Gerd Bergmann

Publications (4)13.88 Total impact

• Source

  Available from: ArXiv

  Article: Friedel Oscillation about a Friedel-Anderson Impurity

  Yaqi Tao, Gerd Bergmann
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  ABSTRACT: The Friedel oscillations in the vicinity of a Friedel-Anderson (FA) impurity are investigated numerically. For an FA impurity in the local moment limit the normalized amplitude A({\xi}) is S-shaped, approximately zero at short distances, approaching two at large distances and crossing the value one at the characteristic length {\xi}_{1/2}. Surprisingly, the Friedel oscillations of a simple non-interacting Friedel impurity with a narrow resonance at the Fermi level show a very similar behavior of their amplitude A({\xi}). A comparison correlates the resonance width and the Kondo energy of the FA impurity with the characteristic length {\xi}_{1/2} of the Friedel oscillations.
  10/2011;
• Source

  Available from: ArXiv

  Article: Numerical Calculation of the Fidelity for the Kondo and the Friedel-Anderson Impurities

  Gerd Bergmann, Richard S. Thompson
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  ABSTRACT: The fidelities of the Kondo and the Friedel-Anderson (FA) impurities are calculated numerically. The ground states of both systems are calculated with the FAIR (Friedel artificially inserted resonance) theory. The ground state in the interacting systems is compared with a nullstate in which the interaction is zero. The different multi-electron states are expressed in terms of Wilson states. The use of N Wilson states simulates the use of a large effective number N_{eff} of states. A plot of ln(F) versus N\proptoln(N_{eff}) reveals whether one has an Anderson orthogonality catastrophe at zero energy. The results are at first glance surprising. The ln(F)-ln(N_{eff}) plot for the Kondo impurity diverges for large N_{eff}. On the other hand, the corresponding plot for the symmetric FA impurity saturates for large N_{eff} when the level spacing at the Fermi level is of the order of the singlet-triplet excitation energy. The behavior of the fidelity allows one to determine the phase shift of the electron states in this regime. PACS: 75.20.Hr, 71.23.An, 71.27.+a, 05.30.-d
  The European Physical Journal B - Condensed Matter and Complex Systems. 09/2011; 84(2).
• Article: Template‐based Synthesis and Magnetic Properties of Cobalt Nanotube Arrays

  Dongdong Li, Richard S. Thompson, Gerd Bergmann, Jia G. Lu
  Advanced Materials 10/2008; 20(23):4575 - 4578. · 13.88 Impact Factor
• Source

  Available from: ArXiv

  Article: Spacial Modulation of the Magnetization in Cobalt Nanowires

  Gerd Bergmann, Yaqi Tao, Jia G Lu, Richard S. Thompson
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  ABSTRACT: Cobalt nanowires with a diameter in the range between 50 to 100nm can be prepared as single-crystal wires with the easy axis (the c-axis) perpendicular to the wire axis. The competition between the crystal anisotropy and demagnetization energy frustrates the magnetization direction. A periodic modulation of the angle between M and the wire axis yields a lower energy.
  08/2007;