499 research outputs found
Vibronic effects on resonant electron conduction through single molecule junctions
The influence of vibrational motion on electron conduction through single
molecules bound to metal electrodes is investigated employing first-principles
electronic-structure calculations and projection-operator Green's function
methods. Considering molecular junctions where a central phenyl ring is coupled
via (alkane)thiol-bridges to gold electrodes, it is shown that -- depending on
the distance between the electronic -system and the metal --
electronic-vibrational coupling may result in pronounced vibrational
substructures in the transmittance, a significantly reduced current as well as
a quenching of negative differential resistance effects.Comment: Submitted to Chem. Phys. Lett. (13 pages, 5 figures) this version:
typos and formating correcte
Conical intersections in an ultracold gas
We find that energy surfaces of more than two atoms or molecules interacting
via dipole-dipole po- tentials generically possess conical intersections (CIs).
Typically only few atoms participate strongly in such an intersection. For the
fundamental case, a circular trimer, we show how the CI affects adiabatic
excitation transport via electronic decoherence or geometric phase
interference. These phe- nomena may be experimentally accessible if the trimer
is realized by light alkali atoms in a ring trap, whose dipole-dipole
interactions are induced by off-resonant dressing with Rydberg states. Such a
setup promises a direct probe of the full many-body density dynamics near a
conical intersection.Comment: 4 pages, 4 figures, replacement to add archive referenc
Ground-state degeneracies leave recognizable topological scars in the one-particle density
In Kohn-Sham density functional theory (KS-DFT) a fictitious system of
non-interacting particles is constructed having the same ground-state (GS)
density as the physical system of interest. A fundamental open question in DFT
concerns the ability of an exact KS calculation to spot and characterize the GS
degeneracies in the physical system. In this article we provide theoretical
evidence suggesting that the GS density, as a function of position on a 2D
manifold of parameters affecting the external potential, is "topologically
scarred" in a distinct way by degeneracies. These scars are sufficiently
detailed to enable determination of the positions of degeneracies and even the
associated Berry phases. We conclude that an exact KS calculation can spot and
characterize the degeneracies of the physical system
MeV-scale seesaw and leptogenesis
We study the type-I seesaw model with three right-handed neutrinos and Majorana masses below the pion mass. In this mass range, the model parameter space is not only strongly constrained by the requirement to explain the light neutrino masses, but also by experimental searches and cosmological considerations. In the existing literature, three disjoint regions of potentially viable parameter space have been identified. In one of them, all heavy neutrinos decay shortly before big bang nucleosynthesis. In the other two regions, one of the heavy neutrinos either decays between BBN and the CMB decoupling or is quasi-stable. We show that previously unaccounted constraints from photodisintegration of nuclei practically rule out all relevant decays that happen between BBN and the CMB decoupling. Quite remarkably, if all heavy neutrinos decay before BBN, the baryon asymmetry of the universe can be quite generically explained by low-scale leptogenesis, i.e. without further tuning in addition to what is needed to avoid experimental and cosmological constraints. This motivates searches for heavy neutrinos in pion decay experiments
Origin of Unexpectedly Simple Oscillatory Responses in the Excited-State Dynamics of Disordered Molecular Aggregates
Resonant Auger decay of the core-excited CO molecule in intense X-ray laser fields
The dynamics of the resonant Auger (RA) process of the core-excited
CO(1s) molecule in an intense X-ray laser field is
studied theoretically. The theoretical approach includes the analogue of the
conical intersections of the complex potential energy surfaces of the ground
and `dressed' resonant states due to intense X-ray pulses, taking into account
the decay of the resonance and the direct photoionization of the ground state,
both populating the same final ionic states coherently, as well as the direct
photoionization of the resonance state itself. The light-induced non-adiabatic
effect of the analogue of the conical intersections of the resulting complex
potential energy surfaces gives rise to strong coupling between the electronic,
vibrational and rotational degrees of freedom of the diatomic CO molecule. The
interplay of the direct photoionization of the ground state and of the decay of
the resonance increases dramatically with the field intensity. The coherent
population of a final ionic state via both the direct photoionization and the
resonant Auger decay channels induces strong interference effects with distinct
patterns in the RA electron spectra. The individual impact of these physical
processes on the total electron yield and on the CO electron
spectrum are demonstrated.Comment: 13 figs, 1 tabe
Theory of Vibrationally Inelastic Electron Transport through Molecular Bridges
Vibrationally inelastic electron transport through a molecular bridge that is
connected to two leads is investigated. The study is based on a generic model
of vibrational excitation in resonant transmission of electrons through a
molecular junction. Employing methods from electron-molecule scattering theory,
the transmittance through the molecular bridge can be evaluated numerically
exactly. The current through the junction is obtained approximately using a
Landauer-type formula. Considering different parameter regimes, which include
both the case of a molecular bridge that is weakly coupled to the leads,
resulting in narrow resonance structures, and the opposite case of a broad
resonance caused by strong interaction with the leads, we investigate the
characteristic effects of coherent and dissipative vibrational motion on the
electron transport. Furthermore, the validity of widely used approximations
such as the wide-band approximation and the restriction to elastic transport
mechanisms is investigated in some detail.Comment: Submited to PRB, revised version according to comments of referees
(minor text changes and new citations
Resonance solutions of the nonlinear Schr\"odinger equation in an open double-well potential
The resonance states and the decay dynamics of the nonlinear Schr\"odinger
(or Gross-Pitaevskii) equation are studied for a simple, however flexible model
system, the double delta-shell potential. This model allows analytical
solutions and provides insight into the influence of the nonlinearity on the
decay dynamics. The bifurcation scenario of the resonance states is discussed,
as well as their dynamical stability properties. A discrete approximation using
a biorthogonal basis is suggested which allows an accurate description even for
only two basis states in terms of a nonlinear, nonhermitian matrix problem.Comment: 21 pages, 14 figure
Not gate in a cis-trans photoisomerization model
We numerically study the implementation of a NOT gate by laser pulses in a
model molecular system presenting two electronic surfaces coupled by non
adiabatic interactions. The two states of the bit are the fundamental states of
the cis-trans isomers of the molecule. The gate is classical in the sense that
it involves a one-qubit flip so that the encoding of the outputs is based on
population analysis which does not take the phases into account. This gate can
also be viewed as a double photo-switch process with the property that the same
electric field controls the two isomerizations. As an example, we consider
one-dimensional cuts in a model of the retinal in rhodopsin already proposed in
the literature. The laser pulses are computed by the Multi Target Optimal
Control Theory with chirped pulses as trial fields. Very high fidelities are
obtained. We also examine the stability of the control when the system is
coupled to a bath of oscillators modelled by an Ohmic spectral density. The
bath correlation time scale being smaller than the pulse duration the dynamics
is carried out in the Markovian approximation.Comment: 29 pages, 7 figure
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