Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T11: Spin and Transport in Low Dimensional Semiconductors |
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Sponsoring Units: FIAP Chair: Nikolai Klimov, National Institute of Standards and Technology Room: D222 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T11.00001: Noble Metals and Transition Metals Adsorbed on Graphene: The Pursuit of Graphene Spintronics Matheus Paes Lima, Adalberto Fazzio Via first-principles simulations we study the structural, magnetic and electron transport properties of 2D graphene in the presence of single atoms. We consider Cu, Ag and Au, as well as Mn, Fe, Co and Ni adsorbed on pristine and defective graphene containing vacancies and divacancies. To obtain the transport properties we perform ab-initio calculations based on Density Functional Theory (DFT) coupled to Non-Equilibrium Greens` Function (NEGF) formalism using the Landauer-B\"uttiker formula within the Meir-Wingreen approach. Our results show that graphene+noble metals systems have a gate controllable spin polarized current, allowing the fabrication of switchable spin filters with a moderated efficiency. In the particular case of Gold adsorbed on pristine graphene, a positive gate leads to a polarized current with excess of up electrons, while a negative gate the converse. Despite the high-spin configuration of Mn, the d levels are very far from the Fermi level. The Ni atom prefers a non-magnetic configuration. Therefore, graphene+transition metals systems present a polarized current only for Fe and Co atoms, allowing the fabrication of spin filters with very high efficiency. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T11.00002: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T11.00003: Hole-induced Dynamic Nuclear Polarization in Quantum Dots Wen Yang, L.J. Sham We present a microscopic theory showing that through the non- collinear hole-nuclear dipolar hyperfine interaction, an optically excited heavy hole in a quantum dot can induce a steady-state nuclear polarization. The preferential direction of the nuclear spin flip is set by the energy mismatch of relevant transitions instead of thermal relaxation. The induced nuclear polarization shows a sign dependence on the product of the nuclear Zeeman splitting and the frequency detuning of the pumping laser, leading to bidirectional hysteretic locking of the optical absorption peak onto resonance or bidirectional hysteretic shift of the peak away from zero detuning. This sheds light on a puzzling observation of bidirectional hysteretic locking of the neutral exciton absorption peak in Faraday geometry [C. Latta et al., Nature Phys. 5, 758 (2009)]. By solving the Fokker-Planck equation for the nuclear polarization distribution, we found a ~10-fold suppression of the steady- state nuclear fluctuation, in reasonable agreement with the single pump experiment in Voigt geometry [X. Xu et al., Nature 459, 1105 (2009)]. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T11.00004: Beating of Friedel Oscillations in Spin-Orbit Coupled System Samvel Badalyan, Alex Matos-Abiague, Giovanni Vignale, Jaroslav Fabian The interplay of different spin-orbit interaction mechanisms induces highly anisotropic modifications of the static dielectric function of a two-dimensional electron system. One of the main changes of the static polarization function is the induced shift of its singularity position, which is in opposite directions for orthogonal momentum orientations. More interestingly, we have found that in certain situations the polarization function exhibits a \textit{doubly-singula}r behavior. This new property generates a novel phenomenon--\textit{the beating of Friedel oscillation}s, which can be controlled by an external electric field. This effect is a general feature of systems with Bychkov-Rashba and Dresselhaus spin-orbit fields and should be directly observable through tunneling microscopy imaging of the density distribution around an impurity. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T11.00005: Cyclotron resonance in graphene and Kohn's theorem Keshav Shrivastava In 1961 Kohn has shown [1] that the cyclotron frequency is independent of the interaction. In the case of graphene there is some effort to suggest that the electron dispersion is linear in k, instead of (h/2$\pi $ )$^{2}$k$^{2}$/2m so that the Kohn theorem may not apply [2]. We find that the Kohn theorem does not use the dispersion relation and applies to graphene the same way as in some other material. We find that if e is replaced by e*=(1/2)ge, the Kohn theorem applies with the cyclotron frequency (h/2$\pi )\omega _{c}$= (1/2)geB/mc. Hence there is no interaction and all of the interaction is contained in g = (2j+1)/(2l+1) which is used only in the unperturbed Hamiltonian. The degeneracy of the levels is found to be related to the flux quantization. We have explained [3] the plateaus observed in the Hall effect resistivity of graphene without