Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session S33: Focus Session: Mostly III-V Semiconductors |
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Sponsoring Units: GMAG FIAP DMP Chair: Steve von Molnar, Florida State University Room: Morial Convention Center 224 |
Wednesday, March 12, 2008 2:30PM - 2:42PM |
S33.00001: Strain-induced Fermi contour anisotropy of GaAs (311)A 2D holes Javad Shabani, Mansour Shayegan, Roland Winkler There is considerable current interest in electronic properties of two-dimensional (2D) carriers whose energy bands are spin-split at finite values of in-plane wave vector, thanks to the spin-orbit interaction and the lack of inversion symmetry. We report experimental and theoretical results revealing that the spin-subband Fermi contours of the heavy and light heavy-holes (HHh and HHl) can be tuned in high mobility GaAs (311)A 2D hole systems via the application of symmetry-breaking in-plane strain. Our calculations show that the HHl spin-subband Fermi contour is circular but the HHh spin-subband Fermi contour is distorted. Experimentally, we probe the Fermi contour anisotropy by measuring the magneto-resistance commensurability peaks induced by square arrays of antidots. When the spin splitting is sufficiently large, the magneto-resistance trace exhibits two peaks, providing clear evidence for spin-resolved ballistic transport. The experimental results are in good agreement with the calculations, and confirm that the majority spin-subband (HHh) has a severely distorted Fermi contour whose anisotropy can be tuned with strain while Fermi contour of the minority spin-subband (HHl) remains nearly isotropic. [Preview Abstract] |
Wednesday, March 12, 2008 2:42PM - 2:54PM |
S33.00002: Strain-induced spin splittings in III-V and II-VI semiconductors B.J. Moehlmann, Michael E. Flatt\'e We have calculated the strain-induced spin splittings in III-V and II-VI semiconductors using a fourteen-band basis for a strain-dependent $k \cdot p$ Hamiltonian. Using deformation potentials from pseudopotential calculations we find quantitative agreement with the precessional rates observed in [1]. For GaAs, the contribution of upper-conduction-band deformation potentials to the strain-induced spin splitting is not negligible. The ratio of the strain-induced spin precession frequency to the drift velocity is similar for GaAs and InAs, but is an order of magnitude larger for GaSb and InSb. For ZnSe it is a factor of 2 smaller than GaAs. This work was supported by an ONR MURI. [1] Kato et al., Nature 427, 50 (2004). [Preview Abstract] |
Wednesday, March 12, 2008 2:54PM - 3:06PM |
S33.00003: Electric field dependent $g$ tensor modulation for a hydrogenic Si donor in bulk GaAs Amrit De, Craig E. Pryor, Michael E. Flatt\'e We present calculations demonstrating the feasibility of electrical manipulation of a donor bound electron spin using $g$ tensor modulation resonance. We calculate the electron $g$ factor as a function of applied electric field, and show that $dg/dE$ is largest when the applied magnetic and electric fields are parallel. Unlike quantum dots, the donor's $g$ factor is highly nonlinear as a function of the applied magnetic field, and the anisotropy of the $g$ tensor also has a strong magnetic field dependence. The calculations are for a Si impurity in GaAs, and are done using real-space 8-band k.p theory in the envelope approximation. [Preview Abstract] |
Wednesday, March 12, 2008 3:06PM - 3:18PM |
S33.00004: Gilbert Damping in (Ga, Mn) As Ion Garate, Allan MacDonald We examine Gilbert damping in (Ga,Mn)As by using a p-d mean-field theory model for the ferromagnetic ground state and a four-band spherical model for the host semiconductor band structure. Within this model it is possible to calculate impurity vertex- corrections to the long-wavelength spin-spin response functions to all orders. Because of spin-orbit coupling in the band structure, beyond leading order vertex corrections make significant contribution to the damping. We comment on the non-monotonic dependence of damping on impurity strength. [Preview Abstract] |
Wednesday, March 12, 2008 3:18PM - 3:30PM |
