105 research outputs found
Magnetic transitions induced by tunnelling electrons in individual adsorbed M-Phthalocyanine molecules (M Fe, Co)
We report on a theoretical study of magnetic transitions induced by
tunnelling electrons in individual adsorbed M-Phthalocyanine (M-Pc) molecules
where M is a metal atom: Fe-Pc on a Cu(110)(21)-O surface and Co-Pc
layers on Pb(111) islands. The magnetic transitions correspond to the change of
orientation of the spin angular momentum of the metal ion with respect to the
surroundings and possibly an applied magnetic field. The adsorbed Fe-Pc system
is studied with a Density Functional Theory (DFT) transport approach showing
that i) the magnetic structure of the Fe atom in the adsorbed Fe-Pc is quite
different from that of the free Fe atom or of other adsorbed Fe systems and ii)
that injection of electrons (holes) into the Fe atom in the adsorbed Fe-Pc
molecule dominantly involves the Fe orbital. These results fully
specify the magnetic structure of the system and the process responsible for
magnetic transitions. The dynamics of the magnetic transitions induced by
tunnelling electrons is treated in a strong-coupling approach. The Fe-Pc
treatment is extended to the Co-Pc case. The present calculations accurately
reproduce the strength of the magnetic transitions as observed by magnetic IETS
(Inelastic Electron Tunnelling Spectroscopy) experiments; in particular, the
dominance of the inelastic current in the conduction of the adsorbed M-Pc
molecule is accounted for
Quenching of magnetic excitations in single adsorbates at surfaces: Mn on CuN/Cu(100)
The lifetimes of spin excitations of Mn adsorbates on CuN/Cu(100) are
computed from first-principles. The theory is based on a strong-coupling
T-matrix approach that evaluates the decay of a spin excitation due to
electron-hole pair creation. Using a previously developed theory [Phys. Rev.
Lett. {\bf 103}, 176601 (2009) and Phys. Rev. B {\bf 81}, 165423 (2010)], we
compute the excitation rates by a tunneling current for all the Mn spin states.
A rate equation approach permits us to simulate the experimental results by
Loth and co-workers [Nat. Phys. {\bf 6}, 340 (2010)] for large tunnelling
currents, taking into account the finite population of excited states. Our
simulations give us insight into the spin dynamics, in particular in the way
polarized electrons can reveal the existence of an excited state population. In
addition, it reveals that the excitation process occurs in a way very different
from the deexcitation one. Indeed, while excitation by tunnelling electrons
proceeds via the s and p electrons of the adsorbate, deexcitation mainly
involves the d electrons
R-matrix calculation of electron collisions with electronically excited O2 molecules
Low-energy electron collisions with O molecules are studied using the
fixed-bond R-matrix method. In addition to the O ground
state, integrated cross sections are calculated for elecron collisions with the
and excited states of O molecules. 13
target electronic states of O are included in the model within a valence
configuration interaction representations of the target states. Elastic cross
sections for the and excited states are
similar to the cross sections for the ground state. As in
case of excitation from the state, the O
resonance makes the dominant contribution to excitation cross sections from the
and states. The magnitude of excitation
cross sections from the state to the
state is about 10 time larger than the corresponding cross sections from the
to the state. For this
transition, our cross section at
4.5 eV agrees well with the available experimental value. These results should
be important for models of plasma discharge chemistry which often requires
cross sections between the excited electronic states of O.Comment: 26 pages, 10 figure
Inelastic effects in electron transport studied with wave packet propagation
A time-dependent approach is used to explore inelastic effects during
electron transport through few-level systems. We study a tight-binding chain
with one and two sites connected to vibrations. This simple but transparent
model gives insight about inelastic effects, their meaning and the
approximations currently used to treat them. Our time-dependent approach allows
us to trace back the time sequence of vibrational excitation and electronic
interference, the ibrationally introduced time delay and the electronic phase
shift. We explore a full range of parameters going from weak to strong
electron-vibration coupling, from tunneling to contact, from one-vibration
description to the need of including all vibrations for a correct description
of inelastic effects in transport. We explore the validity of single-site
resonant models as well as its extension to more sites via molecular orbitals
