Session T17: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Quantum Transport II

AC Bias Spectroscopy of the Kondo Singlet in a Single Electron Transistor

We have measured the nonlinear differential conductance, G, of a single electron transistor in the spin 1/2 Kondo regime in presence of an oscillating source voltage. In two distinct regimes, hf $>$ k$_B$T$_K$ and hf $\ll$ k$_B$T$_K$, where f is the oscillation frequency and T$_K$ is the Kondo temperature, comparison to the static model of Kondo transport reveals agreement at very low frequencies and an increasing systematic departure at high frequencies. When hf > k$_B$T$_K$, the G defined as the derivative of the time averaged current through the device with respect to the average bias drastically differs from the static model. We show that the effect cannot be explained by an increase in the electron temperature.   

Connection between Local moment and Underscreened Kondo effect in parallel double quantum dots

Double quantum dots connected in parallel to a single channel, have been studied theoretically in two disparate limits: (I) Systems in which each dot has strong Coulomb interactions, exhibiting an underscreened spin-1 Kondo effect [1]; (II) An interacting dot 1 and a non-interacting dot 2, showing a quantum phase transition between Kondo phase and non-Kondo local-moment [2]. In this work, we use the numerical renormalization group approach to study a strongly interacting ``quantum dot 1'' and a weakly interacting ``dot 2'' connected in parallel to metallic leads. Gate voltages can drive the system between Kondo-quenched and free-moment phases separated by Kosterlitz-Thouless quantum phase transitions. As interactions in dot 2 become stronger relative to the dot-lead coupling, the free moment evolves from an isolated spin-1/2 in dot 1 to a many-body doublet arising from an underscreened Kondo effect. These limits, which feature very different entanglements between dot and lead electrons, can be distinguished by conductance measurements at finite temperatures. \\[4pt] [1] D. E. Logan, C. J. Wright, and M. R. Galpin, PRB 80, 125117 (2009).\\[0pt] [2] L. G. G. V. Dias da Silva et al., PRL 97, 096603 (2006).   

Temperature Dependence of a Double Quantum Dot Kondo Effect

Lateral quantum dots are highly tunable experimental systems ideal for exploring the interplay of orbital, spin, and charge correlations. We present studies of a double quantum dot system in a GaAs/AlGaAs heterostructure where transport through each dot may be measured independently. In the limit of negligible inter-dot tunneling, the conductance through both dots is enhanced along inter-dot charge degeneracy lines, where the energy cost for an electron to be on either dot is the same [A. H\"{u}bel, et al. PRL 101, 186804 (2008)]. With spin degeneracy, there are expected to be four or five-fold degenerate states, depending on the parity of the electron occupation number of each dot. We attribute the enhanced conductance to a double-dot Kondo effect that screens these localized, degenerate states. The temperature dependence of this Kondo effect is studied as a function of the coupling strength of each dot to its leads and the parity of the electron occupation numbers.   

Phase-dependent coherence and transport effects in a double-dot Aharonov-Bohm interferometer

We study coherence dynamics between two sites comprising a double-dot interferometer attached to non-equilibrium leads. We demonstrate via numerical simulation that a magnetic flux passing through the interferometer results in phase-dependent decoherence in the case of degenerate dots. The precise details of these effects relies on an interplay between Markovian and non-Markovian dynamics. We employ various methods to further investigate these effects analytically, including the derivation of a quantum Langevin equation and a direct calculation of the relevant correlation functions. In addition, we investigate multi-particle scattering states in the same system.   

Electrostatic modulation of periodic potentials in a two-dimensional electron gas: from antidot lattice to quantum dot lattice

We use a dual gated device structure to introduce a gate-tunable periodic potential in a GaAs/AlGaAs two dimensional electron gas (2DEG). Using a suitable choice of gate voltages we can controllably alter the potential landscape in the 2DEG, thereby inducing either a periodic array of antidots or quantum dots. Antidots are artificial scattering centers, and therefore allow for a study of electron dynamics. On the other hand, a quantum dot lattice provides the opportunity to study correlated electron physics. We use a variety of electrical measurements such as magneto-resistance, thermo-voltage and current-voltage characteristics to probe these two contrasting regimes.   

