Bulletin of the American Physical Society
75th Annual Meeting of the Southeastern Section of APS
Volume 53, Number 13
Thursday–Saturday, October 30–November 1 2008; Raleigh, North Carolina
Session DA: Forefront Materials Physics I |
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Chair: Frank Tsui, University of North Carolina Room: Holiday Inn Brownstone Roosevelt |
Thursday, October 30, 2008 1:30PM - 2:00PM |
DA.00001: Generation, Modulation and Electrical Detection of Spin Currents in Silicon in Lateral Transport Invited Speaker: The electron's spin angular momentum is one of several alternative state variables under consideration on the International Technology Roadmap for Semiconductors. Electrical injection / transport of spin-polarized carriers is prerequisite for developing such an approach. While significant progress has been realized in GaAs , little has been made in Si. Electrical injection of spin-polarized electrons is demonstrated in Fe/Al2O3/Si (001) n-i-p structures by measuring the circular polarization of the electroluminescence (EL). The EL polarization tracks the Fe magnetization, confirming spin injection into the Si, and reflects Fe majority spin, consistent with the common delta-1 symmetry of the Fe and Si bands. The Si spin polarization is at least 30{\%} at 5K, with significant polarization extending to at least 125K. These results are confirmed in Fe/Al2O3/Si/AlGaAs/GaAs quantum well structures -- the GaAs EL shows that spin injection occurs despite poor crystalline quality of Si epilayers on GaAs, the 0.3 eV Si/AlGaAs CB offset, and air exposure of the interfaces. Lateral transport structures and non-local detection techniques are used to create a pure spin current which flows separately from the spin-polarized charge current. This spin diffusion current is sensitive to the relative magnetizations of the injecting and detecting contacts, and can be modulated by a perpendicular magnetic field (Hanle effect) which causes precession in the transport channel. The generation of spin currents, coherent spin precession and electrical detection using magnetic tunnel barrier contacts and a simple lateral device geometry compatible with ``back-end'' silicon processing will facilitate development of silicon-based spintronic devices. Refs: Nature Physics 3, 542 (2007); Appl. Phys. Lett 91, 212109 (2007). [Preview Abstract] |
Thursday, October 30, 2008 2:00PM - 2:30PM |
DA.00002: Mode locking of electron spin coherence in singly charged quantum dots Invited Speaker: Fast dephasing of electron spins in an ensemble of quantum dots is detrimental for applications in quantum-information processing. We show that dephasing can be overcome by using a periodic train of light pulses to synchronize the phases of the precessing spins, and demonstrate this effect in an ensemble of singly charged (In,Ga)As/GaAs quantum dots. A periodic train of circularly polarized light pulses from a mode-locked laser synchronizes the precession of the spins to the laser repetition rate, transferring the mode-locking into the spin system. The mode-locking technique allows us to measure the single-spin coherence time to be 3 microseconds, which is four orders of magnitude longer than the ensemble dephasing time of 400 picoseconds. The technique also offers the possibility of achieving all-optical coherent manipulation of spin ensembles, in which electron spins can be clocked by two trains of pump pulses with a fixed temporal delay. The nuclei in these experiments act constructively, leading to the nuclear-induced frequency-focusing effect, which moves the electron-spin precession into dephasing-free subspace. [Preview Abstract] |
Thursday, October 30, 2008 2:30PM - 3:00PM |
DA.00003: From computational materials science to nanoscale device physics Invited Speaker: I will outline formal, computational and device level challenges for modeling and simulation of nanoelectronic devices and systems. \textit{Formal} challenges involve developing the basic equations for quantum transport in the presence of strong many-body correlations (Coulomb Blockade), incoherent scattering (phonons) and time-dependent effects at the nano-micro interface (hysteretic switching and random telegraph noise). \textit{Computational} challenges involve translating these equations into quantitative, predictive models, particularly at surfaces and interfaces, where we need practical semi-empirical descriptions with transferable parameters to handle hybrid regions. In addition, we need multiscaling and embedding techniques to merge these models with more detailed ``ab-initio'' descriptions of chemically significant moieties. Finally, \textit{Device} level challenges involve identifying fundamental limits of existing device paradigms, such as molecular FETs, as well as exploring novel device operational principles. I will touch upon the fundamental issues that arise in context of each challenge, and possible means of solving them. I will then apply these ideas to a specific device architecture, namely, an ordered array of quantum dots grown on the surface of a nanoscale silicon transistor. All of the challenges identified above manifest themselves prominently in this geometry that operates at the nano-micro interface. Specifically, I will discuss how the strongly correlated electrons in the nanoscale dots ``talk'' to their weakly interacting macroscopic counterparts, how the interfacial electronic structure captures both long-ranged band correlations and short-ranged chemical correlations, and how the tunable coupling with the localized dot degrees of freedom can lead to novel physics, such as the experimentally observed blocking and unblocking of a nanotube current by correlated interactions between multiple oxide traps. [Preview Abstract] |
Thursday, October 30, 2008 3:00PM - 3:12PM |
DA.00004: Temperature-dependent study of vibration and polymorphism of oligoacenes and their derivatives Zhongqiao Ren, Laurie McNeil, Christian Kloc Raman measurements have been performed on a series of oligoacenes and their derivatives (anthracene, tetracene, pentacene and diphenyl-anthracene, rubrene) in a wide temperature range (50-300K). It has been observed that different phases co-exist in several of these crystals depending on sample preparation and history, and that transitions between polymorphs can be observed as a function of temperature. Comparisons among crystals with similar molecular structure will be made to clarify the changes in the inter- and intra-molecular modes as the structure changes with temperature. Simulated calculations of the inter-molecular modes between multiple molecules, and the intra-molecular modes of the isolated molecules will also be presented. [Preview Abstract] |
Thursday, October 30, 2008 3:12PM - 3:24PM |
DA.00005: Band Engineering in C/BN Nanoribbons and Stacks Jeffrey Mullen, Marco Buongiorno Nardelli Using electronic structure calculations from first principles we have studied the electronic characteristics of graphene/BN sheets in planar ``super-striped'' and layered stacks geometries. Similarly to Hydrogen-terminated graphene nanoribbons, C/BN super-stripes and stacks show a variation of band gaps. Moreover, the bonding with BN introduces confinement effects that can be potentially exploited to enhance the electronic transport properties of these systems. We have characterized these effects by evaluating the band offsets and the electrostatic potential profile across the structures. [Preview Abstract] |
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