Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J11: Quantum Dots and Quantum Point Contacts |
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Sponsoring Units: DCMP Chair: Sergio Ulloa, Ohio University Room: 305 |
Tuesday, March 17, 2009 11:15AM - 11:27AM |
J11.00001: Imaging a One Dimensional Quantum Dot in an InAs/InP Nanowire Erin E. Boyd, Halvar J. Trodahl, R.M. Westervelt, Linus E. Froberg, Kristian Nilsson, Lars Samuelson Nanowires are promising contenders for use in novel spintronic and nanoelectronic devices. An InAs/InP nanowire containing a long InAs quantum dot (length$>$2xdiameter), is an ideal system to use a liquid He-4 cooled scanning gate microscope tip to probe electron behavior. This increased understanding would help in the design of quantum devices. For a few electron long dot, no excited states in the transverse direction will be occupied, making the system 1D. The electron density in these systems will change their state, as a function of dot length, from a liquid state to a Wigner-crystal like state[1]. Using a weak tip potential, it should be possible to probe this spatial distribution of the electron probability. By applying a large tip potential, and using the transitions between different partitionings of the dot, information on the relative strength of the electron interaction could be obtained. [1] Jiang Qian et. al arXiv:0809.0834 (September 2008) [Preview Abstract] |
Tuesday, March 17, 2009 11:27AM - 11:39AM |
J11.00002: Imaging Double Quantum Dots in InAs/nP Nanowires Halvar J. Trodahl, Erin E. Boyd, R. M. Westervelt, Kristian Nilsson, Linus E. Froberg, Lars Samuelson Coupled quantum dots formed in InAs/InP heterostructure nanowires are attractive candidates for nanoelectronics, spintronics and quantum information processing. The ability to manipulate the charge state of a single quantum dot defined in these nanowire systems using a low temperature scanning probe microscope (SPM) tip has been shown previously [1] and provides a tool to investigate the properties of nanowire systems down to the tens of nanometer scale. In order to realize the above applications, multiple InAs quantum dots can be formed in an InAs/InP nanowire system by using InP barriers. Using a conducting SPM tip as a movable electrostatic gate, the charge can be tuned independently on each dot of a double quantum dot defined in a semiconductor nanowire. [1] A. C. Bleszynski-Jayich, et al., PRB 77, 245327 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 11:39AM - 11:51AM |
J11.00003: Phase Coherence and Mesoscopic Coulomb Blockade in Open Quantum Dots Ileana Rau, Michael Grobis, Sami Amasha, Ron Potok, Hadas Shtrikman, David Goldhaber-Gordon The phase coherence of electrons in open systems at low temperatures leads to mesoscopic effects such as universal conductance fluctuations and weak localization of electrons. These effects are encountered in open large quantum dots and are explained by a model of non-interacting electrons. We have investigated the transport properties of a 1.5 $\mu m^2$ and a 3 $\mu m^2$ lateral GaAs/AlGaAs quantum dot in the open regime. The weak localization effect is complicated at low temperatures by the presence of residual interactions (Mesoscopic Coulomb Blockade) that persists even when the dot is coupled by one or two fully transmitting modes to each of the two leads. We present measurements of the electron dephasing rate at low temperatures in the open quantum dots and discuss how they are affected by the suppression of conductance by these Coulomb blockade effects. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J11.00004: Mesoscopic Coulomb Blockade in a Quantum Dot with Two Open QPCs Sami Amasha, Ileana Rau, Michael Grobis, Ron Potok, Hadas Shtrikman, David Goldhaber-Gordon A quantum dot consists of a confined droplet of electrons connected to an electron reservoir by two quantum point contacts (QPCs). When the conductance of each of these QPCs is less than $2e^2/h$, the dot is in the closed regime and Coulomb Blockade effects dominate the transport properties. Open quantum dots, in which QPC conductances are $2e^2/h$ or above, are generally thought to be well-described by non-interacting electron theory. While Mesoscopic Coulomb Blockade (MCB) effects can occur in a quantum dot with one open and one closed QPC, these effects are expected to be absent for quantum dots with two open QPCs. We have investigated the transport properties of a 1.5 $\mu m^2$ and a $3 \mu m^2$ lateral GaAs/AlGaAs quantum dot in the open regime and find a clear signature of MCB. We will discuss the dependence of MCB on various controllable parameters, including magnetic field, temperature, and bias. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J11.00005: Quantum Phase Transition to a Zigzag Wigner Crystal A. C. Mehta, C. J. Umrigar, A. D. Guclu, K. A. Matveev, H. U. Baranger We use Quantum Monte Carlo techniques to map out the phase diagram of interacting electrons in a quantum wire. Interacting quasi-one-dimensional systems provide excellent examples of quantum phase transitions that are tractable. Previous work gave a qualitative description of the phase diagram of a quasi-one-dimensional system [Meyer, Matveev, and Larkin, PRL 2007]. At low density, electrons confined to one dimension by a transverse harmonic potential form a linear one dimensional Wigner crystal; as the density increases, symmetry about the axis of the wire is broken and there is a transition to a quasi-one-dimensional zigzag crystal. We use variational and diffusion Monte Carlo to study the phase diagram of this system quantitatively. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:27PM |
J11.00006: Quantum Phase Transition in a single-molecule Quantum Dot N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer, F. Balestro Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a manyparticle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, can be observed in several strongly correlated materials such as heavy fermion compounds or possibly high-temperature superconductors, and is believed to govern many of their fascinating, yet still unexplained properties. In contrast to these bulk materials with very complex electronic structure, artificial nanoscale devices could offer a new and simpler vista to the comprehension of quantum phase transitions. This long-sought possibility is demonstrated by our work in a fullerene molecular junction, where gate voltage induces a crossing of singlet and triplet spin states at zero magnetic field. N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer \& F. Balestro, Quantum phase transition in a single-molecule quantum dot, Nature, 2008, 453, 633-637. [Preview Abstract] |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J11.00007: Aharonov-Bohm-type quantum interference effects in narrow gap semiconductor heterostructures R.B. Lillianfeld, R.L. Kallaher, J.J. Heremans, Hong Chen, N. Goel, S.J. Chung, M.B. Santos, W. Van Roy, G. Borghs We present experiments on quantum interference phenomena in semiconductors with strong spin-orbit interaction, using mesoscopic parallel ring arrays fabricated on InSb/InAlSb and InAs/AlGaSb heterostructures. Both external electric field effects and temperature dependence of the ring magnetoresistance are examined. Top-gate voltage-dependent oscillations in ring resistance in the absence of an external magnetic field are suggestive of Aharonov-Casher interference. At low magnetic fields the ring magnetoresistance is dominated by oscillations with h/2e periodicity characteristic of Altshuler-Aronov-Spivak (AAS) oscillations, whereas the h/e periodicity characteristic of Aharonov-Bohm (AB) oscillations persists to high magnetic fields. Fourier spectra (FS) reveal AB amplitudes on the same order as AAS amplitudes at low fields, and in some samples reveal a splitting of the AB peaks, which has been interpreted as a signature of Berry's phase. The FS are also used to quantify the temperature dependence of the oscillation amplitudes (NSF DMR-0618235, DOE DE-FG02-08ER46532, NSF DMR-0520550). [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J11.00008: Time-resolved detection of single-electron interference Simon Gustavsson, Matthias Studer, Renaud Leturcq, Thomas Ihn, Klaus Ensslin, D. C. Driscoll, A. C. Gossard We demonstrate real-time detection of single electron interference in a double quantum dot embedded in an Aharonov-Bohm interferometer, with visibility approaching unity [1]. We use a quantum point contact as a charge detector to perform time-resolved measurements of single-electron tunneling. With increased bias voltage across the quantum point contact a back-action is exerted on the