7,626 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
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
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
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
Water governance in two urban African contexts: agency and action through participatory video
This paper describes and critically examines the process and outcomes of a community-based participatory video (PV) research project on issues related to water governance with residents of underserved and informal settlements in Khayelitsha, South Africa and Accra, Ghana. Co-produced videos were used to facilitate communication and to open a dialogue between the participating communities and their respective local governments, with the aims of improving awareness of the issues, enhancing agency and enabling participation in the political and social debates about water governance. Analysing the approach, our research draws on two key principles of participatory governance – recognition and response – to evaluate the application of PV as a potential engagement tool for participatory water governance. We critically discuss the reality and tensions of PV in shifting deep-rooted inequities of power in decision making through two case studies, both of which involved residents and representatives from local governments in the research process
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