Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X33: Focus Session: Magnetic Resonance in Magnetic Semiconductors |
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Sponsoring Units: GMAG FIAP DMP Chair: Roland Kawakami, University of California, Riverside Room: Morial Convention Center 224 |
Friday, March 14, 2008 8:00AM - 8:12AM |
X33.00001: Microscopic mechanism of optically pumped NMR in bulk GaAs Patrick Coles, Jeffrey Reimer In the past decade, optical nuclear polarization in semiconductors has been used as a tool for basic solid-state physics [1], and has been proposed as a means towards sensitivity-enhanced NMR for biological systems [2] and towards information storage in quantum computing architectures [3]. The microscopic mechanism of this process, however, has been debated recently even in the model system of bulk GaAs [4]. We review our modeling and experimental results towards differentiating between recently considered mechanisms for optical nuclear polarization in GaAs based on localized and delocalized electrons. We discuss a simple experiment that quantifies the amount of localization. Results are consistent with localized electrons cross-relaxing with nearby nuclei, and gradual polarization of the bulk by nuclear spin diffusion. [1] A.E. Dementyev et al. Sol. State Comm. 119: 217 (2001) [2] R. Tycko. Sol. State Nuc. Mag. Res. 11:1 (1998) [3] T.D. Ladd et al. Phys. Rev. Lett. 89:017901 (2002) [4] A.K. Paravastu et al. Phys. Rev. B 69:075203 (2004) [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X33.00002: Spin Resolved Cyclotron Resonance and Magneto-absorption in InSb Multiple Quantum Wells Xingyuan Pan, G.D. Sanders, C.J. Stanton, T. Kasturiarachchi, W. Gempel, X.H. Zhang, R.C. Meyer, N. Goel, M. Edirisooriya, T.D. Mishima, R.E. Doezema, M.B. Santos, Y.J. Wang We present calculations of the spin-dependent cyclotron resonance and magneto-absorption spectra in strained, narrow gap AlInSb/InSb multiple quantum wells. Our calculations are based on the 8-band Pidgeon-Brown model generalized to including the effects of the confinement potential and pseudomorphic strain. Optical properties are calculated within the golden rule approximation and compared with experiments. The large g-factors allow one to spin-resolve the transitions at modest magnetic fields ($<$10T). In the magneto-absorption, we see spin-split transitions and identify both bright and dark transitions in agreement with experiment. For cyclotron resonance, we see not only spin-resolved anticrossings between Landau levels with the same spin, but also anticrossings between levels of opposite spin. These opposite spin anticrossings result from spin-orbit coupling and the resonance-enhanced mixing of the second Landau level of the ground state subband and the lowest Landau level of the first excited state of opposite spin. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X33.00003: High-frequency EPR of impurities on diamond Susumu Takahashi, Mark S. Sherwin, Ronald Hanson, David D. Awschalom The nitrogen-vacancy (N-V) impurity center is a promising solid- state spin system for solid-state quantum information processing. Many desirable quantum properties have been found at room temperature, including long spin-coherence times, demonstration of single N-V spin quantum gate operation, discovery of rapid spin polarization and achievement of readout of single N-V spins. There have been many EPR experiments to investigate electronic structures and dynamics of impurities in diamond. Most of the studies were however performed by low- field EPR. High-frequency EPR generally has a great advantage for spectral and time resolution and absolute sensitivity due to very high spin-polarizations in high magnetic fields. High- frequency EPR for the diamond system therefore enables the investigation of ensembles of low-concentration impurities. In this presentation, we will discuss high-field properties of