708 research outputs found
Origin of the scatter in the X-ray luminosity of early-type galaxies observed with ROSAT
Statistical properties of X-ray luminosity and temperature are studied for 52
early-type galaxies based on the ROSAT PSPC data. All of the X-ray luminous
galaxies show largely extended emission with a radius of a few times of
10, while X-ray faint galaxies do not show such a component. This leads to
a division of early-type galaxies into two categories: X-ray extended and X-ray
compact galaxies. Except for a few galaxies in dense cluster environments, the
luminosity and temperature of X-ray compact galaxies are well explained by a
kinematical heating of the gas supplied by stellar mass loss. In contrast,
X-ray extended galaxies indicate large scatter in the X-ray luminosity. We
discuss that X-ray extended galaxies are the central objects of large potential
structures, and the presence and absence of this potential is the main origin
of the large scatter in the X-ray luminosity.Comment: 35 pages, including 8 figures, Accepted for publication in Ap
Effect of the Kondo correlation on thermopower in a Quantum Dot
In this paper we study the thermopower of a quantum dot connected to two
leads in the presence of Kondo correlation by employing a modified second-order
perturbation scheme at nonequilibrium. A simple scheme, Ng's ansatz [Phys. Rev.
Lett. {\bf 76}, 487 (1996)], is adopted to calculate nonequilibrium
distribution Green's function and its validity is further checked with regard
to the Onsager relation. Numerical results demonstrate that the sign of the
thermopower can be changed by tuning the energy level of the quantum dot,
leading to a oscillatory behavior with a suppressed magnitude due to the Kondo
effect. We also calculate the thermal conductance of the system, and find that
the Wiedemann-Franz law is obeyed at low temperature but violated with
increasing temperature, corresponding to emerging and quenching of the Kondo
effect.Comment: 6 pages, 4 figures; accepted for publication in J Phys.: Condensed
Matte
Aharonov-Bohm Effect for Parallel and T-shaped Double Quantum Dots
We investigate the Aharonov-Bohm (AB) effect for the double quantum dots in
the Kondo regime using the slave-boson mean-field approximation. In contrast to
the non-interacting case, where the AB oscillation generally has the period of
4 when the two-subring structure is formed via the interdot tunneling
, we find that the AB oscillation has the period of 2 in the Kondo
regime. Such effects appear for the double quantum dots close to the T-shaped
geometry even in the charge-fluctuation regime. These results follow from the
fact that the Kondo resonance is always fixed to the Fermi level irrespective
of the detailed structure of the bare dot-levels.Comment: 3 pages, 4 figures; minor change
Numerical simulation of the thermal fragmentation process in fullerene C60
The processes of defect formation and annealing in fullerene C60 at
T=(4000-6000)K are studied by the molecular dynamics technique with a
tight-binding potential. The cluster lifetime until fragmentation due to the
loss of a C2 dimer has been calculated as a function of temperature. The
activation energy and the frequency factor in the Arrhenius equation for the
fragmentation rate have been found to be Ea = (9.2 +- 0.4) eV and A = (8 +-
1)10^{19} 1/s. It is shown that fragmentation can occur after the C60 cluster
loses its spherical shape. This fact must be taken into account in theoretical
calculations of Ea.Comment: 12 pages, 3 figure
Microscopic mechanisms of dephasing due to electron-electron interactions
We develop a non-perturbative numerical method to study tunneling of a single
electron through an Aharonov-Bohm ring where several strongly interacting
electrons are bound. Inelastic processes and spin-flip scattering are taken
into account. The method is applied to study microscopic mechanisms of
dephasing in a non-trivial model. We show that electron-electron interactions
described by the Hubbard Hamiltonian lead to strong dephasing: the transmission
probability at flux is high even at small interaction strength. In
addition to inelastic scattering, we identify two energy conserving mechanisms
of dephasing: symmetry-changing and spin-flip scattering. The many-electron
state on the ring determines which of these mechanisms will be at play:
transmitted current can occur either in elastic or inelastic channels, with or
without changing the spin of the scattering electron.Comment: 11 pages, 16 figures Submitted to Phys. Rev.
