Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session X43: Electronic and Spin Properties of Quantum Dots and Quantum Point Contacts 
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Sponsoring Units: DCMP Chair: Stefan Badescu, Naval Research Laboratory Room: Colorado Convention Center 506 
Friday, March 9, 2007 8:00AM  8:12AM 
X43.00001: The Kondo box in the presence of exchange interaction Stefan Rotter, Hakan E. Tureci, Yoram Alhassid, A. Douglas Stone We study the problem of a quantum dot with finite level spacing which is coupled antiferromagnetically to a Kondo spin (``Kondo box''). In particular, we investigate the influence of a ferromagnetic exchange interaction among the dot electrons and the effect of an applied Zeeman field. This problem is addressed with the help of a numerical algorithm that allows for an exact diagonalization of the Hamiltonian in a good total spin basis [1] of the dot plus the Kondo spin. We discuss how the competition between the ferromagnetic exchange and the antiferromagnetic Kondo interaction affects the groundstate spin of the system. \newline [1] H. Tureci and Y. Alhassid, Phys. Rev. B 74, 165333 (2006). [Preview Abstract] 
Friday, March 9, 2007 8:12AM  8:24AM 
X43.00002: Measurements of Universal Kondo Scaling Behavior in a Quantum Dot Michael Grobis, Ileana Rau, Ronald Potok, David GoldhaberGordon, Hadas Shtrikman At zero temperature, a manybody Kondo singlet forms between a spin1/2 quantum dot and electrons in nearby, tunnelcoupled reservoirs. In transport measurements, the characteristic signature of the Kondo singlet is a narrow zero bias conduction enhancement. Finite temperature and bias destroy the Kondo singlet and suppress this enhancement. At low temperature and bias (eV, kT $<<$ kT$_{K})$, the evolution of the Kondo conduction 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 have measured the scaling power law and lowest order expansion coefficient of the universal scaling function, as well as the expected deviations from universality at higher energies. [Preview Abstract] 
Friday, March 9, 2007 8:24AM  8:36AM 
X43.00003: Kondo Screening Cloud and Charge Quantization in Mesoscopic Devices Rodrigo G. Pereira, Nicolas Laflorencie, Ian Affleck We propose that the finite size of the Kondo screening cloud, $\xi_K$, can be probed by measuring the charge quantization in a onedimensional system coupled to a small quantum dot. When the chemical potential in the system is varied at zero temperature, one should observe charge steps that are influenced by the Kondo effect when the system size is comparable to $\xi_K$. We show that the ratio of the width of the plateaus with an odd number of electrons to the width of the plateaus with an even number is a universal scaling function of $\xi_K/L$. [Preview Abstract] 
Friday, March 9, 2007 8:36AM  8:48AM 
X43.00004: Observation of quantum charge pumping in a gateconfined open dot with symmetrically configured pumping gates Shihying Hsu, KaiMing Liu Open quantum dots formed out of a high mobility two dimensional electron gas from negatively biased submicron metal gates are fabricated. Two quantum point contacts (QPCs) with independently adjustable transmission mode numbers are deviced as the two entrances of the quantum dots. Two additional metal gates are introduced to function as the pumping gates. The overall gate configuration is symmetric. We observe a dc current with the two pumping gates ac biased in the quantum charge pumping mode where the ac bias voltages are at the same frequency, 0.1$\sim $10MHz, but maintain a given phase difference $\phi $. The system remains longitudinally symmetric in this quantum pumping mode. The current amplitude drops with the opening up of the quantum dot. Moreover, we find that the current amplitude decreases with increasing the total transmission mode number in an inverse relation. By switching over to a rectification mode of gate biasing, the dc current characteristic is drastically changed. This leads to strong evidence that we have observed the quantum charge pumping. [Preview Abstract] 
Friday, March 9, 2007 8:48AM  9:00AM 
