Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X15: Focus Session: Spins in Semiconductors - Manipulation of Dopant Spins |
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Sponsoring Units: DMP GMAG FIAP Chair: Paul Koenraad, TU/Eindhoven Room: D171 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X15.00001: Quantum information in silicon: Initialization, manipulation, storage and readout Invited Speaker: Spin qubits in silicon are exciting because of their long coherence times [1] and the electrical readout of the state of one electron spin [2]. In a single experiment we demonstrate initialization [3], manipulation, storage and electrical readout of quantum information with a small ensemble of phosphorus electronic and nuclear spins in silicon [4]. Our electrical readout does not destroy the electron spin coherence which is limited instead by naturally-occurring $^{29}$Si nuclear spins. These experiments require a pulsed electron spin resonance spectrometer operating at high magnetic fields [5]. Silicon quantum computers would benefit from having a second dopant species which can be addressed selectively [6-8], and we find that bismuth atoms are well suited for this role [9]. They offer long spin coherence times [9,10] as well as new opportunities [11] when compared with phosphorus. \\[4pt] [1] A M Tyryshkin {\&} S A Lyon, Phosphorus electron spin coherence time can be over 10 s, Private communication (2010) \newline [2] A Morello\textit{ et al}, Nature \textbf{467}, 687 (2010) \newline [3] D R McCamey, J van Tol, G W Morley {\&} C Boehme, Phys Rev Lett \textbf{102}, 027601 (2009) \newline [4] G W Morley\textit{ et al}, Phys Rev Lett \textbf{101}, 07602 (2008) \newline [5] G W Morley, L-C Brunel {\&} J van Tol, Rev Sci Instrum \textbf{79}, 064703 (2008) \newline [6] A M Stoneham, A H Harker {\&} G W Morley, J Phys Condens Matter \textbf{21}, 364222 (2009) \newline [7] A M Stoneham, A J Fisher {\&} P T Greenland, J Phys Condens Matter \textbf{15}, L447 (2003) \newline [8] P T Greenland\textit{ et al}, Nature \textbf{465}, 1057 (2010) \newline [9] G W Morley\textit{ et al}, Nature Mater \textbf{9}, 725 (2010) \newline [10] R E George\textit{ et al}, Phys Rev Lett \textbf{105}, 067601 (2010) \newline [11] M H Mohammady, G W Morley {\&} T S Monteiro, Phys Rev Lett \textbf{105}, 067602 (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X15.00002: Crystal orientation induced spin Rabi beat oscillations of point defects at the c-Si(111)/SiO$_{2}$ interface Seoyoung Paik, Sang-Yun Lee, Christoph Boehme Spin-dependent electronic transitions such as certain charge carrier recombination and transport processes in semiconductors are usually governed by the Pauli blockade within pairs of two paramagnetic centers. One implication of this is that the manipulation of spin states, e.g. by magnetic resonant excitation, can produce changes to electric currents of the given semiconductor material. If both spins are changed at the same time, quantum beat effects such as beat oscillation between resonantly induced spin Rabi nutation becomes detectable through current measurements [1]. Here, we report on electrically detected spin Rabi beat oscillation caused by pairs of $^{31}$P donor states and P$_{b}$ interface defects at the phosphorous doped Si(111)/SiO$_{2 }$interface. Due to the g-factor anisotropy of the P$_{b}$ center we can tune the intra pair Larmor frequency difference (so called Larmor separation) through orientation of the sample with regard to the external magnetic field. As the Larmor separation governs the spin Rabi beat oscillation, we show experimentally how the crystal orientation can influence the beat effect. \\[4pt] [1] D. R. McCamey, et al. \textit{Phys. Rev. Lett.} \textbf{104}, 017601 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X15.00003: Dynamic Response of the Kondo Resonance in a Single-Electron Transistor in the Presence of Magnetic Field Bryan Hemingway, Tai-Min Liu, Andrei Kogan, Steven Herbert, Michael Melloch We report a sharp peak in the differential conductance of a Single-Electron Transistor (SET) in the Kondo regime irradiated with microwaves, plotted as function of an external, in-plane magnetic field, $B$. The peak emerges at frequencies, $hf,$ above $\sim T_K/2$ and shifts approximately linearly with the microwave signal frequency. At frequencies significantly below the Kondo scale, $T_K/h$ (M. Hettler and H. Schoeller Phys. Rev. Lett. 74, 4907-4910 (1995)), no such peak is present and the conductance data agree with the predictions based on static measurements. In the Coulomb Blockade regime, we find good agreement with the photon-assisted resonant tunneling model and experiments (T.H. Oosterkamp et al., Phys. Rev. Lett. 78 (1997)) . Our SETs are fabricated lithographically using GaAs/AlGaAs heterostructure with sheet density $4.8$ x $10^{11}$ cm$^{-2}$ and mobility $5$ x $10^{5}$ cm$^{2}$V$^{-1}$sec$^{-1}$ and have the lithographic dot size approximately $130$ nm in diameter. