Atomically straight steps on vicinal Si (111) surfaces prepared by step-parallel current in the kink-up direction

We demonstrate that annealing of a vicinal Si(111) surface at about 800 C with a direct current in the direction that ascends the kinks enhances the formation of atomically straight step edges over micrometer lengths, while annealing with a current in the opposite direction does not. Every straight step edge has the same atomic configuration U(2,0), which is useful as a template for the formation of a variety of nanostructures. A phenomenological model based on electromigration of charged mobile atoms explains the observed current-polarity dependent behavior.Comment: Accepted for publication in Appl. Phys. Lett. Numbers of pages and figures are 12 and 4, respectivel

Gluon Propagators and Confinement

We present SU(3) gluon propagators calculated on 48*48*48*N_t lattices at beta=6.8 where N_t=64 (corresponding the confinement phase) and N_t=16 (deconfinement) with the bare gauge parameter,alpha, set to be 0.1. In order to avoid Gribov copies, we employ the stochastic gauge fixing algorithm. Gluon propagators show quite different behavior from those of massless gauge fields: (1) In the confinement phase, G(t) shows massless behavior at small and large t, while around 5<t<15 it behaves as massive particle, and (2) effective mass observed in G(z) becomes larger as z increases. (3) In the deconfinement phase, G(z) shows also massive behavior but effective mass is less than in the confinement case. In all cases, slope masses are increasing functions of t or z, which can not be understood as addtional physical poles.Comment: 6 pages in Postscrip

On the preciseness of subtyping in session types

Subtyping in concurrency has been extensively studied since early 1990s as one of the most interesting issues in type theory. The correctness of subtyping relations has been usually provided as the soundness for type safety. The converse direction, the completeness, has been largely ignored in spite of its usefulness to define the greatest subtyping relation ensuring type safety. This paper formalises preciseness (i.e. both soundness and completeness) of subtyping for mobile processes and studies it for the synchronous and the asynchronous session calculi. We first prove that the well-known session subtyping, the branching-selection subtyping, is sound and complete for the synchronous calculus. Next we show that in the asynchronous calculus, this subtyping is incomplete for type-safety: that is, there exist session types T and S such that T can safely be considered as a subtype of S, but T ≤ S is not derivable by the subtyping. We then propose an asynchronous sub-typing system which is sound and complete for the asynchronous calculus. The method gives a general guidance to design rigorous channel-based subtypings respecting desired safety properties

On the Formation Age of the First Planetary System

Recently, it has been observed the extreme metal-poor stars in the Galactic halo, which must be formed just after Pop III objects. On the other hand, the first gas clouds of mass $\sim 10^6 M_{\odot}$ are supposed to be formed at $z \sim$ 10, 20, and 30 for the $1\sigma$, $2\sigma$ and $3\sigma$, where the density perturbations are assumed of the standard $\Lambda$CDM cosmology. If we could apply this gaussian distribution to the extreme small probability, the gas clouds would be formed at $z \sim$40, 60, and 80 for the $4\sigma$, $6\sigma$, and $8\sigma$. The first gas clouds within our galaxy must be formed around $z\sim 40$. Even if the gas cloud is metal poor, there is a lot of possibility to form the planets around such stars. The first planetary systems could be formed within $\sim 6\times 10^7$ years after the Big Bang in the universe. Even in our galaxies, it could be formed within $\sim 1.7\times 10^8$ years. It is interesting to wait the observations of planets around metal-poor stars. For the panspermia theory, the origin of life could be expected in such systems.Comment: 5 pages,Proceedings IAU Symposium No. 249, 2007, Exoplanets:Y-S. Sun, S. Ferraz-Mello and J.-L, Zhou, eds. (p325

Spin-1/2 Triangular Lattice with Orbital Degeneracy in a Metallic Oxide Ag2NiO2

A novel metallic and magnetic transition metal oxide Ag2NiO2 is studied by means of resistivity, magnetic susceptibility, specific heat and X-ray diffraction. The crystal structure is characterized by alternating stacking of a Ni3+O2 layer and a (Ag2)+ layer, the former realizing a spin-1/2 triangular lattice with eg orbital degeneracy and the latter providing itinerant electrons. It is found that the NiO2 layer exhibits orbital ordering at Ts = 260 K and antiferromagnetic spin ordering at TN = 56 K. Moreover, a moderately large mass enhancement is found for the itinerant electrons, suggesting a significant contribution from the nearly localized Ni 3d state to the Ag 5s state that forms a broad band.Comment: 9 pages, 5 figures, to be published in Rapid Communications, Phys. Rev.

Chemical potential jump between hole- and electron-doped sides of ambipolar high-Tc cuprate

In order to study an intrinsic chemical potential jump between the hole- and electron-doped high-Tc superconductors, we have performed core-level X-ray photoemission spectroscopy (XPS) measurements of Y0.38La0.62Ba1.74La0.26Cu3Oy (YLBLCO), into which one can dope both holes and electrons with maintaining the same crystal structure. Unlike the case between the hole-doped system La_2-xSrxCuO4 and the electron-doped system Nd_2-xCexCuO4, we have estimated the true chemical potential jump between the hole- and electron-doped YLBLCO to be ~0.8 eV, which is much smaller than the optical gaps of 1.4-1.7 eV reported for the parent insulating compounds. We attribute the reduced jump to the indirect nature of the charge-excitation gap as well as to the polaronic nature of the doped carriers.Comment: 4 pages, 3 figure

Fabrication of graphene nanoribbon by local anodic oxidation lithography using atomic force microscope

We conducted local anodic oxidation (LAO) lithography in single-layer, bilayer, and multilayer graphene using tapping-mode atomic force microscope. The width of insulating oxidized area depends systematically on the number of graphene layers. An 800-nm-wide bar-shaped device fabricated in single-layer graphene exhibits the half-integer quantum Hall effect. We also fabricated a 55-nm-wide graphene nanoribbon (GNR). The conductance of the GNR at the charge neutrality point was suppressed at low temperature, which suggests the opening of an energy gap due to lateral confinement of charge carriers. These results show that LAO lithography is an effective technique for the fabrication of graphene nanodevices.Comment: 4 pages, 4 figure

Photoemission Spectral Weight Transfer and Mass Renormalization in the Fermi-Liquid System La1−x_{1-x}Srx_xTiO3+y/2_{3+y/2}

We have performed a photoemission study of La$_{1-x}$Sr$_x$TiO$_{3+y/2}$ near the filling-control metal-insulator transition (MIT) as a function of hole doping. Mass renormalization deduced from the spectral weight and the width of the quasi-particle band around the chemical potential $\mu$ is compared with that deduced from the electronic specific heat. The result implies that, near the MIT, band narrowing occurs strongly in the vicinity of $\mu$. Spectral weight transfer occurs from the coherent to the incoherent parts upon antiferromagnetic ordering, which we associate with the partial gap opening at $\mu$.Comment: 4 pages, 3 figure