spin relaxations of impurities on diamond studied with 240 GHz cw and pulsed EPR. [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X33.00004: Electron-nuclear interactions in lightly-doped GaAs, studied through optically-detected magnetic resonance (ODMR) John Colton, Lee Wienkes, Andrew Gierke, Susan Allemann, Allan Bracker Electron-nuclear interactions have been studied in lightly-doped GaAs via optically-detected magnetic resonance (ODMR). Thermally-polarized electrons were resonated with $\sim $10 GHz microwaves; the spin states were detected with cw optical Kerr rotation. The electron-nuclear interaction was manifested through an effective magnetic field produced by the hyperfine interaction between nuclei and donor electrons, which caused a shift in the electron ODMR peak position. The effective nuclear field could be eliminated by simultaneous magnetic resonance of the three nuclear species. A measurement of the nuclear spin relaxation time was obtained (5.6 minutes) by tracking the amount of effective nuclear field, in the absence of nuclear magnetic resonance. Finally, optically-detected electron-nuclear double resonance (ODENDOR) was also performed, by monitoring changes in the electron Kerr rotation signal while sweeping through nuclear resonant frequencies one at a time. [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X33.00005: Pseudospin Resonance in Semiconductor Bilayers Saeed H. Abedinpour, Marco Polini, Mario P. Tosi, Bilal Tanatar, Allan H. MacDonald, Giovanni Vignale The pseudospin degree of freedom in a semiconductor bilayer gives rise to a collective mode analogous to the ferromagnetic resonance mode of a ferromagnet. We present a theory of the dependence of the energy and the damping of this mode on layer separation $d$. Based on these results, we discuss the possibility of realizing transport-current driven pseudospin-transfer oscillators in semiconductors. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X33.00006: Location and Magnetic Hyperfine Properties of Mn$^{2+}$ in Silicon. R.H. Pink, Archana Dubey, S.R. Badu, R.H. Scheicher, M.B. Huang, Lee Chow, T.P. Das Crystalline Silicon doped with the transition metal ion Mn$^{+2}$ is ferromagnetic at room temperature and thus potentially a useful material for spintronic applications. In attempting to understand from first principles the location of Mn$^{+2}$ and the electronic structure of the ferromagnetic system we have started work first on the dilute system. We have used the Hartree-Fock cluster procedure to determine the binding energies of the three likely locations for Mn$^{2+}$, substitutional (S), tetrahedral interstitial (T$_{i})$ and hexagonal interstitial (H$_{i})$ locations allowing for relaxation of the silicon neighbors. Our calculations show that the H$_{i}$ location is unstable and the S and T$_{i}$ are stable. Our nuclear magnetic hyperfine interactions results for $^{55}$Mn nucleus and $^{29}$Si neighbor will be presented and compared with electron spin resonance [1] experimental data. \newline [1] H.H. Wood bury and G. W. Ludwig Phys. Rev. \underline {117},102(1960) [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X33.00007: The Nuclear Spin Relaxation in the Semiconductor Double Quantum Dots: Study of Spin Diffusion Eddy Yusuf, Xuedong Hu We study the relaxation of the nuclear spins in a semiconductor double quantum dot. We focus on spin diffusion as an important channel for nuclear spin relaxation. The diffusion of the nuclear spin polarization arises from the Fermi contact hyperfine interaction between electrons in the quantum dots and the nuclear spins, as well as the magnetic dipolar interaction between nuclear spins. We calculate the nuclear spin polarization relaxation time and the nuclear diffusion constant within the density matrix framework. We explore the behavior of the relaxation time and diffusion constant for a wide range of conditions, including variations in temperature, the initial degree of nuclear polarization, dot sizes, and the strength of the applied magnetic field. We compare our results to the available experimental data and discuss various experimental schemes to further test the outcome of our calculation. [1] A. Greilich, et al., Science 317, 1896 (2007) [2] S. Das Sarma, et. al., Solid State Commun. 133, 737 (2005) [Preview Abstract] |
