4,972 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
A Local One-Zone Model of MHD Turbulence in Dwarf Nova Disks
The evolution of the magnetorotational instability (MRI) during the
transition from outburst to quiescence in a dwarf nova disk is investigated
using three-dimensional MHD simulations. The shearing box approximation is
adopted for the analysis, so that the efficiency of angular momentum transport
is studied in a small local patch of the disk: this is usually referred as to a
one-zone model. To take account of the low ionization fraction of the disk, the
induction equation includes both ohmic dissipation and the Hall effect. We
induce a transition from outburst to quiescence by an instantaneous decrease of
the temperature. The evolution of the MRI during the transition is found to be
very sensitive to the temperature of the quiescent disk. As long as the
temperature is higher than a critical value of about 2000 K, MHD turbulence and
angular momentum transport is sustained by the MRI. However, MHD turbulence
dies away within an orbital time if the temperature falls below this critical
value. In this case, the stress drops off by more than 2 orders of magnitude,
and is dominated by the Reynolds stress associated with the remnant motions
from the outburst. The critical temperature depends slightly on the distance
from the central star and the local density of the disk.Comment: 20 pages, 2 tables, 6 figures, accepted for publication in Ap
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
Superconductivity in a Two-Orbital Hubbard Model with Electron and Hole Fermi Pockets: Application in Iron Oxypnictide Superconductors
We investigate the electronic states of a one-dimensional two-orbital Hubbard
model with band splitting by the exact diagonalization method. The Luttinger
liquid parameter is calculated to obtain superconducting (SC) phase
diagram as a function of on-site interactions: the intra- and inter-orbital
Coulomb and , the Hund coupling , and the pair transfer . In
this model, electron and hole Fermi pockets are produced when the Fermi level
crosses both the upper and lower orbital bands. We find that the system shows
two types of SC phases, the SC \Roman{u'-large} for and the SC
\Roman{u-large} for , in the wide parameter region including both weak
and strong correlation regimes. Pairing correlation functions indicate that the
most dominant pairing for the SC \Roman{u'-large} (SC \Roman{u-large}) is the
intersite (on-site) intraorbital spin-singlet with (without) sign reversal of
the order parameters between two Fermi pockets. The result of the SC
\Roman{u'-large} is consistent with the sign-reversing s-wave pairing that has
recently been proposed for iron oxypnictide superconductors.Comment: 5 pages, 8 figures, to appear in J. Phys. Soc. Jpn., Vol.78, No.12,
p.12470
Interstellar Gas and X-rays toward the Young Supernova Remnant RCW 86; Pursuit of the Origin of the Thermal and Non-Thermal X-ray
We have analyzed the atomic and molecular gas using the 21 cm HI and 2.6/1.3
mm CO emissions toward the young supernova remnant (SNR) RCW 86 in order to
identify the interstellar medium with which the shock waves of the SNR
interact. We have found an HI intensity depression in the velocity range
between and km s toward the SNR, suggesting a cavity in the
interstellar medium. The HI cavity coincides with the thermal and non-thermal
emitting X-ray shell. The thermal X-rays are coincident with the edge of the HI
distribution, which indicates a strong density gradient, while the non-thermal
X-rays are found toward the less dense, inner part of the HI cavity. The most
significant non-thermal X-rays are seen toward the southwestern part of the
shell where the HI gas traces the dense and cold component. We also identified
CO clouds which are likely interacting with the SNR shock waves in the same
velocity range as the HI, although the CO clouds are distributed only in a
limited part of the SNR shell. The most massive cloud is located in the
southeastern part of the shell, showing detailed correspondence with the
thermal X-rays. These CO clouds show an enhanced CO = 2-1/1-0 intensity
ratio, suggesting heating/compression by the shock front. We interpret that the
shock-cloud interaction enhances non-thermal X-rays in the southwest and the
thermal X-rays are emitted by the shock-heated gas of density 10-100 cm.
Moreover, we can clearly see an HI envelope around the CO cloud, suggesting
that the progenitor had a weaker wind than the massive progenitor of the
core-collapse SNR RX J1713.73949. It seems likely that the progenitor of RCW
86 was a system consisting of a white dwarf and a low-mass star with
low-velocity accretion winds.Comment: 19 pages, 15 figures, 4 tables, accepted for publication in Journal
of High Energy Astrophysics (JHEAp
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
Enhancement of Pairing Correlation and Spin Gap through Suppression of Single-Particle Dispersion in One-Dimensional Models
We investigate the effects of suppression of single-particle dispersion near
the Fermi level on the spin gap and the singlet-pairing correlation by using
the exact diagonalization method for finite-size systems. We consider strongly
correlated one-dimensional models, which have flat band dispersions near wave
number k=\pi/2, if the interactions are switched off. Our results for strongly
correlated models show that the spin gap region expands as the single-particle
dispersion becomes flatter. The region where the singlet-pairing correlation is
the most dominant also expands in models with flatter band dispersions. Based
on our numerical results, we propose a pairing mechanism induced by the
flat-band dispersion.Comment: 5 pages, including 5 eps figures, to appear in J.Phys.Soc.Jpn Vol.69
No.
Spin-Gap Phase in the One-Dimensional t-J-J' Model
The spin-gap phase of the one-dimensional t-J-J' model is studied by the
level-crossing of the singlet and the triplet excitation spectra.
The phase boundary obtained between the Tomonaga-Luttinger and the spin-gap
phases is remarkably consistent with the analytical results at the
and the low-density limits discussed by Ogata et al.
The spin-gap phase has a single domain in the phase diagram even if the spin
gap opens at half-filling.
The phase boundary coincides with the line where the
Tomonaga-Luttinger liquid behaves as free electrons, in the low-density region.
The relation between our method and the solution of the two-electron problem
is also discussed.Comment: 4 pages(JPSJ.sty), 5 figures(EPS), to appear in J. Phys. Soc. Jpn.
67, No.3 (1998
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