Deconstructing the Big Valley Search Space Hypothesis
The big valley hypothesis suggests that, in combinatorial optimisation, local optima of good quality are clustered and surround the global optimum. We show here that the idea of a single valley does not always hold. Instead the big valley seems to de-construct into several valleys, also called ‘funnels’ in theoretical chemistry. We use the local optima networks model and propose an effective procedure for extracting the network data. We conduct a detailed study on four selected TSP instances of moderate size and observe that the big valley decomposes into a number of sub-valleys of different sizes and fitness distributions. Sometimes the global optimum is located in the largest valley, which suggests an easy to search landscape, but this is not generally the case. The global optimum might be located in a small valley, which offers a clear and visual explanation of the increased search difficulty in these cases. Our study opens up new possibilities for analysing and visualising combinatorial landscapes as complex networks
Complex Dynamics and Multistability in a Damped Harmonic Oscillator with Delayed Negative Feedback
A center manifold reduction and numerical calculations are used to demonstrate the presence of limit cycles, two-tori, and multistability in the damped harmonic oscillator with delayed negative feedback. This model is the prototype of a mechanical system operating with delayed feedback. Complex dynamics are thus seen to arise in very plausible and commonly occurring mechanical and neuromechanical feedback systems
Aharonov-Bohm interferometry with quantum dots: scattering approach versus tunneling picture
We address the question of how to model electron transport through closed
Aharonov-Bohm interferometers which contain quantum dots. By explicitly
studying interferometers with one and two quantum dots, we establish the
connection between a tunneling-Hamiltonian formulation on the one hand and a
scattering-matrix approach on the other hand. We prove that, under certain
circumstances, both approaches are equivalent, i.e., both types of models can
describe the same experimental setups. Furthermore, we analyze how the
interplay of the Aharonov-Bohm phase and the orbital phase associated with the
lengths of the interferometers' arms affect transport properties.Comment: 8 pages, 8 figures, published versio
Vibrational Excitations in Weakly Coupled Single-Molecule Junctions: A Computational Analysis
In bulk systems, molecules are routinely identified by their vibrational
spectrum using Raman or infrared spectroscopy. In recent years, vibrational
excitation lines have been observed in low-temperature conductance measurements
on single molecule junctions and they can provide a similar means of
identification. We present a method to efficiently calculate these excitation
lines in weakly coupled, gateable single-molecule junctions, using a
combination of ab initio density functional theory and rate equations. Our
method takes transitions from excited to excited vibrational state into account
by evaluating the Franck-Condon factors for an arbitrary number of vibrational
quanta, and is therefore able to predict qualitatively different behaviour from
calculations limited to transitions from ground state to excited vibrational
state. We find that the vibrational spectrum is sensitive to the molecular
contact geometry and the charge state, and that it is generally necessary to
take more than one vibrational quantum into account. Quantitative comparison to
previously reported measurements on pi-conjugated molecules reveals that our
method is able to characterize the vibrational excitations and can be used to
identify single molecules in a junction. The method is computationally feasible
on commodity hardware.Comment: 9 pages, 7 figure
Electrical and thermoelectrical transport in Dirac fermions through a quantum dot
We investigate the conductance and thermopower of massless Dirac fermions
through a quantum dot using a pseudogap Anderson model in the non-crossing
approximation. When the Fermi level is at the Dirac point, the conductance has
a cusp where the thermopower changes its sign. When the Fermi level is away
from the Dirac point, the Kondo temperature illustrates a quantum impurity
transition between an asymmetric strong coupling Kondo state and a localized
moment state. The conductance shows a peak near this transition and reaches the
unitary limit at low temperatures. The magnitude of the thermopower exceeds
, and the thermoelectric figure of merit exceeds unity.Comment: 5 pages, 4 figure
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