Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Q33: Focus Session: Spin Polarization in Compound Semiconductors |
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Sponsoring Units: GMAG FIAP DMP Chair: Stu Wolf, University of Virginia Room: Morial Convention Center 224 |
Wednesday, March 12, 2008 11:15AM - 11:51AM |
Q33.00001: Boundaries Between Current Carrying Semiconductors and Metallic Contacts Invited Speaker: |
Wednesday, March 12, 2008 11:51AM - 12:03PM |
Q33.00002: Current-Induced Spin Polarization in Gallium Nitride W. Koehl, C. Poblenz, M.H. Wong, U. Mishra, J. Speck, D.D. Awschalom Recent experimental studies have shown that when a current is passed through certain non-centrosymmetric semiconductors (GaAs, ZnSe), a net spin polarization may be generated throughout the material. However, the physical mechanism responsible for this phenomenon, known as current-induced spin polarization (CISP), is still poorly understood but known to exist at high temperatures in wide-bandgap semiconductors. [1] In order to further explore the degree to which CISP depends on the band structure and spin lifetimes of a material, we measure the phenomenon in GaN, a wide-bandgap, non-centrosymmetric semiconductor. A series of n-type GaN epilayers are grown in the wurtzite phase via molecular beam epitaxy at a variety of doping densities chosen to modulate the transverse spin lifetime, T$_{2}^{\ast }$, across its full available range. Using the Kerr effect, CISP is then characterized in these epilayers as a function of excitation energy over a range of temperatures. \newline \newline [1] N. P. Stern, S. Ghosh, G. Xiang, M. Zhu, N. Samarth, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{97}, 126603 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 12:03PM - 12:15PM |
Q33.00003: Spin polarization in quantum point contact structures Anh Ngo, Sergio Ulloa One of the important goals in the field of spintronics is to produce spin-polarized currents in semiconductors [1]. The Rashba spin-orbit interaction is useful in this regard, because its strength is controllable by applying an electric field. In this work we study ballistic transport through semiconductor quantum point contact systems under different confinement geometries and applied fields. In particular, we investigate how the lateral spin-orbit coupling, as induced by the lateral confinement potential, plays a non-trivial role on the spin polarization of the current, even in the absence of magnetic field. We find that high spin polarization can be obtained by controlling the asymmetric shape of the confinement potential, and contrast our results with previous work in the literature [2]. This behavior suggests a novel scheme to implement spin-filters without external magnetic fields, and we present its dependence on structural parameters. \newline [1] S. A. Wolf, et al., Spintronics: a spin based electronic vision of the future, Science 294, 1488-1495 (2001). \newline [2] M. Eto, et al., Spin polarization at semiconductor point contacts in absence of magnetic field, J. Phys. Soc. Jpn. 74, 1934 (2005). [Preview Abstract] |
Wednesday, March 12, 2008 12:15PM - 12:27PM |
Q33.00004: All-Electric Quantum Point Contact Spin Polarizer Philippe Debray, Saydur Rahman, Steven Herbert, Marc Cahay, Richard Newrock The conductance of InAs quantum point contacts (QPCs), created by two side gates on InAs/InGaAs quantum-well structures, was measured at low temperatures ($\le $ 4.2K) as a function of Fermi energy. By tuning the bias voltages of the gates, we were able to make appear or disappear on demand a conductance plateau at G $\cong $ 0.5 (2e$^{2}$/h). The presence of this plateau indicates complete spin polarization in the fundamental mode of transport. The 0.5 plateau appears when the transverse confining potentials of the QPC are tuned to be highly asymmetric. We believe the spin polarization responsible for the 0.5 plateau is induced by the lateral spin-orbit coupling, which originates from the transverse electric field of the confining potentials at the edges of the QPC. In a strong perpendicular magnetic field the magnetic confinement screens out the electrostatic confinement and the 0.5 plateau disappears. Our results show that it is possible to use an InAs QPC as a spin polarizer of both spin species through appropriate tuning of the bias voltages of its side gates. [Preview Abstract] |
Wednesday, March 12, 2008 12:27PM - 12:39PM |
Q33.00005: Nanomechanical spin-polarizer Alexey Kovalev, Liviu Z\^arbo, Yaroslav Tserkovnyak, Gerrit Bauer, Jairo Sinova We study the effects of time dependent strain on transport properties in a long semiconductor rod and predict a piezo-spin effect. Torsional oscillations of a free-standing semiconductor beam are shown to cause spin-dependent oscillating with time potentials that spin-polarize an applied charge current in the presence of intentional or disorder scattering potentials. One can apply AC voltage synchronized with the mechanical motion to obtain a DC spin current. The thus created spin accumulation in the leads can be detected by e.g. ferromagnetic side contact or by the optical Kerr rotation. We propose to build the mechanical spin generators and manipulators based on the piezo-spin effect. [Preview Abstract] |
