526 research outputs found
Van der Waals Density Functional for General Geometries
A scheme within density functional theory is proposed that provides a
practical way to generalize to unrestricted geometries the method applied with
some success to layered geometries [H. Rydberg, et al., Phys. Rev. Lett. 91,
126402 (2003)]. It includes van der Waals forces in a seamless fashion. By
expansion to second order in a carefully chosen quantity contained in the long
range part of the correlation functional, the nonlocal correlations are
expressed in terms of a density-density interaction formula. It contains a
relatively simple parametrized kernel, with parameters determined by the local
density and its gradient. The proposed functional is applied to rare gas and
benzene dimers, where it is shown to give a realistic description.Comment: 4 pages, 4 figure
Spin dynamics from Majorana fermions
Using the Majorana fermion representation of spin-1/2 local moments, we show
how it is possible to directly read off the dynamic spin correlation and
susceptibility from the one-particle propagator of the Majorana fermion. We
illustrate our method by applying it to the spin dynamics of a non-equilibrium
quantum dot, computing the voltage-dependent spin relaxation rate and showing
that, at weak coupling, the fluctuation-dissipation relation for the spin of a
quantum dot is voltage-dependent. We confirm the voltage-dependent Curie
susceptibility recently found by Parcollet and Hooley [Phys. Rev. B {\bf 66},
085315 (2002)].Comment: Small modifications added to figure and tex
Parasitic pumping currents in an interacting quantum dot
We analyze the charge and spin pumping in an interacting dot within the
almost adiabatic limit. By using a non-equilibrium Green's function technique
within the time-dependent slave boson approximation, we analyze the pumped
current in terms of the dynamical constraints in the infinite-U regime. The
results show the presence of parasitic pumping currents due to the additional
phases of the constraints. The behavior of the pumped current through the
quantum dot is illustrated in the spin-insensitive and in the spin-sensitive
case relevant for spintronics applications
Adiabatic pumping through a quantum dot in the Kondo regime: Exact results at the Toulouse limit
Transport properties of ultrasmall quantum dots with a single unpaired
electron are commonly modeled by the nonequilibrium Kondo model, describing the
exchange interaction of a spin-1/2 local moment with two leads of
noninteracting electrons. Remarkably, the model possesses an exact solution
when tuned to a special manifold in its parameter space known as the Toulouse
limit. We use the Toulouse limit to exactly calculate the adiabatically pumped
spin current in the Kondo regime. In the absence of both potential scattering
and a voltage bias, the instantaneous charge current is strictly zero for a
generic Kondo model. However, a nonzero spin current can be pumped through the
system in the presence of a finite magnetic field, provided the spin couples
asymmetrically to the two leads. Tunneling through a Kondo impurity thus offers
a natural mechanism for generating a pure spin current. We show, in particular,
that one can devise pumping cycles along which the average spin pumped per
cycle is closely equal to . By analogy with Brouwer's formula for
noninteracting systems with two driven parameters, the pumped spin current is
expressed as a geometrical property of a scattering matrix. However, the
relevant %Alex: I replaced topological with geometrical in the sentence above
scattering matrix that enters the formulation pertains to the Majorana fermions
that appear at the Toulouse limit rather than the physical electrons that carry
the current. These results are obtained by combining the nonequilibrium Keldysh
Green function technique with a systematic gradient expansion, explicitly
exposing the small parameter controlling the adiabatic limit.Comment: 14 pages, 3 figures, revised versio
Steering Magnetic Skyrmions with Nonequilibrium Green's Functions
Magnetic skyrmions, topologically protected vortex-like configurations in
spin textures, are of wide conceptual and practical appeal for quantum
information technologies, notably in relation to the making of so-called
race-track memory devices. Skyrmions can be created, steered and destroyed with
magnetic fields and/or (spin) currents. Here we focus on the latter mechanism,
analyzed via a microscopic treatment of the skyrmion-current interaction. The
system we consider is an isolated skyrmion in a square-lattice cluster,
interacting with electrons spins in a current-carrying quantum wire. For the
theoretical description, we employ a quantum formulation of spin-dependent
currents via nonequilibrium Green's functions (NEGF) within the generalized
Kadanoff-Baym ansatz (GKBA). This is combined with a treatment of skyrmions
based on classical localized spins, with the skyrmion motion described via
Ehrenfest dynamics. With our mixed quantum-classical scheme, we assess how
time-dependent currents can affect the skyrmion dynamics, and how this in turn
depends on electron-electron and spin-orbit interactions in the wire. Our study
shows the usefulness of a quantum-classical treatment of skyrmion steering via
currents, as a way for example to validate/extract an effective,
classical-only, description of skyrmion dynamics from a microscopic quantum
modeling of the skyrmion-current interaction.Comment: 10 pages, 8 figures, contribution to the proceedings of "Progress in
Nonequilibrium Green's Functions VII
Evaluation of New Density Functional with Account of van der Waals Forces by Use of Experimental H2 Physisorption Data on Cu(111)
Detailed experimental data for physisorption potential-energy curves of H2 on
low-indexed faces of Cu challenge theory. Recently, density-functional theory
has been developed to also account for nonlocal correlation effects, including
van der Waals forces. We show that one functional, denoted vdW-DF2, gives a
potential-energy curve promisingly close to the experiment-derived
physisorptionenergy curve. The comparison also gives indications for further
improvements of the functionals
Kondo effect in complex mesoscopic structures
We study the Kondo effect of a quantum dot placed in a complex mesoscopic
structure. Assuming that electronic interactions are taking place solely on the
dot, and focusing on the infinite Hubbard interaction limit, we use a
decoupling scheme to obtain an explicit analytic approximate expression for the
dot Green function, which fulfills certain Fermi-liquid relations at zero
temperature. The details of the complex structure enter into this expression
only via the self-energy for the non-interacting case. The effectiveness of the
expression is demonstrated for the single impurity Anderson model and for the
T-shaped network.Comment: 12 pages 6 figure
Measuring the Kondo effect in the Aharonov-Bohm interferometer
The conductance of an Aharonov-Bohm interferometer (ABI), with a
strongly correlated quantum dot on one arm, is expressed in terms of the dot
Green function, , the magnetic flux and the non-interacting
parameters of the ABI. We show that one can extract from the observed
oscillations of with , for both closed and open ABI's. In the
latter case, the phase shift deduced from depends strongly on the ABI's parameters, and usually
. These parameters may also reduce the Kondo temperature,
eliminating the Kondo behavior
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