Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B35: Focus Session: Spins in Semiconductors -- Quantum Dots |
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Sponsoring Units: GMAG DMP FIAP Chair: Kohei Itoh, Keio University Room: E145 |
Monday, March 15, 2010 11:15AM - 11:51AM |
B35.00001: Hole - Nuclear Spin Interaction in Semiconductor Quantum Dots Invited Speaker: Xavier Marie Spins of localized electrons in semiconductor quantum dots (QDs) are attractive for future spintronic and quantum information devices since they are not subject to the classical spin relaxation mechanisms known for free carriers [1]. It is now well established that the main spin dephasing mechanism in QDs is due to the coupling of conduction electron spin with the randomly fluctuating nuclear spins (Fermi contact term) [2-5]. For a valence electron (or hole), this coupling is expected to be much weaker because of the p-symmetry of the valence band states and no experimental evidence of such a hole-nuclear spin interaction has been reported so far [6]. We have measured the carrier spin dynamics in p-doped InAs/GaAs quantum dots by pump probe and time-resolved photoluminescence experiments. We demonstrate that the hole spin dynamics in these QDs is governed by the interaction with randomly fluctuating nuclear spins [7]. Our calculations based on dipole-dipole coupling between the hole and the quantum dot nuclei lead to a hole spin dephasing time for an ensemble of dots of 15 ns in close agreement with experiments.\\[4pt] In collaboration with B. Eble, C. Testelin, F. Bernardot, and M. Chamarro, Institut des Nanosciences de Paris, Universit\'{e} P. et M. Curie, CNRS, Paris, F-75015 France; A. Balocchi, T. Amand, and B. Urbaszek, Universit\'{e} de Toulouse~; LPCNO, INSA-CNRS-UPS, 135 av. de Rangueil, 31077 Toulouse Cedex 4, France; and A. Lema\^{\i}tre, Laboratoire de Photonique et de Nanostructures, CNRS, Route de Nozay, F-91460, Marcoussis, France. \\[4pt] [1] \textit{Spin Physics in Semiconductors}, edited by M. D'Yakonov (Springer, Berlin, 2008) \\[0pt] [2] I. Merkulov \textit{et al}, Phys. Rev. B \textbf{65}, 205309 (2002) \\[0pt] [3] P.-F. Braun, X. Marie \textit{et al}, PRL \textbf{94}, 116601 (2005) \\[0pt] [4] A. C. Johnson \textit{et al} , Nature \textbf{435}, 925 (2005) \\[0pt] [5] A. Greilich e\textit{t al}, Science \textbf{313}, 341(2006) \\[0pt] [6] S. Laurent \textit{et al}, Phys. Rev. Lett. \textbf{94}, 147401 (2005) \\[0pt] [7] B. Eble \textit{et al}, Phys. Rev. Lett. \textbf{102}, 146601 (2009) [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B35.00002: Optically-Detected Electron Spin Resonance of Self-Assembled InAs Quantum Dots John S. Colton, Aaron Mitchell Jones, Steve Brown, Scott Thalman, Dallas Smith, Allan Bracker Optically-detected magnetic resonance (ODMR) of electron spins has been performed on self-assembled InAs quantum dots. A cw probe laser was used to monitor the Kerr rotation of a subset of the dots (those resonant with the laser) as microwaves induce transitions between spin states, separated by $\sim $12 GHz at $\sim $1.8 T. At high powers (optical and microwave), the ODMR response seems to be a jumble of peaks superimposed on each other. At low powers, however, individual peaks can be resolved. And, at the lowest power a single ODMR peak is evident. The $g$-factor and $T_{2}^{\ast }$ value obtained from the lowest power ODMR peak position and width were $\vert g\vert $ = 0.485 and $T_{2}^{\ast }$ = 3 ns. This lifetime is consistent with hyperfine effects rather than inter-dot inhomogeneity being the limiting factor. By way of comparison, measurements from time-resolved Kerr rotation indicated a T$_{2}^{\ast }$ of 1 ns at small fields, which decreased to less than 0.5 ns at 2 T due to inhomogeneities in the g-factor. Both of these observations are consistent with the hypothesis that at the lowest powers, we are seeing a response from a very small number of homogeneous quantum dots, possibly even an individual quantum dot. [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B35.00003: Spin relaxation in InAs QDs Andreas Russ, Lars Schweidenback, Mesut Yasar, Athos Petrou, George Kioseoglou, Connie Li, Berend