13 research outputs found
Radiation hydrodynamics simulations of wide-angle outflows from super-critical accretion disks around black holes
By performing two-dimensional radiation hydrodynamics simulations with large
computational domain of 5000 Schwarzschild radius, we revealed that wide-angle
outflow is launched via the radiation force from the super-critical accretion
flows around black holes. The angular size of the outflow, of which the radial
velocity (v_r) is over the escape velocity (v_esc), increases with an increase
of the distance from the black hole. As a result, the mass is blown away with
speed of v_r > v_esc in all direction except for the very vicinity of the
equatorial plane, theta=0-85^circ, where theta is the polar angle. The mass
ejected from the outer boundary per unit time by the outflow is larger than the
mass accretion rate onto the black hole, ~150L_Edd/c^2, where L_Edd and c are
the Eddington luminosity and the speed of light. Kinetic power of such
wide-angle high-velocity outflow is comparable to the photon luminosity and is
a few times larger than the Eddington luminosity. This corresponds to
~10^39-10^40 erg/s for the stellar mass black holes. Our model consistent with
the observations of shock excited bubbles observed in some ultra-luminous X-ray
sources (ULXs), supporting a hypothesis that ULXs are powered by the
super-critical accretion onto stellar mass black holes.Comment: 9 pages, 8 figures, accepted for publication in PAS
Atomically Resolved Surface Structure of SrTiO3(001) Thin Films Grown in Step-Flow Mode by Pulsed Laser Deposition
The surface structure of SrTiO3(001) thin films homoepitaxially grown by PLD
in step-flow mode was characterized using low temperature STM. It was found
that one-dimensional (1D) TiOx-based nanostructures were formed on the thin
film surface and their density increased with increasing thin film thickness.
Most of the 1D nanostructures disappeared after a post-deposition annealing,
indicating that this structure is metastable due to the nonequilibrium growth
mode. The resulting surface after annealing exhibited similar features to that
of a thinner film, having a domain structure with (2x1) and (1x2)
reconstructions, but with fewer oxygen-vacancy-type defects. These results
imply that the step-flow growth is likely to produce TiOx-rich surface and Ti
deficiencies in the film. By the post-deposition annealing, the rich TiOx would
diffuse from the surface into the film to compensate defects associated with Ti
vacancies and oxygen vacancies, resulting in the stable surface structure with
fewer oxygen vacancies. Thus, STM measurements can provide us with a
microscopic picture of surface stoichiometry of thin films originating in the
dynamics of the growth process, and can present a new approach for designing
functional oxide films.Comment: 12 pages, 4 figure
Stripe charge ordering in SrO-terminated SrTiO3(001) surfaces
The local electronic structure of the SrO-terminated SrTiO3(001) surface was
explored using scanning tunneling microscopy. At low bias voltages in the empty
states, a unidirectional structure with a periodicity of 3 unit cells,
superimposed on a c(2 x 2) reconstructed structure, was found to develop along
the crystallographic a axis. This structure indicates a charge-ordered stripe
induced by carrier doping from oxygen vacancies in the SrO and the subsurface
TiO2 planes. In the filled states, localized deep in-gap states were observed
in addition to large energy gaps in the tunneling spectra. This result
represents inelastic tunneling due to significant electron-lattice interaction
associated with unidirectional lattice distortion in the SrO-terminated
surface.Comment: 6 pages, 5 figures, accepted for publication in PR
Atomic-scale visualization of initial growth of homoepitaxial SrTiO3 thin film on an atomically ordered substrate
The initial homoepitaxial growth of SrTiO3 on a (\surd13\times\surd13) -
R33.7{\deg}SrTiO3(001) substrate surface, which can be prepared under oxide
growth conditions, is atomically resolved by scanning tunneling microscopy. The
identical (\surd13\times\surd13) atomic structure is clearly visualized on the
deposited SrTiO3 film surface as well as on the substrate. This result
indicates the transfer of the topmost Ti-rich (\surd13\times\surd13) structure
to the film surface and atomic-scale coherent epitaxy at the film/substrate
interface. Such atomically ordered SrTiO3 substrates can be applied to the
fabrication of atom-by-atom controlled oxide epitaxial films and
heterostructures
Near-midgap deep levels in Al0.26Ga0.74N grown by metal-organic chemical vapor deposition
A deep level with an activation energy of 1.0 eV in n-type Al0.26Ga0.74N grown by metal-organic chemical vapor deposition was detected by deep-level transient spectroscopy (DLTS) with a sampling time window of several seconds. The deep-level density was 6×10^15 cm^[-3]. At the temperatures around which the DLTS peaks were observed, capacitance transient was measured. Under the dark condition, a capacitance increase was observed, corresponding to the thermal emission of electrons from the level with 1.0 eV activation energy. After that, we observed a large capacitance increase under illumination with 2.3 eV photon energy. On the basis of potential simulation taking account of deep levels, we found that the photoinduced capacitance change arose from electron emission from additional near-midgap levels in energy ranging from EC-1.5 to EC-2.3 eV