Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J10: Focus Session: Spin Dynamics in Semiconductors |
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Sponsoring Units: DMP GMAG Chair: Scott Crooker, LANL Room: LACC 153B |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J10.00001: Bias Dependent Spin Relaxation in a [110]-InAs/AlSb Two Dimensional Electron System J. Hicks, A. Kocbay, K. Hall, K. Gundogdu, T. Boggess, K. Holabird, A. Hunter, J. Zinck Manipulation of electron spin is a critical component of many proposed semiconductor spintronic devices. One promising approach utilizes the Rashba effect by which an applied electric field can be used to reduce the spin lifetime or rotate spin orientation through spin-orbit interaction. The large spin-orbit interaction needed for this technique to be effective typically leads to fast spin relaxation through precessional decay, which may severely limit device architectures and functionalities. An exception arises in [110]-oriented heterostructures where the crystal magnetic field associated with bulk inversion asymmetry lies along the growth direction and in which case spins oriented along the growth direction do not precess. These considerations have led to a recent proposal of a spin-FET that incorporates a [110]-oriented, gate-controlled InAs quantum well channel [1]. We report measurements of the electron spin lifetime as a function of applied electric field in a [110]-InAs 2DES. Measurements made using an ultrafast, mid-IR pump-probe technique indicate that the spin lifetime can be reduced from its maximum to minimum value over a range of less than 0.2V per quantum well at room temperature. This work is supported by DARPA, NSERC and the NSF grant ECS - 0322021. [1] K. C. Hall, W. H. Lau, K. Gundogdu, M. E. Flatte, and T. F. Boggess, Appl. Phys. Lett. 83, 2937 (2003). [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J10.00002: Nondegenerate time-resolved Faraday rotation in quantum wells: the role of exciton-exciton interactions Yumin Shen, Alexander Goebel, Hailin Wang Time-resolved Faraday rotation (TRFR) has been widely used in studies of spin related phenomena in semiconductors. Understanding the physical origin of TRFR is thus of special importance to these studies. In this paper, we report experimental studies based on the use of nondegenerate TRFR, in which we measured the spectral response of TRFR by varying the detuning between the pump and probe. Nondegenerate TRFR studies in GaAs and InGaAs quantum wells revealed that simple atomic-like model for TRFR fails to describe the spectral TRFR response. Theoretical analysis further indicated that manybody exciton-exciton interactions fundamentally modify the TRFR response in these semiconductor systems. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J10.00003: Inducing electron spin coherence in GaAs quantum well waveguides: Spin coherence without spin precession Susanta Sarkar, Phedon Palinginis, Hailin Wang, Pei-Cheng Ku, Connie Chang-Hasnain, N. Kwong, R. Binder We report the experimental demonstration of inducing and detecting electron spin coherence in a GaAs quantum well without the use of either an \textit{external} or \textit{internal} magnetic field. We have taken advantage of the spin-orbit coupling in the valence band and have used light-hole transitions in a waveguide to induce coherent superposition of the electron spin states. In the absence of spin precession, the induced spin coherence is detected through quantum interference in the spectral domain, instead of time domain, coherent nonlinear optical response. We interpret the experimental results qualitatively using a few-level model with only the optical transition selection rule as its basic ingredients. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J10.00004: Time-resolved Spin Dynamics in Semiconductor Microdisk Lasers Sayantani Ghosh, Yongqing Li, Florian Meier, Roberto Myers, David D. Awschalom, Wei-Hua Wang, Nitin Samarth Optical microcavities offer unique means of controlling the interaction of light and matter, which have led to the development of a wide range of applications in optical communications and have stimulated discussions of quantum computational schemes based on cavity QED. We present a study of the dynamics of optically injected spins in GaAs/AlGaAs multiple quantum well microdisk lasers, where emission intensity and line width measurements reveal very high quality factor modes, using a pump-probe, time-resolved Kerr rotation technique with picosecond resolution. We measure the spin decoherence as a function of the pump wavelength and input power and find that the spins in these structures couple selectively to the cavity modes at the resonant wavelengths. This is manifested by an enhancement of the spin decoherence time at the lasing threshold of the cavity modes followed by a sharp decrease at greater pump power, where the stimulated emission dominates the radiative decay. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J10.00005: Time-resolved spectroscopy of electron spin dynamics in ZnO Vanessa Sih, Sayantani Ghosh, David D. Awschalom, Seung-Young Bae, Shan Wang, George Chapline, Sheila Vaidya The prediction of room temperature ferromagnetism in magnetically-doped wide band gap semiconductors has galvanized interest in using zinc oxide for spintronic applications. In addition, ZnO has small spin-orbit coupling and a naturally low abundance of nuclear spins, which is expected to contribute to long spin coherence times. Time-resolved Faraday rotation is used to monitor electron spin dynamics in commercially available bulk single crystal ZnO wafers and thin films grown by pulsed laser deposition on sapphire substrates\footnote{S. Ghosh, V. Sih, S. Y. Bae, S. Wang, G. Chapline, S. Vaidya and D. D. Awschalom, \textit{in preparation} (2004)}. Measurements are performed over a range of temperatures and magnetic fields, with spin coherence persisting to room temperature in both bulk and thin film samples. We investigate the role of intrinsic defects in ZnO on spin decoherence through a systematic comparison of thin films grown under varying oxygen partial pressures and bulk samples. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J10.00006: Coupling of a Single Nitrogen-Vacancy Center to Nitrogen Spins in Diamond Ryan J. Epstein, David D. Awschalom Confocal microscopy with photon-correlation detection is employed to optically probe single Nitrogen-Vacancy color centers in diamond at room temperature. Photon anti-bunching is measured to distinguish one center from multiple centers within the laser focus. Polarization-dependent excitation in conjunction with magneto-photoluminescence enables the orientation of an N-V center's symmetry axis to be discerned. For a single center with symmetry axis parallel to the magnetic field, a dip in the photoluminescence intensity is observed when the ground-state spin sublevels anti-cross at 0.1 T. We find an additional dip at 0.05 T that is attributed to resonant dipolar coupling to nearby substitutional Nitrogen spins. [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 1:03PM |
J10.00007: Microsecond spin-flip times for localized donors in GaAs Invited Speaker: One of the central tasks in developing spin-based quantum computing is the development of materials which have long spin lifetimes. Observations of long electron spin lifetimes (hundreds of ns) in n-type GaAs dating back to Kikkawa and Awschalom in 1998 [1] have stimulated much excitement in the field, and many groups have similarly made observations of the inhomogeneous T$_{2}^{\ast }$ lifetime of electrons in GaAs in the ns regime. The homogeneous dephasing time, T$_{2}$, has not yet been measured, although it is expected to be much longer. Here, a series of measurements of lifetimes [2] are described for donors in lightly n-type GaAs doped at 3E14, 1E15, and 3E15 cm$^{-3}$ that mimic spin memory in doped quantum dots. Hanle effect measurements yield T$_{2}^{\ast }$ at close to 0T, magnetic resonance measurements provide T$_{2}$* at 40 mT, and Kerr rotation measurements provide T$_{2}^{\ast }$at higher fields. The measured T$_{2}^{\ast }$ values for the 3E14 sample are consistent with full electron localization. A new pump-probe technique using electronic delays between pulses has been used to measure spin lifetimes into the $\mu $s range. This time-resolved technique provides measurements of the spin-flip time (often labeled T$_{S}$, which is essentially the same as T$_{1})$ for two of the samples at a range of fields and temperatures. T$_{S}$ is greater than 1 $\mu $s for B$>$0.6T at 1.5K and for B$>$2.5T at 6 K. Since T$_{2}$ is limited by the spin-flip time, these measurements show the range of temperature and magnetic field where very long T$_{2}$'s are possible. [1] Phys Rev Lett \textbf{80}, 4313 (1998). [2] J.S. Colton et al., Phys Stat Sol B \textbf{233}, 445 (2002); Phys Rev B \textbf{67}, 165315 (2003); Phys Rev B \textbf{69}, 121307(R) (2004); Solid State Comm \textbf{132}, 613 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J10.00008: Optical Excitation of Spins in Lightly Doped GaAs layers and Quantum Wells T.A. Kennedy, A.S. Bracker, D. Gammon, A. Shabaev, Al.L. Efros, M. Scheibner The transfer of a quantum state from light to localized spins in a solid is of great interest for potential applications in quantum information and optoelectronics. Here, ultrafast pump-probe experiments have been used to excite and detect spin states in n-doped GaAs layers and remotely doped wide quantum wells. Strong signals whose g-factors indicate electron spin precession were observed with 1.5 ps circularly polarized exciting pulses and linearly polarized probe pulses analyzed through Time-Resolved Kerr Rotation (TRKR). We use a simple theory for the trion in the quantum well to describe the results with resonant light. The trion recombination time and hole spin relaxation time are important parameters. However, the functional form of the data partially disagrees with the theory. Reasons for this, such as a contribution from four-wave mixing, are discussed along with explanations of the results for other wavelengths and samples. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J10.00009: Interface-sensitive study of ultrafast spin dynamics in multilayer semiconductors Y.D. Glinka, T.V. Shahbazyan, J.K. Miller, N.H. Tolk, X. Liu, Y. Sasaki, J.K. Furdyna We report the first application of pump--probe second harmonic generation (SHG) measurements to characterize optically induced magnetization in non-magnetic multilayer semiconductors GaAs/GaSb/InAs. A circularly-polarized pump beam has been used to inject electrons into the conduction band of GaAs, where the photons impart their angular momentum to electron-hole pairs. Because of the interface-sensitive method, the spins accumulated at the GaSb/InAs interface have been monitored. Subsequent precession of these spins about the applied magnetic field has then been detected by a time-delayed probe pulse as an interfacial magnetic field induced SHG response. The electron and spin transport through the heterostructure takes place on the time frame of 15-20 ps, and it is followed by the relaxation of interfacial magnetic and electric fields on the time scale of 100 ps. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J10.00010: Far-infrared photo-response of 2D electrons in a single InAs/AlGaSb quantum well in the region of EDSR C.J. Meining, V.R. Whiteside, B.D. McCombe, P. Grabs, I. Chado, G. Schmidt, L.W. Molenkamp Motivated by predictions of strong electric dipole spin resonance (EDSR) and possible spin manipulation by electric fields$^{1}$ we have studied the far-infrared (FIR) photo-response of a two-dimensional electron gas (2DEG) in an asymmetric 15\,nm InAs quantum well (Al$_{0.35}$Ga$_{0.65}$Sb barriers). Changes in $R_{xx}$ induced by a FIR laser beam ($E_{FIR}$ = 3.15\,meV) in Hall-bar geometry were measured vs. magnetic field magnitude ($<$10\,T) and tilt angle $\theta$ of the sample normal with a double lock-in technique. We have observed a sharp minimum (20\,mT wide) in a non-resonant background photo-response for $\theta \approx$ 38$^{\circ}$. This line splits into two sharp minima with increasing angle $\theta$ and vanishes for $\theta < 38^{\circ}$. The dip occurs close to the field (odd filling factor $\nu$ = 7) at which we expect EDSR in this highly non-parabolic system. Possible explanations for the splittings will be discussed.\\ $^{1}$E. I. Rashba, Al. L Efros, Appl. Phys. Lett. {\bf{83}}, 5295 (2003).\\ Work supported by DARPA ONR\# N00014-00-1-0951. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J10.00011: Observation of Spin-Spin Interaction in InSb Quantum Wells R.C. Meyer, T. Kasturiarachchi, X.H. Zhang, N. Goel, S.J. Chung, R.E. Doezema , M.B. Santos , Y.J. Wang For 2D electron systems in InSb, the energy of spin-conserving transitions between neighboring Landau levels (LLs) depends on spin orientation. The spin up transition will have a higher energy than the spin down transition due to the non-parabolicity of the InSb conduction band. These spin-resolved cyclotron resonance features allow us to probe the spin dependence of avoided-level crossings between LLs associated with different subbands. Previous studies in GaAs, where cyclotron resonance peaks were not spin resolved, demonstrated that the anti-crossing energy gaps depend linearly on the tilt of the magnetic field. In our spin-resolved studies, we do observe a linear dependence for anti-crossings between levels with the same spin. However, the strength of this dependence is different for the two spin orientations. More notably, we observe anti-crossings between levels with opposite spin, as would be expected for a spin-spin interaction. The gap for these opposite-spin crossings is essentially tilt-angle independent. None of the observed spin dependencies are expected from the conventional theory behind subband LL anti-crossings. Although spin-orbit effects are strong in InSb, the origin of the observed spin dependencies has not yet been definitively identified. This work is supported by the NSF under Grants No. DMR-0080054 and DMR-0209371. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J10.00012: Hole spin relaxation in diluted mag\-netic semi\-conductors Yongke Sun, Fedir Kyrychenko, Gary Sanders, Christopher Stanton Diluted Magnetic Semiconductors are postulated to serve as efficient spin injectors for various spintronic applications. Hence, a detailed theoretical investigation of carrier spin dynamics in these materials is of particular importance. The introduction of Mn ions influences the itinerant carrier spin dynamics in many ways. First, the orbital scattering of the itinerant carriers with the Mn impurities can affect spin relaxation through Elliot-Yaffet and/or Dyakonov-Perel' mechanisms. Secondly, the diagonal part of exchange interaction results in the large spin splitting of energy bands. Finally, the off-diagonal part of the carrier-magnetic ion exchange interaction forms an additional channel of spin relaxation. In this work we theoretically investigate the hole spin dynamics in III, Mn-V bulk semiconductors such as InMnAs or GaMnAs. We study the changes in spin-relaxation with the increase of Mn concentration. In our model we take into account the valence band complexity and treat the exchange interaction within the mean field approximation. We consider the Elliot-Yafet spin relaxation mechanism for impurity and phonon scattering. Results of our calculations are compared with experiment. [Preview Abstract] |
Tuesday, March 22, 2005 2:03PM - 2:15PM |
J10.00013: Two-dimensional magnetoexcitons in the presence of spin-orbit interactions Oleg Olendski, Tigran V. Shahbazyan We study excitonic energy spectrum and optical absorption in narrow-gap semiconductor quantum wells in strong magnetic field. We find that, in the presence of an in-plane field, the interplay between Zeeman, Coulomb, and spin-orbit terms leads to a drastic change in the magnetoexciton energy spectrum. When separation between adjacent Landau levels with opposite spins becomes of the order of the magnetoexciton binding energy, the bright and dark exciton dispersions exhibit anticrossing, resulting in a pronounced minimum at finite momentum for the higher-energy eigenstate. With varying in-plane field, the anticrossing moves to zero momentum leading to a spin-orbit-induced splitting of the excitonic absorption spectrum. In the presence of both Rashba and Dresselhaus spin-orbit terms, the spectrum is anisotropic and it depends explicitly on the in-plane orientation of the magnetic field. In particular, by varying the azimuthal angle, the splitting of excitonic absorption peak can be tuned in a wide interval. Experimental implications for InAs and InSb quantum wells are discussed. [Preview Abstract] |
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