Wednesday, March 12, 2008 12:39PM - 12:51PM |
Q33.00006: Generation and Decay of Spin Flip Waves in a CdMnTe Quantum Well P. Jacobs, R. Merlin, C. Aku-Leh, F. Perez, G. Karczewski Recently, two dimensional electron gases (2DEG) embedded in semimagnetic Cd$_{1-x}$Mn$_{x}$Te quantum wells have been introduced as a model for spin-polarized systems. The addition of a small amount of Mn induces a giant Zeeman splitting such that, under moderate magnetic fields, spin effects dominate over orbital quantization, providing the reverse situation to that of GaAs. Using Raman spectroscopy, both collective and single-particle spin excitations have been observed in CdMnTe [1]. We have used ultrafast pump-probe spectroscopy to study collective spin-flip excitations in modulation doped CdMnTe quantum wells. Oscillations due to the zone center spin-flip wave were generated by circularly-polarized 70fs pump pulses and detected by Kerr rotation of linearly-polarized probe pulses. The spin-flip lifetime has a strong dependence on the applied magnetic field and is also affected by weak heating due to laser absorption. The dependence of the lifetime on excitation energy and magnetic field indicate that the optically-excited heavy holes and their spin orientation play an important role in the decay of the collective mode. [1] F. Perez et al., Phys. Rev. Lett. \textbf{99}, 026403 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 12:51PM - 1:03PM |
Q33.00007: Quantum Anomalous Hall Effect in HgMnTe Quantum Wells Chaoxing Liu, Xiaoliang Qi, Xi Dai, Zhong Fang, Shoucheng Zhang Quantum Hall effect is usually observed in the two-dimensional electron gas with an external magnetic field, where the electronic states form Landau levels. In this work, we show that a new phenomenon, the quantum anomalous Hall effect, can be realized in HgMnTe quantum wells, without the external magnetic field and associated Landau levels. This effect originates purely from the magnetization of Mn atoms, and is closely related to the quantum spin Hall effect observed in HgTe quantum wells recently. The opposite signs of sp-d exchange coupling between the Mn atoms and conduction or valence band electrons is crucial for realizing this effect. The quantized Hall conductance is predicted for a range of quantum well thickness and concentration of Mn atoms. Within the experimentally accessible parameter regime, we propose an experiment to demonstrate that the quantized Hall conductance indeed arises from the Mn magnetic moments rather than Landau levels. This effect enables dissipationless charge current in spintronics devices. [Preview Abstract] |
Wednesday, March 12, 2008 1:03PM - 1:15PM |
Q33.00008: The Effect of Detector Bias on Non-Local Measurements of Spin Transport E.S. Garlid, T. Kondo, R. Mohr, K.S.M. Reddy, Q. Hu, P.A. Crowell, C.J. Palmstr{\O}m Previous studies of spin transport in Fe/GaAs heterostructures have observed a strong non-monotonic dependence of the spin polarization on the bias across the injector contact in a non-local measurement. We have studied the dependence of the non-local voltage signal in Fe/GaAs/Fe spin valves as a function of detector bias. Measurements were made on lateral devices fabricated from epitaxial Fe/n$^+$/n-GaAs heterostructures with channel dopings ranging from $2\times 10^{16}-6\times 10^{16}$~cm$^{-3}$. Measurements were performed by biasing the detector electrode with respect to a reference electrode and using lock-in techniques to measure the spin polarization generated from an AC biased injector electrode. The non-local voltage signal was found to have a very different dependence on detector bias than on injector bias. A sign change was observed at both forward and reverse detector bias, and the detector sensitivity was enhanced by up to a factor of five under large forward bias. This enhanced sensitivity may reflect the energy dependence of the Fe/GaAs interfacial density of states, although the connection between charge and spin transport in the semiconductor channel must also be considered. This work was supported by ONR and the NSF MRSEC, IGERT, and NNIN programs. [Preview Abstract] |
Wednesday, March 12, 2008 1:15PM - 1:27PM |
Q33.00009: Growth and Characterization of VLS-type (Zn,Mn)Se nanowires Benjamin Cooley, Trevor Clark, Nitin Samarth Magnetically-doped semiconductor nanowires offer an interesting regime for exploring carrier-induced ferromagnetism in the presence of a 1D density of states [Dietl {\it et al}, Phys. Rev. B {\bf 55}, R3347 (1997)]. We discuss the growth and structural characterization of ZnSe nanowires doped with Mn. With diameters ranging from $\sim $30 nm down to $\sim$5 nm, these wires potentially provide 1D or quasi-1D systems in which to study collective magnetic behavior. The wires are grown via the vapor- liquid-solid mechanism on GaAs substrates using an ultrahigh vacuum molecular beam epitaxy system, with gold nanoparticles as the seeds for wire growth. The wires form in a dense random array, as shown by scanning electron microscopy. Transmission electron microscopy is used to study the structure and growth direction of individual wires released from the substrate, and the composition of individual wires is studied with energy dispersive x-ray spectroscopy. Supported by NSF MRSEC and ONR MURI. [Preview Abstract] |
