1,021 research outputs found
UD\u27s Big Read
As part of orientation, all first-year students will receive a copy of Mark Kurlansky\u27s book Nonviolence: Twenty-five Lessons from the History of a Dangerous Idea, recipient of the 2007 Dayton Literary Peace Prize. During their first semester, small groups will gather weekly to discuss it
Resonant tunneling through ultrasmall quantum dots: zero-bias anomalies, magnetic field dependence, and boson-assisted transport
We study resonant tunneling through a single-level quantum dot in the
presence of strong Coulomb repulsion beyond the perturbative regime. The level
is either spin-degenerate or can be split by a magnetic field. We, furthermore,
discuss the influence of a bosonic environment. Using a real-time diagrammatic
formulation we calculate transition rates, the spectral density and the
nonlinear characteristic. The spectral density shows a multiplet of Kondo
peaks split by the transport voltage and the boson frequencies, and shifted by
the magnetic field. This leads to zero-bias anomalies in the differential
conductance, which agree well with recent experimental results for the electron
transport through single-charge traps. Furthermore, we predict that the sign of
the zero-bias anomaly depends on the level position relative to the Fermi level
of the leads.Comment: 27 pages, latex, 21 figures, submitted to Phys. Rev.
Zero-bias anomalies and boson-assisted tunneling through quantum dots
We study resonant tunneling through a quantum dot with one degenerate level
in the presence of a strong Coulomb repulsion and a bosonic environment. Using
a real-time approach we calculate the spectral density and the nonlinear
current within a conserving approximation. The spectral density shows a
multiplet of Kondo peaks split by the transport voltage and boson frequencies.
As a consequence we find a zero-bias anomaly in the differential conductance
which can show a local maximum or minimum depending on the level position. The
results are compared with recent experiments.Comment: 4 pages, revtex, 5 postscript figures, submitted to Phys. Rev. Let
The 2-Channel Kondo Model I: Review of Experimental Evidence for its Realization in Metal Nanoconstrictions
Certain zero-bias anomalies (ZBAs) in the voltage, temperature and magnetic
field dependence of the conductance of quenched Cu point contacts
have previously been interpreted to be due to non-magnetic 2-channel Kondo
(2CK) scattering from near-degenerate atomic two-level tunneling systems (Ralph
and Buhrman, 1992; Ralph et al. 1994), and hence to represent an experimental
realization of the non-Fermi-liquid physics of the T=0 fixed point of the
2-channel Kondo model. In this, the first in a series of three papers
(I,II,III) devoted to 2-channel Kondo physics, we present a comprehensive
review of the quenched Cu ZBA experiments and their 2CK interpretation,
including new results on ZBAs in constrictions made from Ti or from metallic
glasses. We first review the evidence that the ZBAs are due to electron
scattering from stuctural defects that are not static, but possess internal
dynamics. In order to distinguish between several mechanisms proposed to
explain the experiments, we then analyze the scaling properties of the
conductance at low temperature and voltage and extract from the data a
universal scaling function . The theoretical calculation of the
corresponding scaling function within the 2CK model is the subject of papers II
and III. The main conclusion of our work is that the properties of the ZBAs,
and most notably their scaling behavior, are in good agreement with the 2CK
model and clearly different from several other proposed mechanisms.Comment: 35 pages RevTeX, 19 encapsulated postscript figures; this final
published version features two additional authors, an additional section
reviewing recent experiments on Ti nanoconstrictions that agree very well
with the 2-channel Kondo model, 6 new figures (and is much shorter the
previous 53 page version, due to reformatting
A new perturbation treatment applied to the transport through a quantum dot
Resonant tunnelling through an Anderson impurity is investigated by employing
a new perturbation scheme at nonequilibrium. This new approach gives the
correct weak and strong coupling limit in by introducing adjustable
parameters in the self-energy and imposing self-consistency of the occupation
number of the impurity. We have found that the zero-temperature linear response
conductance agrees well with that obtained from the exact sum rule. At finite
