Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q22: Focus Session: Spins in Quantum Dots and Mn in Arsenides |
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Sponsoring Units: GMAG DMP FIAP Chair: Xinyu Liu, University of Notre Dame Room: 324 |
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q22.00001: Intershell Exchange and Sequential Electrically Injected Spin Populations of InAs Quantum-Dot Shell States Invited Speaker: Quantum dots (QDs) are attractive for a variety of spintronic applications. Their electronic structure exhibits the $s,p,d,f$ shells characteristic of atoms. We report electrical injection of spin-polarized electrons from Fe contacts into the individual shells of highly uniform self-assembled InAs QDs, and we determine the s-p and p-d inter-shell exchange energies. The electron population and polarization of each shell are controlled by the spin bias current. The circular polarization of the electroluminescence (EL) spectra exhibits maxima red-shifted with respect to the EL intensity peaks, in contrast with simple models of shell occupation. Using exact diagonalization techniques, calculations of spectra from multi-exciton complexes show that this is due to inter-shell exchange [1]. We determine exchange energies for the $s-p $shells $\sim $ 6-7 meV, and for the $p-d$ shells $\sim $ 13-14 meV. These results are significant to our understanding of QD behavior, and provide a mechanism for electrical control of spins in QDs.\\[4pt] [1] M. Korkusinski and P. Hawrylak, Phys. Rev. Lett. 101, 027205. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q22.00002: Magnetically induced spin relaxation in InAs QDs M. Yasar, I. Khan, T. Ali, A. Petrou, G. Kioseoglou, C. Li, A. Hanbicki, B. Jonker, M. Korkusinski The circular polarization $P$ of light emitted by Fe/InAs QDs spin LEDs has been studied as function of magnetic field $B $and temperature $T$. $P $shows a pronounced decrease around $B_{o }$= 5 T, in the form of a resonance with a full width of $\approx $ 0.75 T with the following characteristics: (i) The resonance strength is quite sensitive to the bias voltage $V$. At low $V $ the resonance is strong, but, as $V$ is increased, it becomes progressively weaker. (ii) The resonance is pronounced at $T$ = 5 K but loses strength with increasing temperature, and disappears above 60 K. The decrease in $P$ around $B_{o }$ is attributed to a spin relaxation mechanism that is induced by magnetic field. The sensitivity of the resonance to $V$ suggests that the origin of the spin relaxation mechanism is connected to the spin-orbit interaction. By changing $B$ we tune the energies of different electron states and thereby change the rate of spin relaxation in the system. We compare experimental results with calculations, in which many-body energies and wave functions are obtained using the effective-mass configuration-interaction approach; the spin-orbit interaction, is treated perturbatively. Work at SUNY was supported by ONR and NSF. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q22.00003: Mn doping of InAs quantum dots studied by X-STM. Paul Koenraad, Murat Bozkurt, Jens Garleff, Vicky Grant, Richard Campion, Tom Foxon, Euclydes Marega, Greg Solomon We report on the X-STM analysis of Mn doped quantum dots. The X-STM technique allows for the atomic scale analysis of single Mn acceptors and their incorporation in III/V nanostructured materials. We will show the detrimental effect of segregation which complicates the doping process of InAs quantum dots in GaAs. Several routes to attain doping of quantum dots are addressed. Only in structures with extremely high doping concentrations we could show the incorporation of a few or a single Mn atom(s) in the InAs dots. We will present low temperature (5 K) spectroscopic measurements that allow studying the interaction of the Mn acceptor state with the quantum dot. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q22.00004: Thermo-spin effects in quantum dots connected to ferromagnetic leads Yonatan Dubi, Massimiliano Di Ventra We study a system composed of a quantum dot in contact with ferromagnetic leads held at different temperatures, which we suggest can be used as a source of spin-voltage. Spin analogs to the thermopower and thermoelectric figure of merit are defined and studied as a function of junction parameters. It is shown that in contrast to bulk ferromagnets, the spin thermopower coeffcient in a junction can be as large as the Seebeck coefficient, resulting in a large spin figure of merit. In addition, it is demonstrated that the junction can be tuned to supply only spin current but no charge current. We also discuss experimental systems where our predictions can be verified. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q22.00005: Tunnel magnetoresistance in mesoscale (Ga,Mn)As magnetic tunnel junctions. Partha Mitra, Mark J. Wilson, Meng Zhu, Peter Schiffer, Nitin Samarth, Kiran V. Thadani, Dan C. Ralph We recently demonstrated exchange-biased magnetic tunnel junctions (MTJs) built from the ferromagnetic semiconductor (Ga,Mn)As [Phys. Rev. B. {\bf 78}, 195307 (2008)]. Here, we report measurements of the tunnel magnetoresistance (TMR) in mesoscale (Ga,Mn)As MTJ devices with areas that range from $\sim 1 - 100 \mu \rm{m}^2$, mapping out the TMR as a function of the magnetic field vector and the sample temperature. The vector field measurements provide insights into the interplay between TMR and the magnetic anisotropies characteristic of (Ga,Mn)As. In contrast to our earlier studies large area devices, we find that the TMR in these mescoscale devices increases exponentially with decreasing temperature, with a form exp (-T/T*). At temperatures lower than T*, the conductance-voltage characteristics show a $\sqrt{V}$ dependence, suggesting the role of Coulomb interactions in the spin- dependent tunneling process in these small area MTJs. Work supported by the ONR MURI program. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q22.00006: Spectra broadening in Point-Contact Andreev Reflection Measurement on GaMnAs Shang-Fan Lee, T.W. Chiang, Y.H. Chiu, S.Y. Huang, J.J. Liang, H. Jaffres, J.M. George, A. Lemaitre Point-Contact Andreev Reflection (PCAR) technique has been considered as a reliable method for determining electron spin polarization of novel metallic ferromangets. For dilute magnetic semiconductors, this technique is less applicable due to the resistive nature of the material. We investigate PCAR spectra of Ga$_{0.94}$Mn$_{0.06}$As using Pb tips. The observed spectrum exhibits behaviors described in the Modified Blonder-Tinkham-Klapwijk (MBTK) model but with a significant spectrum broadening. Modified BTK theory is commonly applied to analyze PCAR spectra with electron polarization, superconducting energy gap, and interface transparency as parameters. We present an analysis based on the introduction of spreading resistance and the inelastic scattering at the interface. In as-grown and annealed samples, we extract spin polarization of 76{\%}/74{\%} from our analysis, both smaller than the value obtained from approach of effective temperature$^{3}$, 90{\%}/82{\%}. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q22.00007: Ultrafast Photoinduced Coherent Spin Dynamics in Ferromagnetic Ga$_{1-x}$Mn$_x$As/GaAs Structure Jingbo Qi, Ying Xu, Andrew Steigerwald, Norman Tolk, Xinyu Liu, Jacek Furdyna, Ilias Perakis Ultrafast pump-probe magneto-optical spectroscopy is used to study
coherent spin dynamics in the ferromagnetic semiconductor
Ga$_{1-x}$Mn$_x$As systems. Above GaAs bandgap $E_g$, the
temporal Kerr signal is found to be strongly dependent on pump
photon polarization. This polarization dependence is attributed
to spins of electrons photoexcited to the conduction band, and
disappears for $E_{ph} |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q22.00008: Temperature Dependence of Anomalous Hall Effect in Metallic (Ga,Mn)As films Xinyu Liu, Zhiguo Ge, Shaoping Shen, Margaret Dobrowolska, Jacek Furdyna We present a systematic study of the temperature dependence of anomalous Hall effect (AHE) in metallic (Ga,Mn)As films. The Hall effect in (Ga,Mn)As is described as $\rho_{xy}$ =$R_{0}B$+$c\rho_{xx}^{n}M_{z}$, where $R_{0}$ ($1/ep$) is the ordinary Hall coefficient, $\rho_{xy}$ and $\rho_{xx}$ are the transverse and longitudinal resistivities, and $n$ is a scaling parameter. In this work we have developed a self-consistent method to determine $R_{0}$, $c$ and $n$ simultaneously. We use this method to analyze the Hall and resistivity data measured up to B = 6 T at various temperatures. We find that for the metallic samples, a distinct evolution of the AHE occurs as temperature increases, as evidenced by the temperature or magnetization dependence of the parameter c. We propose that the correlation between the AHE and the resistivity should be reconsidered by using a two- component model in order to separate contributions due to different scattering mechanisms. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q22.00009: Antiferromagnetic interlayer exchange coupling in Ga$_{1-x}$Mn$_x$As/GaAs diluted ferromagnetic semiconductor multilayers Jae-Ho Chung, S.J. Chung, Sanghoon Lee, B.J. Kirby, J.A. Borchers, Y.J. Cho, X. Liu, J.K. Furdyna We use neutron reflectometry to investigate the interlayer exchange coupling between Ga$_{0.97}$Mn$_{0.03}$As ferromagnetic semiconductor layers separated by non-magnetic Be-doped GaAs spacers. Polarized neutron reflectivity measured below the Curie temperature of Ga$_{0.97}$Mn$_{0.03}$As reveals a characteristic splitting at the wave vector corresponding to twice the multilayer period, indicating that the coupling between the ferromagnetic layers are antiferromagnetic (AFM). When the applied field is increased to above the saturation field, this AFM coupling is suppressed. This behavior is not observed when the spacers are undoped, suggesting that the observed AFM coupling is mediated by charge carriers introduced via Be doping. The behavior of magnetization of the multilayers measured by DC magnetometry is consistent with the neutron reflectometry results. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q22.00010: Engineering the interlayer exchange coupling in hybrid ferromagnetic metal/semiconductor heterostructures Mark J. Wilson, Meng Zhu, Peter Schiffer, Nitin Samarth, Roberto C. Myers, David D. Awschalom, Michael E. Flatte The systematic engineering of exchange coupling in ferromagnetic semiconductor heterostructures is important for developing proof-of-concept spin transfer semiconductor devices. We recently demonstrated interlayer exchange coupling between a ferromagnetic semiconductor (Ga$_{1-x}$Mn$_x$As) and a ferromagnetic metal (MnAs) [APL {\bf 91}, 192503 (2007)]. Here, we report a comprehensive magnetometry study of the underlying exchange coupling in this hybrid system. We vary key parameters such as the thicknesses of both the ferromagnetic layers and the composition of the Ga$_{1-x}$Mn$_x$As layer, and explain our observations using an ``exchange spring'' model. We also demonstrate the propagation of the exchange coupling through a non-magnetic spacer layer (p-doped GaAs) and examine the variation of this coupling as a function of the spacer layer thickness and doping. Work supported by the ONR MURI program and by NSF. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q22.00011: A novel random fixed point in diluted magnetic semiconductors Donald Priour Jr, Sankar Das Sarma We examine the critical behavior of strongly disordered Heisenberg models at the Curie Temperature $T_{c}$, where the disorder is manifest as missing magnetic ions (i.e. as in diluted magnetic semiconductors) with the aid of large-scale Monte Carlo. We calculate the magnetic susceptibility critical exponent, obtaining a value ($\gamma = 1.1 \pm 0.05$) lower than that of the corresponding pure model. We obtain the same reduced value for $\gamma$ even for weak disorder. We find that both the amplitude and exponent of the first correction to leading order scaling are invariant with respect to the strength of the disorder, suggesting that critical behavior is controlled by a single random fixed point. Nevertheless, we show that for reduced temperature $t = (T - T_{c})/T_{c}$ regimes accessible in experiment, one would actually measure an effective exponent $\gamma_{\textrm{eff}}$ markedly higher than that of either the random fixed point or the pure Heisenberg model. We find self-averaging parameters to be non-monotonic in system size, initially increasing with the number of magnetic ions $N$, and ultimately decreasing beyond $N \sim 10^{3}$ as per the Harris Criterion. We acknowledge support from US-ONR and NRI-NSF. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q22.00012: Solubility control in dilute magnetic semiconductors by using the co-doping method Kazunori Sato, Hitoshi Fujii, Lars Bergqvist, Peter H. Dederichs, Hiroshi Katayama-Yoshida To overcome low solubility limit of magnetic impurities in dilute magnetic semiconductors (DMS) and realize room temperature ferromagnetism, we propose a co-doping method to increase solubility of magnetic impurities in DMS [1]. We calculate electronic structure of (Ga, Mn)As, (Ga, Mn)N, (Ga, Cr)N and (Zn, Cr)Te with interstitial impurities, such as Li, Na and Cu, from first-principles by using the Korringa-Kohn- Rostoker coherent potential approximation (KKR-CPA) method. From the total energy results, it is shown that the mixing energy of magnetic impurity becomes negative and the solubility of magnetic impurities is strongly enhanced under the existence of interstitials [1]. In general, the co-dopants compensate hole carriers, thus the system becomes paramagnetic. However, owing to the large diffusivity of these interstitial impurities, we can anneal out the co-dopants after the crystal growth to recover the ferromagnetism. As an example, kinetic Monte Carlo simulations for the diffusion of interstitial co-dopants in DMS will be shown. [1] K. Sato et al., Jpn. J. Appl. Phys. 46 L1120 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q22.00013: Time Resolved Spectroscopy of InMnAs Using Differential Transmission Technique in Mid-Infrared Region M. Bhowmick, K. Nontapot, G.A. Khodaparast, B.W. Wessels The emergence of III-Mn-V magnetic semiconductors, such as GaMnAs, InMnAs, and InMnSb has led to a number of exciting results relevant to the new field of spintronics. In contrast to earlier MBE work, InMnAs structures grown by MOVPE at the Northwestern University are room temperature ferromagnetic semiconductors with a {\it T$_C$} of 330 K. The origins of ferromagnetism and the interactions between itinerant carriers and localized spins in these structures are open and interesting questions. The samples are grown on GaAs substrates with the Mn content ranging from 1-4{\%}. The carrier and spin life time in these structures were probed using a differential transmission technique by tuning the pump-probe radiations from 3-3.6 microns. The relaxation times are in order of 2-4 ps similar to the observations in the MBE grown structures. The temperature dependence of the carrier and spin lifetimes will be presented and related to recent models for spin recombination. [Preview Abstract] |
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