Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K35: Electronic Properties and the Kondo Effect in Quantum Dots |
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Sponsoring Units: DCMP Chair: Michael Scheibner, Naval Research Laboratory Room: Baltimore Convention Center 338 |
Tuesday, March 14, 2006 2:30PM - 2:42PM |
K35.00001: Exotic few-particle ground states in charged self-assembled InAs/GaAs quantum dots Lixin He, Gabriel Bester, Alex Zunger Three-dimensionally confined quantum dots can be loaded with a few carriers. However, unlike real atoms, in self-assembled InAs/GaAs quantum dots, the Coulomb repulsion between holes $J_{hh} \sim $ 15 -25 meV is comparable to the single-particle energy spacing between levels $\Delta\epsilon \sim $ 10 - 20 meV. This opens the possibility of observing stable, exotic spin configurations that defy the rules of atomic physics (Hund's rule and the Aufbau principle). We have applied a combination of atomistic pseudopotential description for the single-particle level structure, with a many-body configuration interaction (CI) description of many-particle effects to predict both electron and hole charged states in InGaAs/GaAs self-assembled quantum-dots. We find that while electron charging follow both the Aufbau principle and Hund's rule, hole charging gives rise to stable but unusual spin configurations. Our theory[1] offer a self-consistent interpretation to recent experimental observations[2], where the simple 2D effective mass parabolic models fail. [1] L. He, G. Bester and A. Zunger, Phys. Rev. Lett. (in press), cond-mat/0505330. [2] Reuter {\it et al}, Phys. Rev. Lett. {\bf 94}, 026808 (2005) [Preview Abstract] |
Tuesday, March 14, 2006 2:42PM - 2:54PM |
K35.00002: A variational wave function for two-dimensional quantum-dot helium M. Golam Faruk, Orion Ciftja, Kevin Storr We introduce a variational wave function for two-dimensional quantum dot helium (a system of two interacting electrons in a two-dimensional parabolic confinement potential) in an arbitrary perpendicular magnetic field. This variational wave function contains a Jastrow pair correlation factor of displaced Gaussian form in addition to the commonly used Laughlin correlation factor. The Gaussian correlation factor assures the overall quality of the variational wave function at all values of the magnetic field including weak (or zero) and strong magnetic fields. The accuracy of the variational wave function is gauged through exact numerical diagonalization results. We find out that this trial wave function is a very accurate representation of the true ground state for the whole range of magnetic fields. [Preview Abstract] |
Tuesday, March 14, 2006 2:54PM - 3:06PM |
K35.00003: Spin-Orbit effects in Quantum Dots: Interplay of disorder and interactions Hakan E. T\"ureci, Y. Alhassid, A. Douglas Stone Disordered or chaotic quantum dots with a large Thouless conductance g provide a tractable experimental and theoretical system for studying electron-electron interactions within the framework of the universal Hamiltonian. In particular, good agreement between theory and experiment is observed for the mesoscopic fluctuations of the conductance in dots with negligible spin-orbit scattering[1]. Here we focus on the effects of a tunable spin-orbit scattering within the universal Hamiltonian model. The presence of spin-orbit scattering was shown to introduce new symmetry limits of the single-particle Hamiltonian[2]. We present results for ground state and finite temperature properties in the crossover regime between symmetries. Furthermore, we study the signatures of a quantum critical regime dominated by collective critical fluctuations[3]. \begin{thebibliography}{3} \bibitem{1}Y. Alhassid and T. Rupp, Phys. Rev. Lett. 91, 056801 (2003). \bibitem{2} I.L. Aleiner and V.I. Fal'ko, Phys. Rev. Lett. {\bf 87}, 256801 (2001). \bibitem{3} G. Murthy, Phys. Rev. B {\bf 70}, 153304 (2004). \end{thebibliography} [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K35.00004: A many-polaron system in a background-charge potential F. Brosens, S. N. Klimin, J.T. Devreese The ground state energy of an $N$-polaron system confined to a quantum dot with a neutralizing background charge is investigated within an all-coupling many-body path-integral variational principle taking into account both Fermi statistics of polarons and the electron-electron interaction. The treatment of the ground-state energy is performed for both closed-shell and open-shell systems. The electron-phonon contribution to the ground-state energy as a function of the number of fermions demonstrates a trend to a constant value when increasing $N$. For a finite number of polarons, the dependencies of the ground-state energy and of the polaron contribution on the parameter $r_{s}^{\ast},$ which determines the average fermion density in a quantum dot, are very similar to those for a polaron gas in bulk. Herefrom, we can conclude that the ground-state properties of a polaron gas in bulk can be qualitatively described using a model of a finite number of polarons in a confinement potential provided by a background charge. [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K35.00005: Spectroscopy of the Kondo Problem in a Box Denis Ullmo, Ribhu Kaul, Gergely Zarand, Shailesh Chandrasekharan, Harold Baranger We study the spin quantum numbers and energy eigenvalues of the ground state and low lying excitations of a quantum dot with a single spin-S impurity. We prove an exact theorem that allows us to infer the ground state spin for an arbitrary spin-S of the impurity and for even/odd electrons on the quantum dot. Additionally, strong and weak coupling perturbation theory backed up with quantum Monte-Carlo simulations enable us to map out the spin structure of the excited states and the energy splittings between them as the ratio $\Delta/T_K $ is tuned. We propose a simple transport experiment involving the tunneling spectroscopy of a double quantum dot, where these results can be observed. Finally, we make explicit contact with experiment by calculating the transport properties of the proposed double dot device that exhibit signatures of tunneling into the Kondo-correlated state. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K35.00006: Charge Fluctuations in a Nanoscale Structure: Interplay of Kondo and Luttinger Liquid Physics Paata Kakashvili, Henrik Johannesson We propose a setup which allows to study the interplay of Kondo and Luttinger liquid physics. It consists of a quantum box, biased by a gate voltage, and side-coupled to a quantum wire by a point contact. Close to the degeneracy points of the Coulomb blockaded box the setup can be described as a Luttinger liquid interacting with an effective Kondo impurity. By applying a magnetic field one drives the system from an effective two-channel to a one-channel situation, allowing for the study of the crossover between the two. Using bosonization and boundary conformal field theory we predict that for the case of spin-polarized electrons the differential capacitance of the box will exhibit distinctive Luttinger liquid scaling with temperature and gate voltage. In the limit of zero field the Luttinger liquid behavior gets masked by two-channel Kondo screening, leading to a logarithmic scaling of the differential capacitance with temperature and gate voltage. These effects should be possible to study experimentally using the recently developed SET electrometer technique. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K35.00007: Universality and non-universality for a time-dependent quantum dot in the Kondo regime Ali Goker, Peter Nordlander We employ time-dependent non-crossing approximation for single electron transistor and calculate the transient response of the conductance for a variety of temperatures in infinitesimal bias when the dot-lead tunneling constant is suddenly changed such that the Kondo effect is present in the final state. In non-universal timescale, we see rapid oscillations. The frequency of these oscillations depends on the dot level and their amplitude is modulated by initial and final tunneling constants. We explain this observation qualitatively by exact solution of the resonant level model. In universal timescale, we compute the conductance for two systems which have different Kondo temperatures and test universality. Our results indicate that universality is well preserved as a function of T/T$_{K}$ in spite of tiny deviations at low temperatures. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K35.00008: Universal Scaling Behavior of the Kondo Effect in a Quantum Dot. David Goldhaber-Gordon, Ileana Rau, Ronald Potok, Michael Grobis At zero temperature, a many-body Kondo singlet forms between a spin-1/2 quantum dot and electrons in nearby, tunnel-coupled reservoirs. This Kondo singlet can be broken by sufficiently large temperature T or bias V across the dot. Differential conductance G(V,T) is predicted to show universal scaling in eV/kT. However, such behavior has not been examined thoroughly in experiments. To address this issue, we have performed detailed transport measurements through a Kondo quantum dot at finite bias and temperature. We will present our results and discuss the scaling behavior seen in this system. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K35.00009: The Two-channel Kondo Effect in a Semiconductor Nanostructure Ileana Rau, Ron Potok, David Goldhaber-Gordon Quantum dots have proven to be excellent systems for studying the single-channel Kondo effect, the many-body ground state resulting from the interaction between a spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ quantum dot and a reservoir of conduction electrons. The two-channel Kondo effect is achieved by coupling the dot with equal strength to two independent reservoirs. In this case the reservoirs compete to independently form a Kondo state with the dot, resulting in overscreening of the excess spin. In a new semiconductor double quantum dot geometry, proposed by Y. Oreg and D. Goldhaber-Gordon (1), we observe spin two-channel Kondo physics. Low temperature transport measurements through the smaller of the two dots reveals that the double dot -- leads system undergoes a quantum phase transition from one competing single-channel Kondo state to another. By tuning between these two Fermi liquid regimes, we observe the non-Fermi liquid signature of the two-channel Kondo state. I will explore recent experimental progress and open questions. (1) Y. Oreg, D. Goldhaber-Gordon, PRL \textbf{90 }136602 (2003) [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K35.00010: Jahn-Teller / Kondo Interplay in a Three-Terminal Quantum Dot R.C. Toonen, H. Qin, A.K. Huettel, S. Goswami, D.W. van der Weide, K. Eberl, R.H. Blick The Jahn-Teller effect is the spontaneous geometric distortion of a nonlinear molecular entity. The Kondo effect, an expression of asymptotic freedom, arises from the hybridization between localized states of a magnetic impurity and the itinerant states of its environment. The interplay of these two phenomena has attracted the attention of theorists studying the growth and interactions of heavy-fermion systems. Because of the technical difficulties associated with probing isolated impurities in bulk materials, this composite effect has