Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W37: Nanoscale Conductance Theory II |
Hide Abstracts |
Sponsoring Units: DMP Chair: Harold Baranger, Duke University Room: Baltimore Convention Center 340 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W37.00001: Variable-range cotunneling and non-Ohmic transport in a chain of one-dimensional quantum dots Michael M. Fogler, Sergey V. Malinin, Thomas Nattermann A 1D wire with a finite density of strong random impurities is modeled as a chain of weakly coupled quantum dots. The resistance of such a system is shown to exhibit a rich dependence on bias voltage $V$ and temperature $T$ due to the interplay of Coulomb blockade, Luttinger-liquid, and disorder effects. At low $T$ and $V$ electrons propagate through the wire by means of thermal activation and a multiple cotunneling. In this regime the resistance is limited by the ``breaks'': randomly occurring clusters of dots with a special length distribution pattern that inhibits the transport no matter how the activation and tunneling are combined. As $T$ or $V$ increases, the breaks become shorter and less resistive. The resistance can exhibit a (stretched) exponential and a quasi power-law dependence on $T$ and $V$ depending on the position at the $T$-$V$ diagram. Unlike the case of a single impurity the effect of $T$ and $eV$ is not symmetric. The Ohmic resistance of a macroscopic wire is always dictated by breaks not single impurities. Our results imply that the power-laws reported in several recent transport measurements of one-dimensional systems may reflect not only intrinsic Luttinger parameters but also impurity distribution statistics. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W37.00002: Pair tunneling through single molecules Jens Koch, Mikhail E. Raikh, Felix von Oppen By a polaronic energy shift, the effective charging energy of molecules can become negative, favoring ground states with even numbers of electrons. Here, we show that charge transport through such molecules near ground-state degeneracies is dominated by tunneling of electron pairs which coexists with (featureless) single-electron cotunneling. Due to the restricted phase space for pair tunneling, the current-voltage characteristics exhibits striking differences from the conventional Coulomb blockade. In asymmetric junctions, pair tunneling can be used for gate-controlled current rectification and switching. We find that pair tunneling also has interesting consequences for the shot noise. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W37.00003: Spin Related Effects in Transport Properties of "Open" Quantum Dots Yashar Ahmadian, Gianluigi Catelani, Igor Aleiner We study the interaction corrections to the transport coefficients in open quantum dots (i.e. dots connected to leads of large conductance $G \gg e^2/\pi\hbar$), via a quantum kinetic equation approach. The effects of all the channels of the universal (in the Random Matrix Theory sense) interaction Hamiltonian are accounted for at one loop approximation. For the electrical conductance we find that even though the magnitude of the triplet channel interaction is smaller than the charging energy, the differential conductance at small bias is greatly affected by this interaction. Furthermore, the application of a magnetic field can significantly change the conductance due to the Zeeman splitting, producing finite bias anomalies. For the thermal conductance we find that the Wiedemann-Franz law is violated by the interaction corrections, and we investigated the effect of magnetic field on the Lorentz ratio for contacts of finite reflection. The charge and triplet channel corrections to the electrical and thermal conductance vanish for reflectionless contacts. In the latter case the temperature and magnetic field dependence of the conductance is determined by the Maki-Thompson correction in the Cooper channel. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W37.00004: Numerical studies of the dynamics of interacting electrons confined in nanostructures Rok Zitko, Janez Bonca, Anton Ramsak, Tomaz Rejec At low temperatures electrons have long phase-relaxation time. They tunnel coherently through nanostructures in a wave-like manner, which leads to various interference effects. We presently have adequate knowledge about the transport phenomena that can be described using single-electron models. The transport in the presence of interactions is, however, still a subject of intensive research. More refined theoretical tools are required to tackle problems such as that of the transport through systems of coupled quantum dots in the Kondo regime. We present our studies using complementary methods: the quantum Monte Carlo, the variational method and the numerical renormalization group. We show the phase diagram of the triple quantum dot and explain the various regimes of enhanced conductance. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W37.00005: Non-equilibrium conductance of a three-terminal quantum dot in the Kondo regime: Perturbative Renormalization Group Nayana Shah, Achim Rosch Motivated by recent experiments, we consider a single-electron transistor in the Kondo regime which is coupled to three leads in the presence of large bias voltages. Such a steady-state non-equilibrium system is to a large extent governed by a decoherence rate induced by the current through the dot. As the two-terminal conductance turns out to be rather insensitive to the decoherence rate, we study the conductance in a three-terminal device using perturbative renormalization group and calculate the characteristic splitting of the Kondo resonance. The interplay between potential biases and anisotropy in coupling to the three leads determines the decoherence rate and the conditions for strong coupling. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W37.00006: Dissipative quantum phase transition in a single electron transistor Alfred Zawadowski, Laszlo Borda, Gergely Zarand, David Goldhaber-Gordon We study the transport properties of a single electron transistor
(SET) with highly resistive gate electrodes, and show that the SET
displays a quantum phase transition analogous to the famous
dissipative phase transition studied by Leggett. At
temperature $T=0$, the charge on the central island of a
conventional SET changes smoothly as a function of gate voltage,
dueto quantum fluctuations. However, sufficiently-strong
dissipation, $R_g>R_C$, can freeze out charge fluctuations on the
island even at the degeneracy point, causing the charge on the
island to change in sharp steps as a function of gate voltage.
