294 research outputs found
Isotope effects and possible pairing mechanism in optimally doped cuprate superconductors
We have studied the oxygen-isotope effects on T_{c} and in-plane penetration
depth \lambda_{ab}(0) in an optimally doped 3-layer cuprate
Bi_{1.6}Pb_{0.4}Sr_{2}Ca_{2}Cu_{3}O_{10+y} (T_{c} \sim 107 K). We find a small
oxygen-isotope effect on T_{c} (\alpha_{O} = 0.019), and a substantial effect
on \lambda_{ab} (0) (\Delta \lambda_{ab} (0)/\lambda_{ab} (0) = 2.5\pm0.5%).
The present results along with the previously observed isotope effects in
single-layer and double-layer cuprates indicate that the isotope exponent
\alpha_{O} in optimally doped cuprates is small while the isotope effect on the
in-plane effective supercarrier mass is substantial and nearly independent of
the number of the CuO_{2} layers. A plausible pairing mechanism is proposed to
explain the isotope effects, high-T_{c} superconductivity and tunneling spectra
in a consistent way.Comment: 5 pages, 4 figure
Nuclear Spin-Isospin Correlations, Parity Violation, and the Problem
The strong interaction effects of isospin- and spin-dependent nucleon-nucleon
correlations observed in many-body calculations are interpreted in terms of a
one-pion exchange mechanism. Including such effects in computations of nuclear
parity violating effects leads to enhancements of about 10%. A larger effect
arises from the one-boson exchange nature of the parity non-conserving nucleon-
nucleon interaction, which depends on both weak and strong meson-nucleon
coupling constants. Using values of the latter that are constrained by
nucleon-nucleon phase shifts leads to enhancements of parity violation by
factors close to two. Thus much of previously noticed discrepancies between
weak coupling constants extracted from different experiments can be removed.Comment: 8 pages 2 figures there should have been two figures in v
Large scale shell model calculations for odd-odd Mn isotopes
Large scale shell model calculations have been carried out for odd-odd
Mn isotopes in two different model spaces. First set of calculations
have been carried out in full shell valence space with two recently
derived shell interactions namely GXPF1A and KB3G treating Ca
as core. The second set of calculations have been performed in
valence space with the interaction treating Ca as core and
imposing a truncation by allowing up to a total of six particle excitations
from the 0f orbital to the upper orbitals for protons and
from the upper orbitals to the 0g orbital for neutron. For
low-lying states in Mn, the KB3G and GXPF1A both predicts good results
and for Mn, KB3G is much better than GXPF1A. For negative parity and
high-spin positive parity states in both isotopes interaction is
required. Experimental data on Mn is sparse and therefore it is not
possible to make any definite conclusions. More experimental data on negative
parity states is needed to ascertain the importance of 0g and higher
orbitals in neutron rich Mn isotopes.Comment: 5 pages, 4 figures, Submitted to Eur. Phys. J.
The H-Dibaryon and the Hard Core
The H dibaryon, a single, triply magic bag containing two up, two down and
two strange quarks, has long been sought after in a variety of experiments. Its
creation has been attempted in , proton and most recently in relativistic
heavy ion induced reactions. We concentrate on the latter, but our conclusions
are more generally applicable. The two baryons coalescing to form the single
dibaryon, likely in the case of heavy ions, must penetrate
the short range repulsive barrier which is expected to exist between them. We
find that this barrier can profoundly affect the probability of producing the H
state, should it actually exist.Comment: 9 pages including 4 figure
The moon as a recorder of organic evolution in the early solar system: a lunar regolith analog study
The organic record of Earth older than ∼3.8 Ga has been effectively erased. Some insight is provided to us by meteorites as well as remote and direct observations of asteroids and comets left over from the formation of the Solar System. These primitive objects provide a record of early chemical evolution and a sample of material that has been delivered to Earth's surface throughout the past 4.5 billion years. Yet an effective chronicle of organic evolution on all Solar System objects, including that on planetary surfaces, is more difficult to find. Fortunately, early Earth would not have been the only recipient of organic matter–containing objects in the early Solar System. For example, a recently proposed model suggests the possibility that volatiles, including organic material, remain archived in buried paleoregolith deposits intercalated with lava flows on the Moon. Where asteroids and comets allow the study of processes before planet formation, the lunar record could extend that chronicle to early biological evolution on the planets. In this study, we use selected free and polymeric organic materials to assess the hypothesis that organic matter can survive the effects of heating in the lunar regolith by overlying lava flows. Results indicate that the presence of lunar regolith simulant appears to promote polymerization and, therefore, preservation of organic matter. Once polymerized, the mineral-hosted newly formed organic network is relatively protected from further thermal degradation. Our findings reveal the thermal conditions under which preservation of organic matter on the Moon is viable
Optical detection of NMR J-spectra at zero magnetic field
Scalar couplings of the form J I_1 \cdot I_2 between nuclei impart valuable
information about molecular structure to nuclear magnetic-resonance spectra.