remained experimentally unexplored. We have investigated co-tunneling transport phenomena in a three-terminal quantum dot with triangular symmetry. Our measurements of anomalous spectral signatures reveal interplay between the Jahn-Teller and Kondo effects. This discovery suggests a means of controlling the correlation of spatially separated pairs of entangled electrons (EPR pairs)---a necessary condition for the physical realization of a quantum computer (DiVincenzo's 7th requirement). [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K35.00011: Quantum Dot Coupled to a Mesoscopic Fabry-Perot Resonator. Michael Grobis, Ileana Rau, Ronald Potok, David Goldhaber-Gordon We will present our study of a quantum dot coupled to a Fabry-Perot resonator. The Fabry-Perot resonator is created by tunnel coupling a lithographically defined two-lead quantum dot to an additional finite reservoir of tunable length. Fabry-Perot oscillations are observed in transport through the quantum dot caused by interference between the direct tunneling path through the dot and the multi-step path that includes tunneling in and out of the finite reservoir. These oscillations are dependent on the phase shifts acquired during the transmission events and the nature of the electron path through the Fabry-Perot cavity. Tuning the quantum dot through a Kondo resonance allows us to extract information about the Kondo transmission phase shift and the nature of Kondo screening cloud in this system. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K35.00012: Electron Dynamics in AC-Driven Quantum Lenses Arezky Hern\'andez-Rodr\'{\i}guez, Lilia Meza-Montes, Carlos Trallero-Giner, Sergio Ulloa We have applied the Floquet formalism to study the effects of an ac electric field applied to quantum dots with lens shape. The electric field is applied along and perpendicular to the direction of the axial symmetry and the time-dependent Hamiltonian in the effective mass approximation is solved. A complete set of orthonormal functions is found to characterize the physical problem while keeping the full lens symmetry. When the electric field is along the axial symmetry axis, we show that the Hilbert space of solutions is separated into orthogonal subspaces with different $z$-component of the angular momentum. In case of a perpendicular field, the appearance of new dynamical symmetry is studied. We give an explicit analytical representation for the quasi-energy spectrum and electronic states, and show numerical results for different lens parameters and intensity and direction of the electric field. The interplay between the lens shape geometry and the ac driven field (intensity and frequency) are analyzed. Our approach applies to the full range of intensities. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K35.00013: Toward Strong Interactions in Circular Quantum Dots: Correlation Induced Inhomogeneity Amit Ghosal, Alev Guclu, Cyrus Umrigar, Denis Ullmo, Harold Baranger Physical properties of the electron gas, which describes conduction electrons interacting via Coulomb forces, change dramatically depending on the balance between the strength of the kinetic energy and the Coulomb repulsion. For weak interactions (high density), the system behaves as a Fermi liquid, with delocalized electrons. In contrast, in the strongly interacting limit (low density), the electrons localize and order in a Wigner crystal phase. The physics at intermediate densities is phenomenally rich and not adequately understood. Here we present a study of the intermediate density electron gas (up to $r_s=18$) confined to a circular quantum dot containing up to 20 electrons. Using an accurate quantum Monte Carlo technique, we show that the correlation induced by increasing interaction strength smoothly causes, first, ring structure and, then, angular modulation in the pair-density without any signature of a cross-over. The excitation energy for certain high-spin states decreases significantly with $r_s$. Dots with smaller number of electrons are typically more affected by correlation effects. [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K35.00014: Kondo Physics in Exactly Solvable SU(4) Double Dots Robert Konik I study a dot-lead system composed of two quantum dots arranged in series. I consider dots that are both capacitive and tunnel coupled and where the applied gate voltage to the individual dots is unequal. This system admits an exact solution via the Bethe ansatz even in the presence of finite (as opposed to infinite) Coulomb repulsion. From this solution, the transport properties of the dot system can be ascertained analytically. In particular transport can be determined everywhere from the empty orbital to the mixed valence to an SU(4) Kondo regime. While double dots are the focus, natural generalizations to systems with $N>2$ dots are similarly integrable. [Preview Abstract] |
Tuesday, March 14, 2006 5:18PM - 5:30PM |
K35.00015: Kondo Shadows in Hybrid Magnetic Molecular Solids Mikhail Kiselev, Konstantin Kikoin We discuss the properties of layered Anderson/Kondo lattices with metallic electrons confined in 2D xy planes and local spins in insulating layers forming chains in z direction. Each spin in this model possesses its own 2D Kondo cloud, so that the Nozieres' exhaustion problem does not occur. The high-temperature perturbational description is matched to exact low-T Bethe-ansatz solution. The excitation spectrum of the model is gapless both in charge and spin sectors. The disordered phases and possible experimental realizations of the model are briefly discussed. [Preview Abstract] |
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