X43.00005: Transient current in a quantum dot asymmetrically coupled to metallic leads Ali Goker, Peter Nordlander We use the timedependent noncrossing approximation to study the transient current for a single electron transistor attached asymmetrically to two metallic leads. We investigate the effects of the bandwidth of the leads, the effect of dot energy level position, the effect of asymmetry in the couplings, and the effects of temperature. In the short timescale, the current reaches a maximum before it starts decaying. In the long timescale, we observe sinusoidal modulations of the current. The frequency of these oscillations is linearly proportional to the bandwidth of the conduction electrons in the leads. The amplitude of these oscillations are found to increase as the temperature is reduced and saturate for temperatures below the Kondo temperature. We discuss the microscopic nature of these oscillations and comment on the possibilities for their experimental detection. [Preview Abstract] 
Friday, March 9, 2007 9:00AM  9:12AM 
X43.00006: Positive correlation in multilevel transport through a tunable quantum dot Yiming Zhang, Leonardo DiCarlo, Douglas McClure, Michihisa Yamamoto, Seigo Tarucha, Charles Marcus, Micah Hanson, Art Gossard We report measurements of shot noise auto and cross correlation in a tunable quantum dot with two or three leads. As the Coulomb blockade is lifted at finite sourcedrain bias, the current noise evolves from superPoissonian to sub Poissonian in the twolead case, and the crosscorrelation evolves from positive to negative in the threelead case. The observed superPoissonian noise and positive crosscorrelation are shown to be consistent with transport through excited states. [Preview Abstract] 
Friday, March 9, 2007 9:12AM  9:24AM 
X43.00007: Electron Localization in Strongly Correlated Quantum Dots A. D. G\"u\c{c}l\"u, Amit Ghosal, C. J. Umrigar, Harold U. Baranger We investigate the electronic properties of quantum dots in the low density regime up to $r_s\sim60$ using variational and diffusion quantum Monte Carlo methods. Quantum dots are highly tunable systems that allow the study of the physics of strongly correlated electrons. With decreasing electronic density, interactions become stronger and electrons are expected to localize at their classical positions, as in Wigner crystallization in an infinite twodimensional system. We have studied several multideterminental wave functions each built from singleparticle states of very different nature  LDA, Hartree, or floating Gaussian orbitals  all optimized using an energy minimization technique. We study the density, pairdensity, power spectrum, and addition energy as a function of increasing interaction strength. The main physical picture that emerges is: The system (i) first experiences a competition between different possible classical configurations, namely either the magic angular momentum states of the shell structure or those of the quantum mechanical symmetry, and (ii) then finally reaches the strongly localized regime consistent with the classical ground state. [Preview Abstract] 
Friday, March 9, 2007 9:24AM  9:36AM 
X43.00008: Magneticfield evolution of collective excitations in AlGaAs/GaAs fewelectron quantum dots in the mK regime. Sokratis Kalliakos, Cesar Pascual Garcia, Vittorio Pellegrini, Aron Pinczuk, Brian S. Dennis, Loren N. Pfeiffer, Ken W. West, Massimo Rontani, Guido Goldoni, Elisa Molinari Spin transitions and interactions in fewelectron quantum dots (QDs) are investigated by resonant inelastic light scattering (ILS). Here we present the observation of intershell excitations in GaAs/AlGaAs QDs that are fabricated by combining ebeam nanolithography with high quality reactive ion etching. The interpretation of the experimental results by numerical evaluations within a full configuration interaction approach highlights the importance of the exchange and correlation effects in these systems. We show that, under the impact of a perpendicular magnetic field, the evolution of electronic spin and charge intershell excitations at mK temperatures reveal that significant changes in the ground state occur even at moderate magnetic fields. These experiments demonstrate that ILS enables the study of fewelectron effects in QDs under the extreme conditions of low temperatures and high magnetic fields. [Preview Abstract] 

X43.00009: ABSTRACT WITHDRAWN 

X43.00010: ABSTRACT WITHDRAWN 
Friday, March 9, 2007 10:00AM  10:12AM 