For $R_g |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W37.00007: The Interplay of Spin and Charge Channels in Zero Dimensional Systems Mikhail Kiselev, Yuval Gefen We study the interplay of charge and spin (zero-mode) channels in quantum dots. The latter affects the former in the form of a distinct signature on the differential conductance. We also obtain both longitudinal and transverse spin susceptibilities. All these observables, underlain by spin fluctuations, become accentuated as one approaches the Stoner instability. The non-perturbative effects of zero-mode interaction are described in terms of the propagation of gauge bosons associated with charge (U(1)) and spin (SU(2)) fluctuations in the dot, while transverse spin fluctuations are analyzed perturbatively. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W37.00008: The 0.7 anomaly in quantum point contacts: a scattering approach Caio Lewenkopf, Paulo Bonfim The conductance steps observed in the electronic transport through quantum point contacts (QPCs) became a paradigm of the Landauer conductance formula. For this reason, the ubiquitous experimental observation of the 0.7 anomaly in the first conductance step of QPCs, that defied the single-particle scenario, raised a lot of attention. The most successful theoretical explanation of this transport feature is in terms of Kondo physics: It builds on an Anderson-like model, whose parameters, namely, the resonance position, its couplings to the reservoirs and the charging energy are adjusted to give meaningful results. Starting from a scattering approach, that uses the Feshbach projection formalism, we construct a single-particle basis that allows us to directly calculate the resonance position and its coupling to left and right reservoirs. We then include an electron-eletron interaction term and proceed as standard. This approach unveils a novel interpretation for the underlying physics of the 0.7 anomaly. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W37.00009: Electron transport in the presence of a magnetic field and the absence of translational invariance Tobias Kramer, Robert E. Parrott, Eric J. Heller Recent experimental techniques in 2DEGs using scanning probe microscope tips allow one to spatially image electron flow directly (see also the talk by Kathy Aidala). These developments motivate theoretical consideration of (localized) magnetic edge states in position space. For the case of a parabolic confinement, a semi-analytic expression of the Green function is given. The underlying physics differs from a conventional edge-state model by the absence of translational invariance. It is also possible to derive a semiclassical interpretation of the current density, which provides additional physical insight into the nature of transport in position space. For additional information, see also http://people.deas.harvard.edu/$\sim$tkramer [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W37.00010: Divergent beams of nonlocally entangled electrons emitted from NS structures Fernando Sols, Elsa Prada We propose the use of normal and Andreev resonances in normal-superconducting structures to generate divergent beams of nonlocally entangled electrons. Resonant levels are tuned to selectively transmit electrons with specific values of the perpendicular energy, thus fixing the magnitude of the exit angle. When the normal metal is a ballistic two-dimensional electron gas, the proposed scheme guarantees arbitrarily large spatial separation of the entangled electron beams emitted from a finite interface. We perform a quantitative study of the linear and nonlinear transport properties of some suitable structures, taking into account the large mismatch in effective masses and Fermi wavelengths. Numerical estimates confirm the feasibility of the proposed beam separation method. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W37.00011: Non-equilibrium Entanglement and Noise in Coupled Double Quantum Dots Ramon Aguado, Neill Lambert, Tobias Brandes We study charge entanglement in two capacitively-coupled double quantum dots in thermal equilibrium and under stationary non-equilibrium transport conditions. In the transport regime, the entanglement exhibits a clear switching threshold and various limits due to suppression of tunneling by Quantum Zeno localisation or by an interaction induced energy gap. We also calculate quantum noise spectra and discuss current cross-correlations as an indicator of the entanglement in transport experiments. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W37.00012: Conductance Fano lineshapes for Kondo impurities on surfaces: A numerical renormalization group description. Nancy Sandler, Luis Dias da Silva, Sergio Ulloa Scanning tunneling microscopy (STM) measurements of Kondo impurities on metallic surfaces has been an active field in recent years. For a flat density-of-states (DoS) near the Fermi energy in the host metal, the low-bias STM conductance acquires the characteristic Fano lineshape, with width proportional to the Kondo temperature $T_K$. In this work, we study how this picture is modified when a \textit{structured} DoS (non-flat) is considered. A variety of physical effects can introduce peak/dips in the DoS, including the presence of a second impurity, hybridization between surface and bulk conduction states, and a magnetic impurity embedded in a molecule. Using numerical renormalization group techniques, we calculate the low-temperature conductance for this system. The zero-bias dip in the Fano conductance is modified by the presence of resonances or anti-resonances in the DoS near $E_F$. In particular, for DoS with pseudogaps and impurities in the mixed-valence regime, zero-bias Fano-like dips appear {\em even when no Kondo state has developed}, but governed by energy scales much larger than $T_K$. We further show that measurements of the scattering phase could be used as an additional probe into the Kondo regime. Supported by NFS-NIRT. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W37.00013: Effects of the electron-phonon interaction on the electron transport in low-dimensional disordered semiconductor structures Andrei Sergeev, Michael Reizer, Vladimir Mitin We investigate the effects of the interference between electron-phonon scattering and elastic electron scattering in heterostructures and nanotubes. Interference strongly enhances the effective electron-phonon coupling in semiconductor structures and strengthens the electron-phonon relaxation [1]. Employing the quantum transport equitation, we calculate the interference contribution to the electrical conductivity and phonon drag thermopower. Our results show that the interference term follows to the logT-law and dominates in the temperature dependence of the conductivity. Phonon drag is also enhanced due to disorder. [1] A. Sergeev et al., Phys. Rev. Lett., 94, 136602 (2005). [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W37.00014: Mesoscopic and nanoscopic physics of molecular-scale electronics Yongqiang Xue Going from the mesoscopic regime of quantum semiconductor device to nanoscopic molecular device, the dominant or first-order transport mechanism remains quantum mechanical coherent transport due to the small size. A large part of the theoretical efforts in nanoelectronics is thus to recast the accumulated knowledge about mesoscopic physics into forms that are suitable for evaluating quantum transport phenomena at the atomic-scale. On the other hand, electrical conduction is intrinsically a dynamical phenomenon. Since the different degrees of freedom (electronic, mechanical, phonon{\ldots}) in the nanostructures can be strongly coupled to each other and to their nano-environment, the measured electrical signal is often the result of complex dynamic coupling processes without requiring ensemble average. New theoretical principles and computational techniques may be needed to unravel the rich physics involved in molecular-scale transport. In this talk, we discuss our efforts in moving from mesoscopic theory to nanoscopic theory of molecular-scale electronics. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W37.00015: Phonon Broadening of Spectral Lines in Scanning Tunneling Spectroscopy J. W. Gadzuk The observation and interpretation of spectral lines associated with quasi-localized states in condensed matter systems has provided a rich source of information pertaining to the transient coupling of these states to their dynamic environment.$^{1}$ While polaron/Franck-Condon models in which a transient localized potential excites the ambient phonon system have formed the basis for phonon broadening in a wide variety of core level spectroscopies, Sunjic and Lucas have put forth an elegantly simple solution to the problem in terms of driven harmonic oscillators which easily incorporates the time scales for both the switching on and the decay of the localized potential.$^{2}$ In recent STS studies of thin NaCl films on Cu substrates, Repp et al. have observed Gaussian-broadened lines that are signatures of bound electrons at Cl vacancies (F-centers).$^{3}$ These resonance tunneling line shapes are here analyzed within the context of the SL model, properly accounting for lifetime effects due to both tip-to-vacancy and also vacancy-to-substrate tunneling, thus enabling determination of the actual electron-phonon interaction. $^{1}$J.W.Gadzuk, PRB \textbf{44}, 13466 (1991). $^{2}$M.Sunjic and A.A.Lucas, CPL \textbf{42}, 462 (1976). $^{3}$J.Repp et al., PRL \textbf{95}, 225503 (2005). [Preview Abstract] |
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