4,113 research outputs found
Electronic States and Superconducting Transition Temperature based on the Tomonaga-Luttinger liquid in PrBaCuO
An NQR experiment revealed superconductivity of
PrBaCuO (Pr247) to be realized on CuO double chain
layers and suggests possibility of novel one-dimensional(1D) superconductivity.
To clarify the nature of the 1D superconductivity, we calculate the band
dispersions of Pr247 by using the generalized gradient approximation(GGA). It
indicates that Fermi surface of CuO double chains is well described to the
electronic structure of a quasi-1D system.
Assuming the zigzag Hubbard chain model to be an effective model of the
system, we derive tight binding parameters of the model from a fit to the
result of GGA. Based on the Tomonaga-Luttinger liquid theory, we estimate
transition temperature () of the quasi-1D zigzag Hubbard model from the
calculated value of the Luttinger liquid parameter . The result of
is consistent with that of experiments in Pr247 and it suggests that the
mechanism of the superconductivity is well understood within the concept of the
Tomonaga-Luttinger liquid.Comment: 4 pages, 5 figure
Ferromagnetism and Superconductivity in the multi-orbital Hubbard Model: Hund's Rule Coupling versus Crystal-Field Splitting
The multi-orbital Hubbard model in one dimension is studied using the
numerical diagonalization method. Due to the effect of the crystal-field
splitting , the fully polarized ferromagnetism which is observed in the
strong coupling regime becomes unstable against the partially polarized
ferromagnetism when the Hund's rule coupling is smaller than a certain
critical value of order of . In the vicinity of the partially polarized
ferromagnetism, the orbital fluctuation develops due to the competition between
the Hund's rule coupling and the crystal-field splitting. The superconducting
phase with the Luttinger liquid parameter is observed for the
singlet ground state in this region.Comment: 4 pages,5 figures,submitted to J.Phys.Soc.Jp
Orbital Order, Structural Transition and Superconductivity in Iron Pnictides
We investigate the 16-band d-p model for iron pnictide superconductors in the
presence of the electron-phonon coupling g with the orthorhombic mode which is
crucial for reproducing the recently observed ultrasonic softening. Within the
RPA, we obtain the ferro-orbital order below TQ which induces the
tetragonal-orthorhombic structural transition at Ts = TQ, together with the
stripe-type antiferromagnetic order below TN. Near the phase transitions, the
system shows the s++ wave superconductivity due to the orbital fluctuation for
a large g case with TQ > TN, while the s+- wave due to the magnetic fluctuation
for a small g case with TQ < TN. The former case is consistent with the phase
diagram of doped iron pnictides with Ts > TN.Comment: 5 pages, 5 figures, minor changes, published in J. Phys. Soc. Jp
Modeling the gamma-ray emission produced by runaway cosmic rays in the environment of RX J1713.7-3946
Diffusive shock acceleration in supernova remnants is the most widely invoked
paradigm to explain the Galactic cosmic ray spectrum. Cosmic rays escaping
supernova remnants diffuse in the interstellar medium and collide with the
ambient atomic and molecular gas. From such collisions gamma-rays are created,
which can possibly provide the first evidence of a parent population of runaway
cosmic rays. We present model predictions for the GeV to TeV gamma-ray emission
produced by the collisions of runaway cosmic rays with the gas in the
environment surrounding the shell-type supernova remnant RX J1713.7-3946. The
spectral and spatial distributions of the emission, which depend upon the
source age, the source injection history, the diffusion regime and the
distribution of the ambient gas, as mapped by the LAB and NANTEN surveys, are
studied in detail. In particular, we find for the region surrounding RX
J1713-3946, that depending on the energy one is observing at, one may observe
startlingly different spectra or may not detect any enhanced emission with
respect to the diffuse emission contributed by background cosmic rays. This
result has important implications for current and future gamma-ray experiments.Comment: version published on PAS
Molecular Clouds as Cosmic-Ray Barometers
The advent of high sensitivity, high resolution gamma-ray detectors, together
with a knowledge of the distribution of the atomic hydrogen and especially of
the molecular hydrogen in the Galaxy on sub-degree scales creates a unique
opportunity to explore the flux of cosmic rays in the Galaxy. We here present
the new data on the distribution of the molecular hydrogen from a large region
of the inner Galaxy obtained by the NANTEN Collaboration. We then introduce a
methodology which aims to provide a test bed for current and future gamma-ray
observatories to explore the cosmic ray flux at various positions in our
Galaxy. In particular, for a distribution of molecular clouds, as provided by
the NANTEN survey, and local cosmic ray density as measured at the Earth, we
estimate the expected GeV to TeV gamma-ray signal, which can then be compared
with observations and use to test the cosmic ray flux.Comment: PASJ (in press
Origin of spin-gap in CaVO: effect of frustration and lattice distortion
We study the origin of spin-gap in recently discovered material CaVO.
We analyze the spin- Heisenberg model on the depleted square lattice
with nearest neighbor (nn) and next nearest neighbor (nnn) interactions, in
terms of the singlet and triplet states of the 4-spin plaquettes and 2-spin
dimers. Phase diagram of the model is obtained within a linear
``spin-wave"-like approximation, and is shown to agree well with the earlier
results of QMC simulations for nn interactions. We further propose that the
special lattice structure of CaVO naturally leads to lattice
distortions, which enhances the spin-gap via a spin-Peierls mechanism.Comment: 4 pages, RevTex, 2 postscript figures. Latex file and figures have
been uuencode
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