X43.00011: Electronic Structure of PbSe/PbS CoreShell Quantum Dots Adam Bartnik, Efrat Lifshitz, Frank Wise The electronic structure of PbSe/PbS coreshell Quantum Dots (QDs) is calculated within a 4band envelope function theory and compared to experimentally observed absorption spectra [1]. Our theory extends the isotropic effective mass approximation used successfully in PbS and PbSe core QDs [2] to be valid across discontinuous barriers in material parameters. Even though the band gaps of PbSe and PbS present a TypeII heterostructure, the model predicts that at typical QD sizes, these TypeII effects will not be seen. In fact, the wavefunctions are predicted to extend evenly over both materials except in the largest of sizes. This unusual lack of confinement is explained, and is shown to agree well with recent experimental results. [1] E. Lifshitz, et. al. Airstable PbSe/PbS and PbSe/PbSe$_{x}$S$_{1x }$coreshell nanocrystal quantum dots and their applications. \textit{Journal of Physical Chemistry B}, 2006. [2] I. Kang and F. W. Wise. Electronic structure and optical properties of PbS and PbSe quantum dots. \textit{J. Opt. Soc. Am. B}, 14(7):1632, July 1997. [Preview Abstract] 
Friday, March 9, 2007 10:12AM  10:24AM 
X43.00012: Investigation of Individual InGaAs Quantum Dots by CrossSectional Ballistic Electron Emission Microscopy (BEEM) S.Y. Lehman, J.P. Pelz, C. Marginean, J.G. Cederberg Quantum dots (QDs) in IIIV semiconductors are of great technological interest, but electronic properties of individual QDs are difficult to measure. We are using crosssectional BEEM for this purpose. Multiple layers of InAs and In$_{0.4}$Ga$_{0.6}$As QDs were grown by organometallic vapor phase epitaxy, with the thickness of deposited material ranging from subcritical to that generating 5 x 10$^{10}$ dots/cm$^{2}$. The QD layers were separated by ndoped (5 x 10$^{16}$ cm$^{3})$ GaAs/Al$_{0.3}$Ga$_{0.7}$As layers of combined thickness ranging from 130 to 190 nm. The sample was cleaved \textit{ex situ} and 5nmthick Au Schottky barrier (SB) contacts were deposited on the cleaved edge using a shadow mask [1]. UHV BEEM at 300 K was used to locate and image the QDs and adjacent layers. The local conduction band energy (as measured by the SB height) ranged from $\sim $0.7 eV at cleaved or nearsurface QDs up to $\sim $1 eV over the AlGaAs layers. Surprisingly, the BEEM current amplitude over particular QD layers appears to vary nonmonotonically with In content. Ongoing work will be discussed to use low temperature BEEM with an applied reverse bias to measure the energy depth and physical depth of the cleaved QDs. Work supported by NSF Grant No. DMR0505165. [1] C. Tivarus \textit{et al}., PRL \textbf{94}, 206803 (2005). [Preview Abstract] 

X43.00013: ABSTRACT WITHDRAWN 
Friday, March 9, 2007 10:36AM  10:48AM 
X43.00014: Composite fermion solid and liquid states in two component quantum dots Chuntai Shi, Gun Sang Jeon, Jainendra K. Jain We consider correlated states of a quantum dot, at high magnetic fields, assuming electrons with two components. This model has possible relevance to quantum dots in a bilayer system, a two valley system, in graphene, or for ordinary GaAs based quantum dots in the limit of small Zeeman energy. We show that both the liquid states and crystallites (the latter occurring at large angular momenta) are accurately described in terms of composite fermions. The residual interaction between composite fermions is important, however, and causes complex nearest and nextnearest neighbor spin correlations in the composite fermion crystallite. [Preview Abstract] 
Friday, March 9, 2007 10:48AM  11:00AM 
X43.00015: Dephasing of an electron MachZehnder interferometer capacitively coupled to a quantum dot SeokChan Youn, HeungSun Sim, HyunWoo Lee We theoretically investigate an electron MachZehnder interferometer capacitively coupled to a quantum dot. We derive the current and connect its interference behavior to the statistics of the charge fluctuation of the dot. The interference is analyzed in the two limiting cases of fast and slow charge fluctuations: For the case of fast fluctuations where the dwell time of the dot is much smaller than the flight time of the interferometer, the visibility is reduced due to the charge fluctuations, while in the opposite limit the behavior of the interference can be understood by screening effects. The connection to recent experimental and theoretical works will be discussed. [Preview Abstract] 
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