Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q15: Focus Session: Spins in Semiconductors - Quantum Dots and Nuclear Spins |
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Sponsoring Units: DMP GMAG FIAP Chair: Ezekiel Johnston-Halperin, Ohio State University Room: D171 |
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q15.00001: Protecting the electron spin coherence in a quantum dot with inhomogeneously polarized nuclear spins via dynamic nuclear polarization Wenxian Zhang, Jun Zhuang, J.Q. You An electron spin in a quantum dot is decohered by its surrounding nuclear spins via hyperfine coupling. During the dynamic nuclear polarization process, when the nuclear spins are polarized inhomogeneously by repeatedly injected polarized electron spins, a fully polarized nuclear spin core is formed. As a consequence, the polarized nuclear spin core can be harnessed to protect the electron spin coherence. In this way, we find that the electron spin coherence time can be extended tens times with the total nuclear polarization as low as 20\%, in contrast to the usual requirement of 90\% polarization, in a quantum dot with 256 nuclear spins. The total nuclear polarization goes even lower for larger quantum dots. The effect of the dipolar interaction between nuclear spins is also discussed. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q15.00002: Electrically detected nuclear magnetic resonance in GaAs/AlGaAs-based quantum point contacts Zachary Keane, Matthew Godfrey, Adam Burke, Jason Chen, Sebastian Fricke, Oleh Klochan, Adam Micolich, Harvey Beere, Dave Ritchie, Kirill Trunov, Dirk Reuter, Andreas Wieck, Alex Hamilton Nuclear magnetic resonance (NMR) is a well-known technique with widespread applications in physics, chemistry and medicine. Conventional NMR studies use inductive coils to detect the magnetic field produced by precessing nuclear spins; this approach requires on the order of $10^{12}$ spins for detection. Recently, resistive detection of NMR through the hyperfine interaction has been demonstrated with electrons in mesoscopic 2- and 1-dimensional devices based on high-quality GaAs/AlGaAs heterostructures. These studies are typically sensitive to $10^8$ spins, enabling NMR on much smaller sample volumes. Holes are predicted to have much weaker nuclear spin coupling than electrons, which could be relevant to the emerging fields of spintronics and quantum information processing. We present a preliminary comparison between the magnitude of the NMR signal in electron and hole quantum point contacts. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q15.00003: Enhancing 29Si Dynamic Nuclear Polarization Through Microwave Frequency Modulation Maja Cassidy, Men Young Lee, Charles Marcus We demonstrate up to a four-fold enhancement in the dynamic nuclear polarization (DNP) of silicon particles by applying an a.c. modulation to the microwave frequency used for irradiation of the electron spin system. The DNP enhancement is studied at temperatures ranging from 2-20 K and across a range of microwave powers. The total nuclear polarization is found to increase with decreasing temperature and increasing microwave power however, surprisingly, the polarization enhancement increases as the temperature is increased. The DNP enhancement is seen to increase with polarization time and is highest in spin-diffusion regime of polarization. By varying the frequency and amplitude of the applied modulation, dynamics of the electron spin system are probed. We find that the highest polarization enhancements are achieved with the frequency is modulated at a rate much greater than the electron spin lattice relaxation rate, where higher order electron spin processes can contribute to the polarization process. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q15.00004: Dynamic Nuclear Polarization in Double Quantum Dots Invited Speaker: We theoretically investigate the controlled dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons.\footnote{M. Gullans, et. al., Phys. Rev. Lett. 104, 226807 (2010).} Three regimes of long-term dynamics are identified, including the build up of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with formation of so-called ``dark states,'' and the elimination of the difference field. We show that in the case of unequal dots, build up of difference fields generally accompanies the nuclear polarization process, whereas for nearly identical dots, build up of difference fields competes with polarization saturation in dark states. The elimination of the difference field does not, in general, correspond to a stable steady state of the polarization process. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q15.00005: Spin-dependent tunneling into an empty lateral quantum dot Peter Stano, Philippe Jacquod In a recent experiment [Phys. Rev. B {\bf 78}, 041306(R) (2008)] Amasha {\it et al.