the use of interaction. Hence the Kohn's them applies to graphene. \\[4pt] [1] W. Kohn, Phys. Rev. 123, 1242-1244 (1961);\\[0pt] [2] E. A. Henriksen, et al., Phys. Rev. Lett. 104, 067404(2010). \\[0pt] [3] K. N. Shrivastava, AIP Conf. Proc. 1150, 59-67(2009); 1017,422-428(2008); Proc. SPIE 7155, 71552F(2008). [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T11.00006: Fluctuation-dissipation theorem for chiral systems in nonequilibrium steady states Dima Feldman, Chenjie Wang We consider a three-terminal system with a chiral edge channel connecting the source and drain terminals. Charge can tunnel between the chiral edge and a third thermal. The third terminal is maintained at a different temperature and voltage than the source and drain. We prove a general relation for the current noises detected in the drain and third terminal. It has the same structure as an equilibrium fluctuation-dissipation relation with the nonlinear response $\partial I/\partial V$ in place of the linear conductance. The result applies to a general chiral system and may be useful for detecting ``upstream'' modes on the quantum Hall edges. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T11.00007: Spontaneous Quantum Hall States in Chirally-Stacked Few-Layer Graphene Systems Fan Zhang, Jeil Jung, Gregory Fiete, Qian Niu, Allan MacDonald Chirally stacked N-layer graphene systems with N$\geq$2 exhibit a variety of distinct broken symmetry states in which charge density contributions from different spins and valleys are spontaneously transferred between layers. We explain how these states are distinguished by their charge, spin, and valley Hall conductivities, by their orbital magnetizations, and by their edge state properties. We argue that valley Hall states have [N/2] edge channels per spin-valley. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T11.00008: Re-entrant Negative Coulomb Drag in a 1D Quantum Circuit Dominique Laroche, Guillaume Gervais, Mike P. Lilly, John L. Reno We report Coulomb drag measurements between tunable vertically- coupled quantum wires. The wires are fabricated in a GaAs/AlGaAs double quantum well heterostructure with a 15 nm barrier separating the quantum wells and are non-ballistic. The Coulomb drag signal is mapped out versus the number of subbands occupied in each wire, and regions of both positive and negative drag are observed. Negative Coulomb drag signals are measured in two regimes: one at low electronic density when the drag wire is close to or beyond depletion, and one at higher electronic density when the drag wire has more than a single 1D subband occupied. A discussion of the negative drag signal in terms of electron-hole asymmetry and localization is presented. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04- 94AL85000. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T11.00009: Dynamic control over nanoparticle plasmon resonance through variation of refractive index Hari P. Paudel, Michael N. Leuenberger In a semiconductor material it is possible to vary the index of refraction by exciting electron-hole pairs through a laser pulse. Generally the change in refractive index changes linearly with the carrier density and also increases with the lattice temperature. We present our results on the variation in index of refraction in the TiO2 shell of an Ag/TiO2 core-shell nanoparticle by exciting electron-hole pairs in TiO2 through a laser pulse. We performed bandstructure calculations using VASP to determine the variation of the optical dielectric tensor as a function of photon frequency. This change in refractive index not only affects the refraction of photons with frequency below bandgap, but also affects strongly the resonance peaks of the surface plasmons due to the Ag core. This effect can be used to dynamically control the plasmon resonance of a hybrid metal-semiconductor nanoparticle, for example for use in cancer therapy or nanoplasmonic circuits. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T11.00010: Transport in Coherent Quantum Hall Bilayers Allan MacDonald, Dmytro Pesin We develop an approach to describe transport in bilayer quantum Hall systems in which coherence is established spontaneously between layers. We use Landauer-Buttiker theory with phenomenological parameters which can be fit to experimental data to describe quasiparticle transport in bilayers with strong coherence. We use the above approach to calculate two-probe conductances for various experimental configurations. We also apply the formalism to describe high-current transport in a bilayer with a time-dependent condensate. To describe the transition from strong to weak coherence, we use a pheonomenological single ``relaxation length'' ansatz for contact-to-contact transmission coefficients. As an application, we consider longitudinal drag, and find a good agreement with experiment in the regime of well- developed Quantum Hall Effect. [Preview Abstract] |
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