interferometer leading to decoherence. We attribute this to emission of radiation from the quantum point contact, which drives electronic transitions in the quantum dots [2]. Surprisingly, the efficiency of this process depends strongly on external magnetic field, with variations occurring on a small fraction of the magnetic field scale associated with one flux quantum penetrating the ring. The unexpected features demonstrate the complex interplay between radiation, absorption and coherence in mesoscopic systems. [1] S. Gustavsson et al., Nano Lett. 8, 2547 (2008). [2] S. Gustavsson et al., PRL 99, 206804 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J11.00009: Non-equilibrium Charge Fluctuations as a Source of Inelastic Back-action in Quantum Point Contact Qubit Detectors Carolyn Young, Aashish Clerk Many recent experiments make use of a quantum point contact (QPC) as a qubit readout (e.g., of a double quantum dot (DQD) qubit). It has long been realized that QPC current fluctuations can give rise to inelastic back-action effects on the DQD [1] [2]. In contrast, the role of QPC charge fluctuations in generating inelastic back-action has not been fully studied, despite the fact that this is a more fundamental mechanism. We provide a full theoretical study of charge-noise induced inelastic back-action effects in a QPC plus DQD system, showing that these effects should be appreciable in typical experimental setups. We also discuss a novel contribution to the charge noise associated with the physics of Friedel oscillations. Finally, we discuss how the effects of charge noise back-action can be distinguished from current noise back-action in experiment. \newline [1] S. Gustavsson et al., Phys. Rev. Lett., 99, 206804 (2007).\newline [2] R. Aguado and L.P. Kouwenhoven, Phys. Rev. Lett., 84, 091987 (2000). [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J11.00010: Hole Spin Filtering by quantum point contacts Taisuke Minagawa, Yuli Lyanda-Geller We calculate the charge carrier spectra in two-dimensional hole systems (2DHS) and in quantum point contacts (QPC) formed in the 2DHS in an in-plane magnetic field B. The origin of the spin splitting for holes differs significantly from that for electrons. For bulk holes, the g-factor is defined not only by the constant of coupling of the angular momentum 3/2 to magnetic field, but also by the Luttinger constants $\gamma_1$, $ \gamma_2$ and $\gamma_3$ defining the heavy and light hole masses. In the high mobility 2DHS, the width of the quantum well (QW) L becomes comparable to the magnetic length $\lambda$ for the in-plane $B > 3$T. We find that the spin splitting for 2D holes and for holes in QPC is strongly affected by the orbital motion in the presence of the in-plane B. We developed the new approach to spectra based on confluent hypergeometric functions. We take into account the anisotropy of the Hamiltonian and calculate the spin splitting for [113] orientation of the 2DHS. For QPC spectra, configurations of in-plane B along and perpendicular to the direction of the current are studied. Our results explain many of the features of spin-resolved QPC conductance observed by Rokhinson group (PRL, ${\bf 100}$, 126401) and by Hamilton group (PRL, ${\bf 97}$, 026403). Our analysis also resolves the puzzling red shift of the Fermi energy discovered in optical spectra for QW in-plane magnetic field by Crooker group (Physica E, ${\bf 22}$:624). [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J11.00011: Ferromagnetic vs. Antiferromagnetic Correlations in a Double Dot System Manas Kulkarni, Robert Konik, Alexei Tsvelik We study a double dot system in a parallel geometry using both a large-N diagrammatic and a SBMFT approach. We consider the role of interdot ferromagnetic correlations upon the conductance. We find at the particle-hole symmetric point that the Friedel sum rule holds and the conductance vanishes. We find that the ground state of the double dot system is a singlet although the correlations between the two dots is primarily ferromagnetic. Hence we observe that the RKKY interaction does not bind the two electrons on the dots into a triplet. We compare our results to a Bethe ansatz analysis of the same system [1]. [1] R.Konik PRL 99, 076602 (2007) [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J11.00012: Magnetic Splitting of the Zero