Jonker, Marek Korkusinski We have studied the circular polarization of the electroluminescence emitted from Fe spin-LEDs which incorporate a single layer of InAs QDs at the center of the device as function of magnetic field applied along the growth axis. The circular polarization below 50 K shows a dramatic drop around 5 tesla, indicating the presence of an efficient spin relaxation mechanism that is tuned by the externally applied magnetic field. Calculations indicate that such a mechanism exists at $B$ = 5 tesla if the QDs are populated by 3 electron-hole pairs. The spin relaxation mechanism consists of a two step process. In the first step we have spin relaxation via spin-orbit interaction; in the second step we have energy relaxation via phonon emission. It is assumed that the QDs are anisotropic. This model was put to the test by studying a spin-LED which has a lower QD density and in which we can control the number of electron-hole pairs occupying each QD by changing the bias voltage. The predicted resonance was observed in this experiment verifying the proposed theoretical model. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:27PM |
B35.00004: Measurement of spin relaxation time in InGaAs double quantum dots Victoria Russell, Karl Petersson, Ian Farrer, Francois Sfigakis, Stuart Holmes, Crispin Barnes, David Anderson, Geb Jones, Charles Smith, David Ritchie, Michael Pepper A proposed spin qubit consists of a single electron residing in a material of selected electron g-factor. For successful implementation the spin relaxation time, T$_{1}$, must be sufficiently long to enable multiple operations. Previously, double quantum dots (DQD) have been used for the detection of spin states. We report measurements of T$_{1}$ in a DQD in In$_{x}$Ga$_{1-x}$As (x = 0.1, 0.2) with g-factors of $\approx$ -0.9 and -1.6 respectively, displaying larger spin-orbit (SO) coupling effects. DQDs are measured in the Pauli spin blockade regime using RF charge detection techniques. Using high frequency pulses applied to electrostatically defined gates T$_{1}$ is calculated from resulting visibility of the triplet spin state. Magnetic field probes the effect of the SO compared to the hyperfine interaction on T$_{1}$. With larger g-factor, splitting of the singlet-triplet energy levels, so supressing hyperfine-mediated relaxation, occurs at comparatively smaller magnetic field and may compensate for increased SO effects. Thus T$_{1}$ in InGaAs is shown to be sufficient for quantum computation applications. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B35.00005: Two-electron spin relaxation in a double quantum dot Chia-Wei Huang, Massoud Borhani, Xuedong Hu Motivated by recent experiments done on GaAs gate-defined double quantum dots (DQD)\footnote{J. R. Petta, \textit{et al}. Science 309,2180(2005)}, we study the two-electron spin relaxation in DQD at various energy detuning $\varepsilon$ and magnetic fields. In particular, we focus on two limits of interdot energy detuning. For $\varepsilon>0$, we investigate the spin blockade by evaluating the electron spin relaxation rate from low energy (1,1) triplet states to the ground (0,2) singlet state. For $\varepsilon<0$, we calculate the electron spin relaxation time at the S(1,1)-$\mathtt{T}{}_{0}$ degeneracy to identify the leakage of an ST$_{0}$ qubit. Particularly, we explore the relaxation channel due to the hyperfine interaction and the electron-phonon interaction. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B35.00006: A quantitative study of spin-flip cotunneling transport in a quantum dot Tai-Min Liu, Anh T Ngo, Bryan Hemingway, Sergio Ulloa, Michael Melloch, Steven Herbert, Andrei Kogan We report detailed transport measurements in a quantum dot in a spin-flip cotunneing regime, and a quantitative comparison of the data to microscopic theory [1]. The quantum dot is fabricated by lateral gating of a GaAs/AlGaAs heterostructure, and the conductance is measured in presence of an in-plane Zeeman field. We are focusing on the ratio of the nonlinear conductance values at bias voltages exceeding the Zeeman threshold, a regime that permits a spin flip on the dot, to those below the Zeeman threshold, when