Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z27: Focus Session: Semiconductor Qubits - Theory and Experiment |
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Sponsoring Units: GQI Chair: Xuedong Hu, SUNY Buffalo Room: C155 |
Friday, March 25, 2011 11:15AM - 11:27AM |
Z27.00001: Quantum dot charge stability diagram from a generalized Hubbard model Xin Wang, Shuo Yang, Sankar Das Sarma We develop a theory for the charge stability diagram in solid state quantum dot spin qubits using a general form of the Hubbard model. We argue that the extended Hubbard model (with both on-site and inter-site Coulomb repulsion) is the minimal model to describe the system. The appropriate parameters of the Hubbard model can be read off by comparing our theoretically derived results with the experimental charge stability plots. We make predictions on how the charge stability diagram depends on various parameters of the Hubbard model, especially the spin-exchange and hopping energies. [Preview Abstract] |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z27.00002: Microscopic theory for the charge stability diagram of coupled quantum dot systems Shuo Yang, Xin Wang, Sankar Das Sarma We present a quantitative microscopic theory for the charge stability diagram of coupled quantum dot systems. Using the configuration interaction method we obtain a generalized Hubbard model, from which the charge stability diagram is calculated and compared with experiments. We establish an exact connection between experimental measurements and the microscopic theory, and predict some experimentally observable quantum effects. We also map the classical capacitance model to the extended Hubbard model, and argue that the effect of spin-exchange and various hopping terms cannot be expressed in the capacitance model. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z27.00003: Formation and electrical characterization of directed self-assembled Ge/Si quantum dot Dongyue Yang, Chris Petz, Jeremy Levy, Jerrold Floro Directed self-assembly of sub-10 nm Ge islands is a candidate for producing laterally coupled quantum dot molecules with geometrically-defined spin exchange couplings. The islands are created by the nucleation of Ge islands on nanoscale SiC templates defined by direct-write electron-beam lithography.\footnote{O. Guise, J. Ahner, J. John T. Yates, V. Vaithyanathan, D. G.Schlom, J. Levy, Appl. Phys. Lett. 87, 1902 (2005).} Ge islands are coupled through ohmic contacts to the Si capping layer, and geometries can be defined that are suitable for either vertical or lateral transport.We describe low-temperature magneto-transport measurements on individual and small arrays of Ge islands grown on semi-insulating silicon substrates. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z27.00004: The Kondo Effect in a Double Quantum Dot Sami Amasha, Ileana Rau, Andrew Keller, Jordan Katine, Hadas Shtrikman, David Goldhaber-Gordon A quantum dot consists of a confined droplet of electrons connected to electron reservoirs by tunnel barriers. When the dot has an odd number of electrons it has a net spin. The electrons in the reservoir can screen this spin via the Kondo effect, which corresponds to a many-body, highly correlated electron state. We study a lateral GaAs/AlGaAs double quantum dot, where one or both of the dots can be in the Kondo regime. The dots are also coupled to each other, and this inter-dot interaction can compete with the Kondo effect. We report transport measurements in this system at low electron temperatures and for a variety of inter-dot couplings. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z27.00005: Exhibition of tunnel coupling of negatively charged dangling bonds on Si Surface Using Scanning Tunneling Microscope M. Baseer Haider, L. Livadaru, J. Pitters, R. Wolkow We have performed Scanning tunneling microscopy study of hydrogen terminated Si (100). We will show that single Si atoms in a solid state environment can be served as quantum dots. These negatively charged quantum dots can be tunnel coupled to the nearby Si quantum dots. We will demonstrate that this tunnel coupling can be controlled by adjusting the separation between the two Si atomic quantum dots. Moreover electron occupation in the tunnel coupled Si quantum dots can be controlled. We have used this tunnel coupling effect of Si atomic quantum dots to fabricate Quantum Cellular Automata Cells. Quantum Cellular Automata are used to transmit binary information through electrostatic interaction between adjacent cells without the transfer of charge from one cell to the next. Devices based on Quantum Cellular Automata will consume much less power compared to the conventional transistor based devices. Moreover, since there is no transfer of charge so power dissipation during its operation is minimal compared to conventional semiconductor devices. This Si based Quantum Cellular Automat Cell works at room temperature. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z27.00006: The RKKY Interaction and the Nature of the Ground State of Double Dots in Parallel Manas Kulkarni, Robert Konik We argue through a combination of slave boson mean field theory and the Bethe ansatz that the ground state of closely spaced double quantum dots in parallel are Fermi liquids. We do so by studying the dots conductance, impurity entropy, and spin correlation. In particularly we find that the zero temperature conductance is characterized by the Friedel sum rule, a hallmark of Fermi liquid physics, and the impurity entropy vanishes in the limit of zero temperature, indicating the ground state is a singlet. This conclusion is in contradistinction to a number of numerical renormalization group studies. We suggest a possible reason for the discrepancy. Our findings are also consistent with a 1/N diagrammatic approach to the same setup. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z27.00007: Frequency-dependent Fano factor of multilevel systems with inelastic decay processes Farzad Qassemi, Bill Coish, Joakim Bergli, Frank K. Wilhelm We study the frequency-dependent noise of electrons passing through a multilevel quantum dot or molecule accounting for ``dark'' states through which current is prohibited and inelastic transitions between the levels. Our theory results in simple closed-form expressions directly relating the frequency-dependent noise to inelastic decay rates in the limit where the rates are widely separated. To demonstrate the method, we apply it to evaluate the shot noise for electrons passing through single and double quantum dots in the presence of multiple spin decay mechanisms. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z27.00008: Electron Localization in the Inhomogeneous Electron Gas: Quantum Point Contacts Abhijit C. Mehta, Cyrus J. Umrigar, A. Devrim Guclu, Harold U. Baranger We use Quantum Monte Carlo (QMC) techniques to investigate the behavior of electrons in an inhomogeneous quasi-one-dimensional wire as a model of quantum point contact geometries. Previous QMC work by Guclu et al. demonstrated that electrons can be strongly localized in quantum point contacts, and this result was reproduced by Welander et al. using LSDA calculations. We model a quantum point contact as a constriction in a quantum ring, and we use variational and diffusion Monte Carlo to investigate the effects of different point contact lengths and geometries on the electronic properties of the QPC. A key issue is how robust the previous results are to the length of the constriction, the depth and steepness of the confining potential, and to increasing the density of the electrons in the high-density lead region. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z27.00009: Electron pair tunneling resonance in a double-dot interferometry Jinhong Park, H.-S. Sim It is difficult to experimentally detect an electron pair tunneling resonance in a quantum dot with repulsive Coulomb interactions, since it is usually masked by lower-order single-electron tunneling processes. We propose to use an Aharonov-Bohm interferometry consisting of two quantum dots for the detection. We find that in the second harmonics of the interference current, pair tunneling processes give a leading non-monotonous contribution around the bias voltages at which pair tunneling resonances appear. The second-harmonics differential conductance shows the signal of a pair tunneling resonance as well as the destructive interference of two pair tunneling resonances. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z27.00010: Giant current fluctuations in an overheated single-electron transistor Matti Laakso, Tero Heikkil\"a, Yuli Nazarov Interplay of cotunneling and single-electron tunneling in a thermally isolated single-electron transistor leads to peculiar overheating effects. In particular, there is an interesting crossover interval where the competition between cotunneling and single-electron tunneling changes to the dominance of the latter. In this interval, the current exhibits anomalous sensitivity to the effective electron temperature of the transistor island and its fluctuations. We present a new theoretical method for the study of the temperature fluctuations and induced fluctuations of other quantities, e.g., current, based on the Fokker--Planck equation. We apply this method to the study of the current and temperature fluctuations in an overheated SET around the crossover interval. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z27.00011: Electron exchange between quantum dot and ring by jumping in magnetic field Igor Filikhin, Sergei Matinyan, James Nimmo, Branislav Vlahovic Semiconductor heterostructures as quantum dots (QD) or quantum rings (QR) demonstrate discreet atom-like energy level configuration. In the presented work we show that in the weak coupled Double Concentric Quantum Ring (DCQR) electron position jumping can exist due to the energy level crossing. We study DCQR composed of GaAs in an Al$_{0.70}$Ga$_{0.30}$As substrate under influence of magnetic field. In our model the DCQR is considered in three dimensional space within single sub-band effective mass approach [1]. Magnetic field is applied in z direction, perpendicular to the DCQR plane. The electron position in DCQR is defined by effective radius which is radius of most probable localization of a single electron. We study electron structure of QD located at the center of QR. The electron position jumping between QD and QR is considered. Discussed will be possibility of experimental implementations of the jumping effect for composite object of QD and QR. \\[4pt] [1] I. Filikhin, V. M. Suslov and B. Vlahovic, Phys. Rev. B 73, 205332 (2006). [Preview Abstract] |
Friday, March 25, 2011 1:27PM - 1:39PM |
Z27.00012: Quantum phase transition of light as a control of the entanglement between interacting quantum dots Angela Barragan, Carlos Vera-Ciro, Ian Mondragon-Shem We study coupled quantum dots arranged in a photonic crystal, interacting with light which undergoes a quantum phase transition. At the mean-field level for the infinite lattice, we compute the concurrence of the quantum dots as a measure of their entanglement. We find that this quantity smoothly changes in the vicinity of the phase transition, and in a step-like fashion in the Mott-insulator phase. This behavior can be externally monitored through the second-order correlation function for the light in each lattice site. For the finite case, we discuss boundary induced effects using a mean-field ansatz, as well as the impact of having finite temperatures on the entanglement of the quantum dots. [Preview Abstract] |
Friday, March 25, 2011 1:39PM - 1:51PM |
Z27.00013: First-principles study of the energy and spin structure of excited states of NV$^{- }$center in diamond and its corresponding Hubbard model parameters Sangkook Choi, Manish Jain, Steven G. Louie A negatively charged nitrogen-vacancy pair defect (NV) in diamond is one of the promising candidates to embody a qubit for quantum computation in solid states. It is an individually addressable quantum system that may be initialized, manipulated, and measured with high fidelity at room temperature due to a long coherence time of the spin in the ground states and long-life time of the excited states. The knowledge of the electronic and spin structures of the NV center in the ground as well as excited state is crucial in understanding them. Here, we evaluate the energies and spin structures of its excited states employing the first-principles GW-BSE methods. We further obtain the Hubbard model parameters for this defect system by comparing the excited-state energies from our ab-initio GW-BSE calculation with those from the model Hamiltonian. This work was supported by NSF Grant No. DMR10-1006184, the U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by DOE at LBNL's NERSC facility. [Preview Abstract] |
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