Observational Constraints on Phantom Crossing DGP Gravity

We study the observational constraints on the Phantom Crossing DGP model. We demonstrate that the crossing of the phantom divide does not occur within the framework of the original Dvali-Gabadadze-Porrati (DGP) model or the DGP model developed by Dvali and Turner. By extending their model in the framework of an extra dimension scenario, we study a model that realizes crossing of the phantom divide. We investigate the cosmological constraints obtained from the recent observational data of Type Ia Supernovae, Cosmic Microwave Background anisotropies, and Baryon Acoustic Oscillations. The best fit values of the parameters with 1$\sigma$ (68%) errors for the Phantom Crossing DGP model are $\Omega_{m,0}=0.27^{+0.02}_{-0.02}$, $\beta=0.54^{+0.24}_{-0.30}$. We find that the Phantom Crossing DGP model is more compatible with the observations than the original DGP model or the DGP model developed by Dvali and Turner. Our model can realize late-time acceleration of the universe, similar to that of $\Lambda$CDM model, without dark energy due to the effect of DGP gravity. In our model, crossing of the phantom divide occurs at a redshift of $z \sim 0.2$.Comment: 17 pages, 9 figures, 1 table, Accepted for publication in International Journal of Modern Physics

Zn-impurity effects on quasi-particle scattering in La2-xSrxCuO4 studied by angle-resolved photoemission spectroscopy

Angle-resolved photoemission measurements were performed on Zn-doped La2-xSrxCuO4 (LSCO) to investigate the effects of Zn impurities on the low energy electronic structure. The Zn-impurity-induced increase in the quasi-particle (QP) width in momentum distribution curves (MDC) is approximately isotropic on the entire Fermi surface and energy-independent near the Fermi level (EF). The increase in the MDC width is consistent with the increase in the residual resistivity due to the Zn impurities if we assume the carrier number to be 1-x for x=0.17 and the Zn impurity to be a potential scatterer close to the unitarity limit. For x=0.03, the residual resistivity is found to be higher than that expected from the MDC width, and the effects of antifferomagnetic fluctuations induced around the Zn impurities are discussed. The leading edges of the spectra near (pi,0) for x=0.17 are shifted toward higher energies relative to EF with Zn substitution, indicating a reduction of the superconducting gap.Comment: 7 pages, 7 figure

Doping Evolution of the Underlying Fermi Surface in La2-xSrxCuO4

We have performed a systematic doping dependent study of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) (0.03$\leq x \leq$0.3) by angle-resolved photoemission spectroscopy. In the entire doping range, the underlying ``Fermi surface" determined from the low energy spectral weight approximately satisfies Luttinger's theorem, even down to the lightly-doped region. This is in strong contrast to the result on Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ (Na-CCOC), which shows a strong deviation from Luttinger's theorem. The differences between LSCO and Na-CCOC are correlated with the different behaviors of the chemical potential shift and spectral weight transfer induced by hole doping.Comment: 4 pages, 4 figure

Spatial Periodicity of Galaxy Number Counts, CMB Anisotropy, and SNIa Hubble Diagram Based on the Universe Accompanied by a Non-Minimally Coupled Scalar Field

We have succeeded in establishing a cosmological model with a non-minimally coupled scalar field $\phi$ that can account not only for the spatial periodicity or the {\it picket-fence structure} exhibited by the galaxy $N$-$z$ relation of the 2dF survey but also for the spatial power spectrum of the cosmic microwave background radiation (CMB) temperature anisotropy observed by the WMAP satellite. The Hubble diagram of our model also compares well with the observation of Type Ia supernovae. The scalar field of our model universe starts from an extremely small value at around the nucleosynthesis epoch, remains in that state for sufficiently long periods, allowing sufficient time for the CMB temperature anisotropy to form, and then starts to grow in magnitude at the redshift $z$ of $\sim 1$, followed by a damping oscillation which is required to reproduce the observed picket-fence structure of the $N$-$z$ relation. To realize such behavior of the scalar field, we have found it necessary to introduce a new form of potential $V(\phi)\propto \phi^2\exp(-q\phi^2)$, with $q$ being a constant. Through this parameter $q$, we can control the epoch at which the scalar field starts growing.Comment: 19 pages, 18 figures, Accepted for publication in Astrophysics & Space Scienc

Multiple Bosonic Mode Coupling in Electron Self-Energy of (La_2-xSr_x)CuO_4

High resolution angle-resolved photoemission spectroscopy data along the (0,0)-($\pi$,$\pi$) nodal direction with significantly improved statistics reveal fine structure in the electron self-energy of the underdoped (La$_{2-x}$Sr$_x$)CuO$_4$ samples in the normal state. Fine structure at energies of (40$\sim$46) meV and (58$\sim$63)meV, and possible fine structure at energies of (23$\sim$29)meV and (75$\sim$85)meV, have been identified. These observations indicate that, in LSCO, more than one bosonic modes are involved in the coupling with electrons.Comment: 4 pages, 3 figures, Fig. 2 update

Doping dependence of the (π,π)(\pi,\pi) shadow band in La-based cuprates studied by angle-resolved photoemission spectroscopy

The $(\pi,\pi)$ shadow band (SB) in La-based cuprate family (La214) was studied by angle-resolved photoemission spectroscopy (ARPES) over a wide doping range from $x=0.01$ to $x=0.25$. Unlike the well-studied case of the Bi-based cuprate family, an overall strong, monotonic doping dependence of the SB intensity at the Fermi level ($E_F$) was observed. In contrast to a previous report for the presence of the SB only close to $x=1/8$, we found it exists in a wide doping range, associated with a doping-independent $(\pi,\pi)$ wave vector but strongly doping-dependent intensity: It is the strongest at $x\sim 0.03$ and systematically diminishes as the doping increases until it becomes negligible in the overdoped regime. This SB with the observed doping dependence of intensity can in principle be caused by the antiferromagnetic fluctuations or a particular form of low-temperature orthorhombic lattice distortion known to persist up to $x\sim 0.21$ in the system, with both being weakened with increasing doping. However, a detailed binding energy dependent analysis of the SB at $x=0.07$ does not appear to support the former interpretation, leaving the latter as a more plausible candidate, despite a challenge in quantitatively linking the doping dependences of the SB intensity and the magnitude of the lattice distortion. Our finding highlights the necessity of a careful and global consideration of the inherent structural complications for correctly understanding the cuprate Fermiology and its microscopic implication.Comment: Note the revised conclusion and author list; To appear in New J. Phy

Low-Energy Electronic Structure of the High-Tc Cuprates La2-xSrxCuO4 Studied by Angle-resolved Photoemission Spectroscopy

We have performed a systematic angle-resolved photoemission spectroscopy (ARPES) study of the high-Tc cuprates La2-xSrxCuO4, ranging from the underdoped insulator to the superconductor to the overdoped metal. We have revealed a systematic doping evolution of the band dispersions and (underlying) Fermi surfaces, pseudogap and quasi-particle features under the influence of strong electron-electron interaction and electron-phonon interaction. The unusual transport and thermodynamic properties are explained by taking into account the pseudogap opening and the Fermi arc formation, due to which the carrier number decreases as the doped hole concentration decreases.Comment: 27 pages, 17 figures, accepted in Journal of Physics Condensed Matte