the spin flip on the dot is energetically prohibited. The data obtained in three different oddly-occupied dot states show a good quantitative agreement with the theory with no adjustable parameters. We also compare the theoretical results to the predictions of a phenomenological form used previously for the analysis of non-linear co-tunneling conductance and specifically the determination of a heterostructure g-factor, and find a good agreement between the two. [1] J. Lehmann and D. Loss, Phys. Rev. B. 73,045328 (2006). [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B35.00007: Kondo effect in a quantum dot doped with a $Mn^{2+}$ ion Edson Vernek, Fanyao Qu, Fabricio F.M. Souza, Enrique V. Anda, J. Carlos Egues We present a study of a quantum dot (QD) with a single $Mn^{2+}$ ion implanted and coupled to normal metallic leads [1]. As a result of the coupling to the leads, single QD is known to exhibit the well known Kondo effect, which emerges at temperatures below the so called Kondo temperature ($T_K$). In the present work we investigate the interplay between the Kondo effect and the ferromagnetic (FM) coupling ($J$) between a spin of the electrons in the QD and the spin of the Mn ion. We show that depending of the ratio $J/T_K$ the ground state of the system is a local FM (non-Kondo) state or a Kondo state of the QD, in which case the $Mn^{2+}$ decouples from the rest of the system.\\[4pt] [1] L. Besombes et al Phys. Rev. Lett. {\bf 93} 207403. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B35.00008: Coupling an electron spin to a cavity Xuedong Hu, Yuxi Liu, Franco Nori We investigate coupling an electron spin in a semiconductor quantum dot to a cavity via the electrically driven spin resonance technique. Spin degree of freedom is accessed here through either spin-orbit interaction or inhomogeneous magnetic field. We derive the spin-photon coupling Hamiltonian, and assess the coupling strength from cavity and quantum dot parameters. Based on these considerations, we identify parameter regimes that are best suited for reaching the strong-coupling regime for the spin-cavity system. [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B35.00009: Coherence time enhancement of spin qubits in quantum dots Ramin Abolfath, Thomas Brabec We present the magnetic phase diagram of artificial H2 molecule in lateral quantum dots doped with magnetic impurities and investigate the possibilities in using their magnetic moments as qubit for the source of entanglement in quantum computer devices. An exact diagonalization method shows that the exchange coupling between magnetic impurities mediated by electrons changes sign following the electron singlet-triplet transition as a function of external magnetic field and plunger gate voltage [1]. We investigate the possibilities in increasing the qubit coherence time, using magnetic impurities. The localized d-electrons in magnetic impurities interact more weakly with host semiconductor nuclei and spin-orbit coupling in comparison with electrons confined in quantum dots. As a result, an increase in coherence time of up to an order of magnitude appears to be feasible. [1] Ramin M. Abolfath, PRB 80, 165332 (2009) [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B35.00010: Measurement of the Spin Relaxation Time T$_{1}$ of Single Electrons in a Silicon MOS-Based Quantum Dot Ming Xiao, Matthew House, Hongwen Jiang Spin relaxation time T$_{1}$ is an important measure of the interaction between a two-level quantum system and its environment. Measurement of T$_{1}$ for individual electrons in silicon based quantum dots has been long awaited. In this talk, we present such a measurement in an electrostatically-confined quantum dot (QD) on Si MOS based materials. Excited-state spectroscopy of the QD was performed using a charge sensing technique for identifying energy levels. T$_{1}$ was subsequently measured in the time-domain with a pump-and-probe method. We measured T$_{1}$ for spin-flip transitions between two magnetic field induced Zeeman sublevels and between singlet-triplet states, for an odd and even number of electrons respectively. For the QD that contains an unpaired spin, we found that T$_{1}$ depended strongly on the applied field. Possible mechanisms leading to the observed spin relaxation will be discussed. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B35.00011: Single-shot readout of an electron spin in silicon Andrea Morello, Jarryd Pla, Floris Zwanenburg, Kok Wai Chan, Hans Huebl, Christopher Nugroho, Changyi Yang, Jessica van Donkelaar, Andrew Alves, David Jamieson, Christopher Escott, Lloyd Hollenberg, Robert Clark, Andrew Dzurak The electron spin of a donor in silicon is an excellent candidate for a solid-state qubit. It is known to have very long coherence and relaxation times in bulk, and several architectures have been proposed to integrate donor spin qubits with classical silicon microelectronics. Here we show the first experimental proof of single-shot readout of an electron spin in silicon. This breakthrough has been obtained with a device consisting of implanted phosphorus donors, tunnel-coupled to a silicon Single-Electron Transistor (Si-SET), where the SET island is used as a reservoir for spin-to-charge conversion. The charge transfer signals are exceptionally large, and allow time-resolved measurements of spin-dependent tunneling on a $\sim $10 $\mu $s scale. By measuring the occurrence of excited spin states as a function of wait time, we find spin lifetimes up to $\sim $1 s at B=1.75 T. Further experiments are underway to integrate this readout method with coherent spin control. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B35.00012: Shot-Noise in a Quantum Dot as a Spin-current Diode F.M. Souza, P.H. Penteado, C.A. Merchant, N. Markovic, J.C. Egues Shot-noise is an unavoidable non-equilibrium current fluctuation that arises from the granularity of the electron charge. In the present work, we investigate shot-noise for the recently proposed spin diode system (1,2). This consists of a quantum dot coupled to two metallic leads, one nonmagnetic (NM) and another ferromagnetic (FM). In the Coulomb blockade regime this system displays a spin-diode effect (1,2), which has recently been probed in a carbon nanotube based quantum dot (2). Our calculation shows that the shot-noise provides a robust signature for this spin-polarization rectification effect. In the bias range for which the current polarization is zero the shot-noise is super-Poissonian. In contrast, for voltages such that the current is spin polarized, the shot-noise becomes sub-Poissonian. Hence shot noise can provide an interesting additional tool to probe spin-polarized transport in these systems. We shall also discuss recent experimental progress in this direction (3). (1) F. M. Souza, J. C. Egues, and A. P. Jauho, Phys. Rev. B 75, 165303 (2007). (2) C. A. Merchant and N. Markovic, Phys. Rev. Lett. 100, 156601 (2008). (3) C. A. Merchant and N. Markovic, J. Appl. Phys. 105, 07C711 (2009). [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B35.00013: Geometric Correlations and Breakdown of Mesoscopic Universality in Spin Transport I. Adagideli, Ph. Jacquod, M. Scheid, M. Duckheim, D. Loss, K. Richter Although the spin Hall effect is by now relatively well understood in bulk diffusive systems, an extension of the theory to mesoscopic quantum dots have proven elusive. Because of its underlying geometric structure, average spin Hall conductance is not described by conventional statistical methods that are used to describe charge transport in quantum dots, such as random matrix theory. In this work, we develop a unified semiclassical theory of transport that is capable of describing how charge currents in mesoscopic systems, be they diffusive or ballistic, induce spin currents and vice versa. Using this theory, we show that, while the charge transport is universal in spin-orbit coupled quantum dots, the geometrical spin correlations break the universality and generate finite average spin conductances. Our results on spin conductance extend beyond the well-known diffusive limit to the strong spin-orbit regime (i.e.~when the spin rotation time is shorter than the momentum relaxation time), for which there have been no previously available analytical results. [Preview Abstract] |
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