Friday, March 14, 2008 9:24AM - 9:36AM |
X33.00008: Microscopic theory of electron spin relaxation in N@C$_{60}$ Z.G. Yu Endohedral N@C$_{60}$ exhibits an extremely long electron spin relaxation time and offers a great potential in storing and processing quantum information. Here we present a microscopic theory of electron spin relaxation in N@C$_{60}$. The theory combines (1) the spin-orbit interaction of N $2p$ electrons, which mixes the ground state $^4S$ with excited $^2P$ and $^2D$ states, and (2) the coupling between the N $2p$ electrons and C$_{60}$ $H_g$ vibrations, which facilitates transitions between $^2P$ and $^2D$ states. The spin relaxation occurs via a two-phonon (Raman) process by absorbing a $H_g$ phonon and emitting another at the (approximately) same frequency. The theory consistently explains measured spin relaxation time $T_1$ and its temperature dependence, and predicts two distinct spin decoherence $T_2$ constants. In addition, the excellent agreement between theory and experiment suggests a universal importance of the two-phonon Raman process in determining spin relaxation in nanostructures such as quantum dots, where a one-phonon process is ineffectual in flipping electron spins because of the lack of low-energy phonons in nanostructures. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X33.00009: Electronic and transport properties of a lateral triple quantum dot molecule in a magnetic field F. Delgado, Y.-P. Shim, M. Korkusinski, P. Hawrylak We present a theory of spin, electronic and transport properties of a few-electron lateral triangular triple quantum dot molecule in a magnetic field. Our theory is based on a Hubbard model and the Linear Combination of Harmonic Orbitals combined with Configuration Interaction method to arbitrary magnetic fields. The one-particle spectra obtained as a function of the magnetic field exhibit Aharonov-Bohm oscillations. As a result, by changing the magnetic field it is possible to engineer the degeneracies of single-particle levels, and thus control the total spin of the many-electron system. For the triple dot with two and four electrons we find oscillations of total spin due to the singlet-triplet transitions occurring periodically in the magnetic field. In the three-electron system we find a transition from a magnetically frustrated to the spin-polarized state. The impact of these phase transitions on the addition spectrum are analyzed and the qualitative behaviour of the current through the quantum molecule under spin blockade conditions is studied as a function of the applied magnetic field [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X33.00010: All optical FMR of ferromagnetic (Ga,Mn)As with various Mn contents Satoi Kobayashi, Yusuke Hashimoto, Hiro Munekata We have investigated all-optical FMR of non-thermal origin [1] on (Ga,Mn)As
with various Mn contents $x$ with time-resolve MO signals obtained by a
single-wavelength pump and probe (P{\&}P) technique, and found clear
dependence of $x$ on the effective magnetic field on ferromagnetically coupled
Mn spins.
(Ga,Mn)As samples $x$ = 0.02-0.11 were grown by molecular beam epitaxy.
Measurements were performed for both as-grown and annealed samples. In-plane
magnetization easy axis was [1-10] for the $x$ = 0.11 sample, whereas it was
$<$100$>$ for the rest. Samples were magnetized prior to P{\&}P
measurements. Polarization of probe pulses was carefully controlled with
respect to the direction of $M$. Pump power was varied between 3.4 - 34 $\mu
$J/cm$^{2}$.
