Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y22: Focus Session: Spin Resonance in Semiconductors |
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Sponsoring Units: GMAG DMP FIAP Chair: Stephen Lyon, Princeton University Room: 324 |
Friday, March 20, 2009 8:00AM - 8:36AM |
Y22.00001: Dynamic Nuclear Polarization in Silicon Invited Speaker: Silicon is a promising material for spintronics and spin-based quantum information processing. However, the highly mixed state of the nuclear spins can be a significant limitation, whether the nuclear spins are used as qubits or act as an environment for the electronic spins. We report the results of recent experiments to hyperpolarize the $^{29}$Si spins in silicon. We used microwave-induced dynamic nuclear polarization to achieve 5\% polarization of the $^{29}$Si in micro-crystalline silicon powder [1], and 5--8\% polarization in antimony- and phosphorus-doped silicon wafers. Since silicon has long T$_1$ relaxation times, polarized silicon micro- and nanoparticles could be of use in magnetic resonance imaging. In the powders the $^{29}$ Si nuclei in the amorphous region (containing unpaired electrons) are polarized by forced electron-nuclear spin flips driven by off-resonant microwave radiation while nuclei in the crystalline region are polarized by spin diffusion across crystalline boundaries. In the wafers the DNP is driven by an Overhauser mechanism within exchange-coupled clusters of donors. \\[4pt] [1] A. Dementyev, D. G. Cory, C. Ramanathan, {\em Phys. Rev. Lett.}, {\bf 100}, Article 127601 (2008). [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y22.00002: Electrical detection of dynamic nuclear polarization and nuclear magnetic resonance in ferromagnet-semiconductor heterostructures Mun Chan, J. Zhang, Q. Hu, T. Kondo, E. Garlid, C.J. Palmstrom, P.A. Crowell We report all-electrical measurements of dynamic nuclear polarization (DNP) and nuclear magnetic resonance (NMR) in ferromagnet-semiconductor heterostructures. Nuclei in GaAs are polarized by electron spins injected through an epitaxial Fe/GaAs Schottky tunnel barrier. In an oblique Hanle geometry, electron spin depolarization due to the hyperfine field is detected by measuring the change in spin-dependent electrochemical potential with a Fe contact.We also report electrical measurements of both magnetic and current driven NMR. A transverse magnetic field or the injection current is modulated at the NMR frequency to suppress DNP, resulting in changes of up to 80{\%} in the spin-dependent voltage. We measured resonant frequencies of different isotopes in the GaAs channel. Harmonics of the fundamental transitions are observed, reflecting the existence of either quadrupolar coupling or dipole-dipole interactions. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y22.00003: Electric-field control of a hydrogenic donor's spin in a semiconductor Amrit De, Craig E. Pryor, Michael E. Flatt\'e The orbital wave function of an electron bound to a single donor in a semiconductor can be modulated by an applied AC electric field, which affects the electron spin dynamics via the spin-orbit interaction. Numerical calculations of the spin dynamics of a single hydrogenic donor (Si) using a real-space multi-band $k\cdot p$ formalism show that in addition to breaking the high symmetry of the hydrogenic donor state, the g-tensor has a strong nonlinear dependence on the applied fields. By explicitly integrating the time dependent Schr\"odinger equation it is seen that Rabi oscillations can be obtained for electric fields modulated at sub-harmonics of the Larmor frequency. The Rabi frequencies obtained from sub-harmonic modulation depend on the magnitudes of the AC and DC components of the electric field. For a purely AC field, the highest Rabi frequency is obtained when E is driven at the 2nd sub-harmonic of the Larmor frequency. Apart from suggesting ways to measure g-tensor anisotropies and nonlinearities, these results also suggest the possibility of direct frequency domain measurements of Rabi frequencies. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y22.00004: Electronic spin polarization and spin-dependent band structure in GaAs probed by optically-pumped NMR (OPNMR) Sophia Hayes, Kannan Ramaswamy, Stacy Mui, Scott Crooker, Xingyuan Pan, Gary Sanders, Christopher Stanton Traditionally, magnetic fields have played an important role in determining the band structure of a material (de Haas-van Alphen techniques for metals and cyclotron resonance or magneto-absorption for semiconductors). We report optically pumped NMR (OPNMR) spectra of 69Ga spins in bulk semi-insulating GaAs generated by a narrowband laser. OPNMR involves optical orientation of electrons in a semiconductor with NMR detection of the polarized nuclear spins to which they are coupled. These experiments resolve fine details of the spin-dependent electronic structure of the valence bands. By comparing the oscillations in the OPNMR signal intensity with theoretical calculations, we have mapped out the conduction band electronic spin polarization. We show that OPNMR experiments in combination with theoretical simulations have the potential to reveal information about spin polarization and electronic structure of bulk semiconductors with far greater sensitivity than conventional techniques such as magneto-absorption. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y22.00005: Theory of the Spin-Splitting of the Valence Band Landau Levels in GaAs X. Pan, G.D. Sanders, C.J. Stanton, K. Ramaswamy, S. Mui, S.E. Hayes, S.A. Crooker We present calculations of the spin-dependent band structure and magneto-optical absorption spectra in bulk GaAs. Our calculations are based on the 8-band Pidgeon-Brown model. Optical properties are calculated within the golden rule approximation and compared with experiments. Although GaAs has a small $g$-factor leading to nearly spin-degenerate conduction band Landau levels, the valence band Landau levels are spin-split and the spin splitting can be observed in the circularly polarized magneto-absorption spectra. By carefully analyzing the energy band structure and the absorption spectra together, we identify the origins of all the optical transitions. We also separate contributions to the absorption coefficient from spin-up electrons and spin-down electrons to get the conduction band electron spin polarization. This information is used to compute the optically-pumped NMR (OPNMR) signal. We demonstrate that OPNMR can provide unique insight into the spin-dependent valence band electronic states. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y22.00006: High bandwidth EDMR detection H. Huebl, L.H. Willems van Beveren, R.P. Starrett, D.R. McCamey, A.J. Ferguson Several proposals discuss the realization of quantum computation with the help of the spin degree of freedom in semiconductors. Electrically detected magnetic resonance (EDMR) provides a well established tool to investigate spin states in semiconductors which was recently extended to investigate the spin dynamics of phosphorus donors in silicon. Typically, the detection bandwidth of EDMR is limited by the characteristic RC time constant of the sample. In this contribution we show that by embedding the sample in a LRC resonant circuit, a so-called tank circuit, it is possible to overcome this limitations. Here, we investigate a silicon MOSFET where the microwave magnetic field to induce the spin transitions is generated on chip by a shorted coplanar stripline[1]. We monitor the MOSFET resistance with a current preamplifier and in-situ by the response of the LRC resonant circuit and observe a spin resonance signature in both cases. Investigating the detection bandwidth by using frequency modulation of the microwaves applied indicates that the spin signature observed with the tank circuit is limited at the high end currently by the experimental setup. This shows that this method has the expected high bandwidth opening the view to faster phenomena in EDMR in a more direct manner. [1] Willems van Beveren et al., APL {\bf{93}}, 072102 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y22.00007: Optically Detected Electron Spin Resonance of GaAs Spin-LEDs John Colton, Steve Brown, Benjamin Heaton, Daniel Jenson, Michael Johnson, Aaron Jones GaAs ``spin-LED'' samples give off circularly polarized light due to spin-polarized electrons being injected into a quantum well diode structure. The spin dynamics of these electrons have been studied through electron spin resonance (ESR), with the resonance being optically detected by a change in the circular polarization of the emitted light. Results of the ESR experiments will be presented. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y22.00008: Spin lifetime properties of a quantum well GaAs sample measured by optically detected magnetic resonance Benjamin Heaton, John Colton, Steve Brown, Daniel Jenson, Michael Johnson, Aaron Jones Optically detected Kerr rotation techniques were used to measure spin properties in GaAs. The samples studied were MBE-grown 14 nm n-type GaAs quantum wells. Magnetic resonance was observed with great sensitivity as the probe laser was tuned to the exciton resonance. The g-factor was measured to be $\vert $g$\vert $=0.35. The T$_{2}$* lifetime measured from the width of the ODMR peaks was 52 ns. Results from pulsed microwave Rabi oscillation and spin echo experiments (to measure the T$_{2}$ spin coherence lifetime) are presented. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y22.00009: Ballistic Spin Resonance Sergey Frolov, Silvia Luescher, Wing-Wa Yu, Yuan Ren, Joshua Folk, Werner Wegscheider We demonstrate spin resonance driven by ballistic motion of electrons and mediated by spin-orbit interaction in a micron-scale channel of GaAs/AlGaAs two-dimensional electron gas. The resonance is observed when the frequency of electron bouncing trajectories in the channel matches the spin precession frequency set by a large in-plane magnetic field. The resonance is manifested as a suppression of pure spin currents that are generated in the channel by injection through quantum point contacts. The resonant frequency (10-50 GHz) can be tuned by varying electron density or channel width, as well as by bending the electron trajectories with a small out-of-plane magnetic field. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y22.00010: Scaling behavior of spin-dependent scattering off Neutral Donors in Silicon Field-Effect Transistors C. C. Lo, J. Bokor, T. Schenkel, J. He, A. M. Tyryshkin, S.A. Lyon Spin-dependent scattering of conduction electrons by neutral impurities is a promising route towards donor nuclear spin-state readout for donor qubits in silicon. Using electron spin resonance techniques, the donor nuclear spin-state can be extracted from the position of the resonance signal. Contrary to readout schemes involving Coulomb/spin blockade or other single electron phenomenon, spin-dependent scattering can be observed and studied in the presence of an ensemble of donors. In our experiments we study neutral impurity scattering of two-dimensional conduction electrons by donor impurities in field-effect transistors [1]. In this talk, we will discuss the scaling behavior of donor resonance signals using electrically detected magnetic resonance techniques in devices with different sizes. [1] C. C. Lo et al, App. Phys. Lett., 91, 242106 (2007) [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y22.00011: Electron Spin Resonance in Si/SiGe Heterostructures at 350 mK Jianhua He, A.M. Tyryshkin, S.A. Lyon, D.E. Savage, M.A. Eriksson Si/SiGe heterostructures are one of the promising matrices for electron spins as qubits in a silicon-based quantum computer. Many electron spin resonance (ESR) measurements have been done to characterize 2D electron spins embedded in such structures at temperatures above 2 K. Here we report the first CW and pulsed ESR experiments in Si/SiGe heterostructures in a $^{3}$He system at 350 mK. Electron beam lithography was used to pattern a large area (16 mm$^{2})$ of a CVD grown modulation doped Si/SiGe quantum well (QW) into an array of $\sim $100 nm quantum dots (300 nm pitch) which has been wet etched about half-way through the doped layer. In the dark, only one signal is observed, which shows a Curie-like temperature dependence indicative of isolated spins. After brief illumination, two more signals appear: a line having the same g-factor as an unpatterned QW sample (g=2.0003) and another line which disappears upon annealing to 20 K. The first of these lines (g=2.0003) shows a Pauli temperature dependence consistent with many-electron quantum dots, and a T$_{2}$ relaxation time of about 150 ns at 350 mK. The origin of these ESR signals and their relaxation mechanisms will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y22.00012: Electron spin resonance in silicon MOS structures down to 0.36 K S. Shankar, A. M. Tyryshkin, S. A. Lyon While transport of 2-dimensional (2D) electrons has been routinely measured down to few mK, performing electron spin resonance (ESR) at low temperatures is challenging. We report measurements of the paramagnetic susceptibility of 2D electrons in a silicon metal-oxide-semiconductor (MOS) structure using ESR at 0.34 T for temperatures down to 0.36 K. When the MOS gate is biased below the threshold voltage, we measure electrons weakly confined below the conduction band edge and find that the susceptibility follows a Curie-like 1/T temperature dependence. The Curie susceptibility of confined electrons suggests that they are independent electrons confined by disorder at the Si-SiO$_2$ interface. At gate voltages above threshold the signal arises from 2D conduction electrons, whose susceptibility is expected to follow a simple Pauli temperature dependence, i.e., constant at low temperatures. Surprisingly, at an electron density of $2.8 \times 10^{11}$ cm$^{-2}$ (Fermi temperature = 20 K), as the temperature is reduced from 4.2 K to 0.36 K, the susceptibility actually drops by a factor of 2. Furthermore, this effect becomes more pronounced at higher 2D electron densities. The drop in susceptibility for 2D conduction electrons at low temperature is unexpected and remains to be explained. [Preview Abstract] |
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