1,439 research outputs found
Comparison of Heine-Abarenkov and alternative pseudopotentials for electron-phonon interaction in aluminium, lead, lithium and calcium.
Extracting a more realistic pseudopotential for aluminum, lead, niobium and tantalum from superconductor electron tunnelling spectroscopy data
Detection of FeO towards SgrB2
We have observed the J=5-4 ground state transition of FeO at a frequency of
153 GHz towards a selection of galactic sources.
Towards the galactic center source SgrB2, we see weak absorption at
approximately the velocity of other features towards this source (62 km
s LSR).
Towards other sources, the results were negative as they were also for
MgOH(3-2) and FeC(6-5). We tentatively conclude that the absorption seen toward
SgrB2 is due to FeO in the hot ( 500 K) relatively low density absorbing
gas known to be present in this line of sight.
This is the first (albeit tentative) detection of FeO or any iron--containing
molecule in the interstellar gas. Assuming the observed absorption to be due to
FeO, we estimate [FeO]/[SiO] to be of order or less than 0.002 and
[FeO]/[H] of order . This is compatible with our negative
results in other sources.
Our results suggest that the iron liberated from grains in the shocks
associated with SgrB2 remains atomic and is not processed into molecular form.Comment: 1 postscrit figure,10 page
Umklapp scattering of pairs in BCS superconductivity theory
The BCS theory of superconductivity is extended to recognize pairing of
electrons by both normal and umklapp scattering. Application of the variational
approach shows that coexistence of normal and umklapp scattering frustrates
superconductivity.Comment: 9 pages, 5 figures. to be published in Journal of Physics: Condensed
Matte
The abundances of nitrogen-containing molecules during pre-protostellar collapse
We have studied the chemistry of nitrogen--bearing species during the initial stages of protostellar collapse, with a view to explaining the observed longevity of N2H+ and NH3 and the high levels of deuteration of these species. We followed the chemical evolution of a medium comprising gas and dust as it underwent free--fall gravitational collapse. Chemical processes which determine the relative populations of the nuclear spin states of molecules and molecular ions were included explicitly, as were reactions which lead ultimately to the deuteration of the nitrogen--containing species N2H+ and NH3. The freeze-out of `heavy' molecules onto grains was taken into account. We found that the timescale required for the nitrogen--containing species to attain their steady--state values was much larger than the free--fall time and even comparable with the probable lifetime of the precursor molecular cloud. However, it transpires that the chemical evolution of the gas during gravitational collapse is insensitive to its initial composition. If we suppose that the grain--sticking probabilities of atomic nitrogen and oxygen are both less than unity (S less than 0.3), we find that the observed differential freeze--out of nitrogen- and carbon--bearing species can be reproduced by the model of free--fall collapse when a sufficiently large grain radius (a_{g}= 0.5 micron) is adopted. Furthermore, the results of our collapse model are consistent with the high levels of deuteration of N2H+ and NH3 which have been observed in L1544 providing that 0.5<a_{g}<1 micron. We note that the o/p H2D+ ratio and fractional abundance of ortho-H2D+ should be largest when ND3 is most abundant
Modematching an optical quantum memory
We analyse the off-resonant Raman interaction of a single broadband photon,
copropagating with a classical `control' pulse, with an atomic ensemble. It is
shown that the classical electrodynamical structure of the interaction
guarantees canonical evolution of the quantum mechanical field operators. This
allows the interaction to be decomposed as a beamsplitter transformation
between optical and material excitations on a mode-by-mode basis. A single,
dominant modefunction describes the dynamics for arbitrary control pulse
shapes.
Complete transfer of the quantum state of the incident photon to a collective
dark state within the ensemble can be achieved by shaping the control pulse so
as to match the dominant mode to the temporal mode of the photon. Readout of
the material excitation, back to the optical field, is considered in the
context of the symmetry connecting the input and output modes. Finally, we show
that the transverse spatial structure of the interaction is characterised by
the same mode decomposition.Comment: 17 pages, 4 figures. Brief section added treating the transverse
spatial structure of the memory interaction. Some references added. A few
typos fixe
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