Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N10: Focus Session: Spin Transport and Dynamics in Quantum Dots |
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Sponsoring Units: DMP GMAG Chair: Michael Flatte, University of Iowa Room: LACC 153B |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N10.00001: g tensor modulation resonance and single-spin manipulation in semiconductor quantum dots Joseph Pingenot, Craig E. Pryor, Michael E. Flatt\'e We explore how electric fields can be used to drive single spin resonance in quantum dots without AC magnetic fields. We calculate the g tensor for a single electron in a semiconductor quantum dot as a function of electric field along the growth direction of the dot. The calculations are based on an eight-band envelope-function formalism[1]. The growth-direction g factor is relatively insensitive to this electric field, but for InAs/GaAs dots with transition energies around 1.2 eV the in-plane g factor changes by 20\% for an electric field of 150kV/cm. For a DC magnetic field oriented at 45 degrees to the growth direction the spin precession axis for an electron changes by 6 degrees from zero electric field to 150 kV/cm. Thus an AC pseudo-magnetic field almost 10\% the size of the DC magnetic field can be generated. This is sufficient to drive g-tensor modulation resonance[2] in the dot and perform single-spin manipulation. 1. C. E. Pryor and M. E. Flatt\'e, cond-mat/0410678. 2. Y. Kato, et al., Science 299, 1201 (2003). [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N10.00002: Land\'e $g$ factors and orbital angular momentum quenching in semiconductor quantum dots Craig E. Pryor, Michael E. Flatt\'e We present calculations of g-factors for nanocrystal and self-assembled quantum dots. We find that in addition to the effects of dot geometry and strain, quantization quenches the orbital angular momentum of the dot states, pushing the electron $g$ factor towards $2$ even when all the semiconductor constituents of the dot have negative $g$ factors. This leads to trends in the dot's electron $g$ factors that are the opposite of those expected from the effective $g$ factors of the dot and barrier material. Both electron and hole $g$ factors are strongly dependent on the magnetic field orientation; hole $g$ factors for InAs/GaAs quatum dots have large positive values along the growth direction and small negative values in-plane. The approximate shape of a quantum dot can be determined from measurements of this $g$ factor asymmetry. This work was supported by DARPA/ARO DAAD19-01-1-0490. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N10.00003: Spin Dynamics of Charged Colloidal Quantum Dots N.P. Stern, M. Poggio, M.H. Bartl, E.L. Hu, G.D. Stucky, D.D. Awschalom Colloidal semiconductor quantum dots are promising structures for controlling spin phenomena because of their highly size- tunable physical properties, ease of manufacture, and nanosecond-scale spin lifetimes at room temperature. Recent experiments have succeeded in controlling the charging of the lowest electronic state of colloidal quantum dots \footnote{C. Wang, B. L. Wehrenberg, C. Y. Woo, and P. Guyot-Sionnest, \textit{J. Phys. Chem B} \textbf{108}, 9027 (2004).}. Here we use time-resolved Faraday rotation measurements in the Voigt geometry to investigate the spin dynamics of colloidal CdSe quantum dot films in both a charged and uncharged state at room temperature. The charging of the film is controlled by applying a voltage in an electrochemical cell and is confirmed by absorbance measurements. Significant changes in the spin precession are observed upon charging, reflecting the voltage- controlled electron occupation of the quantum dot states and filling of surface states. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N10.00004: Spin Dynamics and Energy Levels in Quantum Shells J. Berezovsky, M. Ouyang, F. Meier, D.D. Awschalom, D. Battaglia, X. Peng In a zero-dimensional analogue to planar quantum wells, nanoparticle heterostructures known as quantum-dot quantum wells (QDQWs) allow for the study of single quantum-confined electrons in an engineered potential energy landscape. We have characterized colloidal CdS/CdSe/CdS QDQWs using two-color time-resolved Faraday rotation (TRFR). The spin dynamics show that the electron g-factor is tunable with quantum well width and the transverse spin lifetime of several nanoseconds is robust up to room temperature. As a function of probe energy, the amplitude of the TRFR signal shows pronounced resonances, which allow one to identify individual exciton transitions. While the TRFR data are inconsistent with the conduction and valence band level scheme of spherical QDQWs, a model in which broken spherical symmetry is taken into account captures the essential features. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N10.00005: Spin Dynamics in InAs/GaAs Quantum Structures J. Whitaker, M. Ware, D. Gammon, A.S. Bracker, T.A. Kennedy Arrays of InAs SAQD's with narrow size distributions are being developed for applications in optoelectronics and quantum information. Here we describe initial measurements of spin dynamics from a wafer with a varying dot-density. We measured T$_{2}^{\ast }$ using Time Resolved Kerr Rotation (TRKR) for a wavelength resonant with the 2D InAs wetting layer in a region of the sample where the Stranski-Krastanow strain mediated quantum dots have not formed and found a g-factor of 0.42 and the lifetime to be 125 ps. We attribute this relatively short lifetime, compared to the recombination time of $\sim $1 ns, to the inhomogeneity of the wetting layer. However, when performing the same measurement in a region of the sample where the dots were present the electron lifetime decreased by an order of magnitude to 12 ps. The reduction in lifetime is attributed to the exciton created in the wetting layer being captured by the dot. Weak signals were observed when resonant with the dots. Work supported in part by ONR, NSA/ARDA, and DARPA/SPINS. JW is an NRC/NRL Postdoctoral Research Associate. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N10.00006: Electron spin relaxation by hyperfine interaction in a double quantum dot Alex Johnson, Jason Petta, Jacob Taylor, Charles Marcus, Mikhail Lukin, Amir Yacoby, Micah Hanson, Art Gossard We use a pulsed-gate technique to measure singlet-triplet relaxation in a GaAs/AlGaAs few-electron double quantum dot at low magnetic field. Electrostatic pulses are applied to probe the time dynamics of the (1,1) to (0,2) charge state transition, while average dot occupation is measured by nearby quantum point contact charge sensors. In the (0,2) configuration only a spin singlet is allowed, blocking the transition from (1,1) if a triplet state is initially formed, but relaxation is strongly enhanced near zero magnetic field. We attribute this enhancement to different nuclear environments in each dot, and extract an average effective Overhauser field of ~3 mT. This implies a spin dephasing time of ~30 ns in this system. [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:48AM |
N10.00007: Orbital Kondo Effect and Spin Polarization in Carbon Nanotubes Invited Speaker: In this talk I will review our recent experiments on low-temperature electronic transport in carbon nanotube (CNT) quantum dots (QDs). I will focus, in particular, on strongly coupled quantum dots exhibiting Kondo effect. By means of a magnetic field we are able to modify the energy spectrum of CNT QDs, such that we can tune two orbital states with equal spin polarization into degeneracy. This purely orbital degeneracy enables the observation of an orbital Kondo effect and shows that carbon nanotubes can potentially act as low-impedance spin filters. At zero magnetic field, the four-fold degeneracy of the nanotube spectrum allows to observe a strong Kondo effect, with new transport properties in the non-linear regime and described theoretically by a so-called SU(4) symmetry. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N10.00008: Spin coherence in CdS quantum dots P. Nemec, P. Nahalkova, D. Sprinzl, M. Simurda, F. Trojanek, P. Maly, J.T. Devreese, V.N. Gladilin Spin coherence in CdS quantum dots (QDs) in a glass matrix has been investigated. Time-resolved differential transmission experiments were performed to measure the decay of the degree of circular (linear) polarization DCP (DLP). We show that due to the nearly spherical shape of our QDs the properties of DCP and DLP are considerably different compared to the most often investigated self-assembled QDs that are of pyramidal shape. Namely, we observed a decay of DCP with two distinct time components (300 fs and 10 ns at 300 K) and a strong dependence of the initial values of DCP on the laser wavelength. Our theoretical analysis of the experiments implies that the slow component in the observed decay of DCP is dominated by intralevel exciton transitions with electron spin flip, which are driven by the electron--hole exchange interaction and assisted by two LO phonons. We show that two-phonon processes significantly contribute also to exciton transitions without electron spin flip, which lead to the appearance of a fast component in the decay of DCP. This work was supported by the Grant agency of the Czech Republic (grant 202/03/P150), by the Ministry of Education of the Czech Republic (project 1K03022), by IUAP and FWO-V projects G.0274.01N, G.0435.03, the WOG WO.025.99N (Belgium). [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N10.00009: SU(4) Kondo effect in carbon nanotube quantum dots Ramon Aguado, Mahn-Soo Choi, Rosa Lopez We investigate theoretically the non-equilibrium transport properties of carbon nanotube quantum dots. Owing to the two-dimensional band structure of graphene, a double orbital degeneracy plays the role of a pseudo-spin, which is entangled with the spin. Quantum fluctuations between these four degrees of freedom result in an SU(4) Kondo effect at low temperatures. This exotic Kondo effect manifests as a four-peak splitting in the non-linear conductance when an axial magnetic field is applied [1]. Recent transport experiments in carbon nanotube quantum dots [2] clearly support our theoretical findings. [1] M. S. Choi, R. Lopez and R. Aguado, cond-mat/0411665 (2004). [2] P. Jarillo-Herrero, J. Kong, H. S. J. van der Zant, C. Dekker, L. P. Kouwenhoven and S. De Franceschi, to be published (2004). [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N10.00010: Spin polarization of current passing through a double level quantum dot in magnetic field Lam Chuen Wong, Tai Kai Ng The spin polarization of the current passing through a GaAs quantum dot is found experimentally by Potok \textit{et. al}.$^{1}$ to be always polarized in the same direction as external field when varying the gate voltage. This is in disagreement with theories that assume single energy level on the quantum dot. We investigate the problem by considering a model of double level quantum dot with strong exchange coupling between the electrons on the two levels. A generalized 1/N expansion method was used to construct the approximate ground state, spectral function and then the current. Our results are compared with experimental result by Potok \textit{et.al}.. \newline \newline $^1$R. M. Potok, J. A. Folk, C. M. Marcus, V. Umansky, M. Hanson, and A. C. Gossard, Phys. Rev. Lett. \textbf{91}, 016802(2003) [Preview Abstract] |
