Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D14: Focus Session: Transport Properties of Nanostructures II: Non-Equilibrium and Correlated Electron Phenomena |
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Sponsoring Units: DMP Chair: Mark Hybertsen, Brookhaven National Laboratory Room: B113 |
Monday, March 15, 2010 2:30PM - 3:06PM |
D14.00001: Understanding Nonequilibrium and Correlated Electron Behavior in Molecular Junctions Invited Speaker: Maarten Wegewijs I present an overview of the effects of the strong correlations in single-molecule junctions on non-linear transport, focusing on theory while comparing with several recent experiments. In the brief introduction I outline our real-time diagrammatic transport theory and its renormalization group extensions. In this approach a kinetic equation (generalized master equation) for the molecular density matrix incorporates both the quantum coherence and the strong correlations between electronic, vibrational and spin degrees of freedom of the device. The molecular state and non-linear current are calculated perturbatively beyond the lowest order in the coupling to the electrodes. As a first example, a detailed comparison with recent measurements on carbon-nanotube ``peapod'' devices is presented, indicating non-trivial hybridization and Coulomb interaction with the host nanotube quantum dot. The remainder of the talk focuses on predictions for specific electromechanical (electron-vibration coupling) and magnetic effects (spin-orbit coupling). I discuss non-linear transport signatures of vibrations when going beyond the simplified pictures of sequential tunneling (which breaks down due to quantum fluctuations) and the Born-Oppenheimer separation (its breakdown resulting in pseudo-Jahn-Teller coupling). Both effects have recently been observed. Finally, I address the interplay of transport with various aspects of molecular magnetism, such as antisymmetric (Dzyaloshinskii-Moriya) exchange and magnetic anisotropy. A comparison with recent transport experiments reveals the possibility of electric-field tunable molecular magnetism in an ``ferric-star'' molecular device. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D14.00002: Mechanical control of spin states in single molecules J.J. Parks , A.R. Champagne , T.A. Costi , A.N. Pasupathy , W.W. Shum , E. Neuscamman , G.K.-L. Chan , H.D. Abru\~na , D.C. Ralph We study individual Co(tpy-SH)$_2$ complexes by connecting them within mechanically controllable break-junction devices that allow us to controllably stretch the molecule while measuring its electrical conductance. At low temperature, this molecule produces the Kondo effect, observed as a peak in the conductance at zero bias. We find that as a function of stretching the Kondo peak splits in two, in distinct contrast to behavior observed in spin-1/2 molecules. The temperature dependence of the Kondo signal for the unstretched molecule is in agreement with the scaling prediction for an underscreened $S$ = 1 Kondo effect. The splitting of the Kondo resonance by mechanical stretching can be explained by a spin-orbit-induced lifting of the degeneracy of the $S$ = 1 triplet upon distortion from octahedral symmetry of the Co ion. We observe evidence of the resultant spin anisotropy in the magnetic-field dependence of the Kondo peaks. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D14.00003: Anomalous Transport and Possible Phase Transition in Palladium Nanojunction Gavin D. Scott , Juan J. Palacios , Douglas Natelson Many phenomena in condensed matter are thought to result from competition between different ordered phases. Palladium is a paramagnetic metal close to both ferromagnetism and superconductivity, and is therefore a potentially interesting material to consider. Structuring matter on the nanometer scale is one means of modifying relevant physical energy scales, with nanoscale confinement already known to favor locally modified magnetic interactions. We present transport measurements in electromigrated palladium break junction devices showing the emergence at low temperatures of anomalous sharp features in the differential conductance. These features appear symmetrically in applied bias and exhibit a temperature dependence of their characteristic voltages reminiscent of a mean field phase transition. The systematic variation of these voltages with zero-bias conductance, together with density functional theory calculations illustrating the relationship between the magnetization of Pd and atomic coordination, suggest that the features may result from the onset of spontaneous magnetization in the nanojunction electrodes. We propose that the characteristic conductance features are related to inelastic tunneling involving magnetic excitations. [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D14.00004: Kondo anomalies in magnetic nanocontacts from first principles Procolo Lucignano , Pierpaolo Baruselli , Michele Fabrizio , Riccardo Mazzarello , Alexander Smogunov , Erio Tosatti A realistic calculation of electron transport through magnetic nanocontacts should connect together DFT based electronic structure with many body