Field-induced magnetic behavior in quasi-one-dimensional Ising-like antiferromagnet BaCo2V2O8: A single-crystal neutron diffraction study

BaCo2V2O8 is a nice example of a quasi-one-dimensional quantum spin system that can be described in terms of Tomonaga-Luttinger liquid physics. This is explored in the present study where the magnetic field-temperature phase diagram is thoroughly established up to 12 T using single-crystal neutron diffraction. The transition from the N\'eel phase to the incommensurate longitudinal spin density wave (LSDW) phase through a first-order transition, as well as the critical exponents associated with the paramagnetic to ordered phase transitions, and the magnetic order both in the N\'eel and in the LSDW phase are determined, thus providing a stringent test for the theory.Comment: 17 pages with 15 figure

Resolving Gas Dynamics in the Circumnuclear Region of a Disk Galaxy in a Cosmological Simulation

Using a hydrodynamic adaptive mesh refinement code, we simulate the growth and evolution of a galaxy, which could potentially host a supermassive black hole, within a cosmological volume. Reaching a dynamical range in excess of 10 million, the simulation follows the evolution of the gas structure from super-galactic scales all the way down to the outer edge of the accretion disk. Here, we focus on global instabilities in the self-gravitating, cold, turbulence-supported, molecular gas disk at the center of the model galaxy, which provide a natural mechanism for angular momentum transport down to sub-pc scales. The gas density profile follows a power-law scaling as r^-8/3, consistent with an analytic description of turbulence in a quasi-stationary circumnuclear disk. We analyze the properties of the disk which contribute to the instabilities, and investigate the significance of instability for the galaxy's evolution and the growth of a supermassive black hole at the center.Comment: 16 pages (includes appendix), submitted to ApJ. Figures here are at low resolution; for higher resolution version, download http://casa.colorado.edu/~levinerd/ms.pd

Image Sampling with Quasicrystals

We investigate the use of quasicrystals in image sampling. Quasicrystals produce space-filling, non-periodic point sets that are uniformly discrete and relatively dense, thereby ensuring the sample sites are evenly spread out throughout the sampled image. Their self-similar structure can be attractive for creating sampling patterns endowed with a decorative symmetry. We present a brief general overview of the algebraic theory of cut-and-project quasicrystals based on the geometry of the golden ratio. To assess the practical utility of quasicrystal sampling, we evaluate the visual effects of a variety of non-adaptive image sampling strategies on photorealistic image reconstruction and non-photorealistic image rendering used in multiresolution image representations. For computer visualization of point sets used in image sampling, we introduce a mosaic rendering technique.Comment: For a full resolution version of this paper, along with supplementary materials, please visit at http://www.Eyemaginary.com/Portfolio/Publications.htm

AGN Jet-induced Feedback in Galaxies. I. Suppression of Star Formation

(Abridged) We study the interaction of relativistic jets from AGNs with the ISM in their host galaxy, using a series of Adaptive Mesh Refinement simulations of the evolution of the interaction between the cocoon produced by the jet with a dense cloud, placed very near the cocoon's path. We vary only the jet's input power between P_{jet} = 10^{41}-10^{47} {\rm erg/sec}. The density Probability Distribution Function (PDF) within the cocoon can be described in terms of two distinct components, which are also spatially distinct: a low- and a high-density component. The PDF of the post-shocked region is well approximated by a modified lognormal distribution, for all values of $P_{jet}$. During the active phase, when the jet is fed by the AGN, the cloud is subject both to compression and stripping, which tend to increase its density and diminish its total mass. When the jet is switched off (i.e. during the passive phase) the shocked cloud cools further and tends to become more filamentary, under the action of a back-flow which develops within the cocoon. We study the evolution of the star formation rate within the cloud, assuming that is determined by a Schmidt-Kennicutt law, and we analyze the different physical factors which have an impact on the star formation rate. We show that, although the star formation rate can occasionally increase, on time scales of the order of $10^{5}-10^{6}$ yrs, star formation will be inhibited and the cloud fragments. The cooling time of the environment within which the cloud is embedded is however very long: thus, star formation from the fragmented cloud remains strongly inhibited.Comment: 15 pages, 13 figures, accepted by MNRAS. Version with high-resolution color figures available at http://web.ct.astro.it/cosmoct/web_group/papers/0806.4570.pd

Ferromagnetic Ordering in CeIr2B2: Transport, magnetization, specific heat and NMR studies

We present a complete characterization of ferromagnetic system CeIr2B2 using powder x-ray diffraction XRD, magnetic susceptibility chi(T), isothermal magnetization M(H), specific heat C(T), electrical resistivity rho(T,H), and thermoelectric power S(T) measurements. Furthermore 11B NMR study was performed to probe the magnetism on a microscopic scale. The chi(T), C(T) and rho(T) data confirm bulk ferromagnetic ordering with Tc = 5.1 K. Ce ions in CeIr2B2 are in stable trivalent state. Our low-temperature C(T) data measured down to 0.4 K yield Sommerfeld coefficient gamma = 73(4) mJ/molK2 which is much smaller than the previously reported value of gamma = 180 mJ/molK2 deduced from the specific heat measurement down to 2.5 K. For LaIr2B2 gamma = 6(1) mJ/molK2 which implies the density of states at the Fermi level D(EF) = 2.54 states/(eV f.u.) for both spin directions. The renormalization factor for quasi-particle density of states and hence for quasi-particle mass due to 4f correlations in CeIr2B2 is 12. The Kondo temperature TK ~ 4 K is estimated from the jump in specific heat of CeIr2B2 at Tc. Both C(T) and rho(T) data exhibit gapped-magnon behavior in magnetically ordered state with an energy gap Eg ~ 3.5 K. The rho data as a function of magnetic field H indicate a large negative magnetoresistance (MR) which is highest for T = 5 K.While at 5 K the negative MR keeps on increasing up to 10 T, at 2 K an upturn is observed near H = 3.5 T. On the other hand, the thermoelectric power data have small absolute values (S ~ 7 {\mu}V/K) indicating a weak Kondo interaction. A shoulder in S(T) at about 30 K followed by a minimum at ~ 10 K is attributed to crystal electric field (CEF) effects and the onset of magnetic ordering. 11B NMR line broadening provides strong evidence of ferromagnetic correlations below 40 K.Comment: 10 pages, 11 figure