Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H33: Focus Session: Optical Properties of Magnetic Semiconductors |
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Sponsoring Units: GMAG FIAP DMP Chair: Jim Allen, University of California, Santa Barbara Room: Morial Convention Center 224 |
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H33.00001: Full band structure calculations of optical spin injection in Si and CdSe Julien Rioux, Fred Nastos, John E. Sipe We present a theoretical study of optical electron spin injection (optical orientation) in the bulk semiconductors Si and CdSe from direct optical excitation with circularly polarized light.\footnote{F. Nastos, J. Rioux, M. Strimas-Mackey, B.S. Mendoza, and J.E. Sipe, Phys. Rev. B \textbf{76}, 205113 (2007).} To describe excitation at energies significantly above the band edge, we use full-zone band structures from pseudopotential calculations. For Si, we find that there can be up to 30\% spin polarization from direct transitions. The relatively low symmetry of wurtzite CdSe leads to an orientation dependent spin injection, which can be up to 100\% polarized at the band edge. Averaging over crystal orientation gives a 50\% spin polarization for band edge excitation. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H33.00002: Ultrafast optical injection of magnetization in non-magnetic semiconductors F. Nastos, R. Newson, H. M. van Driel, J. E. Sipe We discuss the optical injection of magnetization into a nonmagnetic semiconductor by absorption of circularly polarized light. A microscopic approach, based on Fermi's golden rule and $\mathbf{k}\cdot\mathbf{p}$ band models, is used to quantify the magnetization-injection rate in GaAs. We find that under usual experiment conditions, relevant to optical orientation, the magnetization-injection rate of holes is approximately 20 times larger than it is for electrons, which reflects the large hole magnetic moment. We then turn to the ultrafast excitation regime and explore the possibility that the injected magnetization can radiate a detectable THz field. Using a phenomenological approach for the magnetization relaxation dynamics, we predict that the THz field from optical orientation is at the limit of current THz detection technology. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H33.00003: Direct and non-demolition optical detection of pure spin currents in semiconductors R.-B. Liu, J. Wang, B.-F. Zhu We put forward a scheme of direct and non-demolition measurement of a pure spin current in a direct-gap semiconductor by a polarized light beam, which may be view as a ``photon spin current'' [1]. The effective coupling between the ``hoton spin current'' and the electron spin current is realized via the spin-orbit coupling in valence bands, but involves neither Rashba effect from structure inversion asymmetry nor Dresselhaus effect from bulk inversion asymmetry. Thus a pure spin current, though bearing no net magnetization, induces Voigt and Faraday rotation of a polarized light beam. For the pure spin current studied in Ref. [2], a light beam, if oblique instead of normal incident, would present Voigt and Faraday rotation in the order of 1 millionth rad in the center region of the sample where the spin current flows without spin accumulation. [1] J. Wang, B. F. Zhu, and R. B. Liu, cond-mat/0708.0881. [2] Y. K. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, Science 306, 1910 (2004). [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 9:12AM |
H33.00004: Theory of Optical Manipulation of Electron Spins in Semiconductors Invited Speaker: |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H33.00005: Carrier Dynamics in Microdisk Photonic Molecules Felix M. Mendoza, Roberto C. Myers, Greg Calusine, Arthur C. Gossard, David D. Awschalom, Xia Li, B.J. Cooley, Nitin Samarth Semiconductor microcavities offer unique means of controlling light-matter interactions, which may be important in optical communications and for quantum information processing schemes. The cavities under study here are coupled microdisks that behave like ``photonic molecules'' (PMs) with bonding and antibonding states for the confined photon modes. We study different PM geometries consisting of laterally coupled GaAs/GaAlAs microdisks of both circular and elliptical shape. Steady-state photoluminescence measurements reveal bonding and antibonding modes with distinct polarization characteristics. Additionally, we present direct time-resolved spectroscopy of the carrier and spin dynamics in these structures. The combination of static and dynamic spectroscopies is used to explore the evolution of spin coherence in photonic molecule structures. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H33.00006: Ultrafast Coherent Precession of Interfacial Electron Spins in Fe/AlGaAs (001) Yichun Fan, Haibin Zhao, Gunter Lupke, Aubrey Hanbicki, C. Li, Berend Jonker Magnetic second harmonic generation (MSHG) is used to selectively study the spin dynamics of the interface magnetization for Fe films on AlGaAs(001) in both the dc and time-resolved domain. The interface spin precession is compared with the bulk Fe precession obtained by MOKE. We find: (a) the coherent precession of the interface magnetization is largely non-collinear to the bulk Fe electron spins even at the picosecond time scale; (b) higher frequency spin precession occurs at the interface than in the bulk; (c) the phase of the interface precession is opposite to that of the bulk at low fields; and (d) the interface and bulk precession exhibit different hysteretic behavior. Model calculations of the field dependence of precession frequencies show a large out-of-plane magnetic anisotropy and a large ratio of in plane uniaxial to cubic anisotropy at the interface, attributed to its tetragonal structure and Fe-As bonding. These may account for the significant reduction in exchange coupling between the interface and bulk magnetization. These effects are studied as a function of Fe film thickness and the corresponding evolution of the anisotropy, as well as for the Fe/Si(001) and Fe/Al2O3 interfaces. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H33.00007: Spin galvanic effect between subband of InGaAs/AlGaAs 2DEG Junfeng Dai, Xiaodong Cui We report the electric current induced by optically injected spin current on the vicinity of a crossbar shaped InGaAs/AlGaAs two dimensional electronic gas (2DEG) without external electric or magnetic field at room temperature. The electric current is either both in-ward or both out-ward flowing through the crossbar, while the total Hall voltage or current remains negligibly. The geometric size, temperature and external magnetic field dependence of the electric current have been examined and the corresponding mechanism has been discussed. [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H33.00008: Manipulating nonlinear optical response from electron spins in a 2D electron gas via exciton injection Shannon O'Leary, Hailin Wang The well-known robustness of electron spin coherences in semiconductors has stimulated intense interest in the use of electron spins in semiconductors for spintronics, quantum information processing, and coherent nonlinear optics. Of special importance to these efforts is the understanding and the manipulation of nonlinear optical processes of electron spins. Here, we report experimental studies of coherent nonlinear optical processes of electron spins in a modulation-doped CdTe semiconductor quantum well. These studies elucidate the important roles of trions and excitons and the underlying manybody interactions in the nonlinear optical process. By exploiting a two-color three-pulse pump-control-probe technique, we demonstrate that nonlinear optical responses of electron spins can be effectively manipulated through the injection of an exciton population at an appropriate time. The manipulation of the nonlinear response takes place without electron spin rotation, providing a new approach for the control and applications of electron spins in semiconductors. [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H33.00009: Rate equation modeling of semiconductor spin-polarized lasers and diodes Christian Gothgen, Athos Petrou, Igor Zutic Optically or electrically pumped spin-polarized carriers into semiconductor lasers can provide important advantages as compared to the conventional lasers in which the carriers are unpolarized. Motivated by recent experiments in spin-polarized lasers which demonstrate the feasibility of polarization modulation and threshold current reduction [1,2], we model these structures using rate equations. Our approach allows a direct comparison of the analytical and numerical results applied to the steady-state laser response. In the absence of material gain, our findings describe the behavior of spin-polarized diodes. We calculate the dependence of threshold reduction on the degree of pumped spin polarization and suggest how a change in the spin polarization could provide several useful device functionalities. \newline [1] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007). \newline [2] J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [Preview Abstract] |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H33.00010: Theory of spin-polarized semiconductor lasers Rafal Oszwaldowski, Christian Gothgen, Igor Zutic In semiconductor systems spin-polarized electrons couple to photons with definite angular momentum. This effect is the basis for numerous existing and proposed devices [1]. Quantum-well based Vertical Cavity Surface Emitting Lasers (VCSELs) take advantage of this phenomenon to produce circularly-polarized light by using either optical or electrical pumping [2]. We describe the VCSEL system employing Semiconductor Bloch Equations. We include the influence of spin-orbit coupling and the dependence of dipole matrix elements on carrier's wavevectors. We reduce this description to an effective four-level model, incorporating such effects as different spin lifetimes for electrons/holes and laser-cavity birefringence. Applying this approach to a spin-polarized system, we calculate the threshold current, the polarization of the emitted light and other relevant quantities. [1] I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). [2] M. Holub et al., Phys. Rev. Lett. 98, 146603 (2007); J. Rudolph et al., Appl. Phys. Lett. 82, 4516 (2003). [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H33.00011: Disorder and many-body effects in transport and optical conductivities of diluted magnetic semiconductors Fedir Kyrychenko, Carsten A. Ullrich The nature of itinerant carriers in diluted magnetic semiconductors like GaMnAs is a subject of intense current debate. The valence-band picture has been widely used, but recent experimental results suggest that the carriers reside in impurity bands. Theoretical results have not been fully conclusive. Most studies within the valence-band picture treat band structures in detail, while disorder and many-body effects are only treated with simple relaxation time and static screening models. We present a more complete theory for electron dynamics in DMSs, combining a multiband ${\bf k\cdot p}$ approach with first-principle descriptions of disorder and many- body effects through the memory function formalism and time- dependent density functional theory. This allows us to capture dynamic screening and collective electronic excitations of the itinerant carriers as well as correlations between impurities. We calculate transport properties and optical conductivities for GaMnAs and compare with experimental results. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H33.00012: Theory for Beats Observed in Optical Orientation Experiments on n-GaAs Nicholas Harmon, William Putikka, Robert Joynt At low temperatures in n-type semiconductors optical orientation experiments can create two types of spin states: itinerant states in the conduction band and localized states on the donor sites. These two states will in general have differnt gyromagnetic ratios, and will thus precess at different frequencies when the system is put into a magnetic field. Strong evidence for two types of spins would be to see beats in optical orientation experiments. We have developed a model in the context of coupled Bloch equations to describe this phenomenon. Unpublished data by Awschalom and Kikkawa on n-GaAs at a temperature of 5 K and in a magnetic field of 6 T show beats. We apply our model to this data and interpret it in terms of spins in a Landau level formed from the conduction band states and localized spins on the donor sites. [Preview Abstract] |
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