and the conditions under which multi-orbital, multi-vibrational descriptions
cannot be simplified. We explain the physical meaning of the spectral features
in the second derivative of the electron current with respect to the bias
voltage. This permits us to nuance the meaning of the energy value of dips and
peaks. Finally, we show that finite-band effects lead to electron
back-scattering off the molecular vibrations in the regime of high-conductance,
although the drop in conductance at the vibrational threshold is rather due to
the rapid variation of the vibronic density of states.Comment: 38 pages, 14 figure
Observation of diffractive orbits in the spectrum of excited NO in a magnetic field
We investigate the experimental spectra of excited NO molecules in the
diamagnetic regime and develop a quantitative semiclassical framework to
account for the results. We show the dynamics can be interpreted in terms of
classical orbits provided that in addition to the geometric orbits, diffractive
effects are appropriately taken into account. We also show how individual
orbits can be extracted from the experimental signal and use this procedure to
reveal the first experimental manifestation of inelastic diffractive orbits.Comment: 4 fig
Isotope effect for associative detachment: H(D)−+H(D)→H2(D2)+e
We report experimental and theoretical results for associative detachment (AD) of D−+D→D2+e−. We compare these data to our previously published results for H−+H→H2+e−. The measurements show no significant isotope effect in the total cross section. This is to be contrasted with previously published experimental and theoretical work which has found a significant isotope effect in diatomic systems for partial AD cross sections, i.e., as a function of the rotational and vibrational levels of the final molecule formed. Our work implies that though the rovibrational distribution of flux is different for AD of H− + H and D− + D, the total flux for these two systems is essentially the same when summed over all possible final channels
Room Temperature Kondo effect in atom-surface scattering: dynamical 1/N approach
The Kondo effect may be observable in some atom-surface scattering
experiments, in particular, those involving alkaline-earth atoms. By combining
Keldysh techniques with the NCA approximation to solve the time-dependent
Newns-Anderson Hamiltonian in the infinite-U limit, Shao, Nordlander and
Langreth found an anomalously strong surface-temperature dependence of the
outgoing charge state fractions. Here we employ the dynamical 1/N expansion
with finite Coulomb interaction U to provide a more realistic description of
the scattering process. We test the accuracy of the 1/N expansion in the
spinless N = 1 case against the exact independent-particle solution. We then
compare results obtained in the infinite-U limit with the NCA approximation and
recover qualitative features found previously. Finally, we analyze the
realistic situation of Ca atoms with U = 5.8 eV scattered off Cu(001) surfaces.
Although the presence of the doubly-ionized Ca species can change the absolute
scattered positive Ca yields, the temperature dependence is qualitatively the
same as that found in the infinite-U limit. One of the main difficulties that
experimentalists face in attempting to detect this effect is that the atomic
velocity must be kept small enough to reduce possible kinematic smearing of the
metal's Fermi surface.Comment: 15 pages, 10 Postscript figures; references and typos correcte
FeCoCp3 Molecular Magnets as Spin Filters
Metallorganic molecules have been proposed as excellent spin filters in
molecular spintronics because of the large spin-polarization of their
electronic structure. However, most of the studies involving spin transport,
have disregarded fundamental aspects such as the magnetic anisotropy of the
molecule and the excitation of spin-flip processes during electron transport.
Here, we study a molecule containing a Co and an Fe atoms stacked between three
cyclopentadienyl rings that presents a large magnetic anisotropy and a S=1.
These figures are superior to other molecules with the same transition metal,
and improves the spin-filtering capacities of the molecule. Non-equilibrium
Green's functions calculations based on density functional theory predict
excellent spin-filtering properties both in tunnel and contact transport
regimes. However, exciting the first magnetic state drastically reduces the
current's spin polarization. Furthermore, a difference of temperature between
electrodes leads to strong thermoelectric effects that also suppress spin
polarization. Our study shows that in-principle good molecular candidates for
spintronics need to be confronted with inelastic and thermoelectric effects
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