Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P11: Focus Session: Transport Properties of Nanostructures IV: Correlation Effects |
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Sponsoring Units: DCMP Chair: Harold Baranger, Duke University Room: 305 |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P11.00001: Incorporating Exchange-Correlation Effects in Quantum Transport through Nano-scale Junctions Invited Speaker: State-of-the-art computational methods for modelling electron transport in nano-scale junctions are based on effective single- particle approximations such as the Kohn-Sham theory of density functional theory. This methodology has been successfully applied to junctions with strong coupling to the metallic electrodes, but has proved insufficient for less homogeneous junctions where the distinction between the nano-device and the electrodes is more pronounced. In order to obtain a more accurate and rigorous description of exchange-correlation effects in weakly correlated molecular junctions, we have implemented the many-body GW approximation within a transport framework suitable to treat non-periodic systems consisting of an interacting region coupled to infinite non-interacting leads with different chemical potentials. Fundamental trends in the properties of metal-molecule-metal junctions are identified on the basis of simple model calculations. These include renormalization of molecular QP levels due to dynamical polarization effects both in- and out of equilibrium as well as the reduction of QP life-times due to enhanced QP scattering under finite bias conditions. As will be shown, these genuine many-body effects can have a large influence on the junction IV characteristics even for weakly correlated systems. The importance of using a fully self-consistent GW self-energy for quantum transport calculations will be demonstrated. Finally (preliminary) results for more realistic molecular junctions will be discussed. References: K. S. Thygesen and A. Rubio, PRB 77, 115333 (2008); K. S. Thygesen, PRL 100, 166804 (2008); K. S. Thygesen and A. Rubio, arXiv:0810.5214. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P11.00002: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P11.00003: Two-channel Kondo effect and phonon-assisted transport in single-molecular junctions Luis Dias da Silva, Elbio Dagotto The interplay between vibrational modes and Kondo physics is a fundamental aspect of transport properties of correlated molecular conductors. In this theoretical work, we study such interplay in a system consisting of a single molecule in a metallic break junction tuned (by gate voltages) to be in an ``odd-N'' coulomb blockade valley (Kondo-prone). The connection to left and right metallic leads creates the usual coupling to a conduction channel with left-right symmetry (the ``even"-parity channel). A center-of-mass vibrational mode introduces an additional, phonon-assisted tunneling through the asymmetric (``odd''-parity channel). Our numerical renormalization-group calculations reveal that the phonon-mediated coupling to the odd channel leads to the appearance of a two-channel Kondo (2chK) effect, characterized by a non-Fermi-liquid (NFL) fixed point. The ground-state has NFL properties for a critical value of the phonon-mediated coupling strength and critical lines are present for wide range of parameters, including the regime away from particle-hole symmetry. Signatures of this 2chK non-Fermi-liquid behavior are prominent in the thermodynamic properties as well as in the linear conductance. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P11.00004: Universal Scaling of Zero-Bias Conductance Peaks in Single-Molecule Transistors Incorporating Tetra[2,3-thienylene] Zachary Keane, Gavin Scott, Douglas Natelson There is significant interest in exploring universal scaling laws as they apply to the Kondo state in diverse physical systems. One such system is GaAs quantum dots, in which Grobnis et al. have demonstrated that the conductance follows a universal scaling function in temperature and source-drain bias. More recently, Scott et al. have demonstrated that the same scaling law applies to single molecule transistors incorporating both C60 and bis(2,5-di-[2]pyridyl-3,4-dithiocyanto-pyrrolate)Cu(II), despite the fact that the relevant energy scales in these systems can differ by 3 orders of magnitude. We will report measurements and universal scaling analysis of the Kondo conductance as a function of temperature and source-drain bias in a fourth system, single molecule transistors incorporating tetra[2,3-thienylene]. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P11.00005: Tunneling spectra of individual magnetic endofullerene molecules E. S. Tam, J. E. Grose, J. J. Parks, B. Ulgut, H. D. Abru\~{n}a, D. C. Ralph, C. Timm, M. Scheloske, W. Harneit We report measurements of electron tunneling spectra for individual N@C$_{60}$ molecules, a spin-3/2 endohedral fullerene. The molecules were measured at low temperature in electromigrated break-junctions in the single-electron transistor configuration. We observe that the N@C$_{60}$ devices exhibit a spin-state transition as a function of applied magnetic field which was not observed in C$_{60}$ control devices. The nature of this transition enables us to identify the charge and spin states of the molecule. The spectra of N@C$_{60}$ devices also exhibit low-energy excited states and signatures of non-equilibrium spin excitations predicted for this molecule. The experimental spectra can be reproduced theoretically by accounting for the exchange interaction between the nitrogen spin and electron(s) on the C$_{60}$ cage. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P11.00006: Spin-dependent effects in transport through individual molecules and nanoparticles J. J. Parks, E. S. Tam, S. Flores-Torres, H. D. Abruna, D. C. Ralph We report measurements of electron transport through individual molecules and nanoparticles incorporated into electromigrated break junction devices. In low-temperature studies of a thiol-terminated organometallic complex using a mechanically controllable break junction, we have studied the effects of molecular distortions. We find that as a function of stretching the molecule, a zero-bias Kondo peak can split into two finite-bias peaks, reminiscent of singlet-triplet transitions in other types of quantum dots. We discuss possible mechanisms in terms of coupling between broken spatial symmetries and the spin state of the molecule. We also measure devices in which molecules and nanoparticles are contacted by ferromagnetic electrodes, so as to study the interplay of spin polarization with single-electron charging effects. [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P11.00007: Universal Scaling of Nonequilibrium Transport in the Kondo Regime of Single Molecule Devices Gavin Scott, Zachary Keane, Jacob Ciszek, James Tour, Douglas Natelson Scaling laws and universality are often associated with systems exhibiting emergent phenomena possessing a characteristic energy scale. We report nonequilibrium transport measurements on two different types of single-molecule transistor (SMT) devices in the Kondo regime. The conductance at low bias and temperature adheres to a scaling function characterized by two parameters. This result, analogous to that reported recently in semiconductor dots with Kondo temperatures two orders of magnitude lower, demonstrates the universality of this scaling form. We compare the extracted values of the scaling coefficients to previous experimental and theoretical results. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P11.00008: Spatial range of the Kondo effect C.A. B\"usser, G. B. Martins, L. Costa Ribeiro, E. V. Anda, E. Dagotto The objective of this work is to discuss the spatial range of the effect caused by the Coulomb interaction localized at an impurity center. The numerical method we use, the embedded cluster approximation (ECA) and the finite U slave bosons mean field (FU-SBMF), are developed to treat localized impurity systems. It is important to note that, contrary to other techniques, ECA and FUSB can work in real space. Instead of using the spin-spin correlation to determine the length of the Kondo cloud, we will use the local density of states (LDOS) on the lead, far from the impurity. The presence of the impurity produce a disturbance in the LDOS of sites away from it. In this work, we propose to use this distortion to evaluate the spatial range of the Kondo effect. We observe that the effect of the distortion decays exponentially as a function of the distance from the impurity. With that in mind, a characteristic length $\hat{R}_{\rm K}$ can be easily defined. When the coupling between the impurity and the metal is increased, we verify that $\hat{R}_{\rm K} \sim 1/ T_{\rm K}$. We will also discuss how the magnetic field and temperature affect the length $R_{\rm K}$. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P11.00009: 1/N expansion of the nonequilibrium single-impurity Anderson Model Zurab Ratiani, Aditi Mitra Results are presented for the nonequilibrium single-impurity Anderson model using a large-N approach, where N is the degeneracy of the impurity level. Using the Keldysh formalism, we extend the slave-boson functional integral method of Read and Newns to the out of equilibrium current carrying case. The correlation function for the slave boson is shown to exhibit a long time power law behavior along with an exponential decay whose origin is current induced decoherence, a result consistent with nonequilibrium X-ray edge physics. Expressions for the impurity susceptibility and the conductance through the device are presented to O(1/N) and for an applied voltage less than the Kondo temperature. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P11.00010: Interplay of Rashba and lateral spin-orbit interactions on the spin polarized conductance of quantum point contacts.