S33.00005: Magnetic patterning of GaMnAs layers by hydrogen passivation L. Thevenard, A. Lemaitre, G. Faini, N. Vernier, J. Ferre, S. Fusil In order to study the magnetic switching behavior of diluted magnetic semiconductor (DMS) microstructures, we have patterned thin layers of (Ga,Mn)As by an original method. We have used local hydrogen passivation to locally suppress the carrier-mediated ferromagnetic phase. This purely diffusive process maintains the continuity of the film and smoothes border effects. Two types of structures were examined by Kerr microscopy, and their magnetic behavior compared to that of structures made by dry etching$^{1}$. On hydrogenated arrays of micron-sized magnetic dots, the switching fields were closer to the continuous film coercivity, and with a smaller dispersion. On micron-wide magnetic stripes, current-induced domain wall (DW) propagation was observed with typical currents as low as j = 4.10$^{5}$ A.cm$^{-2}$, a result of the low edge roughness induced by the patterning, and the low density of magnetic atoms. Local hydrogen passivation therefore appears as a viable route towards lower injection currents in DMS spintronics devices based on DW manipulation. $^{1}$L. Thevenard et al., Appl. Phys. Lett. 91, 142511 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 3:30PM - 3:42PM |
S33.00006: Infrared longitudinal and Hall conductivities in Ga$_{1-x}$Mn$_{x}$As films Gheorghe Acbas, M.-H. Kim, J. Cerne, M. Cukr, V. Novak, T. Jungwirth, J. Sinova We determine the complete infrared (0.1-1.2 eV) magneto-conductivity tensor of a series of Ga$_{1-x}$Mn$_{x}$As films from the complex Faraday and Kerr angles as outlined in M.-H. Kim, et al., Phys. Rev. B 75, 214416 (2007). A systematic series of samples with varying Mn and hole concentrations is studied. The samples range from insulating to metallic. The frequency dependence of the real part of the longitudinal conductivity $\sigma _{xx}$ is consistent with the values determined from transmission and reflection measurements. The complex transverse (Hall) conductivity $\sigma _{xy}$ shows resonances associated with the inter-valence band transitions. As the Mn concentration decreases these transitions become broadened due to increased disorder. The temperature dependence shows non-monotonic behavior with sign changes at certain wavelengths. The data is compared with predictions from a disordered valence band model (T. Jungwirth, et al., Phys. Rev. B 76, 125206 (2007)). This work is supported by the Research Corporation Cottrell Scholar Award (Buffalo and Texas A$\backslash ${\&}M), NSF-CAREER-DMR0449899 (Buffalo), an instrumentation award from the CAS, Univ. at Buffalo, ERAS-CT-2003-980409 (Prague)and NSF-CAREER-DMR-0547875 (Texas A$\backslash ${\&}M). [Preview Abstract] |
Wednesday, March 12, 2008 3:42PM - 3:54PM |
S33.00007: An n-type tunable two-dimensional ferromagnetic semiconductor Angelo Bove, F. Altomare, N. Kundtz, A. Chang, Y.J. Cho, X. Liu, J. Furdyna In the past two decades ferromagnetic semiconductors have been the focus of intense studies because of their potential technological application for spintronics. Particular attention has been dedicated to III-V Diluted Magnetic Semiconductors (DMS), where the ferromagnetism (FM) is hole-mediated and the Curie temperature can therefore be tuned by changing the concentration of free carriers\footnote{T. Dietl \textit{et al.}, Phys. Rev. B \textbf{63}, 195205 (2001)}. In these structures, the Anomalous Hall Effect (AHE) has played a key role in establishing that FM is hole-mediated. We will present data that show the first evidence of electron-mediated FM in GaMnAs. Our heterostructure has a low carrier density ($\sim$ $1.1E12 cm^-2$), a mobility of $\sim$ $600 cm^2/(Vs)$ and excellent gating capabilities. We will also present data that show the first clear bound on the AHE in an electron-mediated DMS and find it much reduced in magnitude when compared to the case of hole-mediated FM. [Preview Abstract] |
Wednesday, March 12, 2008 3:54PM - 4:06PM |
