Browsing by Author "Achenefe Yohannes"

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    Electron Scattering in Graphene by Electric and Magnetic Dipole
    (Addis Ababa University, 2016-02) Achenefe Yohannes; Malnev Vadim N. (Professor)
    The two-dimensional form of graphite (graphene) has been obtained only very re- cently, and immediately attracted great deal of attention. Electrons in graphene, obeying linear dispersion relation, behave like massless relativistic particles. It is be- lieved that graphene can replace silicon in electronics world. But the biggest obstacle is that graphene itself does not have a band gap. This property makes graphene always conducting and conduction can not be completely turned o . One of the most challenging task is to learn how to control the electron behavior using electric elds in this two-dimensional layer. In this work we wanted to learn (and to propose mechanisms) "How to control the electron motion in graphene using magnetic dipole and electric dipole?". The second idea was to investigate if it is possible to use mag- netic elds of large suspended nanomagnet to open a gap in the energy spectrum of graphene. The elastic electron scattering by impurities with electric and magnetic dipoles in graphene is studied with the help of Born approximation. Both types of scatterers give the nonzero cross section of backscattering. The scattering by the impurities with electric dipoles is more efcient even comparing to the scattering by the nanomagnets with anomalous magnetic moments. A comparison of the electron scattering transport cross sections by charged impurities and impurities with electric dipole moments shows that they can be comparable. The scattering by the impurities electric dipoles can be important in limiting the electron mobility in graphene along with the Coulomb scattering. xii xiii The elastic electron scattering by a nonuniform magnetic eld of a remote nano- magnets in the graphene is also considered with the help of the modied Born ap- proximation. The nanomagnets are modeled by point like magnetic dipoles oriented transversally and parallel to the graphene plane. They can form rather high magnetic elds without any damage of the graphene plane. The electron scattering cross sections are obtained in the closed form and analyzed numerically. It is shown that this mech- anism of scattering has nonzero backscattering cross section and can considerably a ect the graphene conductivity
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    Quantum hall Effect in Two Dimensional Electron Gas
    (Addis Ababa University, 2009-06) Achenefe Yohannes; Mal’nev Vadim (Professor)
    In this work we analyze the modern state of the problem connected with quantum Hall effect (integral and fractional). The quantum Hall effect is a quantum-mechanical version of the Hall effect, observed in two dimensional electron systems subjected to low temperatures (< 1K) and strong magnetic fields ( 10T), in which the Hall conductance ( H) takes on the quantized values ( e2/h) with an integer (integer quantized Hall effect) or a rational fraction (fractional quantized Hall effect), independent of the detail of the sample geometry. The fractional quantum Hall effect is not complectly understood at the time being. Recently the idea of chiral heat transport in quantum Hall regime was pushed forward [33], we also analyze thermal transport in the fractional quantum Hall effect (FQHE). As an original part we consider the problem of two electrons in a uniform magnetic field with the account of the center mass: (1) Quantum Mechanical Description and (2) Classical Description