the conductance, mobility, dissipation spectra are analyzed by solving the master equation with the transition rates estimated by the golden rule. We clarify obtained in a uniform systematic way, how the transport mode changes from a dominantly coherent transport to the dissipative hopping transport. Friday, March 9, 2007 10:48AM - 11:00AM X44.00013: Quantum open systems approach to single-molecule devices Yongqiang Xue , William Kennerly Experimental advances in electrically and optically probing individual molecules have provided new insights into the behavior of single quantum objects and their interaction with the nanoenvironments without requiring ensemble average. Single-molecule devices are open quantum systems whose dynamics are intrinsically stochastic and are subject to dissipation and decoherence through system-environment correlation. New concepts and computational techniques may be needed to unravel the rich physics underlying single-molecule measurements. In this talk, we discuss our efforts in developing quantum open systems theory of single-molecule electronics and optics, building on concepts and techniques from quantum optics and quantum measurement.