7,918 research outputs found
The suppression of superconductivity in MgCNi3 by Ni-site doping
The effects of partial substitution of Cu and Co for Ni in the intermetallic
perovskite superconductor MgCNi3 are reported. Calculation of the expected
electronic density of states suggests that electron (Cu) and hole (Co) doping
should have different effects. For MgCNi3-xCux, solubility of Cu is limited to
approximately 3% (x = 0.1), and Tc decreases systematically from 7K to 6K. For
MgCNi3-xCox, solubility of Co is much more extensive, but bulk
superconductivity disappears for Co doping of 1% (x = 0.03). No signature of
long range magnetic ordering is observed in the magnetic susceptibility of the
Co doped material.Comment: submitted, Solid State Communication
Energy Associated with Schwarzschild Black Hole in a Magnetic Universe
In this paper we obtain the energy distribution associated with the Ernst
space-time (geometry describing Schwarzschild black hole in Melvin's magnetic
universe) in Einstein's prescription. The first term is the rest-mass energy of
the Schwarzschild black hole, the second term is the classical value for the
energy of the uniform magnetic field and the remaining terms in the expression
are due to the general relativistic effect. The presence of the magnetic field
is found to increase the energy of the system.Comment: RevTex, 8 pages, no figures, a few points are clarified, to appear in
Int. J. Mod. Phys. A. This paper is dedicated to Professor G. F. R. Ellis on
the occasion of his 60th birthda
Construction and enlargement of traversable wormholes from Schwarzschild black holes
Analytic solutions are presented which describe the construction of a
traversable wormhole from a Schwarzschild black hole, and the enlargement of
such a wormhole, in Einstein gravity. The matter model is pure radiation which
may have negative energy density (phantom or ghost radiation) and the
idealization of impulsive radiation (infinitesimally thin null shells) is
employed.Comment: 22 pages, 7 figure
Gravitational waves, black holes and cosmic strings in cylindrical symmetry
Gravitational waves in cylindrically symmetric Einstein gravity are described
by an effective energy tensor with the same form as that of a massless Klein-
Gordon field, in terms of a gravitational potential generalizing the Newtonian
potential. Energy-momentum vectors for the gravitational waves and matter are
defined with respect to a canonical flow of time. The combined energy-momentum
is covariantly conserved, the corresponding charge being the modified Thorne
energy. Energy conservation is formulated as the first law expressing the
gradient of the energy as work and energy-supply terms, including the energy
flux of the gravitational waves. Projecting this equation along a trapping
horizon yields a first law of black-hole dynamics containing the expected term
involving area and surface gravity, where the dynamic surface gravity is
defined with respect to the canonical flow of time. A first law for dynamic
cosmic strings also follows. The Einstein equation is written as three wave
equations plus the first law, each with sources determined by the combined
energy tensor of the matter and gravitational waves.Comment: 10 pages, revtex. Published version with further detail
Production and decay of evolving horizons
We consider a simple physical model for an evolving horizon that is strongly
interacting with its environment, exchanging arbitrarily large quantities of
matter with its environment in the form of both infalling material and outgoing
Hawking radiation. We permit fluxes of both lightlike and timelike particles to
cross the horizon, and ask how the horizon grows and shrinks in response to
such flows. We place a premium on providing a clear and straightforward
exposition with simple formulae.
To be able to handle such a highly dynamical situation in a simple manner we
make one significant physical restriction, that of spherical symmetry, and two
technical mathematical restrictions: (1) We choose to slice the spacetime in
such a way that the space-time foliations (and hence the horizons) are always
spherically symmetric. (2) Furthermore we adopt Painleve-Gullstrand coordinates
(which are well suited to the problem because they are nonsingular at the
horizon) in order to simplify the relevant calculations.
We find particularly simple forms for surface gravity, and for the first and
second law of black hole thermodynamics, in this general evolving horizon
situation. Furthermore we relate our results to Hawking's apparent horizon,
Ashtekar et al's isolated and dynamical horizons, and Hayward's trapping
horizons. The evolving black hole model discussed here will be of interest,
both from an astrophysical viewpoint in terms of discussing growing black
holes, and from a purely theoretical viewpoint in discussing black hole
evaporation via Hawking radiation.Comment: 25 pages, uses iopart.cls V2: 5 references added; minor typos; V3:
some additional clarifications, additional references, additional appendix on
the Viadya spacetime. This version published in Classical and Quiantum
Gravit
Absence of a structural transition up to 40 Gpa in MgB2 and the relevance of magnesium non-stoichiometry
We report measurements on MgB2 up to ~40GPa. Increasing pressure yields a
monotonous decrease of the lattice parameters and of the c/a ratio, but no
structural transition down to parameters smaller than those of AlB2. The
transition superconducting temperature also decreases with temperature in a
sample dependent way. The results are explained by an increase of the filling
of the 2D pxy bands with pressure, the Mg stoichiometry determining the
starting position of the Fermi level. Our measurements indicate that these hole
bands are the relevant ones for superconductivity.Comment: submitted March 9th 2001, PRB accepte
Temperature dependence of the structural parameters of the non-oxide perovskite superconductor MgCNi3
We report the structural parameters of superconducting MgCxNi3 (x=0.96,
TC=7.3 K) as a function of temperature, from 2 K to 295 K, determined by
neutron powder diffraction profile refinement. The compound has the perovskite
structure over the whole temperature range, with symmetry Pm3m and a=3.81221(5)
A at 295 K: no structural or long range magnetic ordering transitions were
observed. The lattice parameter a and the Debye-Waller factors for the
individual atoms decrease smoothly with decreasing temperature. There are no
unusual changes of the structural parameters near TC
Superconductivity in the three-leg Hubbard ladder: a Quantum Monte Carlo study
Quantum Monte Carlo method is used to look into the superconductivity in the
three-leg Hubbard ladder. The enhanced correlation for the pairing across the
central and edge chains, which has been predicted in the weak-coupling
renormalization as an effect of coexistence of gapful and gapless spin modes,
is here shown to persist for intermediate interaction strengths.Comment: 10 pages, RevTeX, 3 figures in PostScript file
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