Wednesday, March 23, 2005 10:24AM - 10:36AM |
N10.00011: Auger recombination of excitons in semimagnetic quantum dot structure in a magnetic field Alexander Chernenko, Pavel Dorozhkin, Vladimir Kulakovskii, Anrey Brichkin, Sergey Ivanov, Alexey Toropov We present the results of magnetoluminescence study of ZnSe:CdMnSe quantum dots samples in a magnetic field up to 11 T both in the Faraday and Voigt geometries at liquid He temperatures and various levels of laser excitation. We found that the intensity of the quantum dot photoluminescence strongly increases (up to two orders of magnitude) in the Faraday geometry and only slightly ($\sim$ 1.5 times) in the Voigt geometry within the range of B=0-11 T . We explain the strong increase of the photoluminescence in the Faraday geometry within the frame of the spin-dependent Auger recombination of excitons through Mn ions. We relate the observed anisotropy of the quantum dot emission with the high anisotropy of the hole spins in QDs. We present a theoretical model which allows us to obtain selection rules for the Auger transition and thoroughly explains experimental results. The selections rules allow to explain characteristic fitures in single quantum dot spectra. [Preview Abstract] |
Wednesday, March 23, 2005 10:36AM - 10:48AM |
N10.00012: Single exciton spectroscopy in a single semimagnetic quantum dot J. Fernandez-Rossier Motivated by recent experiments of single spin detection [1,2] and optically induced magnetization [3], the problem of a photoexcited (II,Mn)VI Diluted Magnetic Semiconductor quantum dot is studied. The Hamiltonian of N spins exchange-coupled to an electron hole pair is solved both exactly (for N$\sigma$ 7) and in mean field approximation, using numerical diagonalization. The Hamiltonian involves as well the electron-hole pair exchange, the spin orbit interaction of the holes and the Mn-Mn antiferromagnetic exchange. The ground state and spin wave excitation are obtained both with and without the exciton present in the dot. Using linear response theory in the light-matter coupling, the absorption and emission spectrum are calculated and compared with those of the experiments. The optical signatures of the exciton induced spin order are discussed. Prospects for the optical detection of the number of Mn spins in a single are commented. [1] G. Bacher et al. Phys. Rev. Lett. {\bf 89}, 127201 (2002) [2] L. Besombes, et al. Phys. Rev. Lett. {\bf 93}, 207403 (2004) [3] S. Mackowski et al., Appl. Phys. Lett. {\bf 84}, 3337 (2004) [Preview Abstract] |
Wednesday, March 23, 2005 10:48AM - 11:00AM |
N10.00013: Magnetically-controlled impurities in quantum wires with strong Rashba coupling Rodrigo Pereira, Eduardo Miranda We investigate the effect of strong spin-orbit interaction on the electronic transport through non-magnetic impurities in one-dimensional systems. When a perpendicular magnetic field is applied, the electron spin polarization becomes momentum-dependent and spin-flip scattering appears, to first order in the applied field, in addition to the usual potential scattering. By tuning the Fermi level and the Rashba coupling, it is possible to suppress the potential scattering and the spin-flip scattering will dominate at low temperatures. As a result, the resistance of the wire will strongly depend on the magnetic field. [Preview Abstract] |
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N10.00014: Oscillatory spin-flip rates and anisotropic g-factor in quantum dots Carlos Destefani, Sergio Ulloa We study phonon-induced electron spin relaxation rates in parabolic quantum dots (QDs) as function of in-plane and perpendicular magnetic fields as well as of QD lateral and vertical sizes. Rashba and Dresselhaus spin-orbit (SO) couplings are included via exact diagonalization of the model. Deformation and piezoelectric couplings for acoustic phonons are considered, and we show how the former (latter) yields the dominant mechanism in a narrow (wide) gap material. We also report an oscillatory spin-flip rate between QD Zeeman sublevels. In the minima of such rates, quite large spin relaxation times can be obtained in properly designed QDs. The rich interplay between external magnetic fields and intrinsic SO interactions is studied, where two distinct phases are visible in the spectrum of GaAs QDs in perpendicular fields if their vertical width is narrow. We also discuss the QD g-factor strong anisotropy and show how even a sign change can be induced for large magnetic field [1]. Good agreement with available experimental findings is obtained. [1] C. F. Destefani and Sergio E. Ulloa, cond-mat/0411071. [Preview Abstract] |
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