* A. Ngo, P. Debray, S. E. Ulloa In this work, we study the conductance properties of semiconductor quantum point contacts (QPCs) created by laterally confining a two-dimensional electron gas via side-gating. The electric field due to the gradients of the lateral confining potential results in lateral spin-orbit coupling. Our experimental observations in QPCs fabricated in InGaAs/InAs exhibit a plateau in conductance at half-quantization, G $\cong $ 0.5(2e$^{2}$/h)$,$ in the \textit{absence} of applied magnetic field. To understand our experimental results, we carry out calculations of ballistic transport through QPCs in the presence of Rashba \textit{and} lateral spin-orbit coupling. Using a scattering matrix approach, we calculate the spin-dependent conductance for different confinement and applied electric fields. High spin polarization can be obtained in the absence any external magnetic field by controlling the tunable perpendicular applied electric field and the shape of lateral confining potential, but only at high spin-orbit interaction strength. We also study the possibility that the strong asymmetric confining potential creates an effective spin-dependent term due e.g. to electron-electron interactions. This term breaks the time reversal symmetry and is able to produce the 0.5 conductance plateau, similar to that seen in our experiments. Our results might provide a new approach to explore spin polarized electron sources. * Supported by NSF-DMR. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P11.00011: Spin control in Rashba-Aharonov-Bohm quantum dot ring in the Kondo regime. E. Vernek, N. Sandler, S. E. Ulloa Application of small magnetic fields in QDs embedded in Aharonov-Bohm (AB) ring geometries, as well as gate voltages that modify the Rashba spin-orbit interaction (RSOI), are possible experimental probes to control spin transport. One important feature of charge transmission through QDs is the Kondo effect, resulting from the strong Coulomb interactions in the dot and carrier hopping between dot and current leads. Although much work has focused on the Kondo regime in QDs, not much is known on how RSOI modifies charge transport through the dot or its role in spin-transport. Full understanding of RSOI on the Kondo regime is fundamental, as it studies the competition of different coherent phenomena and has potential applications in devices such as spin-filters. A study of this geometry included the role of RSOI perturbatively [1]. However, the full features of Kondo physics are subtle and not captured in perturbation theory. In this work, we present a numerical renormalization group study that addresses charge and spin transport properties in the zero-bias regime, and allows comparisons with perturbation results. We find that the presence of both AB fields and RSOI results in an intrinsic polarizing field that breaks the spin degeneracy. This allows a delicate control of spin polarization of the conductance in the system, while strong RSOI suppresses the Kondo effect. [1] R. J. Heary et al., PRB 77, 115132 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P11.00012: Decoherence due to contacts in ballistic nanostructures I. Knezevic In quasiballistic nanoscale electronic structures, the process of relaxation towards a steady state cannot be attributed to carrier scattering. Rather, the active region of a nanostructure is an open quantum-mechanical system, whose nonunitary evolution (decoherence) toward a nonequilibrium steady state is determined by carrier injection from the rapidly dephasing contacts. I will present a technique for the treatment of contact-induced decoherence in ballistic nanostructures, which is established within the framework of the open system theory. Efficient electron-electron scattering in the contacts enables one to consider them nearly ``memoryless'' and derive a Markovian kinetic equation for the active region's statistical operator through coarse graining over the contacts' short memory- retention time. By incorporating a first-principles model interaction between the active region and the contacts into the Markovian dynamics derived, nonequilibrium steady-state distribution functions of the forward- and backward-propagating states in the active region are derived analytically. The approach is illustrated on several two- terminal nanostructures. I will also discuss the relationship between the present approach and the Landauer-B\"{u}ttiker formalism, as well as the inclusion of scattering. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P11.00013: High frequency measurements of shot noise suppression in atomic-scale metal contacts Patrick J. Wheeler, Kenneth Evans, Jeffrey Russom, Nicholas King, Douglas Natelson Shot noise provides a means of assessing the number and transmission coefficients of transmitting channels in atomic- and molecular-scale junctions. Previous experiments at low temperatures in metal and semiconductor point contacts have demonstrated the expected suppression of shot noise when junction conductance is near an integer multiple of the conductance quantum, $G_{0}\equiv 2e^2/h$. Using high frequency techniques, we demonstrate the high speed acquisition of such data at room temperature in mechanical break junctions. In clean Au contacts conductance histograms with clear peaks at $G_{0}$, $2G_{0}$, and $3G_{0}$ are acquired within hours, and histograms of simultaneous measurements of the shot noise show clear suppression at those conductance values. We describe the dependence of the noise on bias voltage and analyze the noise vs. conductance histograms in terms of a model that averages over transmission coefficients. [Preview Abstract] |
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