5,912 research outputs found
Ultrafast Transient Dynamics of Adsorbates on Surfaces Deciphered: The Case of CO on Cu(100)
Time-resolved vibrational spectroscopy constitutes an invaluable experimental
tool for monitoring hot-carrier induced surface reactions. However, the absence
of a full understanding on the precise microscopic mechanisms causing the
transient spectral changes has been limiting its applicability. Here we
introduce a robust first-principles theoretical framework that successfully
explains both the nonthermal frequency and linewidth changes of the CO internal
stretch mode on Cu(100) induced by femtosecond laser pulses. Two distinct
processes engender the changes: electron-hole pair excitations underlie the
nonthermal frequency shifts, while electron-mediated vibrational mode coupling
gives rise to linewidth changes. Furthermore, the origin and precise sequence
of coupling events are finally identified.Comment: Article as accepted for publication in Physical Review Letters; 5
pages, 2 figures, 1 tabl
Influence of the binding mode and many-body interactions
In the present contribution, the ultrafast photoinduced electron migration
dynamics at the interface between an alizarin dye and an anatase TiO2 thin
film is investigated from first principles. Comparison between a time-
dependent many-electron configuration interaction ansatz and a single active
electron approach sheds light on the importance of many-body effects, stemming
from uniquely defined initial conditions prior to photoexcitation. Particular
emphasis is put on understanding the influence of the binding mode on the
migration process. The dynamics is analyzed on the basis of a recently
introduced toolset in the form of electron yields, electronic fluxes, and flux
densities, to reveal microscopic details of the electron migration mechanism.
From the many-body perspective, insight into the nature of electron-electron
and hole-hole interactions during the charge transfer process is obtained. The
present results reveal that the single active electron approach yields
quantitatively and phenomenologically similar results as the many-electron
ansatz. Furthermore, the charge migration processes in the dye-TiO2 model
clusters with different binding modes exhibit similar mechanistic pathways but
on largely different time scales
Spiral Magnets as Gapless Mott Insulators
In the large limit, the ground state of the half-filled, nearest-neighbor
Hubbard model on the triangular lattice is the three-sublattice
antiferromagnet. In sharp contrast with the square-lattice case, where
transverse spin-waves and charge excitations remain decoupled to all orders in
, it is shown that beyond leading order in the three Goldstone modes
on the triangular lattice are a linear combination of spin and charge. This
leads to non-vanishing conductivity at any finite frequency, even though the
magnet remains insulating at zero frequency. More generally, non-collinear spin
order should lead to such gapless insulating behavior.Comment: 10 pages, REVTEX 3.0, 3 uuencoded postscript figures, CRPS-94-0
Superintegrability of the Tremblay-Turbiner-Winternitz quantum Hamiltonians on a plane for odd
In a recent FTC by Tremblay {\sl et al} (2009 {\sl J. Phys. A: Math. Theor.}
{\bf 42} 205206), it has been conjectured that for any integer value of ,
some novel exactly solvable and integrable quantum Hamiltonian on a plane
is superintegrable and that the additional integral of motion is a th-order
differential operator . Here we demonstrate the conjecture for the
infinite family of Hamiltonians with odd , whose first member
corresponds to the three-body Calogero-Marchioro-Wolfes model after elimination
of the centre-of-mass motion. Our approach is based on the construction of some
-extended and invariant Hamiltonian \chh_k, which can be interpreted
as a modified boson oscillator Hamiltonian. The latter is then shown to possess
a -invariant integral of motion \cyy_{2k}, from which can be
obtained by projection in the identity representation space.Comment: 14 pages, no figure; change of title + important addition to sect. 4
+ 2 more references + minor modifications; accepted by JPA as an FT
Diffusion of hydrogen interstitials in the near-surface region of Pd(111) under the influence of surface coverage and external static electric fields
Past scanning tunneling microscopy (STM) experiments of H manipulation on
Pd(111), at low temperature, have shown that it is possible to induce
diffusion of surface species as well as of those deeply buried under the
surface. Several questions remain open regarding the role of subsurface site
occupancies. In the present work, the interaction potential of H atoms with
Pd(111) under various H coverage conditions is determined by means of density
functional theory calculations in order to provide an answer to two of these
questions: (i) whether subsurface sites are the final locations for the H
impurities that attempt to emerge from bulk regions, and (ii) whether
penetration of the surface is a competing route of on-surface diffusion during
depletion of surface H on densely covered Pd(111). We find that a high H
coverage has the effect of blocking resurfacing of H atoms travelling from
below, which would otherwise reach the surface fcc sites, but it hardly alters
deeper diffusion energy barriers. Penetration is unlikely and restricted to
high occupancies of hcp hollows. In agreement with experiments, the Pd lattice
expands vertically as a consequence of H atoms being blocked at subsurface
sites, and surface H enhances this expansion. STM tip effects are included in
