5,210 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
Excited-state Forces within a First-principles Green's Function Formalism
We present a new first-principles formalism for calculating forces for
optically excited electronic states using the interacting Green's function
approach with the GW-Bethe Salpeter Equation method. This advance allows for
efficient computation of gradients of the excited-state Born-Oppenheimer
energy, allowing for the study of relaxation, molecular dynamics, and
photoluminescence of excited states. The approach is tested on photoexcited
carbon dioxide and ammonia molecules, and the calculations accurately describe
the excitation energies and photoinduced structural deformations.Comment: 2 figures and 2 table
Quantum resonance, Anderson localisation and selective manipulations in molecular mixtures by ultrashort laser pulses
We demonstrate that the current laser technology used for field-free
molecular alignment via a cascade of Raman rotational transitions allows for
observing long-discussed non-linear quantum phenomena in the dynamics of the
periodically kicked rotor. This includes the scaling of the absorbed energy
near the conditions of quantum resonance and Anderson-like localisation in the
angular momentum. Based on these findings, we suggest a novel approach to
tunable selective rotational excitation and alignment in a molecular mixture,
using trains of short laser pulses. We demonstrate the efficiency of this
approach by applying it to a mixture of two nitrogen isotopologues (14N2 and
15N2), and show that strong selectivity is possible even at room temperature
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
Optimal Topological Test for Degeneracies of Real Hamiltonians
We consider adiabatic transport of eigenstates of real Hamiltonians around
loops in parameter space. It is demonstrated that loops that map to nontrivial
loops in the space of eigenbases must encircle degeneracies. Examples from
Jahn-Teller theory are presented to illustrate the test. We show furthermore
that the proposed test is optimal.Comment: Minor corrections, accepted in Phys. Rev. Let
A Spectroscopic Survey of Electronic Transitions of CH, CH, and CD
Electronic spectra of CH are measured in the cm
domain using cavity ring-down spectroscopy of a supersonically expanding
hydrocarbon plasma. In total, 19 (sub)bands of CH are presented, all
probing the vibrational manifold of the B electronically excited state.
The assignments are guided by electronic spectra available from matrix
isolation work, isotopic substitution experiments (yielding also spectra for
CH and CD), predictions from ab initio calculations as well as
rotational fitting and vibrational contour simulations using the available
ground state parameters as obtained from microwave experiments. Besides the
origin band, three non-degenerate stretching vibrations along the
linear backbone of the CH molecule are assigned: the mode
associated with the C-C bond vibration and the and modes
associated with CC triple bonds. For the two lowest and
bending modes, a Renner-Teller analysis is performed identifying the
() and both () and
() components. In addition, two higher lying bending
modes are observed, which are tentatively assigned as ()
and () levels. In the excitation region below the first
non-degenerate vibration (), some transitions are
observed that are assigned as even combination modes of low-lying bending
vibrations. The same holds for a transition found above the
level. From these spectroscopic data and the vibronic analysis a
comprehensive energy level diagram for the B state of CH is derived
and presented.Comment: Accepted for publication in The Journal of Physical Chemistry A (26
July 2016
A geometrical approach to the dynamics of spinor condensates I: Hydrodynamics
In this work, we derive the equations of motion governing the hydrodynamics
of spin-F spinor condensates. We pursue a description based on standard
physical variables (total density and superfluid velocity), alongside 2F
`spin-nodes': unit vectors that describe the spin F state, and also exhibit the
point-group symmetry of a spinor condensate's mean-field ground state. The
hydrodynamic equations of motion consist of a mass continuity equation, 2F
Landau-Lifshitz equations for the spin-nodes, and a modified Euler equation. In
particular, we provide a generalization of the Mermin-Ho relation to spin one,
and find an analytic solution for the skyrmion texture in the incompressible
regime of a spin-half condensate. These results exhibit a beautiful geometrical
structure that underlies the dynamics of spinor condensates.Comment: 12 pages. First paper in two-part serie
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
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