1,422 research outputs found
Description of Four-Body Breakup Reaction with the Method of Continuum-Discretized Coupled-Channels
We present a method for smoothing discrete breakup -matrix elements
calculated by the method of continuum-discretized coupled-channels (CDCC). This
smoothing method makes it possible to apply CDCC to four-body breakup
reactions. The reliability of the smoothing method is confirmed for two cases,
Ni(, ) at 80 MeV and the transition of He. We apply
CDCC with the smoothing method to He breakup reaction at 22.5 MeV.
Multi-step breakup processes are found to be important.Comment: 19 pages, 7 figures, published in Progress of Theoretical Physic
Effect of Adiabatic Phonons on Striped and Homogeneous Ground States
The effects of adiabatic phonons on a spin-fermion model for high T_c
cuprates are studied using numerical simulations. In the absence of
electron-phonon interactions (EPI), stripes in the ground state are observed
for certain dopings while homogeneous states are stabilized in other regions of
parameter space. Different modes of adiabatic phonons are added to the
Hamiltonian:breathing, shear and half-breathing modes. Diagonal and
off-diagonal electron-phonon couplings are considered. It is observed that
strong diagonal EPI generate stripes in previously homogeneous states, while in
striped ground states an increase in the diagonal couplings tends to stabilize
the stripes, inducing a gap in the density of states (DOS) and rendering the
ground state insulating. The off-diagonal terms, on the other hand, destabilize
the stripes creating inhomogeneous ground states with a pseudogap at the
chemical potential in the DOS. The breathing mode stabilizes static diagonal
stripes; while the half-breathing (shear) modes stabilize dynamical (localized)
vertical and horizontal stripes. The EPI induces decoherence of the
quasi-particle peaks in the spectral functions.Comment: latex, 9 pages,13 figure
Direct observation of the formation of polar nanoregions in Pb(MgNb)O using neutron pair distribution function analysis
Using neutron pair distribution function (PDF) analysis over the temperature
range from 1000 K to 15 K, we demonstrate the existence of local polarization
and the formation of medium-range, polar nanoregions (PNRs) with local
rhombohedral order in a prototypical relaxor ferroelectric
Pb(MgNb)O. We estimate the volume fraction of the PNRs as a
function of temperature and show that this fraction steadily increases from 0 %
to a maximum of 30% as the temperature decreases from 650 K to 15 K.
Below T200 K the PNRs start to overlap as their volume fraction reaches
the percolation threshold. We propose that percolating PNRs and their
concomitant overlap play a significant role in the relaxor behavior of
Pb(MgNb)O.Comment: 4 pages, 3 figure
Can correlations drive a band insulator metallic?
We analyze the effects of the on-site Coulomb repulsion U on a band insulator
using dynamical mean field theory (DMFT). We find the surprising result that
the gap is suppressed to zero at a critical Uc1 and remains zero within a
metallic phase. At a larger Uc2 there is a second transition from the metal to
a Mott insulator, in which the gap increases with increasing U. These results
are qualitatively different from Hartree-Fock theory which gives a
monotonically decreasing but non-zero insulating gap for all finite U.Comment: 4 pages, 5 figure
Metallic phase in the two-dimensional ionic Hubbard model
We investigate the phases of the ionic Hubbard model in a two-dimensional
square lattice using determinant quantum Monte Carlo (DQMC). At half-filling,
when the interaction strength or the staggered potential dominate we find Mott
and band insulators, respectively. When these two energies are of the same
order we find a metallic region. Charge and magnetic structure factors
demonstrate the presence of antiferromagnetism only in the Mott region,
although the externally imposed density modulation is present everywhere in the
phase diagram. Away from half-filling, other insulating phases are found.
Kinetic energy correlations do not give clear signals for the existence of a
bond-ordered phase
Inhomogeneous charge textures stabilized by electron-phonon interactions in the t-J model
We study the effect of diagonal and off-diagonal electron-phonon coupling in
the ground state properties of the t-J model. Adiabatic and quantum phonons are
considered using Lanczos techniques. Charge tiles and stripe phases with mobile
holes (localized holes) are observed at intermediate (large) values of the
diagonal electron-phonon coupling. The stripes are stabilized by half-breathing
modes, while the tiles arise due to the development of extended breathing
modes. Off-diagonal terms destabilize the charge inhomogeneous structures with
mobile holes by renormalizing the diagonal coupling but do not produce new
phases. Buckling modes are also studied and they seem to induce a gradual phase
separation between hole rich and hole poor regions. The pairing correlations
are strongly suppressed when the holes are localized. However, in charge
inhomogeneous states with mobile holes no dramatic changes, compared with the
uniform state, are observed in the pairing correlations indicating that D-wave
pairing and moderate electron-phonon interactions can coexist.Comment: minor changes; to appear in Physical Review
Coulomb breakup effects on the elastic cross section of He+Bi scattering near Coulomb barrier energies
We accurately analyze the He+Bi scattering at 19 and 22.5 MeV
near the Coulomb barrier energy, using the continuum-discretized
coupled-channels method (CDCC) based on the ++He+Bi four-body
model.
The three-body breakup continuum of He is discretized by diagonalizing
the internal Hamiltonian of He in a space spanned by the Gaussian basis
functions.
The calculated elastic and total reaction cross sections are in good
agreement with the experimental data, while the CDCC calculation based on the
di-neutron model of He, i.e., the +He+Bi three-body
model, does not reproduce the data.Comment: 5 pages, 5 figures, uses REVTeX 4, submitted to Phys. Rev.
- …