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X15.00004: Jahn-Teller induced multiple ferromagnetic exchange interactions in magnetic semiconductors Hannes Raebiger, Takeshi Fujita Ferromagnetic interactions among $3d$ impurities in compound semiconductors (II-VI, III-V, etc) are usually rationalized via ``double exchange'', ``$p$-$d$ exchange'', and ``superexchange'' type interactions, which ensue a description of the $3d$ impurity electronic configuration based on host symmetries. Obviously Jahn-Teller distortions break these symmetries and yield completely different, closed shell, electronic configurations for the impurities, which most theories~[1] simply describe as magnetically inactive. Nonetheless, such Jahn-Teller distorted $3d$ impurities e.g. in the AlN host exhibit strong short range ferromagnetic interactions. Superexchange models may offer some insight to ``closed shell'' magnetic interactions, not applicable to the present case, however. We investigate such peculiar magnetic interactions via density-functional calculations, and find that a Jahn-Teller distortion can trigger the formation of a 3d-3d chemical bond that stabilizes ferromagnetism via direct exchange~[2], and further facilitates a double exchange type interaction. This multiple exchange is what aligns parallel the spins of e.g. Cr impurities in AlN.\\[4pt] [1] A. Zunger, S. Lany, and H. Raebiger, Physics {\bf 3}, 53 (2010).\\[0pt] [2] H. Raebiger, S. Lany, and A. Zunger, PRL {\bf 99}, 167203 (2007). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X15.00005: Detecting excitation and magnetization of individual dopants in a semiconductor two-dimensional electron gas Invited Speaker: Magnetic atoms doped into a semiconductor are the building blocks for bottom up spintronic and quantum logic devices. They also provide model systems for the investigation of fundamental effects. In order to correlate the dopant's atomic structure with its magnetism magnetically sensitive techniques with atomic resolution are a prerequisite. Here, I show electrical excitation and read-out $\left[1\right]$ of single magnetic dopant associated spins in a two-dimensional electron gas (2DEG) confined to a semiconductor surface $\left[2\right]$ using spin-resolved scanning tunneling spectroscopy $\left[3\right]$. I will review our real-space study of the quantum Hall transition in the 2DEG $\left[2\right]$ and of the magnetic properties of the dopants $\left[1\right]$. Finally, I will demonstrate that the dopant serves as an atomic scale probe for local magnetometry of the 2DEG. This work was done in collaboration with A. A. Khajetoorians, B. Chillian, S. Schuwalow, F. Lechermann, K. Hashimoto, C. Sohrmann, T. Inaoka, F. Meier, Y. Hirayama, R. A. R{\"o}mer, M. Morgenstern, and R. Wiesendanger.\\[4pt] $\left[1\right]$ A. A. Khajetoorians {\it et al.}, Nature {\bf 467}, 1084 (2010).\\[0pt] $\left[2\right]$ K. Hashimoto {\it et al.}, Phys. Rev. Lett. {\bf 101}, 256802 (2008).\\[0pt] $\left[3\right]$ J. Wiebe {\it et al.}, Rev. Sci. Instrum. {\bf 75}, 4871 (2004). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X15.00006: Coherent spin precession of multiple spins in an inhomogeneous environment V. Kortan, M.E. Flatt\'e Mn dopants in GaAs, whose core spins are bound anti-aligned to a hole, forming a J=1 ground state of the neutral acceptor, are very sensitive to their environment, including strain [1] and electric fields [2]. This sensitivity affects spin precession by broadening resonance lines and shifting/adding resonant frequencies. Using a low energy Hamiltonian developed for a single Mn ion-hole complex in GaAs [1,2] we have studied spin dynamics of a small collection of spins in the presence of bias electric fields and strain fields. Each Mn ion-hole complex is locally subject to a random electric field in addition to bias fields to determine if coherent spin precession persists. Using these calculations we predict the possible observation of coherent spin precession of small numbers of Mn spins via optical polarization measurements [3], and estimate the strength of the random field necessary to destroy the signal of coherent spin precession. This work supported by NRI through WIN.\\[4pt] [1] A. M. Yakunin, et al, Nature Mat. 6, 512 (2007).\\[0pt] [2] J.-M Tang, Jeremy Levy, and M. E. Flatt\'e, Phys. Rev. Lett. 97, 106803 (2006).\\[0pt] [3] R. C. Myers, et al, Nature Mat. 7, 203 (2008). [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X15.00007: Study of spin interactions between InAs surface electrons and local magnetic moments by antilocalization measurements Yao Zhang, R.L. Kallaher, V. Soghomonian, J.J. Heremans Spin-orbit coupled electrons in the InAs surface accumulation layer can be used as a sensitive system to experimentally study the interactions and exchange between electrons and local magnetic moments in semiconductors. We use antilocalization measurements as a probe of quantum states, by comparing measurements on patterned InAs accumulation layers where Sm$^{3+}$, Gd$^{3+}$ and Ho$^{3+}$ have been deposited, with those where no magnetic species are deposited. The randomly distributed ions modify the spin-orbit scattering time and the magnetic spin-flip time, identified via the antilocalization signal and characterized over temperature. The magnetic spin-flip time carries information about magnetic interactions. Experiments indicate that the spin-orbit scattering times display a weak temperature dependence. The Sm$^{3+}$ and Gd$^{3+}$ cases yield temperature-independent magnetic spin-flip times, while Ho$^{3+}$ shows a spin-flip time obeying T$^{-1/2}$ at low temperatures. Similar results as observed in the Ho$^{3+}$ case have in the literature been attributed to Kondo-like behavior. We thus interpret the results as indicative of a Kondo interaction with a Kondo temperature considerably above 5 K, of which antilocalization measurements can identify the low-temperature tail (partial support from DOE DE-FG02-08ER46532). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X15.00008: Spin relaxation dynamics for an electron gas with localized magnetic impurities near the ferromagnetic transition Matthew Mower, Giovanni Vignale We study the spin relaxation dynamics of electrons in Mn-doped GaAs. Modeling the Mn as magnetic impurities embedded in an electron gas, we construct effective electron-electron and impurity-impurity interactions. This model exhibits a ferromagnetic transition as the temperature is lowered. Near the ferromagnetic transition, strong spin fluctuations cause an enhancement of the electron scattering rate, which affects the spin relaxation time of spin polarized electrons. This is especially notable in the D'yakonov-Perel' spin relaxation time which is proportional to the electron scattering rate. We will elucidate the behavior of the spin relaxation time and other spin-dependent kinetic coefficients near the ferromagnetic transition. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X15.00009: Intrinsic Magnetism at Silicon Surfaces Steven Erwin, Franz Himpsel It has been a long-standing goal to create magnetism in a nonmagnetic material by manipulating its structure at the nanometer scale. Many structural defects have unpaired spins; an ordered arrangement of such defects can give rise to a magnetically ordered state. Here we predict theoretically [1] that stepped silicon surfaces stabilized by adsorbed gold realize this goal by self-assembly, creating linear chains of polarized electron spins with virtually perfect structural order. The spins are localized at the silicon step edges, which have the form of graphitic hexagonal ribbons. The predicted magnetic state is indirectly supported by recent experimental observations, such as the coexistence of double- and triple-period distortions and the absence of edge states in photoemission. Ordered arrays of spins at a surface offer access to local probes with single spin sensitivity, such as spin-polarized scanning tunneling microscopy. The integration of structural and magnetic order is crucial for technologies involving spin-based computation and storage at the atomic level. \\[4pt] [1] S.C. Erwin and F.J. Himpsel, Nature Communications 1:58 (2010). [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X15.00010: Magnetic Si Atoms at the Step Edges of Si(553)-Au Paul Snijders, Nathan Guisinger, Phillip Johnson, Steven Erwin, Franz Himpsel A recent calculation predicts the possibility of magnetism at step edges of vicinal Si(111) surfaces decorated with gold [1]. Graphene-like Si ribbons are formed, which contain spin-polarized Si atoms at their edges. Those atoms form a six-fold superlattice at low temperature. Scanning tunneling spectroscopy (STS) of the magnetic broken bond orbitals reveals two peaks below 50 K. They match the calculated majority and minority spin states. The peaks merge into a single, broad peak at 300 K due to rapid spin fluctuations. \\[4pt] [1] Steven C. Erwin and F. J. Himpsel, Nature Communications 1:58 (2010). [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X15.00011: Electronic structure of Gd impurities in GaN on Ga, N and Ga-N adjacent sites and the role of N interstitials Tawinan Cheiwchanchamnangij, Walter Lambrecht Gd-doped GaN is one of the most interesting dilute magnetic semiconductors. However, the origins of its magnetic properties are still unclear. Previous studies have focused on the role of intrinsic defects, such as Ga vacancies and N or O interstitials. Here, we study Gd doped in pairs on adjacent Ga and N sites, which were suggested to be required to explain the X-ray linear dichroism signals in Gd L-edge spectra by Ney et al. JMMM 322, 1162 (2010). By using the FP-LMTO method in the LSDA+U, we find that the Gd on N site is pushed to the interstitial site after the relaxation and there is no extra magnetic moment besides the seven Bohr magneton from the 4f half-filled shell on each Gd atom. In spite of the relaxation, we find the energy of formation of this cluster to be of order 10 eV, which shows that the Gd doped on Ga-N adjacent sites is unlikely to occur. We also study Gd doped on a single N site and find an excess magnetic moment of 3 Bohr magneton which is spread over Gd, the nearest neighbor Ga atoms, and second nearest neighbor N atoms. However, its energy of formation is also large that this kind of impurity is unlikely to occur. We critically examine previous work on the role of N interstitials in the magnetism of Gd-doped GaN by studying the magnetic properties of the split interstitials, their interaction with each other and with Gd. [Preview Abstract] |
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