} reported a strong spin dependence of the rate for electrons to tunnel into an empty quantum dot in a Zeeman field. Such dependence is intriguing, as one expects tunneling rates to depend on the orbital structure of the wavefunction, over which a Zeeman field has no effect. In search for an explanation, we find two mechanisms leading to a spin-dependent tunneling rate. The first originates from different electronic $g$-factors in the lead and in the dot, and favors the tunneling into the spin ground (excited) state when the $g$-factor magnitude is larger (smaller) in the lead. The second is triggered by spin-orbit interactions via the opening of off-diagonal spin-tunneling channels. It systematically favors the spin excited state. Numerically modeling the experimental setup, we find that in GaAs the spin-orbit interaction is unable to explain the experimental results, as it leads to no more than a $\sim$10\% discrepancy in the spin up vs spin down tunneling rates. We conjecture that the significantly larger discrepancy observed experimentally originates from the enhancement of the $g$-factor in the laterally confined lead. Reference: P. Stano and Ph. Jacquod, Phys. Rev. {\bf B} 82, 125309 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q15.00006: Theory of spin blockade in a triple quantum dots Chang-Yu Hsieh, Yun-Pil Shim, Pawel Hawrylak We present a theory of electronic properties and spin blockade in a linear triple quantum dots. We use micoroscopic LCHO-CI and double-band Hubbard model to analyze the electronic and spin properties of a triple quantum dots near a symmetrical quadruple point involving the (1,1,1) configuration which is essential for implementing quantum information processing with electron spin. We calculate spectral functions and relate them via the rate equation, including coupling with a phonon bath, to current as a function of applied bias. We show that the spin blockade in a triple quantum dots can serve as a spectroscopic tool to distinguish spin polarized states from spin depolarized states. We also show that a spin blockade is developed only at high bias when an onsite triplet state on the edge quantum dot connected to the source lead becomes accessible in the transport window. In contradiction to the case of double quantum dot molecule, the onsite triplet is not only essential for lifting spin blockade but also important for building up spin polarisation and spin blockade in the system. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q15.00007: Spin-polarized current generation in multiterminal quantum dot in Kondo regime Mikio Eto, Tomohiro Yokoyama We theoretically study the generation of spin-polarized current in a quantum dot with strong spin-orbit interaction, such as InAs quantum dot. As a minimal model, we consider two energy levels in a quantum dot, which is connected to $N$ leads through tunnel barriers. When an unpolarized current is injected from a lead, spin-polarized currents are ejected to other leads in the case of $N \ge 3$. First, we show that the spin polarization of the output currents is markedly enhanced by resonant tunneling, around current peaks of Coulomb oscillation, when the level spacing in the dot is smaller than the level broadening. Next, we examine the many-body resonance induced by the Kondo effect in the Coulomb blockade regime. A large spin current is created in the presence of the SU(4) Kondo effect when the level spacing is less than the Kondo temperature.\footnote{M.\ Eto and T.\ Yokoyama, J.\ Phys.\ Soc.\ Jpn., in press; arXiv:1010.5956.} [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q15.00008: Spin blockade in the optical response of a charged quantum dot Eleftheria Kavousanaki, Guido Burkard We theoretically model the population dynamics in a semiconductor quantum dot charged with a single electron in an optical pump-probe setup when the two lowest quantum dot levels are photoexcited. We calculate the differential transmission spectrum as a function of the time delay between the two circularly polarized optical pulses by using a density matrix formalism and treating intraband relaxation with the Lindblad equation. Taking into account both spin conserving and spin-flip relaxation processes we investigate the possibility for spin-dependent blocking of intraband relaxation due to the presence of the ground state electron for zero and finite magnetic fields. We show that the differential transmission spectrum is initially dominated by the fast spin-conserving mechanism before the slower spin-flip processes start to contribute at longer time scales. As a consequence of spin conservation for short time scales, we find a spin blockade effect in the optical recombination process. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q15.00009: Simulating electron spin entanglement in a double quantum dot M.A. Rodriguez-Moreno, A.D. Hernandez de la Luz, Lilia Meza-Montes One of the biggest advantages of having a working quantum-computing device when compared with a classical one, is the exponential speedup of calculations. This exponential increase is based on the ability of a quantum system to create and operate on entangled states. In order to study theoretically the entanglement between two electron spins, we simulate the dynamics of two electron spins in an electrostatically-defined double quantum dot with a finite barrier height between the dots. Electrons are initially confined to separated quantum dots. Barrier height is varied and the spin entanglement as a function of this variation is investigated. The evolution of the system is simulated by using a numerical approach for solving the time-dependent Schr\"{o}dinger equation for two particles. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q15.00010: Exchange coupling between hole qubits and between electron qubits in quantum dot molecules Alex Greilich, Stefan C. Badescu, Danny Kim, Allan S. Bracker, Daniel Gammon The exchange interaction between electron spins has been a paradigm for solid-state implementation of quantum gates. Holes are receiving an increasing attention for their reduced hyperfine coupling as compared to electrons in III-V semiconductors. Besides the isotropic exchange, both electrons and holes couple through spin-nonconserving interactions. Here we present detailed experimental evidence of these interactions for electrons and for holes in stacked InAs/GaAs quantum dots, achieved through electrical and magnetic fields that induce energy level resonances. Particularly large spin-mixing effects are found for holes, which involve their multi-band structure. We provide a theoretical understanding of the essential mixing mechanisms involved, tracing them down to system asymmetries and inhomogeneities. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q15.00011: Chiral spin currents and spectroscopically addressable single merons in quantum dots Catherine Stevenson, Jordan Kyriakidis We provide unambiguous theoretical evidence for the formation of correlation-induced isolated merons in rotationally-symmetric quantum dots beyond the lowest-Landau-level approximation. For experimentally accessible system parameters, unbound merons condense in the ground state at magnetic fields as low as B* = 0.3 T and for as few as N = 3 confined fermions. The four-fold degenerate ground-state at B* corresponds to four orthogonal merons characterised by their winding number $\pm$1 and topological charge $\pm$1. This degeneracy is completely lifted by the Rashba and Dresselhaus spin-orbit interactions, yielding spectroscopic accessability to individual merons. We further derive a closed-form expression for the topological chirality in the form of a chiral spin current and use it to both characterise our states and predict the existence of other topological textures in other regions of phase space. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q15.00012: Magnetoconductance of a Single-Electron Transistor in the Kondo Regime Tai-Min Liu, Bryan Hemingway, Andrei Kogan, Steven Herbert, Michael Melloch, Theo A. Costi We have measured the zero-bias conductance of a Single-Electron Transistor (SET) in the Kondo regime as a function of temperature, $T,$ and magnetic field, $B,$ oriented parallel to the plane of the device. Our SETs are fabricated on a GaAs/AlGaAs heterostructure with electron sheet density $4.8\times 10^{11}$ cm$^{-2}$ and mobility $5\times 10^5$ cm$^2$V$^{-1}$s$^{-1}$. Scaled plots of both the $T$ and $B$-dependent data show universal behavior. At moderate and high $B$, the magnetoconductance data show good agreement with renormalization group calculations in the spin-1/2 Kondo regime. At very low $B$, we observe a non-monotonic behavior: as $B$ increases, the conductance initially increases and only starts to decrease at a finite $B$. A possible explanation of this effect due to the presence of multiple orbital dot levels with similar energies will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q15.00013: Measuring mesoscopic spin currents by spin-to-charge conversion Philippe Jacquod, Peter Stano A number of theoretical investigations show that spin currents can be magneto-electrically generated by passing electric currents through spin-orbit coupled quantum dots. Measuring these currents has however not been achieved to date. In this talk, we present a theoretical proposal for measuring such mesoscopic spin currents with a voltage probe connected to the quantum dot via a single channel quantum point contact. We demonstrate that a spin current flowing through the quantum point contact results in an odd dependence of the charge current $I_{\rm qpc}$ on an externally applied Zeeman field, while this response is even in the absence of the spin current. The magnitude of the spin current is proportional to the zero field derivative of $I_{\rm qpc}$, with a constant of proportionality depending weakly on the geometry of the point contact. Numerical estimates suggest that in this way, mesoscopic spin currents can successfully be measured in GaAs quantum dots. [Preview Abstract] |
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