methods like NRG. We recently moved a first step in this direction [1,2]. Identifying symmetry-dictated conduction channels, we calculate first the DFT channel- and spin-dependent impurity scattering phase shifts; then build an Anderson model whose symmetry and parameters are forced to reproduce at the Hartree Fock level the phase shifts; and finally solve the Anderson model by NRG. This yields much more than just the Kondo temperature. As exemplified by a Ni impurity in a Au nanocontact, we uncover the orbital origin the Fano interference in the predicted zero bias Kondo anomalies; the origin of their large structural dependence; and the likely occurrence of the so far ignored ``ferro'' Kondo effect. We are presently extending calculations to other nanocontacts including magnetic impurities on surfaces and nanotubes.\\[4pt] [1] P. Lucignano et al., Nat. Mat. 8, 563 (2009). [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D14.00005: GW approach to Anderson model in and out of equilibrium: scaling properties in the Kondo regime Catalin D. Spataru The low-energy properties of the Anderson model for a single impurity coupled to two leads are studied using the GW approximation. We find that quantities such as the spectral function at zero temperature, the linear-response conductance as function of temperature or the differential conductance as function of bias voltage exhibit universal scaling behavior in the Kondo regime. We show how the form of the GW scaling functions relates to the form of the scaling functions obtained from the exact solution at equilibrium. We also compare the energy scale that goes inside the GW scaling functions with the exact Kondo temperature, for a broad range of the Coulomb interaction strength in the asymptotic regime. This analysis allows to clarify a presently suspended question in the literature, namely whether or not the GW solution captures the Kondo resonance. [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D14.00006: Lateral spin-orbit coupling and the Kondo effect in quantum dots Edson Vernek , Anh Ngo , Sergio Ulloa We present studies of the Coulomb blockade and Kondo regimes of transport of a quantum dot connected to current leads through \textit{spin-polarizing} quantum point contacts (QPCs) [1].~ This configuration, arising from the effect of \textit{lateral} spin-orbit fields, results in spin-polarized currents \textit{even in the absence of external magnetic fields} and greatly affects the correlations in the dot. Using an equation-of-motion technique and numerical renormalization group calculations we obtain the conductance and spin polarization for this system under different parameter regimes. Our results show that both the Coulomb blockade and Kondo regimes exhibit non-zero spin-polarized conductance. We analyze the role that the spin-dependent tunneling amplitudes of the QPC play in determining the charge and net magnetic moment in the dot. We find that the Kondo regime exhibits a strongly dependent Kondo temperature on the QPC polarizability. These effects, controllable by lateral gate voltages, may provide a new approach for exploring Kondo correlations, as well as possible spin devices. Supported by NSF DMR-MWN and PIRE. [1] P. Debray \textit{et al}., Nature Nanotech. \textbf{4}, 759 (2009). [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D14.00007: Kondo temperature in a quantum dot system Seungjoo Nah , Michael Pustilnik We discuss the dependence of the Kondo temperature $T_K$ of a quantum dot system on the gate voltage $V_g$. We show that due to the finite size of the dot (i.e. the finite single particle level spacing) the dependence $T_{K}(V_g)$ differs from the conventional expression for the Anderson impurity model. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:30PM |
D14.00008: Kondo effects in a triangular triple quantum dot II: ground-state properties for deformed configurations Akira Oguri , Shinichi Amaha , Yunori Nisikawa , A.C. Hewson , Seigo Tarucha , Takahide Numata We study transport through a triangular triple quantum dot (TTQD) connected to two noninteracting leads, using the numerical renormalization group. The system has been theoretically revealed to show a variety of Kondo effects depending on the electron filling of the triangle [1]. For instance, the SU(4) Kondo effect takes place at three-electron filling, and a two-stage Kondo screening of a high-spin $S=1$ Nagaoka state takes place at four-electron filling. Because of the enhanced freedom in the configurations, however, the large parameter space of the TTQD still has not been fully explored, especially for large deformations. We report the effects of the inhomogeneity in the inter-dot couplings and the level positions in a wide region of the filling. [1] T.~Numata, Y.~Nisikawa, A.~Oguri, and A. C. Hewson: PRB {\bf 80}, 155330 (2009). [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 4:42PM |
