Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session Y12: Focus Session: Spin Relaxation and Dynamics |
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Sponsoring Units: GMAG DMP FIAP Chair: Sayantani Ghosh, University of Cailfornia, Merced Room: Colorado Convention Center Korbel 3C |
Friday, March 9, 2007 11:15AM - 11:27AM |
Y12.00001: Spontaneous polarization in classical spin transport far from equilibrium Tobias Reichenbach, Thomas Franosch, Erwin Frey Spintronic devices like the Datta-Das spin-field effect transistor aim to exploit the spin of electrons for information processing. Based on an exclusion process, we present a classical model for the spin transport in such systems [1]. Particles move unidirectionally on a one-dimensional lattice under the constraint of Pauli's exclusion principle. Analyzing the emerging non-equilibrium steady state, we find that it can be tuned by controlling the particle fluxes at the boundaries. In particular, a spontaneous polarization may occur at a certain spatial position and, upon changing the fluxes at the boundaries, be driven in or out of the system. We derive the shape of the density profiles as well as resulting phase diagrams analytically by a mean-field approximation and a continuum limit. \newline \newline [1] T. Reichenbach, T. Franosch, E. Frey, Phys. Rev. Lett. 97, 050603 (2006) [Preview Abstract] |
Friday, March 9, 2007 11:27AM - 11:39AM |
Y12.00002: Ultrafast hole-spin dynamics in bulk GaAs Hans Christian Schneider, Michael Krauss This talk presents theoretical results on hole-spin dynamics in bulk GaAs after optical excitation. The coupled dynamics of spin and orbital angular momentum is determined by solving dynamical Boltzmann equations for carrier-carrier scattering, which include the effect of spin-orbit coupling on the level of a 4-band Luttinger Hamiltonian. Hole-spin relaxation takes place in two stages. In the first regime, on a timescale of a few hundred femtoseconds, pure momentum scattering dominates the dynamics and the anisotropic contributions to the orbital angular momentum, which are created by the optical excitation, are evened out. In the second regime, on a timescale of a few picoseconds, energy relaxation dominates. The hole-spin dynamics can be approximated by a different relaxation time for each of the two regimes. The fast spin relaxation-time in the first regime is in agreement with experimental results for heavy-hole spin relaxation. [Preview Abstract] |
Friday, March 9, 2007 11:39AM - 11:51AM |
Y12.00003: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 11:51AM - 12:03PM |
Y12.00004: Time Resolved Probe of Spin and Carrier Lifetimes in InSb Based Quantum Wells and Films Rajeev N. Kini, K. Nontapot, M. Frazier, B. Spencer, J.J. Heremans, G.A. Khodaparast, N. Goel, S.J. Chung, M.B. Santos Semiconductor quantum wells (QWs) based on InSb have potential applications in the rapidly growing field of spintronics. Due the large Bychkov-Rashba and Dresselhaus spin-orbit interaction terms, g factor, and the high electron mobility of InSb, it may be possible to realize a number of novel high speed, spin-sensitive electronic, and optoelectronic devices. A recent measurement has indicated a value of $\sim $ 0.5ps for the room temperature spin lifetime in InSb/AlInSb QWs (Litvinenko \textit{et al}, New Journal of Physics, 8, 49, (2006)). Here we present our measurements of the spin and carrier relaxations in InSb based QWs with symmetric and asymmetric doping profile and n-type InSb films. We use pump probe and magneto-optical Kerr effect (MOKE) spectroscopy to study the dynamics at different excitation wavelengths, power densities, and temperatures. We observe relaxations which last longer than 5ps and our measurements can provide important information regarding the relaxation mechanisms in this narrow gap system. [Preview Abstract] |
Friday, March 9, 2007 12:03PM - 12:15PM |
