6,070 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
Frequency stability of maser oscillators operated with cavity Q
The short term frequency stability of masers equipped with an external feedback loop to increase the cavity quality factor was studied. The frequency stability of a hydrogen and a rubidium maser were measured and compared with theoretical evaluation. It is shown that the frequency stability passes through an optimum when the cavity Q is varied. Long term fluctuations are discussed and the optimum mid term frequency stability achievably by small size active and passive H-masers is considered
Spin susceptibility of interacting electrons in one dimension: Luttinger liquid and lattice effects
The temperature-dependent uniform magnetic susceptibility of interacting
electrons in one dimension is calculated using several methods. At low
temperature, the renormalization group reaveals that the Luttinger liquid spin
susceptibility approaches zero temperature with an infinite slope
in striking contrast with the Fermi liquid result and with the behavior of the
compressibility in the absence of umklapp scattering. This effect comes from
the leading marginally irrelevant operator, in analogy with the Heisenberg spin
1/2 antiferromagnetic chain. Comparisons with Monte Carlo simulations at higher
temperature reveal that non-logarithmic terms are important in that regime.
These contributions are evaluated from an effective interaction that includes
the same set of diagrams as those that give the leading logarithmic terms in
the renormalization group approach. Comments on the third law of thermodynamics
as well as reasons for the failure of approaches that work in higher dimensions
are given.Comment: 21 pages, latex including 5 eps figure
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
Breakdown of Fermi liquid behavior at the (\pi,\pi)=2k_F spin-density wave quantum-critical point: the case of electron-doped cuprates
Many correlated materials display a quantum critical point between a
paramagnetic and a SDW state. The SDW wave vector connects points (hot spots)
on opposite sides of the Fermi surface. The Fermi velocities at these pairs of
points are in general not parallel. Here we consider the case where pairs of
hot spots coalesce, and the wave vector (\pi,\pi) of the SDW connects hot spots
with parallel Fermi velocities. Using the specific example of electron-doped
cuprates, we first show that Kanamori screening and generic features of the
Lindhard function make this case experimentally relevant. The temperature
dependence of the correlation length, the spin susceptibility and the
self-energy at the hot spots are found using the Two-Particle-Self-Consistent
theory and specific numerical examples worked out for parameters characteristic
of the electron-doped cuprates. While the curvature of the Fermi surface at the
hot spots leads to deviations from perfect nesting, the pseudo-nesting
conditions lead to drastic modifications to the temperature dependence of these
physical observables: Neglecting logarithmic corrections, the correlation
length \xi scales like 1/T, i.e. z=1 instead of the naive z=2, the (\pi,\pi)
static spin susceptibility \chi like , and the imaginary part of the
self-energy at the hot spots like . The correction to the Korringa NMR relaxation rate is subdominant. We also consider
this problem at zero temperature, or for frequencies larger than temperature,
using a field-theoretical model of gapless SDW fluctuations interacting with
fermions. The imaginary part of the fermionic self-energy close to the hot
spots scales as . This is less singular than earlier
predictions of the form . The difference arises from the
effects of umklapp terms that were not included in previous studies.Comment: 23 pages, 12 figures; (v2) minor changes; (v3) Final published
versio
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
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