Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Y35: Focus Session: Spins in Semiconductors  Qubits and Quantum Wires 
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Sponsoring Units: GMAG DMP FIAP Chair: Paul Crowell, University of Minnesota Room: E145 
Friday, March 19, 2010 8:00AM  8:12AM 
Y35.00001: Gate control of a quantum dot singleelectron spin through Geometric Phases: Feynman Disentangling Method Sanjay Prabhakar, James Raynolds, Akira Inomata Among recent proposals for nextgeneration, nonchargebased logic is the notion that a single electron can be trapped and its spin can be manipulated by moving the quantum dot adiabatically in a closed loop (Berry effect) through the application of gate potentials. In this paper, we present numerical simulations and analytical expressions of such spins in single electron devices for a quantum dot. Using analytical and numerical techniques, we show that spin orbit coupling in IIIV type semiconductor will enhance the transition probability of the electron spin over pure Rashba and or pure Dresselhaus cases. With the help of Feynman Disentangling technique of the nonabelean operator, we found the exact analytical expression for the propagator of an electron moving under the influence of three different cases: pure Rashba, pure Dresselhaus and equal strength of Rashba and Dresselhaus spin orbit coupling. For the most general cases where the solution of the propagator becomes nontrivial, we carry out the numerical simulations of such propagator. [Preview Abstract] 
Friday, March 19, 2010 8:12AM  8:24AM 
Y35.00002: A coherent beam splitter for electronic spin states J. R. Petta, H. Lu, A. C. Gossard Energy level crossings, where two quantum states cross in energy as a function of an external parameter, are ubiquitous in quantum mechanics. We demonstrate coherent control of electronic spin states in a double quantum dot by sweeping an initially prepared spin singlet state through a singlettriplet anticrossing in the energy level spectrum. The anticrossing serves as a beam splitter for the incoming spin singlet state. Consecutive crossings through the beam splitter, when performed within the spin dephasing time, result in controllable coherent quantum oscillations between the singlet state and m$_s$=+1 triplet state, T$_+$. The observed quantum interference patterns are in good agreement with a theoretical model of consecutive LandauZener tunneling events. This allelectrical method for quantum control relies on electronnuclear spin coupling and drives single electron spinflip transitions on nanosecond timescales without electron spin resonance fields, which are difficult to localize on the nanometer scale. [Preview Abstract] 
Friday, March 19, 2010 8:24AM  8:36AM 
Y35.00003: A singlet  triplet T$_{+}$ based qubit Hugo Ribeiro, Jason Petta, Guido Burkard We theoretically show that the electronic twospin states singlet and triplet T$_{+}$ are promising candidates for the implementation of a \emph{qubit} in GaAs double quantum dots (DQD). A coherent superposition of the twospin states is obtained by finite time LandauZenerSt\"uckelberg interferometry [1] and the single qubit rotations are performed by means of an external magnetic field with a typical amplitude of about $100$ mT. In such a system, the coherent manipulation of the qubit takes place in a time scale of about $1$ ns. We also study the nuclear induced decoherence, mainly due to hyperfine contact coupling between the electronic and nuclear spins, and compute the decoherence time $T_2^{*} \sim10$ ns. \\[4pt] [1] H. Ribeiro and G. Burkard, Phys. Rev. Lett. \textbf{102}, 216802 (2009) [Preview Abstract] 
Friday, March 19, 2010 8:36AM  9:12AM 
Y35.00004: Optically controlled spins in semiconductor quantum dots Invited Speaker: Sophia Economou Spins in charged semiconductor quantum dots are currently generating much interest, both from a fundamental physics standpoint, as well as for their potential technological relevance. Being naturally a twolevel quantum system, each of these spins can encode a bit of quantum information. Optically controlled spins in quantum dots possess several desirable properties: their spin coherence times are long, they allow for alloptical manipulationwhich translates into fast logic gatesand their coupling to photons offers a straightforward route to exchange of quantum information between spatially separated sites. Designing the laser fields to achieve the unprecedented amount of control required for quantum information tasks is a challenging goal, towards which there has been recent progress. Special properties of hyperbolic secant optical pulses enabled the design of single qubit rotations, initially developed about the growth axis z [1], and later about an arbitrary direction [2]. Recently we demonstrated our theoretical proposal [1] in an ensemble of InAs/GaAs quantum dots by implementing ultrafast rotations about the z axis by an arbitrary angle [3], with the angle of rotation as a function of the optical detuning in excellent agreement with the theoretical prediction. We also developed twoqubit conditional control in a quantum dot `molecule' using the electronhole exchange interaction [4]. In addition to its importance in quantum dotbased quantum computation, our twoqubit gate can also play an important role in photonic cluster state generation for measurementbased quantum computing [5]. [1] S. E. Economou, L. J. Sham, Y. Wu, D. S. Steel, Phys. Rev. \textbf{74}, 205415 (2006) [2] S. E. Economou and T. L. Reinecke, Phys. Rev. Lett., \textbf{99}, 217401 (2007) [3] A. Greilich, S. E. Economou \textit{et al}, Nature Phys. \textbf{5}, 262 (2009) [4] S. E. Economou and T. L. Reinecke, Phys. Rev. B, \textbf{78}, 115306 (2008) [5] S. E. Economou, N. H. Lindner, and T. Rudolph, in preparation [Preview Abstract] 
Friday, March 19, 2010 9:12AM  9:24AM 
Y35.00005: Antilocalization in quasi1D InAs wires R. L. Kallaher, J. J. Heremans, W. Van Roy, G. Borghs The influence of mesoscopic confinement on spin and phase coherence of carriers can be investigated through measurements of antilocalization. Antilocalization results in a decrease in the resistance at low magnetic field, from which spin and phase coherence lengths can be extracted. We present low temperature magnetotransport measurements performed on quasi1D (Q1D) wires fabricated from InAs/AlGaSb two dimensional electron systems (2DESs). Shubnikovde Haas oscillations indicate strong spinorbit coupling for such 2DESs. Measurements of antilocalization in the unpatterned 2DESs with different electron mobilities (30 vs 10 $m^2/Vs$) and associated Q1D wires of widths ranging from 0.1  1 $\mu m$ are compared. The effect of antilocalization on magnetoresistance is more readily observed in the narrow wires as compared to the unpatterned 2DESs where little, or no, antilocalization was detected. We discuss the enhancement of antilocalization, and the wire width dependence of extracted spin and phase coherence lengths obtained by fitting the magnetoresistance curves to localization theory. Support by DOE DEFG0208ER46532 is acknowledged. [Preview Abstract] 

Y35.00006: ABSTRACT WITHDRAWN 
Friday, March 19, 2010 9:36AM  9:48AM 
Y35.00007: Lateral Spin Injection in Germanium Nanowires EnShao Liu, Junghyo Nah, Kamran Varahramyana, Sanjay Banerjee, Emanuel Tutuc Efficient spin injection from ferromagnetic (FM) contacts into semiconductors (SC) is typically suppressed by the conductivities mismatch between the FM contact and the SC. Spin injection can be achieved however if the contact resistivity at the FM/SC interface is appropriately engineered [1]. Here we report spin injection in ntype, phosphorusdoped germanium nanowires with doping densities above 10$^{19}$ cm$^{3}$, using cobalt as FM contacts and MgO tunnel barriers for contact resistance engineering. The twopoint magneotresistance measurements of Ge nanowires with Co contacts reveal spinvalve effect, with a low (high) resistance for parallel (antiparallel) polarizations of the FM contacts. Nonlocal, fourpoint magnetoresistance measurements, which separate the spin diffusion path from the charge current path, demonstrate that the observed spinvalve effect stems from spin injection in the Ge nanowires. [1] A. Fert and H. Jaffres, Phys. Rev. B. \textbf{64}, 184420 (2001) [Preview Abstract] 
Friday, March 19, 2010 9:48AM  10:00AM 
Y35.00008: Spinengineered hybrid ferromagnet/semiconductor nanowires J. Liang, B.J. Cooley, N. Dellas, D. Rench, D.M. Zhang, A. Kandala, P. Schiffer, S.E. Mohney, N. Samarth, B. Maertz, D.D. Awschalom Hybrid ferromagnet/semiconductor (NWs) provide new opportunities for nanospintronics by integrating heterogeneous materials in the form of axial and radial heterostructures [e.g. APL {\bf 95}, 133126 (2009)]. Here, we report structural, magnetic and optoelectronic investigations of hybrid ferromagnet/semiconductor NWs synthesized by the molecular beam epitaxy of MnAs on the facets of zincblende GaAs NWs. The latter are grown {\it in situ} by a selfseeded process on (111)B GaAs substrates. Transmission electron microscopy reveals the epitaxy of a single crystal MnAs shell on a GaAs NW core. Raman and photoluminescence spectroscopies probe changes in the vibrational and emission spectra of single GaAs NWs imposed by the growth of the MnAs shell. Magnetic force microscopy confirms the presence of ferromagnetism at room temperature. We develop novel approaches to probe the dynamical magnetic response of a single NW using vector magnetometry. Work supported by NSF and ONR. [Preview Abstract] 
