Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin Manipulation and Phenomena in Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Igor Zutic, University at Buffalo Room: 104 |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J8.00001: Magnetotransport and Structural Properties of Mn$_{2}$CoAl Thin Film Spin Gapless Semiconductor Michelle E. Jamer, Badih A. Assaf, Trithep Devakul, Don Heiman Spin gapless semiconductors (SGS) are predicted to have a density of states displaying both half-metallic and zero-gap semiconducting properties. They are being investigated for spintronic devices due to their unique magnetic and electrical properties. Calculations predict several SGS compounds\footnote{S. Skaftouros, K. Ozdogan, E. Sasioglu, and I. Galanakis, App. Phys. Lett. 102, 022402 (2013).}$^,$\footnote{S. Ouardi, G. Fecher, and C. Felser, and J. Kubler, Phys. Rev. Lett. 110, 100401 (2013).} including Mn$_{2}$CoAl, Ti$_{2}$CoSi, V$_{3}$Al, and Ti$_{2}$MnAl. Mn$_{2}$CoAl thin films were grown by MBE on GaAs (100) substrates at 200 $^{\circ}$C.\footnote{M.E. Jamer, B.A. Assaf, T. Devakul and D. Heiman, Appl. Phys. Lett. 103, 142403 (2013).} The as-grown thin films were epitaxial with the substrate, which resulted in a tetragonal distortion. Annealing studies showed that the films lose their epitaxial registration and approach an aligned cubic structure for 325 $^{\circ}$C with a$=$c$=$5.80 {\AA}. The resistivity shows a thermally-activated semiconducting-like negative slope at higher temperatures. The Hall resistivity scales with $\rho _{xx}^{2}$ for all temperatures and magnetic fields, expected for a topological intrinsic anomalous Hall effect computed from the Berry phase curvature. The connection of electrical and spin-gapless properties is discussed. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J8.00002: Quantifying absolute spin polarization with non-magnetic contacts in FM/$n$-GaAs heterostructures Chad Geppert, Lee Wienkes, Kevin Christie, Sahil Patel, Chris Palmstr\O m, Paul Crowell We report on a novel method of quantifying spin accumulation in Co$_2$MnSi/$n$-GaAs and Fe/$n$-GaAs heterostructures using a non-magnetic probe. The presence of a non-equilibrium spin polarization generates a large electrostatic potential shift relative to the equilibrium state. This is due to the combination of (1) the parabolic (non-constant) density of states and (2) the population imbalance between the two spin sub-bands. We observe this shift as a Hanle effect in a non-local, non-magnetic semiconducting contact. Since this signal depends only on experimentally accessible parameters of the bulk semiconductor, its magnitude may be used to quantify the injected spin polarization in absolute terms. By comparison with the (smaller) spin-valve signal observed with a second ferromagnetic contact, we demonstrate that this electrostatic shift scales quadratically with spin polarization, dephases in the presence of both applied and hyperfine fields, and is observable to higher temperatures than traditional non-local measurements. Quantitative modeling allows extraction of absolute polarizations in excess of 50 \% at low temperatures, and further indicates that this contribution constitutes a large fraction of the three-terminal signal observed in these devices. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J8.00003: Electrical detection of FMR in epitaxial FM/$n$-GaAs heterostructures by tunneling anisotropic magnetoresistance Changjiang Liu, Yakov Boyko, Chad Geppert, Kevin Christie, Gordon Stecklein, Paul Crowell, Sahil Patel, Chris Palmstr\O m Electrical detection of ferromagnetic resonance (FMR) is a widely used technique for the detection of spin pumping. We report here a new means for electrical detection of FMR based on tunneling anisotropic magnetoresistance (TAMR). The TAMR is due to the spin-orbit coupling at the FM/$n$-GaAs interface. Because the tunneling resistance in this case depends on the orientation of the magnetization with respect to the crystalline axes, the opening of the cone angle when the FM is driven resonantly produces a dc voltage which is proportional the square of the time-dependent magnetization. Our samples are FM//(001) $n$-GaAs heterostructures grown by MBE, where the FM is Fe, Co$_2$MnSi, or Co$_2$FeSi. All three of these heterostructures show non-local spin transport effects when they are biased. In each case, we observe a symmetric FMR peak with a bias and angular dependence that tracks the TAMR almost exactly. The resonance field depends on the anisotropies as expected, and the linewidth is smallest for Co$_2$MnSi. At low temperatures, the amplitude of the FMR signal is clearly sensitive to the spin accumulation, but this effect is not due to spin pumping. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J8.00004: Experimental Measurements of $^{69/71}$Ga NMR in Optically-pumped NMR (OPNMR) of AlGaAs/GaAs Quantum Wells Sophia Hayes, Erika Sesti, Dustin Wheeler, Matt Willmering, Ryan Wood, Clifford Bowers, Dipta Saha, Christopher Stanton We have conducted photon-energy and helicity-dependent measurements of the $^{69}$Ga and $^{71}$Ga NMR signals that result from optical pumping of states in the conduction band. The sample we have used for these studies is a 60-well multiple quantum well sample of Al$_{0.34}$Ga$_{0.66}$As/GaAs. Our measurements show a particularly strong dependence of the OPNMR signal from the GaAs quantum wells, when irradiating at photon energies consistent with the spin-split light hole within the material. (We use a frequency-stabilized continuous wave Ti:sapphire ring laser, with a very narrow linewidth for these excitation.) The coupling to the light-hole has an important NMR signature which we will discuss in this presesntation. We will show results for multiple external magnetic field strengths (B$_{0})$ and for different laser light intensities. A thorough understanding of the ``fine structure'' observed in the photon energy dependence of these OPNMR signals is afforded through theoretical modeling of these results, which will be shown in a separate presentation. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J8.00005: Spin-Dependent Band Structure Spectroscopy in a Strained Al$_{0.1}$Ga$_{0.9}$As/GaAs Multiple Quantum Well by Optically Pumped Nuclear Magnetic Resonance Ryan Wood, Clifford Bowers, Dipta Saha, Christopher Stanton, Arneil Reyes, Philip Kuhns, Stephen McGill, Sophia Hayes We present the photon energy dependence of optically pumped NMR (OPNMR) signals in a Si-$\delta $-doped Al$_{0.1}$Ga$_{0.9}$As/GaAs multiple quantum well (MQW). Data was acquired at 3.9 T, 4.9 T, and 9.4 T with different polarizations of light. The OPNMR spectra exhibit a strain-induced nuclear quadrupole splitting, caused by differential contraction of the MQW and the Si support to which it is epoxy bonded. The tensile strain in the GaAs quantum well layers, which is estimated from the observed quadrupole splitting, is included in band structure calculations based on the 8-band Pidgeon-Brown model generalized to include the effects of the confinement potential. Our OPNMR photon energy dependence data is compared with calculations of optically pumped electron spin polarization to correlate the OPNMR data with the strained MQW's band structure. Our results demonstrate that the OPNMR photon energy dependence is sensitive to strain-induced light-hole/heavy-hole splitting and quantum confinement effects on the MQW's band structure. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J8.00006: Modelling Optically Pumped NMR and Spin Polarization in AlGaAs/GaAs Quantum Wells Dipta Saha, Chris Stanton, R. Wood, C.R. Bowers, E. Sesti, S.E. Hayes, P.L. Kuhns, S.A. McGill, A.P. Reyes Optically Pumped NMR (OPNMR) is a combination of the optical pumping of semiconductors to create spin-polarized electrons and the direct detection of an enhanced NMR signal as the electron spin polarization is transferred to the nucleus. We present theoretical calculations for the average electron spin polarization at different photon energies for different values of external magnetic field in both unstrained and strained $Al_{x}Ga_{1-x}As/GaAs$ quantum wells. Comparison is made with the experimental OPNMR signal intensity. We identify the Landau level transitions which are responsible for the peaks in the OPNMR signal intensity. Our calculations are based on the 8-band Pidgeon-Brown model generalized to include the effects of the confinement potential as well as strain. In strained wells, the strain is calculated using a relation that associates the experimental value of the nuclear quadrupole splitting with the strain along a given axis. Optical properties are calculated using Fermi's Golden rule. Results show that the strength and sign the OPNMR signal is related to the average electron spin polarization. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J8.00007: Mapping the anisotropic Lande g-factor tensor of 1D GaAs holes in all 3 spatial directions Karina Hudson, Ashwin Srinivasan, Qingwen Wang, LaReine Yeoh, Oleh Klochan, Ian Farrer, David Ritchie, Alex Hamilton We have studied the Zeeman splitting of 1D holes formed on a (100) GaAs/AlGaAs heterostructure on a single cooldown. The strong spin orbit coupling and 1D confinment give rise to a highly anisotropic spin splitting. By use of the high-symmetry (100) crystal, we eliminate the effects of crystal anisotropy on our measurements. In measuring the spin splitting as a function of angle between the wire and the applied magnetic field, we are able to identify the principle axes of the $g$-tensor. We show that the principle axes are defined by the potential confining the 1D holes, and are not affected by the crystal axes. We find that $g_{\parallel}^{\perp} < g_{\parallel}^{\parallel} < g_{\perp}$, where $g_{\parallel}$ refers to the in-plane $g$-factors parallel and perpendicular to the wire, and $g_{\perp}$ refers to the $g$-factor perpendicular to the 2D well. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J8.00008: Optical spin injection in GaAs nanowires Paulo Eduardo Faria Junior, Guilherme Matos Sipahi, Igor Zutic Controlling quantum confinement in semiconductor nanowires (NWs) allow desirable spin-dependent properties and enable novel devices, such as spin-interconnects[1], spin-lasers[2,3] or spin-enhanced phonon lasers[4]. Typically, the key element in such applications is the presence of non-equilibrium spin population. Focusing on GaAs NWs of different cross-sectional areas, we analyze their carrier spin polarization based on k.p band structure calculations[5,6]. We show that shining circularly polarized light propagating along the NW axis provides a robust spin injection, reaching $\sim 100 \%$ and switchable by changing the incident photon energy. For optical spin injection in bulk GaAs near the $\Gamma$-point, we recover previously known results[7]. [1] H. Dery, et al., Appl. Phys. Lett. 99, 082502 (2011). [2] J. Lee, et al., Phys. Rev. B 85, 045314 (2012). [3] J. Sinova and I. Zutic, Nature Mater. 11, 368 (2012). [4] A. Khaetskii, et al., Phys. Rev. Lett. 111, 186601 (2013). [5] G. M. Sipahi, et al., Phys. Rev. B 57, 9168 (1998). [6] P. Li and H. Dery, Phys. Rev. Lett. 107, 107203 (2011). [7] F. Nastos, et al., Phys. Rev. B 76, 205113 (2007). [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J8.00009: Properties of InGaAs/InAlAs double quantum wells toward spin-filtering application Takaaki Koga, Shoichiro Yokota, Takashi Yamashige, Atsushi Sawada, Toru Matsuura, Sebastien Faniel, Satofumi Souma, Yoshiaki Sekine, Hiroki Sugiyama We recently determined the intrinsic constant for the Rashba effect in (001)InP-matched InGaAs/InAlAs quantum wells (QW) [1]. We now apply this knowledge to band-engineer a spin-filter based on the double QW (DQW) [2]. Firstly, we studied the subband energy spectra of the InGaAs/InAlAs DQW from the beatings in the Shubnikov de Haas (SdH) oscillations. The basic properties obtained here provide useful information to refine the device structures for the spin-filter devices based on the DQW system [2]. In our DQW samples, a thin barrier layer of In$_{0.52}$Al$_{0.48}$As ($d_{\rm B}=$ 1.5$\sim$5 nm) is sandwiched with non-doped QW layers of In$_{0.53}$Ga$_{0.47}$As ($d_{\rm QW}=$ 10 nm). By carefully tuning the doping densities above and below the DQW part and the top-gate voltage, the potential profile of this DQW should be ideally made symmetric about the middle barrier layer, though not yet realized in our experiment. The experimental results of our not-yet-ideal DQW samples showed clear beatings in the SdH data, which originate from both the subband and Rashba splittings. We report on our successful separation of these two effects based on the proper band-bending assumptions.\\[4pt] [1] Faniel, et.al., Phys. Rev. B {\bf 83}, 115309 (2011).\\[0pt] [2] Souma, et.al, arXiv:1304.6992. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J8.00010: Spin pumping efficiency in room temperature CdSe nanocrystal quantum dots Ahmad Khastehdel Fumani, Jesse Berezovsky To understand and optimize optical spin initialization in room temperature CdSe nanocrystal quantum dots (QDs) we studied the dependence of the pump energy $E$ on the time-resolved Faraday rotation signal in a series of QD samples with different sizes. In $6.1$-nm-diameter QDs, we observe two peaks in the spin signal vs. $E$. The first peak occurs on resonance at $1.95$eV, with the second peak $\sim 0.17$eV higher in energy before the spin signal falls off to near zero. We calculate the spin-dependent oscillator strengths of optical transitions using an 8-band effective mass model to understand these results. The observed $E$ dependence of the spin pumping efficiency (SPE) arises from the competition between the heavy hole (hh), light hole (lh) and split-off (so) band contributions to transitions to the conduction band. The two latter contributions lead to an electron spin polarization in the opposite direction from the former. At lower $E$ the transitions mainly involve the hh band, giving rise to the two main peaks. At higher $E$, the increasing contributions from the lh and so bands lead to a reduction in SPE. In smaller QDs, both peaks merge while moving to higher $E$, and the overall SPE is reduced. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J8.00011: Proposal for a phonon laser utilizing quantum-dot spin states Alexander Khaetskii, Vitaly Golovach, Xuedong Hu, Igor Zutic We propose [1] a nano-scale realization of a phonon laser utilizing phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long spin relaxation time, the device can be operated in a strong pumping regime, in which the population inversion is close to its maximal value allowed under Fermi statistics. In this regime, the threshold for stimulated emission is unaffected by spontaneous spin flips. Considering a nanowire with quantum dots defined along its length, we show that a further improvement arises from confining the phonons to one dimension, and thus reducing the number of phonon modes available for spontaneous emission. Our work calls for the development of nanowire-based, high-finesse phonon resonators. [1]. A. Khaetskii, V.N. Golovach, X. Hu, and I. Zutic, PRL {\bf 111}, 186601 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J8.00012: Two-State Polariton Lasing in a Planar Semiconductor Microcavity Feng-Kuo Hsu, Yi-Shan Lee, Sheng-Di Lin, Chih-Wei Lai We report room-temperature sequential polariton lasing at two distinct energies in a planar microcavity non-resonantly pumped by a 2-ps pulsed laser. The sample consists of multiple InGaAs/GaAs quantum wells embedded within GaAs/AlGaAs distributed Bragg mirrors. A sub-10-ps high energy (HE) lasing mode with a linewidth $\sim$ 3 meV commences within 10 ps after pump, and is followed by a 20 to 50 ps low energy (LE) mode with a transient linewidth $\sim$ 1 meV. The time-average degree of polarization of both states decrease with increasing photoexcited densities. Near above the lasing threshold, the HE state is spin polarized, resulting in fully circularly polarized radiation under a circularly polarized pump and partially circularly polarized radiation under a linearly polarized pump. In contrast, the LE state is partially linearly polarized with stochastic polarization orientations that are weakly correlated to the [110] crystalline direction. The energy difference between the two lasing modes is controlled by the photoexcited density and pump polarization. With increasing pump flux, the HE state blue-shifts about 5 meV, while the LE state red-shifts less than 1 meV. This two-state lasing effect exemplifies spontaneous symmetry breaking in a microcavity laser. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:30PM |
J8.00013: Putting Spin into Lasers Invited Speaker: Jeongsu Lee Considering circular polarization, an optical analog of electron spin, semiconductor lasers with spin-polarized carriers can open up unexplored possibilities for spin-controlled devices. Once spin-polarized carriers are introduced in the gain region of lasers, by circularly polarized light or electrical spin-injection, the operation of such spin-lasers should be revisited to incorporate their novel properties. Spin-polarized carriers can enhance the performance of lasers for communication and signal processing [1]. In the steady-state, such spin-lasers already demonstrated a lower threshold current for the lasing operation [2] compared to their conventional (spin-unpolarized) counterparts, however, the most exciting opportunities come from their dynamical operation. We reveal that the spin modulation in lasers can lead to an improvement in the two key figures of merit: enhanced bandwidth [3] and reduced parasitic frequency modulation---chirp [4]. Analyses are carried out under generalized modulation regimes we propose. Different mechanisms for quantum dots and quantum wells as a gain medium are also discussed and we provide a mapping between the two gain media. Spin states in quantum dots may also enable elusive phonon lasers [5], which emitts coherent phonons instead of photons. This work was performed in collaboration with R. Oszwa\l dowski, C. G{\o}thgen, G. Boeris, and I. \v{Z}uti\'{c}. \\[4pt] [1] J. Sinova and I. \v{Z}uti\'{c}, Nature Materials 11, 368 (2012); Handbook of Spin Transport and Magnetism, edited by E. Y. Tsymbal and I. \v{Z}uti\'{c} (CRC Press, New York, 2011). \\[0pt] [2] J. Rudolph et al., Appl. Phys. Lett. 87, 241117 (2005); M. Holub et al, Phys. Rev. Lett. 98, 146603 (2007); S. Hovel et al., Appl. Phys. Lett. 92, 041118 (2008); S. Iba, et al., Appl. Phys. Lett. 98, 08113 (2011); M. Holub and B. T. Jonker, Phys. Rev. B 83, 125309 (2011). \\[0pt] [3] J. Lee, W. Falls, R. Oszwa\l dowski, and I. \v{Z}uti\'{c}, Appl. Phys. Lett. 97, 041116 (2010); J. Lee, R. Oszwa\l dowski, C. G{\o}thgen, and I. \v{Z}uti\'{c}, Phys. Rev. B 85, 045314 (2012). \\[0pt] [4] G. Bo\'{e}ris, J. Lee, K. V\'{y}born\'{y}, and I. \v{Z}uti\'{c}, Appl. Phys. Lett. 100, 121111 (2012). \\[0pt] [5] A. Khaetskii, V. N. Golovach, X. Hu, and I. \v{Z}uti\'{c}, Phys. Rev. Lett. 111, 186601 (2013). [Preview Abstract] |
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