5,176 research outputs found
Counting statistics of tunneling through a single molecule: effect of distortion and displacement of vibrational potential surface
We analyze the effects of a distortion of the nuclear potential of a
molecular quantum dot (QD), as well as a shift of its equilibrium position, on
nonequilibrium-vibration-assisted tunneling through the QD with a single level
() coupled to the vibrational mode. For this purpose, we derive an
explicit analytical expression for the Franck-Condon (FC) factor for a
displaced-distorted oscillator surface of the molecule and establish rate
equations in the joint electron-phonon representation to examine the
current-voltage characteristics and zero-frequency shot noise, and skewness as
well. Our numerical analyses shows that the distortion has two important
effects. The first one is that it breaks the symmetry between the excitation
spectra of the charge states, leading to asymmetric tunneling properties with
respect to and . Secondly, distortion (frequency
change of the oscillator) significantly changes the voltage-activated cascaded
transition mechanism, and consequently gives rise to a different nonequilibrium
vibrational distribution from that of the case without distortion. Taken in
conjunction with strongly modified FC factors due to distortion, this results
in some new transport features: the appearance of strong NDC even for a
single-level QD with symmetric tunnel couplings; a giant Fano factor even for a
molecule with an extremely weak electron-phonon interaction; and enhanced
skewness that can have a large negative value under certain conditions.Comment: 29 pages, 11 figures, published versio
Excitation and emission spectra of rubidium in rare-gas thin-films
To understand the optical properties of atoms in solid state matrices, the
absorption, excitation and emission spectra of rubidium doped thin-films of
argon, krypton and xenon were investigated in detail. A two-dimensional
spectral analysis extends earlier reports on the excitation and emission
properties of rubidium in rare-gas hosts. We found that the doped crystals of
krypton and xenon exhibit a simple absorption-emission relation, whereas
rubidium in argon showed more complicated spectral structures. Our sample
preparation employed in the present work yielded different results for the Ar
crystal, but our peak positions were consistent with the prediction based on
the linear extrapolation of Xe and Kr data. We also observed a bleaching
behavior in rubidium excitation spectra, which suggests a population transfer
from one to another spectral feature due to hole-burning. The observed optical
response implies that rubidium in rare-gas thin-films is detectable with
extremely high sensitivity, possibly down to a single atom level, in low
concentration samples.Comment: 7 pages, 5 figure
Non-adiabatic Effects in the Dissociation of Oxygen Molecules at the Al(111) Surface
The measured low initial sticking probability of oxygen molecules at the
Al(111) surface that had puzzled the field for many years was recently
explained in a non-adiabatic picture invoking spin-selection rules [J. Behler
et al., Phys. Rev. Lett. 94, 036104 (2005)]. These selection rules tend to
conserve the initial spin-triplet character of the free O2 molecule during the
molecule's approach to the surface. A new locally-constrained
density-functional theory approach gave access to the corresponding
potential-energy surface (PES) seen by such an impinging spin-triplet molecule
and indicated barriers to dissociation which reduce the sticking probability.
Here, we further substantiate this non-adiabatic picture by providing a
detailed account of the employed approach. Building on the previous work, we
focus in particular on inaccuracies in present-day exchange-correlation
functionals. Our analysis shows that small quantitative differences in the
spin-triplet constrained PES obtained with different gradient-corrected
functionals have a noticeable effect on the lowest kinetic energy part of the
resulting sticking curve.Comment: 17 pages including 11 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Predicting and verifying transition strengths from weakly bound molecules
We investigated transition strengths from ultracold weakly bound 41K87Rb
molecules produced via the photoassociation of laser-cooled atoms. An accurate
potential energy curve of the excited state (3)1Sigma+ was constructed by
carrying out direct potential fit analysis of rotational spectra obtained via
depletion spectroscopy. Vibrational energies and rotational constants extracted
from the depletion spectra of v'=41-50 levels were combined with the results of
the previous spectroscopic study, and they were used for modifying an ab initio
potential. An accuracy of 0.14% in vibrational level spacing and 0.3% in
rotational constants was sufficient to predict the large observed variation in
transition strengths among the vibrational levels. Our results show that
transition strengths from weakly bound molecules are a good measure of the
accuracy of an excited state potential.Comment: 7 pages, 7 figure
On the mutual polarization of two He-4 atoms
We propose a simple method based on the standard quantum-mechanical
perturbation theory to calculate the mutual polarization of two atoms He^4.Comment: 9 pages, 1 table; the article is revised and the calculation is
essentially refined; v4: final version, the Introduction is delete
Quantum initial condition sampling for linearized density matrix dynamics: Vibrational pure dephasing of iodine in krypton matrices
This paper reviews the linearized path integral approach for computing time
dependent properties of systems that can be approximated using a mixed
quantum-classical description. This approach is applied to studying vibrational
pure dephasing of ground state molecular iodine in a rare gas matrix. The
Feynman-Kleinert optimized harmonic approximation for the full system density
operator is used to sample initial conditions for the bath degrees of freedom.
This extremely efficient approach is compared with alternative initial
condition sampling techniques at low temperatures where classical initial
condition sampling yields dephasing rates that are nearly an order of magnitude
too slow compared with quantum initial condition sampling and experimental
results.Comment: 20 pages and 8 figure
CO oxidation at Pd(100): A first-principles constrained thermodynamics study
The possible formation of oxides or thin oxide films (surface oxides) on late
transition metal surfaces is recently being recognized as an essential
ingredient when aiming to understand catalytic oxidation reactions under
technologically relevant gas phase conditions. Using the CO oxidation at
Pd(100) as example, we investigate the composition and structure of this model
catalyst surface over a wide range of (T,p)-conditions within a multiscale
modeling approach where density-functional theory is linked to thermodynamics.
The results show that under the catalytically most relevant gas phase
conditions a thin surface oxide is the most stable "phase" and that the system
is actually very close to a transition between this oxidic state and a reduced
state in form of a CO covered Pd(100) surface.Comment: 13 pages including 7 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Exact Coupling Coefficient Distribution in the Doorway Mechanism
In many--body and other systems, the physics situation often allows one to
interpret certain, distinct states by means of a simple picture. In this
interpretation, the distinct states are not eigenstates of the full
Hamiltonian. Hence, there is an interaction which makes the distinct states act
as doorways into background states which are modeled statistically. The crucial
quantities are the overlaps between the eigenstates of the full Hamiltonian and
the doorway states, that is, the coupling coefficients occuring in the
expansion of true eigenstates in the simple model basis. Recently, the
distribution of the maximum coupling coefficients was introduced as a new,
highly sensitive statistical observable. In the particularly important regime
of weak interactions, this distribution is very well approximated by the
fidelity distribution, defined as the distribution of the overlap between the
doorway states with interaction and without interaction. Using a random matrix
model, we calculate the latter distribution exactly for regular and chaotic
background states in the cases of preserved and fully broken time--reversal
invariance. We also perform numerical simulations and find excellent agreement
with our analytical results.Comment: 22 pages, 4 figure
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