Precession of magnetization was clearly observed at 10-40 K in most of
samples. The period of the oscillation becomes longer with increasing $x$
value, indicating a decrease in an effective magnetic field $H_{eff}$ =
$\hbar \omega $ / \textit{g$\mu $}$_{B}$ on Mn spins from 0.2 to 0.1 Tesla with increasing
$x$. Assuming $H_{eff}=J_{pd} |
Friday, March 14, 2008 10:00AM - 10:12AM |
X33.00011: Nuclear Magnetic Resonance in Semiconductor Nanostructures Ionel Tifrea One measurement for the nuclear spin dynamics in solid state systems is the Knight shift observed in nuclear magnetic resonance experiments. I will present a theoretical investigation of the Knight shift in samples with reduced dimensionality. The nuclear spin dynamics is dominated by the hyperfine interaction between nuclear and electronic spins and depends on the electronic local density of states. As an example, I will discuss the temperature, position, and time dependence of the induced nuclear spin polarization and the resulting Knight shift in semiconductor quantum wells. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X33.00012: Electrically-detected magnetic resonance in accumulation-layer MOSFETs Laurens Willems van Beveren, Dane McCamey, Hans Huebl, Andrew Ferguson, Tim Duty, Robert Clark Spin-dependent transport, originating from neutral-impurity scattering, in silicon accumulation-layer MOSFETs was reported more than a decade ago in an electron-spin resonance (ESR) cavity setup [1]. There, current measurements on the MOSFET showed ESR features with a hyperfine (HF) splitting of 42 G, indicative of electrons whose wavefunctions overlap with phosphorous nuclei in the silicon crystal. Here, we report the observation of electrically-detected magnetic resonance (EDMR) in phosphorous-doped silicon MOSFETs without the constraint of a cavity and down to the mK-regime in a dilution refrigerator with a superconducting magnet. Instead, the ESR-field is generated by an on-chip shorted coplanar stripline (CPS), allowing broadband operation. Continuous-wave EDMR was achieved up to 30 GHz. The EDMR spectra show (i) the two hyperfine-split (42 G) ESR lines and (ii) an EDMR signal that is centered between the hyperfine lines, associated with the `free electron' ESR response. [1] R. Ghosh and W. Silsbee, Phys. Lett. 85, 439 (1992). [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X33.00013: Electrically detected magnetic resonance in Si:P at high magnetic fields (B = 8.5 T) Dane McCamey, Gavin Morley, Louis Claude Brunel, Johan van Tol, Heather Seipel, Christoph Boehme Phosphorus doped silicon (Si:P) is a technologically important material with possible uses in spintronic and quantum information processing devices. A useful way to understand the properties of this material is by investigation of the spin dependence of its transport processes. Whilst numerous studies of this type have been performed on Si:P at low magnetic fields, no systematic investigation has been undertaken at high magnetic fields. We will present an electrically detected magnetic resonance (EDMR) study of Si:P, with a native oxide surface, at $B = 8.5$ T ($f_{resonance} \sim 240$ GHz). The change in the sample photocurrent, $\Delta I/I$, was measured as a function of B using a microwave chopping method. Resonant signals from the P donors, as well as P$_{\textrm{b}}$ defects near the Si-SiO$_2$ interface, were observed. The temperature dependence of the observed signals in the range $T = 3$ K - $10$ K will be presented, and the microscopic processes leading to the signals discussed. Finally, pulsed EDMR (Rabi oscillations, Hahn echos) was performed to investigate spin coherence and manipulation in high fields, and these results will also be discussed. [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X33.00014: Pulsed electrically detected magnetic resonance of phosphorus donors near the Si(111)-SiO$_{\mathrm{2}}$ interface Seoyoung Paik, Heather Seipel, Sang-Yun Lee, Thomas Herring, Dane McCamey, Christoph Boehme Recently, there has been a large effort towards the electrically detection of spin coherence of phosphorus donor electrons in silicon. These studies have been undertaken on the Si (100) interface, due to its wide utilization in the semiconductor industry. Here, we present a pulsed electrically detected magnetic resonance study on P donors near the Si(111) interface. We observe the transient current after a short, coherent microwave pulse as a function of both the magnitude and relative orientation (with respect to the [111] direction) of the applied magnetic field. Similar to previous Si(100) studies, we observe three resonant peaks, which we attribute to a) the two well known hyperfine spilt phosphorus resonances, and b) the P$_{\mathrm{b}}$ defect resonance. The P$_{\mathrm{b}}$ resonance exhibited an anisotropy with field direction, in agreement with conventional ESR studies. In addition, we observe a fourth isotropic resonance, with a g-factor of $g=2.0031 \pm 0.0004$. We conclude that, aside from the anticipated and well know P-P$_{\mathrm{b}} $ transition, at least one additional spin dependent recombination pathway exists at the Si(111)-SiO$_{\mathrm{2}}$ interface. [Preview Abstract] |
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