8,188 research outputs found
Unusual interplay between copper-spin and vortex dynamics in slightly overdoped La{1.83}Sr{0.17}CuO{4}
Our inelastic neutron scattering experiments of the spin excitations in the
slightly overdoped La{1.83}Sr{0.17}CuO{4} compound show that, under the
application of a magnetic field of 5 Tesla, the low-temperature susceptibility
undergoes a weight redistribution centered at the spin-gap energy. Furthermore,
by comparing the temperature dependence of the neutron data with
ac-susceptibility and magnetization measurements, we conclude that the filling
in of the spin gap tracks the irreversibility/melting temperature rather than
Tc2, which indicates an unusual interplay between the magnetic vortices and the
spin excitations even in the slightly overdoped regime of high-temperature
superconductors.Comment: 7 pages, including 5 figure
Thermal Equilibria of Optically Thin, Magnetically Supported, Two-Temperature, Black Hole Accretion Disks
We obtained thermal equilibrium solutions for optically thin, two-temperature
black hole accretion disks incorporating magnetic fields. The main objective of
this study is to explain the bright/hard state observed during the bright/slow
transition of galactic black hole candidates. We assume that the energy
transfer from ions to electrons occurs via Coulomb collisions. Bremsstrahlung,
synchrotron, and inverse Compton scattering are considered as the radiative
cooling processes. In order to complete the set of basic equations, we specify
the magnetic flux advection rate. We find magnetically supported (low-beta),
thermally stable solutions. In these solutions, the total amount of the heating
via the dissipation of turbulent magnetic fields goes into electrons and
balances the radiative cooling. The low- solutions extend to high mass
accretion rates and the electron temperature is moderately cool. High
luminosities and moderately high energy cutoffs in the X-ray spectrum observed
in the bright/hard state can be explained by the low-beta solutions.Comment: 24 pages, 10 figures,accepted for publication in Astrophysical
Journa
Brane in 6D with increasing gravitational trapping potential
A new solution to Einstein equations in (1+5)-spacetime with an embedded
(1+3) brane is given. This solution localizes the zero modes of all kinds of
matter fields and 4-gravity on the (1+3) brane by an increasing, transverse
gravitational potential. This localization occurs despite the fact that the
gravitational potential is not a decreasing exponential, and asymptotically
approaches a finite value rather than zero.Comment: Revised paper. 6 pages, revtex 4. to be published in PR
Pulsed UCN production using a Doppler shifter at J-PARC
We have constructed a Doppler-shifter-type pulsed ultra-cold neutron (UCN)
source at the Materials and Life Science Experiment Facility (MLF) of the Japan
Proton Accelerator Research Complex (J-PARC). Very-cold neutrons (VCNs) with
136- velocity in a neutron beam supplied by a pulsed neutron
source are decelerated by reflection on a m=10 wide-band multilayer mirror,
yielding pulsed UCN. The mirror is fixed to the tip of a 2,000-rpm rotating arm
moving with 68- velocity in the same direction as the VCN. The
repetition frequency of the pulsed UCN is and the time width
of the pulse at production is . In order to increase the UCN
flux, a supermirror guide, wide-band monochromatic mirrors, focus guides, and a
UCN extraction guide have been newly installed or improved. The
-equivalent count rate of the output neutrons with longitudinal
wavelengths longer than is ,
while that of the true UCNs is . The spatial density at
production is . This new UCN source enables us to
research and develop apparatuses necessary for the investigation of the neutron
electric dipole moment (nEDM).Comment: 32 pages, 15 fugures. A grammatical error was fixe
A small angle neutron scattering study of the vortex matter in La{2-x}Sr{x}CuO{4} (x=0.17)
The magnetic phase diagram of slightly overdoped La{2-x}Sr{x}CuO{4} (x=0.17)
is characterised by a field-induced hexagonal to square transition of the
vortex lattice at low fields (~0.4 Tesla) [R. Gilardi et al., Phys. Rev. Lett.
88, 217003 (2002)]. Here we report on a small angle neutron scattering study of
the vortex lattice at higher fields, that reveals no further change of the
coordination of the square vortex lattice up to 10.5 Tesla applied
perpendicular to the CuO2 planes. Moreover, it is found that the diffraction
signal disappears at temperatures well below Tc, due to the melting of the
vortex lattice.Comment: 3 pages, 2 figures. Presented at the New3SC-4 meeting, San Diego,
Jan. 16-21 2003; to be published in Int. J. Mod. Phys.
X-Ray Fluctuations from Locally Unstable Advection-Dominated Disks
The response of advection-dominated accretion disks to local disturbances is
examined by one-dimensional numerical simulations. It is generally believed
that advection-dominated disks are thermally stable. We, however, find that any
disurbance added onto accretion flow at large radii does not decay so rapidly
that it can move inward with roughly the free-fall velocity. Although
disturbances continue to be present, the global disk structure will not be
modified largely. This can account for persistent hard X-ray emission with
substantial variations observed in active galactic nuclei and stellar black
hole candidates during the hard state. Moreover, when the disturbance reaches
the innermost parts, an acoustic wave emerges, propagating outward as a shock
wave. The resultant light variation is roughly (time) symmetric and is quite
reminiscent of the observed X-ray shots of Cygnus X-1.Comment: plain TeX, 11 pages, without figures; to be published in ApJ Lette
Steady Models of Optically Thin, Magnetically Supported Black Hole Accretion Disks
We obtained steady solutions of optically thin, single temperature,
magnetized black hole accretion disks assuming thermal bremsstrahlung cooling.
Based on the results of 3D MHD simulations of accretion disks, we assumed that
the magnetic fields inside the disk are turbulent and dominated by azimuthal
component. We decomposed magnetic fields into an azimuthally averaged mean
field and fluctuating fields. We also assumed that the azimuthally averaged
Maxwell stress is proportional to the total pressure. The radial advection rate
of the azimuthal magnetic flux is prescribed as being proportional
to , where is the radial coordinate and is a
parameter which parameterizes the radial variation of . We found
that when accretion rate exceeds the threshold for the onset of the
thermal instability, a magnetic pressure dominated new branch appears. Thus the
thermal equilibrium curve of optically thin disk has a 'Z'-shape in the plane
of surface density and temperature. This indicates that as the mass accretion
rate increases, a gas pressure dominated optically thin hot accretion disk
undergoes a transition to a magnetic pressure dominated, optically thin cool
disk. This disk corresponds to the X-ray hard, luminous disk in black hole
candidates observed during the transition from a low/hard state to a high/soft
state. We also obtained global steady transonic solutions containing such a
transition layer.Comment: 11 pages, 6 figures, accepted by PAS
Rotating Black Holes at Future Colliders. III. Determination of Black Hole Evolution
TeV scale gravity scenario predicts that the black hole production dominates
over all other interactions above the scale and that the Large Hadron Collider
will be a black hole factory. Such higher dimensional black holes mainly decay
into the standard model fields via the Hawking radiation whose spectrum can be
computed from the greybody factor. Here we complete the series of our work by
showing the greybody factors and the resultant spectra for the brane localized
spinor and vector field emissions for arbitrary frequencies. Combining these
results with the previous works, we determine the complete radiation spectra
and the subsequent time evolution of the black hole. We find that, for a
typical event, well more than half a black hole mass is emitted when the hole
is still highly rotating, confirming our previous claim that it is important to
take into account the angular momentum of black holes.Comment: typoes in eqs(82)-(84) corrected; version to appear in Phys. Rev. D;
references and a footnote added; same manuscript with high resolution
embedded figures available on
http://www.gakushuin.ac.jp/univ/sci/phys/ida/paper
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