500 research outputs found
Spin filters with Fano dots
We compute the zero bias conductance of electrons through a single ballistic
channel weakly coupled to a side quantum dot with Coulomb interaction. In
contrast to the standard setup which is designed to measure the transport
through the dot, the channel conductance reveals Coulomb blockade dips rather
then peaks due to the Fano-like backscattering. At zero temperature the Kondo
effect leads to the formation of broad valleys of small conductance
corresponding to an odd number of electrons on the dot. By applying a magnetic
field in the dot region we find two dips corresponding to a total suppression
in the conductance of spins up and down separated by an energy of the order of
the Coulomb interaction. This provides a possibility of a perfect spin filter.Comment: 5 pages, 4 figures, to be published in European Physical Journal
Theory of Fano-Kondo effect in quantum dot systems: temperature dependence of the Fano line shapes
The Fano-Kondo effect in zero-bias conductance is studied based on a
theoretical model for the T-shaped quantum dot by the finite temperature
density matrix renormalization group method. The modification of the two Fano
line shapes at much higher temperatures than the Kondo temperature is also
investigated by the effective Fano parameter estimated as a fitting parameter.Comment: 2 pages, 2 figures, the proceeding of SCES'0
Anderson impurity in the one-dimensional Hubbard model on finite size systems
An Anderson impurity in a Hubbard model on chains with finite length is
studied using the density-matrix renormalization group (DMRG) technique. In the
first place, we analyzed how the reduction of electron density from
half-filling to quarter-filling affects the Kondo resonance in the limit of
Hubbard repulsion U=0. In general, a weak dependence with the electron density
was found for the local density of states (LDOS) at the impurity except when
the impurity, at half-filling, is close to a mixed valence regime. Next, in the
central part of this paper, we studied the effects of finite Hubbard
interaction on the chain at quarter-filling. Our main result is that this
interaction drives the impurity into a more defined Kondo regime although
accompanied in most cases by a reduction of the spectral weight of the impurity
LDOS. Again, for the impurity in the mixed valence regime, we observed an
interesting nonmonotonic behavior. We also concluded that the conductance,
computed for a small finite bias applied to the leads, follows the behavior of
the impurity LDOS, as in the case of non-interacting chains. Finally, we
analyzed how the Hubbard interaction and the finite chain length affect the
spin compensation cloud both at zero and at finite temperature, in this case
using quantum Monte Carlo techniques.Comment: 9 pages, 9 figures, final version to be published in Phys. Rev.
Interdisciplinary Teaching in the Field of Resilient Energy Systems: Experiences with Expert Lecture Series Combined with Workshops
The increased complexity of renewable energy systems derives in uncertain and vulnerable systems behavior, making necessary for energy experts to understand and apply resilience studies with interdisciplinary approaches. With this aim, we have designed a postgraduate course on resilience of energy systems, pursuing an “exchange interdisciplinarity” level that enables students to (i) become aware of competing approaches in terms of methods and theories stemming from different disciplines, and (ii) to critically argue on the suitability of presented concepts for energy systems design and management. The course aimed at achieving a sound level of exchange interdisciplinarity as defined in relevant literature. We chose the following specific teaching methods and didactic items to facilitate this aim and address the different challenges of interdisciplinary education identified from the literature: fundamental introduction (FI), expert interviews and deepening workshops. The FI aimed at providing a sound common basis for understanding the perspectives and approaches from different disciplines. The expert lecture series exposed students to the broadness of state-of-the-art approaches existing in resilience research for energy systems, while the deepening workshops allowed students to develop a deep and critical appraisal of the disciplinary approaches and their relations. The course impact is evaluated through the standard questionnaire for teaching evaluation from the University of Oldenburg. The evaluation shows that the course fostered a critical and interdisciplinary thinking, with a high and interactive participation through the use of multiple didactic measures. This is supported by the high satisfaction of the students, the high level of engagement and academic performance and the qualitative perception from the lecturers. Topics of similar complexity or interdisciplinarity in energy higher education, such as sustainability, technology assessment or energy systems analysis could also benefit from such a course design
Conductance through an array of quantum dots
We propose a simple approach to study the conductance through an array of
interacting quantum dots, weakly coupled to metallic leads. Using a mapping to
an effective site which describes the low-lying excitations and a slave-boson
representation in the saddle-point approximation, we calculated the conductance
through the system. Explicit results are presented for N=1 and N=3: a linear
array and an isosceles triangle. For N=1 in the Kondo limit, the results are in
very good agreement with previous results obtained with numerical
renormalization group (NRG). In the case of the linear trimer for odd , when
the parameters are such that electron-hole symmetry is induced, we obtain
perfect conductance . The validity of the approach is discussed in
detail.Comment: to appear in Phys. Rev.
Quantum dot with ferromagnetic leads: a density-matrix renormalization group study
A quantum dot coupled to ferromagnetically polarized one-dimensional leads is
studied numerically using the density matrix renormalization group method.
Several real space properties and the local density of states at the dot are
computed. It is shown that this local density of states is suppressed by the
parallel polarization of the leads. In this case we are able to estimate the
length of the Kondo cloud, and to relate its behavior to that suppression.
Another important result of our study is that the tunnel magnetoresistance as a
function of the quantum dot on-site energy is minimum and negative at the
symmetric point.Comment: 4 pages including 5 figures. To be published as a Brief Report in
Phys. Rev.
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