the calculations self-consistently as an external static electric field. The
main contribution to the induced surface electric dipoles originates from the
Pd substrate polarisability. We find that the electric field has a non-
negligible effect on the H-Pd potential in the vicinity of the topmost Pd
atomic layer, yet typical STM intensities of 1-2 VÅ−1 are insufficient to
invert the stabilities of the surface and subsurface equilibrium sites
Neel order, ring exchange and charge fluctuations in the half-filled Hubbard model
We investigate the ground state properties of the two dimensional half-filled
one band Hubbard model in the strong (large-U) to intermediate coupling limit
({\it i.e.} away from the strict Heisenberg limit) using an effective spin-only
low-energy theory that includes nearest-neighbor exchange, ring exchange, and
all other spin interactions to order t(t/U)^3. We show that the operator for
the staggered magnetization, transformed for use in the effective theory,
differs from that for the order parameter of the spin model by a
renormalization factor accounting for the increased charge fluctuations as t/U
is increased from the t/U -> 0 Heisenberg limit. These charge fluctuations lead
to an increase of the quantum fluctuations over and above those for an S=1/2
antiferromagnet. The renormalization factor ensures that the zero temperature
staggered moment for the Hubbard model is a monotonously decreasing function of
t/U, despite the fact that the moment of the spin Hamiltonien, which depends on
transverse spin fluctuations only, in an increasing function of t/U. We also
comment on quantitative aspects of the t/U and 1/S expansions.Comment: 9 pages - 3 figures - References and details to help the reader adde
The microscopic effect of intramolecular vibrational energy redistribution
We investigate the effect of inter-mode coupling on the vibrational relaxation
dynamics of molecules in weak dissipative environments. The simulations are
performed within the reduced density matrix formalism in the Markovian regime,
assuming a Lindblad form for the system-bath interaction. The prototypical
two-dimensional model system representing two CO molecules approaching a
Cu(100) surface is adapted from an ab initio potential, while the diatom-
diatom vibrational coupling strength is systematically varied. In the weak
system-bath coupling limit and at low temperatures, only first order non-
adiabatic uni-modal coupling terms contribute to surface-mediated vibrational
relaxation. Since dissipative dynamics is non-unitary, the choice of
representation will affect the evolution of the reduced density matrix. Two
alternative representations for computing the relaxation rates and the
associated operators are thus compared: the fully coupled spectral basis, and
a factorizable ansatz. The former is well-established and serves as a
benchmark for the solution of Liouville-von Neumann equation. In the latter, a
contracted grid basis of potential-optimized discrete variable representation
is tailored to incorporate most of the inter-mode coupling, while the Lindblad
operators are represented as tensor products of one-dimensional operators, for
consistency. This procedure results in a marked reduction of the grid size and
in a much more advantageous scaling of the computational cost with respect to
the increase of the dimensionality of the system. The factorizable method is
found to provide an accurate description of the dissipative quantum dynamics
of the model system, specifically of the time evolution of the state
populations and of the probability density distribution of the molecular wave
packet. The influence of intra-molecular vibrational energy redistribution
appears to be properly taken into account by the new model on the whole range
of coupling strengths. It demontrates that most of the mode mixing during
relaxation is due to the potential part of the Hamiltonian and not to the
coupling among relaxation operator
Invariants of Triangular Lie Algebras
Triangular Lie algebras are the Lie algebras which can be faithfully
represented by triangular matrices of any finite size over the real/complex
number field. In the paper invariants ('generalized Casimir operators') are
found for three classes of Lie algebras, namely those which are either strictly
or non-strictly triangular, and for so-called special upper triangular Lie
algebras. Algebraic algorithm of [J. Phys. A: Math. Gen., 2006, V.39, 5749;
math-ph/0602046], developed further in [J. Phys. A: Math. Theor., 2007, V.40,
113; math-ph/0606045], is used to determine the invariants. A conjecture of [J.
Phys. A: Math. Gen., 2001, V.34, 9085], concerning the number of independent
invariants and their form, is corroborated.Comment: LaTeX2e, 16 pages; misprints are corrected, some proofs are extende
A shrinking Compact Symmetric Object: J11584+2450?
We present multi-frequency multi-epoch Very Long Baseline Array (VLBA)
observations of J11584+2450. These observations clearly show this source,
previously classified as a core-jet, to be a compact symmetric object (CSO).
Comparisons between these new data and data taken over the last 9 years shows
the edge brightened hot spots retreating towards the core (and slightly to the
west) at approximately 0.3c. Whether this motion is strictly apparent or
actually physical in nature is discussed, as well as possible explanations, and
what implications a physical contraction of J11584+2450 would have for current
CSO models.Comment: 16 pages, 6 figures, 5 tables. Accepted for publication in Ap
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