Here we demonstrate direct detection of J-spectra due to both heteronuclear and
homonuclear J-coupling in a zero-field environment where the Zeeman interaction
is completely absent. We show that characteristic functional groups exhibit
distinct spectra with straightforward interpretation for chemical
identification. Detection is performed with a microfabricated optical atomic
magnetometer, providing high sensitivity to samples of microliter volumes. We
obtain 0.1 Hz linewidths and measure scalar-coupling parameters with 4-mHz
statistical uncertainty. We anticipate that the technique described here will
provide a new modality for high-precision "J spectroscopy" using small samples
on microchip devices for multiplexed screening, assaying, and sample
identification in chemistry and biomedicine.Comment: 15 pages, 4 Figure
Optical Spectra of SNR Candidates in NGC 300
We present moderate-resolution (<5A) long-slit optical spectra of 51 nebular
objects in the nearby Sculptor Group galaxy NGC 300 obtained with the 2.3 meter
Advanced Technology Telescope at Siding Spring Observatory, Australia. Adopting
the criterion of [SII]/Ha>=0.4 to confirm supernova remnants (SNRs) from
optical spectra, we find that of 28 objects previously proposed as SNRs from
optical observations, 22 meet this criterion with six showing [SII]/Ha of less
than 0.4. Of 27 objects suggested as SNRs from radio data, four are associated
with the 28 previously proposed SNRs. Of these four, three (included in the 22
above) meet the criterion. In all, 22 of the 51 nebular objects meet the
[SII]/Ha criterion as SNRs while the nature of the remaining 29 objects remains
undetermined by these observations.Comment: Accepted for publication in Astrophysics & Space Scienc
Berry phases and pairing symmetry in Holstein-Hubbard polaron systems
We study the tunneling dynamics of dopant-induced hole polarons which are
self-localized by electron-phonon coupling in a two-dimensional antiferro-
magnet. Our treatment is based on a path integral formulation of the adia-
batic approximation, combined with many-body tight-binding, instanton, con-
strained lattice dynamics, and many-body exact diagonalization techniques. Our
results are mainly based on the Holstein- and, for comparison, on the
Holstein-Hubbard model. We also study effects of 2nd neighbor hopping and
long-range electron-electron Coulomb repulsion. The polaron tunneling dynamics
is mapped onto an effective low-energy Hamiltonian which takes the form of a
fermion tight-binding model with occupancy dependent, predominant- ly 2nd and
3rd neighbor tunneling matrix elements, excluded double occupan- cy, and an
effective intersite charge interactions. Antiferromagnetic spin correlations in
the original many-electron Hamiltonian are reflected by an attractive
contribution to the 1st neighbor charge interaction and by Berry phase factors
which determine the signs of effective polaron tunneling ma- trix elements. In
the two-polaron case, these phase factors lead to polaron pair wave functions
of either -wave symmetry or p-wave symme- try with zero and
nonzero total pair momentum, respectively. Implications for the doping
dependent isotope effect, pseudo-gap and Tc of a superconduc- ting polaron pair
condensate are discussed/compared to observed in cuprates.Comment: 23 pages, revtex, 13 ps figure
ASEAN and the Dynamics of Resistance to Sovereignty Violation:The Case of the Third Indochina War (1978–1991)
This article investigates the history of ASEAN’s relationship to external intervention in regional affairs. It addresses a specific question: What was the basic cause of the success of ASEAN resistance to the Vietnamese challenge to ASEAN’s sovereignty from 1978-1991? ASEAN’s history is understood in terms of a realist theoretical logic, in terms of the relationship between an ASEAN state with the most compelling interests at stake in a given issue, which I call a ‘vanguard state,’ and selected external powers. Using the Third Indochina War (1978–1991) as a case study, this article contends that ASEAN’s ability to resist violations to the sovereignty of Thailand from a Soviet-backed Vietnam is a consequence of high interest convergence between Thailand, and a designated external power, China
Azimuthal anisotropy of K0s and Lambda prduction at mid-rapidity from Au+Au collisions at root s = 130 GeV
We report STAR results on the azimuthal anisotropy parameter v2 for strange
particles K0S, L and Lbar at midrapidity in Au+Au collisions at sNN = 130 GeV
at RHIC. The value of v2 as a function of transverse momentum of the produced
particles pt and collision centrality is presented for both particles up to pt
3.0 GeV/c. A strong pt dependence in v2 is observed up to 2.0 GeV/c. The v2
measurement is compared with hydrodynamic model calculations. The physics
implications of the pt integrated v2 magnitude as a function of particle mass
are also discussed.Comment: 6 pages, 4 figures, by the STAR collaboratio
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