Y12.00005: Spin relaxations in 2D electron gas determined by the memory in the carrier dynamics. Eugene Sherman, Mikhail Glazov The effects of long memory, in carrier dynamics in a magnetic field, on spin polarization evolution in 2D electron gas are investigated qualitatively and quantitatively. As examples we consider (i) systems with random Rashba-type SO coupling and (ii) quantum wells with rigid short-range scatterers (antidotes) and regular Dresselhaus SO coupling. In both cases the spin dynamics is strongly non-Markovian. In the system with the random SO coupling the time dependence of the spin polarization shows Gaussian rather than exponential behavior with the cusps corresponding to the electron revolutions. The relaxation speeds up with the increase of the magnetic field. In the system with antidotes scattering, the spin polarization shows a long-tail behavior with the relaxation rate determined by inelastic electron-phonon and electron-electron collisions and demonstrates unusual field dependence. [Preview Abstract] |
Friday, March 9, 2007 12:15PM - 12:27PM |
Y12.00006: Spin dynamics in a two-dimensional electron system with generic spin-orbit coupling Tudor Stanescu, Victor Galitski We study the dynamics of a spin density injected into a two-dimensional electron system with generic spin-orbit interactions. We derive the general spin-charge coupled diffusion equation for a model that includes Rashba as well as linear and cubic Dresselhaus spin-orbit interaction terms. We analyze in detail two regimes: The first regime corresponds to negligible spin-charge coupling and is characterized by standard charge diffusion, decoupled from the spin dynamics. The interplay between different spin-orbit interaction terms leads to interesting phenomena such as a finite momentum enhancement of the spin relaxation time, real space oscillatory dynamics, and anisotropic transport. In the second regime, we include the effects of the spin-charge coupling. The coupling depends quadratically on the overall strength of the spin-orbit interaction and, in addition, it strongly depends on the relative magnitude of the various interaction terms. It is shown that the spin-charge coupling leads to an enhancement of the effective charge diffusion coefficient. In the presence of a boundary, spin-charge coupling leads to a current-induced spin accumulation. In the strong spin-charge coupling limit, we find that the relaxation rates formally become complex and the spin/charge dynamics is characterized by real time oscillations, qualitatively similar to those observed in spin-grating experiments [Weber et al., Nature 437, 1330 (2005)]. [Preview Abstract] |
Friday, March 9, 2007 12:27PM - 12:39PM |
Y12.00007: Electrically Detected Hahn Echoes in Si:P Hans Huebl, Benno Grolik, Andre R. Stegner, Martin Stutzmann, Martin S. Brandt Several proposals discuss the realization of quantum computation with the help of the spin degree of freedom in semiconductors, including implementations using the nuclear or electronic spin of phosphorus donors. Recently, Stegner et al. [1] showed in ensemble measurements that the electron spin state of donors can be read out electrically by investigating the current transient after the application of a microwave pulse and by observing Rabi oscillations. In this contribution we show that this technique can be extended to detect Hahn echoes which allow to determine the $T_2^*$ time of the specific spin-to-charge transfer system used. We observe $T_2^* \approx 1.2\rm{\mu}$s at 6~K under illumination with a tungsten lamp. This time is considerably shorter than $T_2$ characteristic of isolated P in Si, most likely due to the interaction with the Si-SiO$_2$ interface and photoexcited carriers. The experiments show that dynamic constants specific to the actual spin read-out technique used can be determined by pulsed electrically detected magnetic resonance. [1] A. R. Stegner et al., Nature Physics doi:10.1038/nphys465 [Preview Abstract] |
Friday, March 9, 2007 12:39PM - 12:51PM |