Universal out-of-equilibrium transport in Kondo-correlated quantum dots: a renormalized superperturbation theory on the Keldysh contour

The non-linear conductance of semiconductor heterostructures and single molecule devices exhibiting Kondo physics has recently attracted attention [1,2]. We address the observed sample-dependence across various systems by considering additional electronic contributions present in the effective low-energy model underlying these experiments. To this end we develop a novel version of the superperturbation theory [3] in terms of dual fermions on the Keldysh contour. We analyze the role of particle hole asymmetry on the transport coefficients. Our approach [4] systematically extends the work of Yamada and Yosida and others to the particle-hole asymmetric Anderson model and reproduce the exactly solvable resonant level model and the special case considered in [5]. It correctly describes the strong coupling physics and is free of internal inconsistencies that would lead to a breakdown of current conservation. \\[4pt] [1] M. Grobis et al., Phys. Rev. Lett. 100, 246601 (2008).\\[0pt] [2] G. D. Scott et al., Phys. Rev. B 79, 165413 (2009).\\[0pt] [3] H. Hafermann et al., EPL 85, 27007 (2009).\\[0pt] [4] Enrique Munoz, C.J. Bolech, and Stefan Kirchner, submitted (2011).\\[0pt] [5] K. Yamada, Prog. Theo. Phys. 62, 354 (1979).   

Muli-state operation in quantum dot channel FETs incorporating spatial wavefunction-switching

Three-state behavior has been demonstrated in Si and InGaAs quantum dot gate (QDG) field-effect transistors\footnote{S. Karmakar, \textit{et al.}, J. Electronic Materials, 40, 1746, 2011.}$^,$\footnote{F Jain, J. Electronic Materials, 40, 1717, 2011.} (FETs). Recently, spatial wavefunction switched\footnote{Ibid.} (SWS) and quantum dot channel\footnote{F. Jain et al., Proc. II-VI Workshop, Oct.2011.} (QDC) FETs have been reported to exhibit four-state operation. This paper presents simulations of versatile combinations of SWS features in QDC channels to optimally design multi-state transport in FETs that have the potential of scaling to sub-12nm regime. A QDC-FET channel is modeled as having superlattice-like mini-energy bands where the carrier wavefunctions are transferred across the channel as drain voltage is changed, producing step-like multi-state electrical characteristics. This behavior is analogous to that of single electron transistors.\footnote{S. J. Shin, et al., Appl. Phys. Lett. 97, 103101, 2010.} The difference is that QDC devices use more than a few electrons and operate at room temperature. The SWS feature additionally provides carrier transfer from lower to upper dot layer(s) in a QDC having more than one layer of quantum dots.   

Tunneling in double quantum dots and rings: search for chaos assistance

Semiconductor heterostructures as quantum dots (QD) or quantum rings (QR) demonstrate discreet atom-like energy level structure. In the case of double QD (DQD) or double concentric QR (DCQR), a single electron spectrum is composed of a set of quasi-doublets [1]. We study influence of these specific spectrum properties on electron tunneling related to the electron transport through DQD (DCQR). The double InAs/GaAs quantum dots (rings) are considered within three dimensional single sub-band effective approach [2]. Tunneling between dots (rings) is evaluated by effect of inter-dot distance and QD (QR) geometry. We show that the quasi-doublets of the electron spectra define tunneling properties of the DCQR. Violation of symmetry of the DCQR geometry leads to increase of tunneling. Discussed will be also the chaos assisted tunneling in the double QD (DCQR). \\[4pt] [1] I. Filikhin, S. G. Matinyan and B. Vlahovic, Phys. Lett. A 375, 620 (2011); I. Filikhin, S. G. Matinyan, J. Nimmo and B. Vlahovic, Physica E 43, 1669 (2011). \\[0pt] [2] I. Filikhin, V. M. Suslov and B. Vlahovic, Phys. Rev. B 73, 205332 (2006).   

First-principles study of orbital-dependent quantum confinement in Si/Ge nanowire superlattices

We study electronic structures of H-passivated Si/Ge nanowire superlattices (NWSLs) oriented along [110] direction, using an \textit{ab-initio} pseudopotential density-functional method with the local density approximation. Obtained electronic structures of the Si/Ge NWSLs show both dispersive and non-dispersive bands in conduction and valence bands due to band-selective quantum confinement: the highest valence band and the lowest conduction band are not confined in either the Si- or Ge-nanowire segment but they are extended throughout the whole NWSLs, while there exist non-dispersive bands confined in either Si- or Ge-nanowire segment below the top of the valence band and above the bottom of the conduction band. This feature originates from strong orbital-dependence of quantum confinement of electronic states, making conventional band-offset diagrams for superlattices invalid in Si/Ge NWSLs. Effects of atomic geometries on the confinement are studied with different diameters and superlattice periodicities. This work was supported by NRF of Korea (Grant Nos. 2009-0081204 and 2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2011-C3-05).   