Wednesday, March 12, 2008 1:27PM - 1:39PM |
Q33.00010: Enhancement of In-Plane Magnetic Anisotropy Through Compensation in Ga$_{1-x}$Mn$_{x}$P:S P.R. Stone, O.D. Dubon, K.M. Yu, J.W. Beeman, C. Bihler, M.S. Brandt Ga$_{1-x}$Mn$_{x}$P is a ferromagnetic semiconductor (FS) in which exchange is mediated by localized holes [Scarpulla \textit{et al.}, Phys. Rev. Lett. \textbf{95,} 207204 (2005)]. As is the case for the prototypical FS Ga$_{1-x}$Mn$_{x}$As, there exists a uniaxial magnetic anisotropy between in-plane $<$110$>$-type directions with the magnetic easy axis lying near the in-plane [1-10] direction [Bihler \textit{et al.}, Phys. Rev. B \textbf{75,} 214419 (2007)]. Here we report the effect of compensation of Mn acceptors by sulfur donors on the in-plane uniaxial magnetic anisotropy in Ga$_{1-x}$Mn$_{x}$P as measured by both ferromagnetic resonance (FMR) and SQUID magnetometry. Raising the S concentration increases the magnitude of the uniaxial magnetic anisotropy between in-plane $<$110$>$-type directions. While the [1-10] direction remains the easy axis in the plane of the film, ``wasp-waisted'' hysteresis loops develop in the [110] direction as the S concentration increases. The wasp-waisted loops are modeled whereby magnetization reversal occurs by a combination of coherent spin rotation and noncoherent spin switching. Finally, by comparing FMR and SQUID data we extract domain wall formation energies as a function of compensation. [Preview Abstract] |
Wednesday, March 12, 2008 1:39PM - 1:51PM |
Q33.00011: Deposition and characterization of highly spin-polarized Co$_{1-x}$Fe$_{x}$S$_{2}$ thin films. Michael Manno, C. Leighton Many spintronic devices could benefit from the use of a highly spin polarized ferromagnet. We have recently demonstrated tunable spin polarization of --56 $< \quad P \quad <$ +85 {\%} by composition control of the Fermi level in bulk Co$_{1-x}$Fe$_{x}$S$_{2}$ [1]. Although this material offers great promise for fundamental studies of spintronic devices, integration in devices such as spin injectors requires deposition of thin films. We present here synthesis details, structural, electronic, and magnetic properties of polycrystalline Co$_{1-x}$Fe$_{x}$S$_{2}$ films successfully fabricated by \textit{ex-situ} sulfidation on Al$_{2}$O$_{3}$ (0001), SrTiO$_{3}$ (001), and GaAs (001). The synthesis involves exposure of sputter or MBE deposited Co and Co-Fe alloy films to a S atmosphere at various reaction temperatures. Significant reaction with S occurs only above 200 C, while at 350 C and above we observe single-phase CoS$_{2}$ XRD patterns, and S contents (from EDS) of 65-70 {\%}. The conversion process has been studied in detail using multiple probes. Optimal conditions produce films that are ferromagnetic with the bulk T$_{C}$, bulk magnetization, metallic transport, and the expected magnetotransport phenomena. [1] L. Wang et al., Phys. Rev. B. \textbf{73} 144402 (2006). [Preview Abstract] |
Wednesday, March 12, 2008 1:51PM - 2:03PM |
Q33.00012: Spin-filtering properties of ultra thin Eu chalcogenide films Martina Mueller, Marius Costache, Jagadeesh Moodera Promising materials for the generation of nearly fully polarized current are the magnetic semiconductors EuO and EuS when used as tunnel barriers. The spin-filter effect is due to exchange splitting of their conductance band below the ferromagnetic transition temperature, T$_{C}$. Combined with a structural and electrical compatibility with Si, Eu chalcogenides can be recognized as potential materials to study spin injection into semiconductors. In this work, special attention was drawn to the magnetic and transport properties of thin ($<$ 6nm) EuO and EuS films to explore the feasibility of their integration into spin-injection devices. We investigated the magnetic, structural and transport behavior of EuO (EuS) thin films with regard to thickness- and substrate-induced changes. The influence of reduced dimensionality on exchange splitting and spin filter efficiency was observed in transport experiments using EuO (EuS) as a tunnel barrier. ``The phenomena of spin filter tunneling'', J. S. Moodera, T. S. Santos and T. Nagahama, J. Phys.: Condens. Matter \textbf{18 }(2007) 1--24 -- A review [Preview Abstract] |
Wednesday, March 12, 2008 2:03PM - 2:15PM |
Q33.00013: Resonant spin dipole induced by an in-plane potential gradient spin-orbit interaction C. S. Chu, K. Y. Chen, A. G. Mal'shukov Spin-orbit interaction (SOI) arising from in-plane potential gradient is invoked for the generation of spin accumulation in a driven electric field. The SOI and a local in-plane potential pattern together bring about resonant spin dependent scatterings to electrons in a nonequilibrium distribution. In the vicinity of a ring- shaped potential barrier pattern, a spin dipole distribution with a resonant dipole strength characteristic is obtained. As the chemical potential $\mu $is increased across one such resonant energy, the dipole strength manifests both reversal and large amplitude enhancement. The scattering resonance, thus, provides an additional knob for the manipulation of the spin accumulation. [Preview Abstract] |
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