Y12.00008: Control and measurements of spin and carrier dynamics in InAs films Kanokwan Nontapot, Rajeev Kini, Giti Khodaparast, Louis Guido, Roger Welser In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors. We report control and measurements of spin and carrier relaxations in InAs (100) and InAs (111) films with doping densities of $\sim $ 3x10$^{12}$ cm$^{-2}$ (mobility $\sim $20,000 cm$^{2}$/Vs at 77 K) and 2.0x10$^{12}$ cm$^{-2}$ (mobility $\sim $33,00 cm$^{2}$/Vs at 77 K) at room temperature and 77K, respectively. We use standard pump-probe and magneto-optical Kerr effect (MOKE) spectroscopy at different excitation wavelengths, power densities, and temperatures. Spin relaxations in these structures demonstrate dynamics which is different from the carrier relaxations under the same experimental conditions. We explain our results using the Elliot-Yafet picture which is considered to be the dominant relaxation process in narrow gap semiconductors. [Preview Abstract] |
Friday, March 9, 2007 12:51PM - 1:03PM |
Y12.00009: Evolution of coherently controlled charge and spin currents injected by optical pulses. R. Abrarov, A. Najmaie, E. Ya. Sherman, J. E. Sipe We consider dynamics of coherently controlled currents injected by short (100 fs) optical pulses with frequencies $\omega $ and 2$\omega $ in multiple GaAs/AlGaAs quantum wells. Our approach is based on the series expansion of the carrier and current densities in an appropriate set of basis functions. The role of space-charge effects (long-range Coulomb interaction between electrons and holes) and current-conserving and non-conserving collisions of the carriers on the dynamics of the quantities observable in pump-and-probe experiments is investigated. We show that under certain conditions, dependent on the relaxations rates and the band structure effects, displacement of electrons and holes from the initial positions can be finite even on a long time scale of the order of few picoseconds. Due to the skew scattering arising during electron-hole collisions, injected charge (spin) currents drive the spin (charge) currents which can be observed experimentally. [Preview Abstract] |
Friday, March 9, 2007 1:03PM - 1:15PM |
Y12.00010: Bias-Dependent Electron Spin Lifetimes in n-Type GaAs and the Role of Donor Impact Ionization M. Furis, D.L. Smith, S.A. Crooker, J.L. Reno We present a study of electron spin lifetimes $\tau_s$ in n-type GaAs as a function of applied lateral electrical bias [1]. Using the magneto- optical Kerr effect, $\tau_s$ is obtained from Hanle depolarization measurements on n-GaAs epilayers doped near the metal-insulation transition ($n_e\sim 0.4 - 5.0$ x $10^{16} $ cm$^{-3}$) . Below 10 K, we observe that applied electric fields larger than 10 V/cm result in a large and sudden decrease of $\tau_s$. This collapse results from impact ionization of the cold donor-bound electrons into free electron states that are subject to Dyakonov-Perel spin relaxation. The effect is less dramatic at higher temperatures and at higher doping concentrations above the metal- insulator transition, where most electrons are delocalized even in the absence of an applied electrical bias. The collapse of $\tau_s$ is shown to strongly influence lateral spin transport in the impact ionization regime. [1] M. Furis, D. L. Smith, S. A. Crooker, and J. L. Reno, Appl. Phys. Lett. \textbf{89}, 102102 (2006) [Preview Abstract] |
Friday, March 9, 2007 1:15PM - 1:27PM |
Y12.00011: Spin precession and spin relaxation in semiconductors Dimitrie Culcer, Roland Winkler In order to achieve a lasting spin polarization a proper understanding of the mechanisms leading to spin polarization decay is critical. We present a general theory for spin polarization decay due to the interplay of spin precession and momentum scattering that is applicable to both spin-1/2 electrons and spin-3/2 holes and that allows us to identify and characterize a wide range of qualitatively different regimes [1]. The spin polarization of ballistic carriers is reduced by spin dephasing, which is characterized by a non-exponential time dependence and results in an incomplete decay of the spin polarization. For weak momentum scattering or fast spin precession, the spin relaxation time is proportional to the momentum relaxation time. For strong momentum scattering and slow spin precession we recover the D'yakonov-Perel result that the spin relaxation time is inversely proportional to the momentum relaxation time. \newline \newline [1] D. Culcer and R. Winkler, cond-mat/0610779. [Preview Abstract] |
