Comment on ``Stripes and the t-J Model''

This is a comment being submitted to Physical Review Letters on a recent letter by Hellberg and Manousakis on stripes in the t-J model.Comment: One reference correcte

Checkerboard patterns in the t-J model

Using the density matrix renormalization group, we study the possibility of real space checkerboard patterns arising as the ground states of the t-J model. We find that checkerboards with a commensurate (pi,pi) background are not low energy states and can only be stabilized with large external potentials. However, we find that striped states with charge density waves along the stripes can form approximate checkerboard patterns. These states can be stabilized with a very weak external field aligning and pinning the CDWs on different stripes.Comment: 4 pages, 5 figure

How do galactic winds affect the Lyalpha forest?

We investigate the effect of galactic winds on the Lyalpha forest in cosmological simulations of structure and galaxy formation. We combine high resolution N-body simulations of the evolution of the dark matter with a semi-analytic model for the formation and evolution of galaxies which includes detailed prescriptions for the long-term evolution of galactic winds. This model is the first to describe the evolution of outflows as a two-phase process (an adiabatic bubble followed by a momentum--driven shell) and to include metal--dependent cooling of the outflowing material. We find that the main statistical properties of the Lyalpha forest, namely the flux power spectrum P(k) and the flux probability distribution function (PDF), are not significantly affected by winds and so do not significantly constrain wind models. Winds around galaxies do, however, produce detectable signatures in the forest, in particular, increased flux transmissivity inside hot bubbles, and narrow, saturated absorption lines caused by dense cooled shells. We find that the Lyalpha flux transmissivity is highly enhanced near strongly wind-blowing galaxies, almost half of all high-redshift galaxies in our sample, in agreement with the results of Adelberger et al. (2005). Finally, we propose a new method to identify absorption lines potentially due to wind shells in the Lyalpha forest: we calculate the abundance of saturated regions in spectra as a function of region width and we find that the number with widths smaller than about 1 Angstrom at z=3 and 0.6 Angstrom at z=2 may be more than doubled. This should be detectable in real spectra.Comment: 14 pages, 11 figures. Minor changes in the text. Accepted for publication in MNRA

Competition Between Stripes and Pairing in a t-t'-J Model

As the number of legs n of an n-leg, t-J ladder increases, density matrix renormalization group calculations have shown that the doped state tends to be characterized by a static array of domain walls and that pairing correlations are suppressed. Here we present results for a t-t'-J model in which a diagonal, single particle, next-near-neighbor hopping t' is introduced. We find that this can suppress the formation of stripes and, for t' positive, enhance the d_{x^2-y^2}-like pairing correlations. The effect of t' > 0 is to cause the stripes to evaporate into pairs and for t' < 0 to evaporate into quasi-particles. Results for n=4 and 6-leg ladders are discussed.Comment: Four pages, four encapsulated figure

Effect of the W-term for a t-U-W Hubbard ladder

Antiferromagnetic and d_{x2-y2}-pairing correlations appear delicately balanced in the 2D Hubbard model. Whether doping can tip the balance to pairing is unclear and models with additional interaction terms have been studied. In one of these, the square of a local hopping kinetic energy H_W was found to favor pairing. However, such a term can be separated into a number of simpler processes and one would like to know which of these terms are responsible for enhancing the pairing. Here we analyze these processes for a 2-leg Hubbard ladder

Deconvolution of ASCA X-ray data: II. Radial temperature and metallicity profiles for 106 galaxy clusters

In Paper-I we presented a methodology to recover the spatial variations of properties of the intracluster gas from ASCA X-ray satellite observations of galaxy clusters. We verified the correctness of this procedure by applying it to simulated cluster datasets which we had subjected to the various contaminants common in ASCA data. In this paper we present the results which we obtain when we apply this method to real galaxy cluster observations. We determine broad-band temperature and cooling-flow mass-deposition rates for the 106 clusters in our sample, and obtain temperature, abundance and emissivity profiles (i.e. at least two annular bins) for 98 of these clusters. We find that 90 percent of these temperature profiles are consistent with isothermality at the 3-sigma confidence level. This conflicts with the prevalence of steeply-declining cluster temperature profiles found by Markevitch et al. (1998) from a sample of 30 clusters. In Paper-III (in preparation) we utilise our temperature and emissivity profiles to determine radial hydrostatic-mass properties for a subsample of the clusters presented in this paper.Comment: MNRAS, accpeted. Postscript copy of paper and individual postscript files for plots in Appendix B can be obtained from: http://www-xray.ast.cam.ac.uk/~da

ROSAT PSPC observations of the outer regions of the Perseus cluster of galaxies

We present an analysis of four off-axis ROSAT PSPC observations of the Perseus cluster of galaxies (Abell~426). We detect the surface brightness profile to a radius of 80 arcmin ($\sim 2.4 h_{50}^{-1}$ Mpc) from the X-ray peak. The profile is measured in various sectors and in three different energy bands. Firstly, a colour analysis highlights a slight variation of $N_{H}$ over the region, and cool components in the core and in the eastern sector. We apply the $\beta$-model to the profiles from different sectors and present a solution to the, so-called, $\beta$-problem. The residuals from an azimuthally-averaged profile highlight extended emission both in the East and in the West, with estimated luminosities of about 8 and 1 $\times 10^{43} erg/s$, respectively. We fit several models to the surface brightness profile, including the one obtained from the Navarro, Frenk and White (1995) potential. We obtain the best fit with the gas distribution described by a power law in the inner, cooling region and a $\beta$-model for the extended emission. Through the best-fit results and the constraints from the deprojection of the surface brightness profiles, we define the radius where the overdensity inside the cluster is 200 times the critical value, $r_{200}$, at $2.7 h_{50}^{-1}$ Mpc. Within $2.3 h_{50}^{-1}$ Mpc ($0.85 r_{200}$), the total mass in the Perseus cluster is $1.2 \times 10^{15} M_{\odot}$ and its gas fraction is about 30 per cent.Comment: 21 pages, 23 figures; accepted for publication in MNRAS; also available at http://www-xray.ast.cam.ac.uk/~settori/paper.htm