31,116 research outputs found
Close-packed structures and phase diagram of soft spheres in cylindrical pores
It is shown for a model system consisting of spherical particles confined in cylindrical pores that the first ten close-packed phases are in one-to-one correspondence with the first ten ways of folding a triangular lattice, each being characterized by a roll-up vector like the single-walled carbon nanotube. Phase diagrams in pressure-diameter and temperature-diameter planes are obtained by inherent-structure calculation and molecular dynamics simulation. The phase boundaries dividing two adjacent phases are infinitely sharp in the low-temperature limit but are blurred as temperature is increased. Existence of such phase boundaries explains rich, diameter-sensitive phase behavior unique for cylindrically confined systems
Strong magnetic field enhancement of spin triplet pairing arising from coexisting spin and charge fluctuations
We study the effect of the magnetic field (Zeeman splitting) on the triplet
pairing. We show generally that the enhancement of spin triplet pairing
mediated by coexisting spin and charge fluctuations can be much
larger than in the case of triplet pairing mediated by ferromagnetic spin
fluctuations. We propose that this may be related to the recent experiment for
(TMTSF)ClO, in which a possibility of singlet to triplet pairing
transition has been suggested.Comment: 5 page
Orbital Symmetry and Electron Correlation in Na_{x}CoO_2
Measurements of polarization-dependent soft x-ray absorption reveal that the
electronic states determining the low-energy excitations of NaCoO
have predominantly symmetry with significant O character. A large
transfer of spectral weight observed in O x-ray absorption provides
spectral evidence for strong electron correlations in the layered cobaltates.
Comparing Co x-ray absorption with calculations based on a cluster model,
we conclude that NaCoO exhibits a charge-transfer electronic
character rather than a Mott-Hubbard character
Semiclassical Tunneling of Wavepackets with Real Trajectories
Semiclassical approximations for tunneling processes usually involve complex
trajectories or complex times. In this paper we use a previously derived
approximation involving only real trajectories propagating in real time to
describe the scattering of a Gaussian wavepacket by a finite square potential
barrier. We show that the approximation describes both tunneling and
interferences very accurately in the limit of small Plank's constant. We use
these results to estimate the tunneling time of the wavepacket and find that,
for high energies, the barrier slows down the wavepacket but that it speeds it
up at energies comparable to the barrier height.Comment: 23 pages, 7 figures Revised text and figure
Non-resonant inelastic x-ray scattering involving excitonic excitations
In a recent publication Larson \textit{et al.} reported remarkably clear
- excitations for NiO and CoO measured with x-ray energies well below the
transition metal edge. In this letter we demonstrate that we can obtain an
accurate quantitative description based on a local many body approach. We find
that the magnitude of can be tuned for maximum sensitivity for
dipole, quadrupole, etc. excitations. We also find that the direction of
with respect to the crystal axes can be used as an equivalent to
polarization similar to electron energy loss spectroscopy, allowing for a
determination of the local symmetry of the initial and final state based on
selection rules. This method is more generally applicable and combined with the
high resolution available, could be a powerful tool for the study of local
distortions and symmetries in transition metal compounds including also buried
interfaces
Fermi Large Area Telescope Detection of Two Very-High-Energy (E>100 GeV) Gamma-ray Photons from the z = 1.1 Blazar PKS 0426-380
We report the Fermi Large Area Telescope (LAT) detection of two
very-high-energy (VHE, E>100 GeV) gamma-ray photons from the directional
vicinity of the distant (redshift, z = 1.1) blazar PKS 0426-380. The null
hypothesis that both the 134 and 122 GeV photons originate from unrelated
sources can be rejected at the 5.5 sigma confidence level. We therefore claim
that at least one of the two VHE photons is securely associated with the
blazar, making PKS 0426-380 the most distant VHE emitter known to date. The
results are in agreement with the most recent Fermi-LAT constraints on the
Extragalactic Background Light (EBL) intensity, which imply a
horizon for 100 GeV photons. The LAT detection of the two VHE
gamma-rays coincided roughly with flaring states of the source, although we did
not find an exact correspondence between the VHE photon arrival times and the
flux maxima at lower gamma-ray energies. Modeling the gamma-ray continuum of
PKS 0426-380 with daily bins revealed a significant spectral hardening around
the time of detection of the first VHE event (LAT photon index \Gamma\
1.4) but on the other hand no pronounced spectral changes near the detection
time of the second one. This combination implies a rather complex variability
pattern of the source in gamma rays during the flaring epochs. An additional
flat component is possibly present above several tens of GeV in the
EBL-corrected Fermi-LAT spectrum accumulated over the ~8-month high state.Comment: 5 pages, 1 table, 4 figures. Accepted by ApJ
A quantitative model for IcR product in d-wave Josephson junctions
We study theoretically the Josephson effect in d-wave superconductor /
diffusive normal metal /insulator/ diffusive normal metal/ d-wave
superconductor (D/DN/I/DN/D) junctions. This model is aimed to describe
practical junctions in high- cuprate superconductors, in which the product
of the critical Josephson current () and the normal state resistance ()
(the so-called product) is very small compared to the prediction
of the standard theory. We show that the product in D/DN/I/DN/D
junctions can be much smaller than that in d-wave superconductor / insulator /
d-wave superconductor junctions and formulate the conditions necessary to
achieve large product in D/DN/I/DN/D junctions. The proposed
theory describes the behavior of products quantitatively in
high- cuprate junctions.Comment: 4 pages, 6 figure
Are the Nuclei of Seyfert 2 Galaxies Viewed Face-On?
We show from modeling the Fe Kalpha line in the ASCA spectra of four X-ray
bright narrow emission line galaxies (Seyfert types 1.9 and 2) that two equally
viable physical models can describe the observed line profile. The first is
discussed by Turner et al. (1998) and consists of emission from a nearly
pole-on accretion disk. The second, which is statistically preferred, is a
superposition of emission from an accretion disk viewed at an intermediate
inclination of about 48 degrees and a distinct, unresolved feature that
presumably originates some distance from the galaxy nucleus. The intermediate
inclination is entirely consistent with unified schemes and our findings
challenge recent assertions that Seyfert 2 galaxies are preferentially viewed
with their inner regions face-on. We derive mean equivalent widths for the
narrow and disk lines of =60 eV and = 213 eV, respectively. The
X-ray data are well described by a geometry in which our view of the active
nucleus intersects and is blocked by the outer edges of the obscuring torus,
and therefore do not require severe misalignments between the accretion disk
and the torus.Comment: 19 pages, 3 postscript figures. Accepted for publication in ApJ
Approximate Analytic Solution for the Spatiotemporal Evolution of Wave Packets undergoing Arbitrary Dispersion
We apply expansion methods to obtain an approximate expression in terms of
elementary functions for the space and time dependence of wave packets in a
dispersive medium. The specific application to pulses in a cold plasma is
considered in detail, and the explicit analytic formula that results is
provided. When certain general initial conditions are satisfied, these
expressions describe the packet evolution quite well. We conclude by employing
the method to exhibit aspects of dispersive pulse propagation in a cold plasma,
and suggest how predicted and experimental effects may be compared to improve
the theoretical description of a medium's dispersive properties.Comment: 17 pages, 4 figures, RevTe
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