Friday, March 9, 2007 1:27PM - 1:39PM |
Y12.00012: Spin relaxation of electrons in bulk CdTe Daniel Sprinzl, Petra Nahalkova, Jan Kunc, Petr Maly, Petr Horodysky, Roman Grill, Eduard Belas, Jan Franc, Petr Nemec We report on the measurements of the spin relaxation time $T_{1}$ of photo-excited electrons in bulk CdTe. The carrier dynamics were investigated by transient absorption experiments using 80 fs circularly polarized laser pulses at sample temperatures from 20 to 300 K. We studied both p and n type doped CdTe samples, which were prepared in the form of thin platelets from the crystals grown by the modified Bridgman method. The obtained results are compared with the spin relaxation times reported for other semiconductors with the same crystal structure (e.g., GaAs [1]). Finally, the relative contributions of the D'yakonov-Perel, Elliott-Yafet, Bir-Aronov-Pikus, and other mechanisms to the measured spin relaxation times in CdTe are discussed. This work was supported by the Grant Agency of the Czech Republic (grant 202/03/H003), by the Ministry of Education of the Czech Republic in the framework of the research centre LC510 and the research plan MSM 0021620834. [1] J. M. Kikkawa and D. D. Awschalom, Phys. Rev. Lett. 80, 4313 (1998). [Preview Abstract] |
Friday, March 9, 2007 1:39PM - 1:51PM |
Y12.00013: Suppression of nuclear polarization in photon-irradiated GaAs M.R. Fitzsimmons, B.J. Kirby, F. Trouw, P.A. Crowell, C. Adelmann, S.D. Flexner, C.J. Palmstrom , M. Erickson, J.A. Borchers, C.F. Majkrzak, W. Chen, T.R. Gentile, R. Pynn We measured the spin dependence of polarized neutron beams reflected by a GaAs sample at 20 K in a magnetic field of 250 Oe applied along the sample's surface normal. Neutron data were acquired with left and right circularly polarized light (0.25 W/cm$^{2}$ and 808 nm) illuminating the sample. These conditions yielded nuclear polarization in the same sample (before and after the neutron experiment) of several percent as evidenced by a shift of a peak in the optical Hanle curve. The neutron data exhibit a correlation with light polarization and thus nuclear polarization. Quantitative analysis of the spin dependence of the polarized neutron reflectivities indicates nuclei within 50 nm of the sample's surface are not polarized, and then nuclear polarization increases to a small value in the bulk. We attribute suppression of nuclear polarization near the sample's surface to the electric field in the depletion layer that inhibits binding of spin polarized carriers to donor sites and to the electric field gradient at the nuclei (induced by the electric field in the depletion layer) that depolarizes nuclei with quadrupole moments such as Ga and As. [Preview Abstract] |
Friday, March 9, 2007 1:51PM - 2:03PM |
Y12.00014: ABSTRACT WITHDRAWN |
Friday, March 9, 2007 2:03PM - 2:15PM |
Y12.00015: Electric field dependence of the electron g-factor for a Si donor in bulk GaAs A. De, Craig E. Pryor, Michael E. Flatt\'e Modulation of the electron g-factor by an applied electric field may be used to coherently manipulate spins for quantum information processing. We present numerical calculations of the electric field dependence of the $g$-factor of an electron bound to a silicon donor in bulk GaAs. The calculations were carried out using 8-band $k.p$ theory in the envelope function approximation, which is implemented using finite differences on a real-space grid. The binding energy of the Si impurity in GaAs was fit to experimental data by adding a central cell correction to the donor site. Electrically modulating the impurity's $g$-factor is possible as the electric field modifies the binding energy for the Si donor. In our calculations, it is seen that the variation in $g$ is nearly quadratic as a function of electric field (up to 0.2 mV/nm) and for $E=0$, ${d^2g} / {dE^2}=1.2~ \rm (mV/nm)^{-2}$. The largest variation in $g$ is obtained when the applied magnetic field and electric fields are in the same direction. The proposed scheme provides a realizable alternative to quantum information processing using quantum dots. [Preview Abstract] |
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