Hydrogen-assisted stabilization of Ni nanowires in solution

We have studied conductance characteristics of mechanically fabricated Ni nanoconstrictions under controlling electrochemical potential and pH of the electrolyte. Conductance histogram showed clear feature peaked at 1-1.5 $G_{0}$ (=$2e^{2}/h$) when the potential of the constriction was kept at more negative potential than -900 mV vs. Ag/AgCl in pH=3.7. Comparable feature also appeared at more positive potential when lower pH solution was used. We have revealed that Ni mono atomic contact or mono atomic wire can be stabilized in solution at room temperature under the hydrogen evolution.Comment: 4 pages, 3 figures; to appear in Appl. Phys. Let

Measurement of the Kerr Spin Parameter by Observation of a Compact Object's Shadow

A black hole casts a shadow as an optical appearance because of its strong gravitational field. We study how to determine the spin parameter and the inclination angle by observing the apparent shape of the shadow, which is distorted mainly by those two parameters. Defining some observables characterizing the apparent shape (its radius and distortion parameter), we find that the spin parameter and inclination angle of a Kerr black hole can be determined by the observation. This technique is also extended to the case of a Kerr naked singularity.Comment: 9 pages, 11 figures; v2: references added, typos corrected; v3: accepted for publication in Physical Review

Heterotic orbifold models on Lie lattice with discrete torsion

We provide a new class of Z_N x Z_M heterotic orbifolds on non-factorisable tori, whose boundary conditions are defined by Lie lattices. Generally, point groups of these orbifolds are generated by Weyl reflections and outer automorphisms of the lattices. We classify abelian orbifolds with and without discrete torsion. Then we find that some of these models have smaller Euler numbers than those of models on factorisable tori T^2 x T^2 x T^2. There is a possibility that these orbifolds provide smaller generation numbers of N=1 chiral matter fields than factorisable models.Comment: 24 pages, 5 figures; v2: a few errors on tables are corrected, typos corrected, version to appear in JHE

The Core-Collapse Supernova with "Non-Uniform" Magnetic Fields

We perform two-dimensional numerical simulations on the core-collapse of a massive star with strong magnetic fields and differential rotations using a numerical code ZEUS-2D. Changing field configurations and laws of differential rotation parametrically, we compute 14 models and investigate effects of these parameters on the dynamics. In our models, we do not solve the neutrino transport and instead employ a phenomenological parametric EOS that takes into account the neutrino emissions. As a result of the calculations, we find that the field configuration plays a significant role in the dynamics of the core if the initial magnetic field is large enough. Models with initially concentrated fields produce more energetic explosions and more prolate shock waves than the uniform field. Quadrapole-like fields produce remarkably collimated and fast jet, which might be important for gamma-ray bursts(GRB). The Lorentz forces exerted in the region where the plasma-beta is less than unity are responsible for these dynamics. The pure toroidal field, on the other hand, does not lead to any explosion or matter ejection. This suggests the presupernova models of Heger et al.(2003), in which toroidal fields are predominant, is disadvantageous for the magnetorotation-induced supernova considered here. Models with initially weak magnetic fields do not lead to explosion or matter ejection, either. In these models magnetic fields play no role as they do not grow on the timescale considered in this paper so that the magnetic pressure could be comparable to the matter pressure. This is because the exponential field growth as expected in MRI is not seen in our models. The magnetic field is amplified mainly by field-compression and field-wrapping in our simulations.Comment: 24 pages, 5 figures, ApJ in press, typos correcte

Creation of a brane world with Gauss-Bonnet term

We study a creation of a brane world using an instanton solution. We analyze a brane model with a Gauss-Bonnet term in a bulk spacetime. The curvature of 3-brane is assumed to be closed, flat, or open. We construct instanton solutions with branes for those models, and calculate the value of the actions to discuss an initial state of a brane universe.Comment: 9 pages, 10 figure

Numerical Simulations of Equatorially-Asymmetric Magnetized Supernovae: Formation of Magnetars and Their Kicks

A series of numerical simulations on magnetorotational core-collapse supernovae are carried out. Dipole-like configurations which are offset northward are assumed for the initially strong magnetic fields together with rapid differential rotations. Aims of our study are to investigate effects of the offset magnetic field on magnetar kicks and on supernova dynamics. Note that we study a regime where the proto-neutron star formed after collapse has a large magnetic field strength approaching that of a ``magnetar'', a highly magnetized slowly rotating neutron star. As a result, equatorially-asymmetric explosions occur with a formation of the bipolar jets. Resultant magnetar's kick velocities are $\sim 300-1000$ km s$^{-1}$. We find that the acceleration is mainly due to the magnetic pressure while the somewhat weaker magnetic tension works toward the opposite direction, which is due to stronger magnetic field in the northern hemisphere. Noted that observations of magnetar's proper motions are very scarce, our results supply a prediction for future observations. Namely, magnetars possibly have large kick velocities, several hundred km s$^{-1}$, as ordinary neutron stars do, and in an extreme case they could have those up to 1000 km s$^{-1}$.Comment: 36 pages, 9 figures, accepted by the Astrophysical Journa

Magnetohydrodynamic Simulations of A Rotating Massive Star Collapsing to A Black Hole

We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of the collapse of a rotating star of 40 Msun and in the light of the collapsar model of gamma-ray burst. Considering two distributions of angular momentum, up to \sim 10^{17} cm^2/s, and the uniform vertical magnetic field, we investigate the formation of an accretion disk around a black hole and the jet production near the hole. After material reaches to the black hole with the high angular momentum, the disk is formed inside a surface of weak shock. The disk becomes in a quasi-steady state for stars whose magnetic field is less than 10^{10} G before the collapse. We find that the jet can be driven by the magnetic fields even if the central core does not rotate as rapidly as previously assumed and outer layers of the star has sufficiently high angular momentum. The magnetic fields are chiefly amplified inside the disk due to the compression and the wrapping of the field. The fields inside the disk propagate to the polar region along the inner boundary near the black hole through the Alfv{\'e}n wave, and eventually drive the jet. The quasi-steady disk is not an advection-dominated disk but a neutrino cooling-dominated one. Mass accretion rates in the disks are greater than 0.01 Msun/sec with large fluctuations. The disk is transparent for neutrinos. The dense part of the disk, which locates near the hole, emits neutrino efficiently at a constant rate of < 8 \times 10^{51} erg/s. The neutrino luminosity is much smaller than those from supernovae after the neutrino burst.Comment: 42 pages, accepted for publication in the Astrophysical Journal. A paper with higher-resolution figures available at http://www.ec.knct.ac.jp/~fujimoto/collapsar/mhd-color.pd