Friday, March 19, 2010 10:00AM  10:12AM 
Y35.00009: Observation of orientation and kdependent Zeeman spinsplitting in hole quantum wires on (100)oriented AlGaAs/GaAs heterostructures Adam Micolich, Jason Chen, Oleh Klochan, Alex Hamilton, Theodore Martin, Laphang Ho, Ulrich Zuelicke, Dirk Reuter, Andreas Wieck We will present our recent study the Zeeman spinsplitting in hole quantum wires oriented along the $[011]$ and $[01\overline{1}]$ crystallographic axes of a high mobility undoped (100)oriented AlGaAs/GaAs heterostructure. Our data shows that the spinsplitting can be switched `on' (finite $g^{*}$) or `off' (zero $g^{*}$) by rotating the field from a parallel to a perpendicular orientation with respect to the wire, and the properties of the wire are identical for the two orientations with respect to the crystallographic axes. We also find that the $g$factor in the parallel orientation decreases as the wire is narrowed. This is in contrast to electron quantum wires, where the $g$factor is enhanced by exchange effects as the wire is narrowed. This is evidence for a $k$dependent Zeeman splitting that arises from the spin$\frac{3}{2}$ nature of holes. [Preview Abstract] 
Friday, March 19, 2010 10:12AM  10:24AM 
Y35.00010: Spin Resonance and dc Current Generation in a Quantum Wire Wayne Saslow, Valery Pokrovsky In a quantum wire of degenerate semiconductor the effective spinorbit field due to the Rashba and Dresselhaus interactions is independent of direction. A consequence is that at low temperatures there is a narrow spin resonance at low temperatures, even in the absence of an external magnetic field. Resonance absorption by linearly polarized radiation gives a dc spin current; resonance absorption by circularly polarized radiation gives a dc electric current or magnetization on the order of a pA. The resonance is typically in the terahertz region. Its heating effects may be observable if the wire is thermally isolated from the substrate. The amplitude of these effects is sensitive to the external magnetic field and to the gate voltage, thus suggesting technological applications. [Preview Abstract] 
Friday, March 19, 2010 10:24AM  10:36AM 
Y35.00011: Magnetoconductance of quantum wires Gerson J. Ferreira, Filipe Sammarco, Carlos Egues At low temperatures the conductance of a quantum wires exhibit characteristic plate\aus due to the quantization of the transverse modes [1]. In the presence of high inplane magnetic fields these spinsplit transverse modes cross. Recently, these crossings were observed experimentally [2] via measurements of the differential conductance as a function of the gate voltage and the inplane magneticfield. These show structures described as either anticrossings or magnetic phase transitions. Motivated by our previous works on magnetotransport in 2DEGs via the Spin Density Functional Theory (SDFT) [3], here we propose a similar model to investigate the magnetoconductance of quantum wires. We use (i) the SDFT via the KohnSham selfconsistent scheme within the local spin density approximation to obtain the electronic structure and (ii) the LandauerBuettiker formalism to calculate the conductance of a quantum wire. Our results show qualitative agreement with the data of Ref.~[2]. \noindent [1] B.~J.~van Wees \textit{et al.}, Phys.~Rev.~Lett.~\textbf{60}, 848 (1988). \noindent [2] A.~C.~Graham \textit{et al.}, Phys.~Rev.~Lett. \textbf{100}, 226804 (2008). \noindent [3] H.~J.~P.~Freire, and J.~C.~Egues, Phys.~Rev.~Lett. \textbf{99}, 026801 (2007); G.~J.~Ferreira, and J.~Carlos Egues, J. Supercond. Nov. Mag., in press; G.~J.~Ferreira, H.~J.~P.~Freire, J.~Carlos Egues, submitted. [Preview Abstract] 
Friday, March 19, 2010 10:36AM  10:48AM 
Y35.00012: Coulomb drag from spinonholon coupling Rodrigo Pereira, Eran Sela We discuss the density and temperature dependence of the Coulomb drag resistivity due to longwavelength scattering between quantum wires, based on an approximation for the dynamic charge response of nonlinear spin1/2 Luttinger liquids. Besides accounting for the broadening of the charge peak in the dynamic charge response due to twoholon excitations, the nonlinearity of the dispersion gives rise to a twospinon peak, which at zero temperature has an asymmetric line shape. When the charge velocity of one wire matches the spin velocity of the other wire, the drag resistivity is enhanced by holonspinon scattering, and its temperature dependence has signatures of spin diffusion. This effect opens the possibility of observing spincharge separation in Coulomb drag experiments. [Preview Abstract] 
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