Possible Implications of Asymmetric Fermionic Dark Matter for Neutron Stars

We consider the implications of fermionic asymmetric dark matter for a "mixed neutron star" composed of ordinary baryons and dark fermions. We find examples, where for a certain range of dark fermion mass -- when it is less than that of ordinary baryons -- such systems can reach higher masses than the maximal values allowed for ordinary ("pure") neutron stars. This is shown both within a simplified, heuristic Newtonian analytic framework with non-interacting particles and via a general relativistic numerical calculation, under certain assumptions for the dark matter equation of state. Our work applies to various dark fermion models such as mirror matter models and to other models where the dark fermions have self interactions.Comment: 20 pages, 6 figure

Interplay of disorder and geometrical frustration in doped Gadolinium Gallium Garnet

The geometrically-frustrated, triangular antiferromagnet GGG exhibits a rich mix of short-range order and isolated quantum states. We investigate the effects of up to 1% Neodymium substitution for Gallium on the ac magnetic response at temperatures below 1 K in both the linear and nonlinear regimes. Substitutional disorder actually drives the system towards a more perfectly frustrated state, apparently compensating for the effect of imperfect Gadolinium/Gallium stoichiometry, while at the same time more closely demarcating the boundaries of isolated, coherent clusters composed of hundreds of spins. Optical measurements of the local Nd environment substantiate the picture of an increased frustration index with doping.Comment: 5 pages, 5 figure

Quantum and Classical Glass Transitions in LiHoxY1−xF4Li Ho_x Y_{1-x} F_4

When performed in the proper low field, low frequency limits, measurements of the dynamics and the nonlinear susceptibility in the model Ising magnet in transverse field, $\text{LiHo}_x\text{Y}_{1-x}\text{F}_4$, prove the existence of a spin glass transition for $x$ = 0.167 and 0.198. The classical behavior tracks for the two concentrations, but the behavior in the quantum regime at large transverse fields differs because of the competing effects of quantum entanglement and random fields.Comment: 5 pages, 5 figures. Updated figure 3 with corrected calibration information for thermometr

Conductivity of Metallic Si:B near the Metal-Insulator Transition: Comparison between Unstressed and Uniaxially Stressed Samples

The low-temperature dc conductivities of barely metallic samples of p-type Si:B are compared for a series of samples with different dopant concentrations, n, in the absence of stress (cubic symmetry), and for a single sample driven from the metallic into the insulating phase by uniaxial compression, S. For all values of temperature and stress, the conductivity of the stressed sample collapses onto a single universal scaling curve. The scaling fit indicates that the conductivity of si:B is proportional to the square-root of T in the critical range. Our data yield a critical conductivity exponent of 1.6, considerably larger than the value reported in earlier experiments where the transition was crossed by varying the dopant concentration. The larger exponent is based on data in a narrow range of stress near the critical value within which scaling holds. We show explicitly that the temperature dependences of the conductivity of stressed and unstressed Si:B are different, suggesting that a direct comparison of the critical behavior and critical exponents for stress- tuned and concentration-tuned transitions may not be warranted

Malin 1: interacting galaxy pair?

Malin 1 is a unique, extraordinarily large low surface brightness galaxy. The structure and the origins of the galaxy are poorly understood. The reason for such a situation is an absence of detailed observational data, especially, of high-resolution kinematics. In this Letter we study the stellar kinematics of the inner part (r < 15 kpc) of Malin 1. We present spectroscopic arguments in favour of a small galaxy - Malin 1B - being a companion probably interacting with the main galaxy - Malin 1. This object is clearly seen in many published images of Malin 1 but is not mentioned in any astronomical databases. Malin 1B is located at the projected distance of 14 kpc from the Malin 1's nucleus and has small - 65$\pm$16 km/s - relative velocity, which we determined for the first time. We suggest that ongoing interaction with Malin 1B can explain main morphological features of the Malin 1's central region - two-armed spiral structure, a bar, and an external one-armed spiral pattern. We also investigated the large scale environment of Malin 1 and postulate that the galaxy SDSS J123708.91+142253.2 might be responsible for the formation of extended low-surface brightness envelope by means of head-on collision with Malin 1 (in the framework of collision scenario proposed by Mapelli et al. 2008). To test the collisional origins of Malin 1 global structure, more observational data and new numerical models are needed.Comment: 5 pages, 4 figures, accepted for publication in MNRA

Quantum spin glass in anisotropic dipolar systems

The spin-glass phase in the \LHx compound is considered. At zero transverse field this system is well described by the classical Ising model. At finite transverse field deviations from the transverse field quantum Ising model are significant, and one must take properly into account the hyperfine interactions, the off-diagonal terms in the dipolar interactions, and details of the full J=8 spin Hamiltonian to obtain the correct physical picture. In particular, the system is not a spin glass at finite transverse fields and does not show quantum criticality.Comment: 6 pages, 2 figures, to appear in J. Phys. Condens. Matter (proceedings of the HFM2006 conference

Insm1 promotes the transition of olfactory progenitors from apical and proliferative to basal, terminally dividing and neuronogenic

<p>Abstract</p> <p>Background</p> <p>Insm1 is a zinc-finger transcription factor transiently expressed throughout the developing nervous system in late progenitors and nascent neurons. Insm1 is also highly expressed in medulloblastomas and other neuroendocrine tumors.</p> <p>Results</p> <p>We generated mice lacking the <it>Insm1 </it>gene and used them to elucidate its role in neurogenic proliferation of the embryonic olfactory epithelium. We found that deletion of <it>Insm1 </it>results in more apical cells and fewer nascent and mature neurons. In the embryonic olfactory epithelium of <it>Insm1 </it>mutants we detect fewer basal progenitors, which produce neurons, and more apical progenitors, which at this stage produce additional progenitors. Furthermore, in the mutants we detect fewer progenitors expressing NEUROD1, a marker of terminally dividing, neuronogenic (neuron-producing) progenitors (immediate neuronal precursors), and more progenitors expressing ASCL1, a marker of the transit amplifying progenitors that migrate from the apical to the basal edges of the epithelium while dividing to generate the terminal, neuronogenic progenitors. Finally, with timed administration of nucleoside analogs we demonstrate that the <it>Insm1 </it>mutants contain fewer terminally dividing progenitors at embryonic day 12.5.</p> <p>Conclusions</p> <p>Altogether, these results suggest a role for <it>Insm1 </it>in promoting the transition of progenitors from apical and proliferative to basal, terminal and neuronogenic. This role appears partially conserved with that of its nematode ortholog, <it>egl-46</it>. The similar effects of <it>Insm1 </it>deletion on progenitors of embryonic olfactory epithelium and cortex point to striking parallels in the development of these neuroepithelia, and particularly between the basal progenitors of olfactory epithelium and the subventricular zone progenitors of cortex.</p

Variation of the Metallic Onset with Magnetic Field in Doped Germanium

We demonstrate substantial magnetic-field tuning of the metal-insulator transition in Ge:Sb for H < 20 kOe. From a fit to the conductivity in a series of samples at millikelvin temperatures in different fields to the critical form σ ~ (n/n_c(H) - 1)^μ, we find that μ ≈ 1 independent of H, but that n_c(H) increases more than 25% by H ~ 20 kOe. Moreover, n_c(H) - n_c(0) α H^(1/2), in agreement with a weak-disorder perturbative scaling approach, but in contrast to recent experimental results in Si:As