One-dimensional physics in transition-metal nanowires

One atom wide transition-metal nanowires can now be fabricated on Cu step edges. We present a theoretical study of the electronic properties of such systems. While the systems fall within the broad class of Luttinger liquids, about which much is known, characteristic features of transition metals including orbital degeneracy and interactions which favor locally high-spin configurations lead to new physics. Density functional calculations indicate that Co nanowires on Cu surfaces are half metals and are not electronically isolated from the substrate material. The multi-orbital and local high-spin physics are elucidated by Hartree-Fock approximations and bosonization calculations of the multi-orbital Hubbard model.   

Detection of spin states in a quantum dot by random telegraph signal analysis with the hidden Markov model

A lateral GaAs quantum dot with an adjacent quantum point contact charge sensor was tuned so that its chemical potential is close to the Fermi level of an adjacent electron reservoir. In this configuration electrons tunnel back and forth between the quantum dot and the reservoir due to thermal fluctuations, characteristic switching known as a random telegraph signal (RTS). The charge state of the quantum dot is directly observable, but spin and orbital state information is not. Such extra states may reveal themselves in the statistics of the timing of tunneling events. We present a statistical analysis approach based on the Hidden Markov Model (HMM) for extracting information about the internal structure of the quantum dot from RTS data, particularly focusing on determining the electronic spin state. We demonstrate on simulated and experimental data that this technique can detect electron spin states and measure their tunneling rates individually when the energy level difference is less than or comparable to the thermal energy scale. This opens a new regime for studying quantum dot spin physics because other experiments require a Zeeman energy difference greater than the thermal energy in order to distinguish the spin states.   

Impact of adsorbed organic monolayers on vacuum electron tunneling contributions to electrical resistance at an asperity contact

Electrical Contact Resistance measurements are reported for RF MEMS switches situated within an ultrahigh vacuum system equipped with \textit{in situ} oxygen plasma cleaning capabilities. Measurements were preformed on Au/Au permanently adhered switches, and functioning Au/RuO$_{2}$ switches in the presence and absence of adsorbed monolayers of pentane and dodecane. The data are analyzed to explore how adsorbed molecules in regions close to the contact may impact vacuum tunneling contributions to the experimentally measured resistance: (1) The resistance associated with direct contact in parallel with a vacuum tunneling path, which upon uptake of the monolayer is replaced by the molecular resistance, and (2) A series connection of the direct contact resistance with the molecular layer after adsorption occurs, with the vacuum tunneling path assumed to be negligible. The results favor scenario (1), whereby uptake of the molecular layer effectively shuts down the vacuum tunneling path, which in this case is effectively $\sim $30 Ohms in the absence of an adsorbed film. The methods constitute a new and original approach to documenting vacuum tunneling levels in regions of close proximity. Funding agencies: NSF, AFOSR MURI, DARPA. \\[4pt] [1] D. Berman, M. Walker, C. Nordquist, J. Krim, \textit{J. Appl. Phys.,} in press   

Dependence of Conductance Resonanoces and Modulations on Channel Length in Asymmetric Quantum Point Contacts (QPCs)

Transport features below $2e^2/h$ show resonance peaks in highly asymmetric QPCs. As we increace the channel length, the number of peaks observable also increases. We characterize the number of peaks and/or oscillations as a function of channel length, when the QPC is tuned to or below the first quantum channel. The the number of peaks/oscillations appears to increase on average as the channel length increases. In addition, we find preliminary evidence that there is a correspondence between the resonance peaks and the zero-bias anomaly(ZBA) in the differential conductance. These behaviors are consistent with an interpretation based on the formation of quasi-bound-states within the QPC channel in the single-mode limit.   

New scenario of shuttling mechanism in magnetic nano-electromechanical single-electron tunneling systems

We investigate a new shuttling scenario in the electro-mechanics of a movable quantum dot between a nonmagnetic lead and a magnetic lead. In this device, the quantum dot has two energy levels due to the Zeeman energy splitting under magnetic field with Coulomb blockade. The electromechanical instability is shown to depend on the external voltage when the vibrating energy overcomes the dissipation energy of the system. In addition to the normal shuttling behavior, the shuttling current can be suppressed and then recovered depending on the external voltage. It is also found that the nano-electromechanical oscillation significantly improves the spin polarized current compared with one in the fixed quantum dot due to the interplay between the spin polarized transport and mechanical degree of freedom.