temperature the conductance shows a nonzero minimum at the Kondo valley, as
shown in recent experiments. The effects of an applied bias voltage on the
single-particle density of states and on the differential conductances are
discussed for Kondo and non-Kondo systems.Comment: 4 pages, 4 figures, submitted to PRB-Rapid Comm. Email addresses
[email protected], [email protected]
Anomalous Kondo-effect in Quantum Dot at non-zero bias
We present measurements on the Kondo-effect in a small quantum dot connected
strongly to one lead and weakly to the other. The conductance of the dot
reveals an offset of the Kondo resonance at zero magnetic field. While the
resonance persists in the negative bias regime, it is suppressed in the
opposite direction. This demonstrates the pinning of the Kondo resonance to the
Fermi levels of the left and right lead. As an outlook we exploit the spin
blockade effect to gain information about the spin state of the quantum dot in
the Kondo regime.Comment: submitted to PRL; revised version: 2 figures change
Power and Biomass-to-Liquid (PBtL): a Promising Approach to Produce Biofuels using Electricity
To achieve the emission mitigation and de-carbonization goals future aviation sector requires alternative sustainable fuels. Synthetic paraffinic kerosene, generated by Fischer-Tropsch synthesis, is certified as a drop-in jet fuel up to 50 %. Potential production routes are via conversion of biomass (Biomass-to-Liquid, BTL), the combination of renewable power and biomass (Power-and-Biomass-to-Liquid, PBTL) and the conversion of CO2 with hydrogen to fuels (Power-to-Liquid, PTL). Even if the BTL route enables the lowest net production costs, it has some drawbacks compared to the alternatives. The fuel yield of the BTL concepts can be increased by a factor of about 3 by the PBTL processes. The highest overall efficiency is found for the PTL (48.1 %) followed by PBTL (45.0 %) and last BTL (29.8 %). For BTL only 27 % of the introduced carbon can be converted into fuels. The total investment costs are highest for BTL, followed by PBTL and PTL. The net production costs are in the reverse order. Low electricity prices and high capacities are advantageous for PBTL, while high electricity prices favor BTL-concept
Numerical analyses of the nonequilibrium electron transport through the Kondo impurity beside the Toulouse point
Nonequilibrium electron transport through the Kondo impurity is investigated
numerically for the system with twenty conduction-electron levels. The electron
current under finite voltage drop is calculated in terms of the `conductance
viewed as transmission' picture proposed by Landauer. Here, we take into
account the full transmission processes of both the many-body correlation and
the hybridization amplitude up to infinite order. Our results demonstrate, for
instance, how the exact solution of the differential conductance by Schiller
and Hershfield obtained at the Toulouse point becomes deformed by more
realistic interactions. The differential-conductance-peak height is suppressed
below e^2/h with the width hardly changed through reducing the Kondo coupling
from the Toulouse point, whereas it is kept unchanged by further increase of
the coupling. We calculated the nonequilibrium local Green function as well.
This clarifies the spectral property of the Kondo impurity driven far from
equilibrium
Nonlinear Response of a Kondo system: Direct and Alternating Tunneling Currents
Non - equilibrium tunneling current of an Anderson impurity system subject to
both constant and alternating electric fields is studied. A time - dependent
Schrieffer - Wolff transformation maps the time - dependent Anderson
Hamiltonian onto a Kondo one. Perturbation expansion in powers of the Kondo
coupling strength is carried out up to third order, yielding a remarkably
simple analytical expression for the tunneling current. It is found that the
zero - bias anomaly is suppressed by an ac - field. Both dc and the first
harmonic are equally enhanced by the Kondo effect, while the higher harmonics
are relatively small. These results are shown to be valid also below the Kondo
temperature.Comment: 7 pages, RevTeX, 3 PS figures attached, the article has been
significantly developed: time - dependent Schrieffer - Wolff transformation
is presented in the full form, the results are applied to the change in the
direct current induced by an alternating field (2 figures are new
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