47,001 research outputs found
Electron-Phonon Interactions for Optical Phonon Modes in Few-Layer Graphene
We present a first-principles study of the electron-phonon (e-ph)
interactions and their contributions to the linewidths for the optical phonon
modes at and K in one to three-layer graphene. It is found that due to
the interlayer coupling and the stacking geometry, the high-frequency optical
phonon modes in few-layer graphene couple with different valence and conduction
bands, giving rise to different e-ph interaction strengths for these modes.
Some of the multilayer optical modes derived from the - mode of
monolayer graphene exhibit slightly higher frequencies and much reduced
linewidths. In addition, the linewidths of K- related modes in
multilayers depend on the stacking pattern and decrease with increasing layer
numbers.Comment: 6 pages,5 figures, submitted to PR
Vibrational Modes in LiBC: Theory Compared with Experiment
The search for other superconductors in the MgB2 class currently is focussed
on Li{1-x}BC, which when hole-doped (concentration x) should be a metal with
the potential to be a better superconductor than MgB2. Here we present the
calculated phonon spectrum of the parent semiconductor LiBC. The calculated
Raman-active modes are in excellent agreement with a recent observation, and
comparison of calculated IR-active modes with a recent report provides a
prediction of the LO--TO splitting for these four modes, which is small for the
B-C bond stretching mode at ~1200 cm^{-1}, but large for clearly resolved modes
at 540 cm^{-1} and 620 cm^{-1}.Comment: 4 pages, two embedded figures. Physica B (in press
Binary pulsars as probes of a Galactic dark matter disk
As a binary pulsar moves through a wind of dark matter particles, the
resulting dynamical friction modifies the binary's orbit. We study this effect
for the double disk dark matter (DDDM) scenario, where a fraction of the dark
matter is dissipative and settles into a thin disk. For binaries within the
dark disk, this effect is enhanced due to the higher dark matter density and
lower velocity dispersion of the dark disk, and due to its co-rotation with the
baryonic disk.We estimate the effect and compare it with observations for two
different limits in the Knudsen number (). First, in the case where DDDM is
effectively collisionless within the characteristic scale of the binary
() and ignoring the possible interaction between the pair of dark
matter wakes. Second, in the fully collisional case (), where a fluid
description can be adopted and the interaction of the pair of wakes is taken
into account. We find that the change in the orbital period is of the same
order of magnitude in both limits. A comparison with observations reveals good
prospects to probe currently allowed DDDM models with timing data from binary
pulsars in the near future. We finally comment on the possibility of extending
the analysis to the intermediate (rarefied gas) case with .Comment: 15 pages, 6 figures. Few comments and references added, version
accepted for publication in Physics of the Dark Universe (PDU
Scheme for remote implementation of partially unknown quantum operation of two qubits in cavity QED
By constructing the recovery operations of the protocol of remote
implementation of partially unknown quantum operation of two qubits [An Min
Wang: PRA, \textbf{74}, 032317(2006)], we present a scheme to implement it in
cavity QED. Long-lived Rydberg atoms are used as qubits, and the interaction
between the atoms and the field of cavity is a nonresonant one. Finally, we
analyze the experimental feasibility of this scheme.Comment: 7 pages, 2 figure
Resolving the pulsations of subdwarf B stars: HS 0039+4302, HS 0444+0458, and an examination of the group properties of resolved pulsators
We continue our program of single-site observations of pulsating subdwarf B
(sdB) stars and present the results of extensive time series photometry of HS
0039+4302 and HS 0444+0458. Both were observed at MDM Observatory during the
fall of 2005. We extend the number of known frequencies for HS 0039+4302 from 4
to 14 and discover one additional frequency for HS 0444+0458, bringing the
total to three. We perform standard tests to search for multiplet structure,
measure amplitude variations, and examine the frequency density to constrain
the mode degree .
Including the two stars in this paper, 23 pulsating sdB stars have received
follow-up observations designed to decipher their pulsation spectra. It is
worth an examination of what has been detected. We compare and contrast the
frequency content in terms of richness and range and the amplitudes with
regards to variability and diversity. We use this information to examine
observational correlations with the proposed pulsation mechanism as
well as alternative theories.Comment: 32 pages, 18 figures, 7 tables. Accepted for publication in MNRA
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