S33.00008: Electrically-induced Spin Coherence by Ultrafast Electrical Spin Injection. B. Beschoten, L. Schreiber, J. Moritz, C. Schwark, G. Guentherodt, X. Lou, P. Crowell, C. Adelmann, C. Palmstrom Efficient electrical spin injection from a ferromagnet into a semiconductor has been demonstrated for various material systems by steady-state experiments. We introduce a novel time-resolved technique based on electrical pumping and optical probing. As a pump we apply ultrafast current pulses ($\sim $200ps) to electrically inject spin packets from an iron layer through a reverse biased Schottky barrier into a n-GaAs layer. Spin coherence in the semiconductor is probed by subsequent spin precession in a transverse magnetic field using time-resolved Faraday rotation. We observe spin precession for current pulse widths down to 200 ps. The spin polarization of the spin packets is directly measured by Faraday rotation and is found to increase linearly with the current pulse width for pulses shorter than 3 ns at small magnetic fields. This finding together with independent measurements of the samples' high frequency bandwidth indicate that even shorter than 200 ps pulses might be used for generating coherent spin currents in our devices. Work supported by BMBF, DFG and HGF. [Preview Abstract] |
Wednesday, March 12, 2008 4:06PM - 4:18PM |
S33.00009: Electrical detection of spin polarized current in semiconductors using Andreev reflection Partha Mitra, Meng Zhu, Nitin Samarth Electron transport across the interface between a superconductor and a normal conductor is sensitive to the spin populations of electrons in the conduction band of the latter, leading to the phenomenon of Andreev reflection. A characteristic non-linear behavior in differential conductance is observed in superconductor/ferromagnet bilayers, with a suppression of the conductivity below the superconducting gap. This allows the carrier spin polarization in the normal conductor to be extracted. We attempt to exploit Andreev reflection for measuring hole spin diffusion lengths in p-GaAs by studying a series of hybrid heterostructures of superconducting metal ( In or Nb) and a ferromagnetic semiconductor (GaMnAs), separated by p-GaAs spacer layers of different thickness. Qualitatively, our data show evidence for a finite spin polarization in the current that decreases with spacer thickness. [Preview Abstract] |
Wednesday, March 12, 2008 4:18PM - 4:30PM |
S33.00010: Reconfigurable spin logic gate in Gallium Arsenide C. Awo-Affouda, O.M.J. van 't Erve, M. Holub, C.H. Li, A.T. Hanbicki, G. Kioseoglou, B.T. Jonker Electrical injection and detection of pure spin currents has recently been shown in semiconductors. Here we concentrate on the realization of spin-based logic circuits in semiconductors. We report on the electrical injection and detection of spin polarized currents using reconfigurable magnetic contacts. Fe/GaAs Schottky contacts are used to create and analyze the spin current in a GaAs transport channel. Non-local detection techniques show that the circuit output voltage can be modulated using current carrying wires that independently switch the magnetization of the contacts. We use this effect to generate a logic function based on pure spin transport in semiconductors. The realization of this integrated spin-based structure may facilitate the development of pure spin-based logic gates. [Preview Abstract] |
Wednesday, March 12, 2008 4:30PM - 4:42PM |
S33.00011: Inversion of Ferromagnetic Proximity Polarization in GaAs by MgO Interlayers Y. Li, Y. Chye, Y.F. Chiang, K. Pi, W.H. Wang, J.M. Stephens, S. Mack, D.D. Awschalom, R.K. Kawakami Ferromagnet/semiconductor hybrid structures are building blocks for spin transport devices and spin-based logic gates for large- scale circuits. Recent experiments achieved success in making a lateral Fe/GaAs spin transport device, but anomalous bias dependence of the spin injection signal demand an understanding of the role of atomic-scale interfacial structure in determining the spin dependent reflection and transmission coefficients. In our studies, we incorporate a spin-filtering material MgO in the Fe/GaAs structure, and directly study the spin dependent reflection properties of the interface, or the ferromagnetic proximity polarization (FPP) effect, through ultrafast optical measurements. We find that the FPP in Fe/MgO/GaAs can be tuned by controlling MgO thickness, and we observe a sign change by MgO interlayers. Through study of the related nuclear spin polarization, we also observed sign change of FPP with laser intensity when MgO thickness is in the transition range of sign change. By modification of the interface, mainly changing oxygen partial pressure during MgO growth, we find that the Fe-Mg bond is a key factor in the sign change. Supported by CNID, ONR and NSF. [Preview Abstract] |
Wednesday, March 12, 2008 4:42PM - 4:54PM |
