Description of Four-Body Breakup Reaction with the Method of Continuum-Discretized Coupled-Channels

We present a method for smoothing discrete breakup $S$-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, $^{58}$Ni($d$, $p n$) at 80 MeV and the $E1$ transition of $^6$He. We apply CDCC with the smoothing method to $^6$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(Mg1/3_{1/3}Nb2/3_{2/3})O3_3 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(Mg$_{1/3}$Nb$_{2/3}$)O$_3$. 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 $\sim$ 30% as the temperature decreases from 650 K to 15 K. Below T$\sim$200 K the PNRs start to overlap as their volume fraction reaches the percolation threshold. We propose that percolating PNRs and their concomitant overlap play a significant role in the relaxor behavior of Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$.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 6^6He+209^{209}Bi scattering near Coulomb barrier energies

We accurately analyze the $^6$He+$^{209}$Bi scattering at 19 and 22.5 MeV near the Coulomb barrier energy, using the continuum-discretized coupled-channels method (CDCC) based on the $n$+$n$+$^4$He+$^{209}$Bi four-body model. The three-body breakup continuum of $^6$He is discretized by diagonalizing the internal Hamiltonian of $^6$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 $^6$He, i.e., the $^2n$+$^{4}$He+$^{209}$Bi three-body model, does not reproduce the data.Comment: 5 pages, 5 figures, uses REVTeX 4, submitted to Phys. Rev.