X-ray and optical observations of three clusters of galaxies: Abell 901, Abell 1437, and Abell 3570

We analyse three clusters of galaxies, Abell 901 (z=0.17), Abell 1437 (z=0.13) and Abell 3570 (z=0.037). They have low to intermediate X-ray fluxes and an irregular morphology in the ROSAT All-Sky Survey (RASS). These clusters are chosen to test the abilities and limitations of the RASS in terms of cluster fluxes and cluster morphologies. Therefore some ``worst'' cases are used here. X-ray observations with the ROSAT/HRI and optical spectroscopic observations are carried out. The ROSAT/HRI observations, which have a much better spatial resolution than the RASS, reveal in two of the three cases a significantly different morphology than seen in the RASS. The reasons are point sources which could not be resolved in the RASS and were therefore confused with the cluster emission. For A3570 we could confirm the relaxed state of the cluster by the optical determination of a small velocity dispersion. In the cluster with the lowest flux (Abell 901) the countrate measurement is strongly affected by point sources, in the two other cases the countrate measurements of the RASS are reliable, i.e. they are reproduced by the ROSAT/HRI measurement. We conclude that for clusters with a flux of a few times 10^{-12} erg/cm^2/s or smaller, which show at the same time a non-relaxed morphology, the flux measurement of the RASS can be seriously affected by fore- or background sources. We point out that an all-sky survey of a second ABRIXAS mission would provide a much clearer source distinction for low-flux clusters and thus a much improved countrate determination.Comment: 8 pages (incl. 6 figures), accepted for publication in A&A (Suppl.

On a filtered multiplicative basis of group algebras

Let $K$ be a field of characteristic $p$ and $G$ a nonabelian metacyclic finite $p$-group. We give an explicit list of all metacyclic $p$-groups $G$, such that the group algebra $KG$ over a field of characteristic $p$ has a filtered multiplicative $K$-basis. We also present an example of a non-metacyclic 2-group $G$, such that the group algebra $KG$ over any field of characteristic 2 has a filtered multiplicative $K$-basis.Comment: 10 pages, AMS-Te

Critical exponents at the superconductor-insulator transition in dirty-boson systems

I obtain the inverse of the correlation length exponent at the superfluid-Bose glass quantum critical point as a series in small parameter $\sqrt{d-1}$, with d being the dimensionality of the system, and compute the first two terms using a novel field-theoretic technique. For d=2 I find $\nu_s = 0.81$ and $\nu_c = 1.03$, for short-range and Coulomb interactions between bosons, respectively. When combined with the exact values of the dynamical critical exponents $z_s = d$ and $z_c = 1$, these results are in quantitative agreement with the experiments on onset of superfluidity in $^4 He$ in porous glasses, and on superconductor-insulator transition in homogeneous metallic films, in support of the dirty-boson theory fot the latter. Higher-order calculation of the exponents and of the universal conductivity is discussed.Comment: 4 pages, RevTex, eq. 10 correcte

Multicomponent mixture of charged hard-sphere chain molecules in the polymer mean-spherical approximation

The analytical solution of the recently proposed ideal chain polymer mean-spherical approximation (Yu.Kalyuzhnyi, Mol.Phys., 94, 735(1998)) is presented for the multicomponent mixture of charged hard-sphere linear chain flexible molecules. The solution apply to any mixture of chain molecules with arbitrary distribution of the charge and size of the beads along the molecular backbone. Closed form analytical expressions for the internal energy, Helmholtz free energy, chemical potentials and pressure are derived. By way of illustration thermodynamical properties of several versions of the fluid of charged chain molecules of different length, including the molecules with uniform, diblock and alternating distribution of the charge are studied. Theoretical predictions are in reasonable agreement with available computer simulation predictions. We present also the liquid-gas phase diagrams for the systems with diblock and alternating distribution of the charge.Comment: Submitted to J. Chem. Phys. December 15, 200

On the Twisted N=2N=2 Superconformal Structure in 2d2d Gravity Coupled to Matter

It is shown that the two dimensional gravity, described either in the conformal gauge (the Liouville theory) or in the light cone gauge, when coupled to matter possesses an infinite number of twisted $N=2$ superconformal symmetries. The central charges of the $N=2$ algebra for the two gauge choices are in general different. Further, it is argued that the physical states in the light cone gauge theory can be obtained from the Liouville theory by a field redefinition.Comment: Plain Tex, 13 pages, IC/93/81, UG-3/9

Kinetics of spin coherence of electrons in an undoped semiconductor quantum well

We study the kinetics of spin coherence of optically excited electrons in an undoped insulating ZnSe/Zn$_{1-x}$Cd$_x$Se quantum well under moderate magnetic fields in the Voigt configuration. After clarifying the optical coherence and the spin coherence, we build the kinetic Bloch equations and calculate dephasing and relaxation kinetics of laser pulse excited plasma due to statically screened Coulomb scattering and electron hole spin exchange. We find that the Coulomb scattering can not cause the spin dephasing, and that the electron-hole spin exchange is the main mechanism of the spin decoherence. Moreover the beat frequency in the Faraday rotation angle is determined mainly by the Zeeman splitting, red shifted by the Coulomb scattering and the electron hole spin exchange. Our numerical results are in agreement with experiment findings. A possible scenario for the contribution of electron-hole spin exchange to the spin dephasing of the $n$-doped material is also proposed.Comment: 12 pages, RevTex, 11 figures, scheduled to publish in PRB Jan. 15, 200