D14.00009: Zero-bias anomaly in resonant tunneling: the role of contact asymmetry Henok Mebrahtu , Yuriy Bomze , Ivan Borzenets , Gleb Finkelstein We study the zero-bias anomaly in resonant tunneling through a carbon nanotube quantum dot caused by dissipative environment. At the base temperature, we find a qualitative difference between the cases of symmetric and asymmetric barriers defining the quantum dot. The observed behavior is compared to the theoretical predictions for a physically similar picture of resonant tunneling in Luttinger liquid. [Preview Abstract] |
Monday, March 15, 2010 4:42PM - 4:54PM |
D14.00010: Effects of spin-orbit interaction in spin-polarized single-electron transistors Javier Nossa , Carlo Canali We consider a model of an artificial atom with interacting electrons having both spin degrees of freedom and orbital degeneracies. The interaction includes both spin and orbital exchange couplings, which favour a spin polarized ground state with nonzero orbital moment. For the two-electron problem with $l=1$ orbital degeneracy we enumerate all the eigenstates of the system with and without spin-orbit interaction. We then study quantum transport for the case in which the atom is weakly connected to metallic leads, focusing in particular on the effect of the spin-orbit interaction on the tunnelling conductance. We also discuss how spin-orbit interaction and an external magnetic field influence the conductance when the leads are spin-polarized and tunnelling magneto-resistance is expected. [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:06PM |
D14.00011: Correlated electron transport through double quantum dots coupled to normal and superconducting leads Yoichi Tanaka , Norio Kawakami , Akira Oguri We study Andreev transport through double quantum dots connected in series to normal and superconducting leads, using the numerical renormalization group. We show that the ground state of this system shows a crossover among a local Cooper-pairing singlet, a Kondo singlet and an inter-dot coupling singlet. The difference between these singlet states is clearly reflected in the transport properties; the conductance for the local Cooper-pairing singlet has a peak with the unitary-limit value $4e^2/h$, while the Andreev reflection is suppressed in the Kondo singlet region by the Coulomb interaction. Furthermore, we find that the conductance has two successive peaks near the crossover between the local Cooper-pairing singlet and the Kondo singlet. It is further elucidated that the gate voltage gives a different variation into the crossover. Specifically, as the energy level of the dot adjacent to the normal lead varies, the Kondo screening cloud around the double dots is deformed to a long-range singlet bond. [Preview Abstract] |
Monday, March 15, 2010 5:06PM - 5:18PM |
D14.00012: Non-local transport through a quantum dot coupled to two normal and one superconducting leads Yasuhiro Yamada , Yoichi Tanaka , Norio Kawakami Using the modified second-order perturbation theory, we study the non-local transport through a quantum dot coupled to two normal and one superconducting leads. The non-local transport between the two normal leads is governed by two competing electron transport processes, the crossed Andreev reflection and the elastic cotunneling. In equilibrium case, the non-local conductance shows the zero-bias anomaly with increasing Coulomb interaction at the quantum dot because the elastic cotunneling is enhanced by the Kondo effect. Even in the strong interaction regime, however, we also observe that the non-local differential conductance drastically decreases with increasing bias voltage between the normal leads, and shows the sign change in some specific nonequilibrium condition. We elucidate that the sign change comes from the alternation of the dominant transport process, which is caused by the enhancement of the crossed Andreev reflection due to the cooperation between the Kondo/proximity effects and the suppression of the elastic cotunneling at finite bias. [Preview Abstract] |
Monday, March 15, 2010 5:18PM - 5:30PM |
D14.00013: Tuning the Coupling of Molecular Species with Metal~Nanoparticle Dimers M. Claudia Troparevsky , Ke Zhao , Di Xiao , Zhenyu Zhang Recently, we have studied the electronic coupling between metal nanoparticles using real-space first-principles calculations within density functional theory [1, 2]. Here, we investigate the effect of placing a molecule in the gap region between the nanoparticles on the linear response of the system to an applied electric field. We find that the values of the static polarizability are significantly larger than in the case where the molecule is absent. We also investigate the role of the electronic coupling between the nanoparticles on the binding energies of oxygen and carbon monoxide. We show that the binding energies change significantly when the separation between the nanoparticles is varied. Finally, we explore the effect of applying an electric field on these binding energies which may emerge as a novel way of tuning the chemical reactivity of metal nanoparticles aggregates. \\[4pt] [1] K. Zhao, M. C. Troparevsky, D. Xiao, A. G. Eguiluz, and Z. Zhang, Phys. Rev. Lett. \textbf{102,} 186804 (2009). \\[0pt] [2] M. C. Troparevsky, K. Zhao, D. Xiao, Z. Zhang, and A. G. Eguiluz, Nano Lett. (2009). [Preview Abstract] |
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