Bias Peak in a Quantum Point Contact with a Variable Aspect Ratio Tai-Min Liu, Bryan Hemingway, Andrei Kogan, Steven Herbert, Michael Melloch We have measured the nonlinear conductance of a four-gate Quantum Point Contact (QPC) device fabricated in a GaAs/AlGaAs heterostructure containing a 2-dimensional electron gas. By continuously varying the longitudinal potential profile of the QPC, we controllably create and destroy a local bound state. The nonlinear transport data show both a characteristic Coulomb blockade diamand and a zero-bias peak similar to the Kondo effect signature peak in quantum dots. We find that even when the bound state is suppressed the zero-bias peak persists. Applying an in-plane magnetic field perpendicular to the direction of the current produces a splitting of the peak which closely matches the $g$-factor data obtained via the cotunneling spectroscopy method in a separate quantum dot on the same chip. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J11.00013: Topological spin textures in strongly interacting quantum dots Jordan Kyriakidis, Catherine J. Stevenson We present results of configuration-interaction calculations on two-dimensional quantum dots confining charges with long-range Coulomb repulsion. We focus on correlation-induced spin textures formed at zero magnetic field. By looking at chiral structures, at two- and three-point spin-correlation functions, and at explicit symmetry-breaking fields, a consistent picture emerges of incipient topological spin textures formed throughout the dot and particularly at annular regions of increased electron density. In addition to singular vortex-type structures, 2$\pi$-windings are observed in the spin field along these annular regions. These textures are solely due to statistics and repulsion. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J11.00014: Spin interference in quantum rings manipulated with quantum point contacts Francisco Mireles, Leo Diago The Aharonov-Bohm (AB) and Aharonov-Casher (AC) effects are two well known interference phenomena that may appear in semiconductor quantum rings (QR's). Although the AB effect has been long observed, its counterpart, the AC effect has been only recently detected in clever magnetoconductance oscillations experiments on HgTe/HgCdTe based QR's exhibiting strong Rashba SO-interaction [1]. In this work, using the S-matrix formalism we study the role of the contacts between the leads and the QR on the AB and AC conductance oscillations of the device in the presence of Rashba and Dresselhaus type of SO interactions. We describe the backscattering and transparence of the conjunctions lead-to-ring through quantum point contacts (QPCs) modelled with gate-controllable saddle-point potentials. The variable transmitivity of the QPCs, adjusted in the experiment by gate voltages and/or applied magnetic fields, is readily incorporated in our approach. It is shown that manipulating electrostatically the confinement strength at the QPCs, may be of utility to implement a novel way to modulate spin interference effects in semiconductor quantum rings. [1] M. K\"onig {\it et al.}, PRL {\bf 96}, 076804 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J11.00015: NEGF Study of the Spontaneous Spin Polarization in a Quantum Point Contact Junjun Wan, Marc Cahay, Richard Newrock, Philippe Debray A non-equilibrium Green function formalism (NEGF) is used to study the conductance of a side-gated quantum point contact (QPC) in the presence of lateral spin-orbit coupling (LSOC) induced by the electric field due to the gradient of the lateral confining potential. A small asymmetry in the confining potential induced by difference of potential between the two side-gates (SGs) leads to an inversion asymmetry in the LSOC which triggers a spontaneous spin polarization in the QPC. In the regime of single-mode transport, the spontaneous spin polarization can reach nearly 100 {\%} when a strong electron-electron (e-e) interaction is taken into account. This leads to the occurrence of a plateau at G $\approx $0.5($e^{2}$/$h)$ in the ballistic conductance without the need of any externally applied magnetic field. Two ingredients are essential for the occurrence of the 0.5 plateau: an asymmetric LSOC and a strong e-e interaction. [Preview Abstract] |
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