S33.00012: GaMnAs-based hybrid multiferroic memory device . M. Overby, A. Chernyshov, L.P. Rokhinson, J.K. Furdyna, X. Liu In a ferromagnetic semiconductor GaMnAs grown on GaAs there are two equivalent easy axes of magnetization along the [100] and [010] crystallographic directions. These two directions can form a basis for a memory device with the binary state being encoded in the direction of the magnetization. The state can be electrostatically controlled by introducing a compressive (tensile) strain along one of the easy axes. We demonstrate a novel non-volatile hybrid multiferroic memory cell with electrostatic control of magnetization based on strain-coupled GaMnAs and a piezoelectric material. ~The magnetization direction is monitored via planar Hall effect, which changes sign when magnetization rotates. At zero voltage on the piezoelectric magnetization can be oriented either along [100] or [010], when voltage on piezoelectric is swept between positive and negative values magnetization forms a hysteresis loop with abrupt transitions between the two orientations. [Preview Abstract] |
Wednesday, March 12, 2008 4:54PM - 5:06PM |
S33.00013: Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: First-principles calculations A.N. Chantis, K.D. Belashchenko, D.L. Smith, E.Y. Tsymbal, M. van Schilfgaarde, R.C. Albers A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin density approximation we calculate spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts [1,2] and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions [3]. \newline [1] S. A. Crooker {\it et al.}, Science {\bf 309}, 2191 (2005) \newline [2] X. Lou {\it et al.}, Nature Phys. 3, {\bf 197} (2007) \newline [3] J. Moser {\it et al.}, Appl. Phys. Lett. {\bf 89}, 162106 (2006) [Preview Abstract] |
Wednesday, March 12, 2008 5:06PM - 5:18PM |
S33.00014: Spin-Polarized Edge-Emitting Lasers M. Holub, A.T. Hanbicki, C.S. Kim, G. Kioseoglou, O.M.J. van 't Erve, C.H. Li, I. Vurgaftman, J.R. Meyer, B.R. Bennett, B.T. Jonker Semiconductor lasers driven by a spin-polarized current are expected to provide a threshold current reduction, optical polarization control, and intensity stabilization. We explore these possibilities in edge-emitting lasers where the low switching fields of in-plane magnetized Fe thin films should enable electronic modulation of the output polarization. Fe/AlGaAs/GaAs heterostructures are designed, grown, and fabricated into surface-emitting light-emitting diodes (LEDs) and double heterostructure lasers. The LED emission is dominated by an H-band feature at 5 K and by the bulk recombination feature at 20 K. An electron spin polarization of 24{\%} is measured in the Faraday geometry. Oxide-stripe lasers are fabricated with and without an Fe capping layer. Lasing is observed at low temperatures with threshold current densities of $\sim $100 A/cm$^{2}$. Magnetic field-dependent studies to examine the effects of spin injection on laser performance will be discussed. [Preview Abstract] |
Wednesday, March 12, 2008 5:18PM - 5:30PM |
S33.00015: Spin Injection into Co$_{2}$MnAl by Optical Absorption in GaAs S. Isber, Y.J. Park, J.S. Moodera, D. Heiman Ferromagnet-semiconductor heterostructure devices have been made with half-metallic ferromagnets, which have larger spin polarizations than transition metal based ferromagnets. Spin-polarized electrons generated in the semiconductor by circularly polarized light are injected across a Schottky barrier into the ferromagnet and detected as photocurrent. In this technique, the optically pumped semiconductor is the source of spin polarized electrons, and the ferromagnet is the detector of spin polarized electrons. The heterostructures consist of MBE-grown Co$_{2}$MnAl Heusler ferromagnets which are lattice matched to n$^{+}$GaAs, forming a Schottky barrier. A magnetic field is applied parallel to the light and perpendicular to the ferromagnetic layer, allowing the magnetization to be switched from parallel to antiparallel to the light direction, thus switching the direction of the detected spin polarization. Spin-dependent photocurrent was measured as a function of applied voltage bias across the Schottky barrier. The injection of spin-polarized electrons was determined after subtracting the magnetic circular dichroism (MCD) effect. [Preview Abstract] |
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