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@ARTICLE{Hell:201463,
      author       = {Hell, Michael and Das, Sourin and Wegewijs, Maarten Rolf},
      title        = {{T}ransport of spin anisotropy without spin currents},
      journal      = {Physical review / B},
      volume       = {88},
      number       = {11},
      issn         = {1098-0121},
      address      = {College Park, Md.},
      publisher    = {APS},
      reportid     = {FZJ-2015-03758},
      pages        = {115435},
      year         = {2013},
      abstract     = {We revisit the transport of spin-degrees of freedom across
                      an electrically and thermally biased tunnel junction between
                      two ferromagnets with noncollinear magnetizations. Besides
                      the well-known charge current and spin current we show that
                      a nonzero spin-quadrupole current flows between the
                      ferromagnets. This tensor-valued current describes the
                      nonequilibrium transport of spin anisotropy relating to both
                      local and nonlocal multiparticle spin correlations of the
                      circuit. This quadratic spin anisotropy, quantified in terms
                      of the spin-quadrupole moment, is fundamentally a
                      two-electron quantity. In spin valves with an embedded
                      quantum dot such currents have been shown to result in a
                      quadrupole accumulation that affects the measurable quantum
                      dot spin and charge dynamics. The spin-valve model studied
                      here allows fundamental questions about spin-quadrupole
                      storage and transport to be worked out in detail, while
                      ignoring the detection by a quantum dot. The physical
                      understanding of this particular device is of importance for
                      more complex devices where spin-quadrupole transport can be
                      detected. We demonstrate that, as far as storage and
                      transport are concerned, the spin anisotropy is only partly
                      determined by the spin polarization. In fact, for a
                      thermally biased spin valve the charge current and spin
                      current may vanish, while a pure exchange spin-quadrupole
                      current remains, which appears as a fundamental consequence
                      of Pauli's principle. We extend the real-time diagrammatic
                      approach to efficiently calculate the average of
                      multiparticle spin observables, in particular the
                      spin-quadrupole current. Although the paper addresses only
                      leading-order and spin-conserving tunneling, we formulate
                      the technique for arbitrary order in an arbitrary,
                      spin-dependent tunnel coupling in a way that lends itself to
                      extension to quantum-dot spin-valve structures.},
      cin          = {PGI-2 / JARA-FIT},
      ddc          = {530},
      cid          = {I:(DE-Juel1)PGI-2-20110106 / $I:(DE-82)080009_20140620$},
      pnm          = {422 - Spin-based and quantum information (POF2-422)},
      pid          = {G:(DE-HGF)POF2-422},
      typ          = {PUB:(DE-HGF)16},
      UT           = {WOS:000324952000002},
      doi          = {10.1103/PhysRevB.88.115435},
      url          = {https://juser.fz-juelich.de/record/201463},
}