2,897 research outputs found
Carbon Concentration Dependence of the Superconducting Transition Temperature and Structure of MgCxNi3
The crystal structure of the superconductor MgCxNi3 is reported as a function
of carbon concentration determined by powder neutron diffraction. The
single-phase perovskite structure was found in only a narrow range of carbon
content, 0.88 < x < 1.0. The superconducting transition temperature was found
to decrease systematically with decreasing carbon concentration. The
introduction of carbon vacancies has a significant effect on the positions of
the Ni atoms. No evidence for long range magnetic ordering was seen by neutron
diffraction for carbon stoichiometries within the perovskite phase stability
range.Comment: 4 figure
(13)C NMR investigation of the superconductor MgCNi_3 up to 800K
We report (13)C NMR characterization of the new superconductor MgCNi_3 (He et
al., Nature (411), 54 (2001)). We found that both the uniform spin
susceptibility and the spin fluctuations show a strong enhancement with
decreasing temperature, and saturate below ~50K and ~20K respectively. The
nuclear spin-lattice relaxation rate 1/(13)T_1T exhibits typical behaviour for
isotropic s-wave superconductivity with a coherence peak below Tc=7.0K that
grows with decreasing magnetic field.Comment: Accepted for publication in Physical Review Letter
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
Entropy of the FRW cosmology based on the brick wall method
The brick wall method in calculations of the entropy of black holes can be
applied to the FRW cosmology in order to study the statistical entropy. An
appropriate cutoff satisfying the covariant entropy bound can be chosen so that
the entropy has a definite bound. Among the entropy for each of cosmological
eras, the vacuum energy-dominated era turns out to give the maximal entropy
which is in fact compatible with assumptions from the brick wall method.Comment: 10 pages, 2 figure
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
Hawking radiation as tunneling and the unified first law of thermodynamics at the apparent horizon in the FRW universe
Relations between the tunneling rate and the unified first law of
thermodynamics at the apparent horizon of the FRW universe are investigated.
The tunneling rate arises as a consequence of the unified first law of
thermodynamics in such a dynamical system. The analysis shows obviously how the
tunneling is intimately connected with the unified first law of thermodynamics
through the principle of conservation of energy.Comment: Latex, 9 pages, no figur
Effect of Boron substitution on the superconductivity of non-oxide perovskite MgCNi3
We report synthesis, structural and magnetic (DC and AC) properties of Boron
substituted MgCNi3 superconductor. A series of polycrystalline bulk samples
Mg1.2C1.6-xBxNi3 (x = 0.0, 0.08 and 0.16) is synthesized through standard
solid-state reaction route, which are found to crystallize in cubic perovskite
structure with space group Pm3m. Rietveld analysis of observed XRD data show
that lattice parameters expand from a = 3.8106 (4) {\AA} for pure, to 3.8164
(2) {\AA} and 3.8173 (5) {\AA} for 5% and 10% Boron substituted samples
respectively. DC magnetization exhibited superconducting transition (Tc) at
around 7.3 K for pure sample, and the same decreases slightly with Boron
substitution. The lower critical field (Hc1) at 2 K is around 150 Oe for pure
sample, which increases slightly with Boron substitution. For pure sample the
upper critical field (Hc2) being determined from AC susceptibility measurements
is 11.6 kOe and 91.70 kOe with 50% and 90% diamagnetism criteria respectively,
which decreases to 5.57 kOe and 42.5 kOe respectively for 10% Boron substituted
sample. 10% Boron substitution at Carbon site has decreased both the Hc2 and
Tc. On the other hand lower critical field (Hc1) at 2 K is slightly increased
from around 150 Oe for pure sample, to 200 Oe for 10% Boron substituted sample.
Seemingly, the Carbon site Boron substitution induced disorder though has
increased slightly the Hc1 but with simultaneous decrease in superconducting
transition temperature (Tc) and upper critical field (Hc2). The high relative
proportion of Ni in studied MgCNi3 suggests that magnetic interactions are
important and non-oxide perovskite structure make it interesting.Comment: 13 Pages Text + Figures: comments/suggestions
([email protected]
Molecular gas and star formation in nearby starburst galaxy mergers
We employ the Feedback In Realistic Environments (FIRE-2) physics model to
study how the properties of giant molecular clouds (GMCs) evolve during galaxy
mergers. We conduct a pixel-by-pixel analysis of molecular gas properties in
both the simulated control galaxies and galaxy major mergers. The simulated
GMC-pixels in the control galaxies follow a similar trend in a diagram of
velocity dispersion () versus gas surface density
() to the one observed in local spiral galaxies in the
Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For
GMC-pixels in simulated mergers, we see a significant increase of factor of 5 -
10 in both and , which puts these pixels
above the trend of PHANGS galaxies in the vs
diagram. This deviation may indicate that GMCs in the simulated mergers are
much less gravitationally bound compared with simulated control galaxies with
virial parameter () reaching 10 - 100. Furthermore, we
find that the increase in happens at the same time as
the increase in global star formation rate (SFR), which suggests stellar
feedback is responsible for dispersing the gas. We also find that the gas
depletion time is significantly lower for high GMCs
during a starburst event. This is in contrast to the simple physical picture
that low GMCs are easier to collapse and form stars on
shorter depletion times. This might suggest that some other physical mechanisms
besides self-gravity are helping the GMCs in starbursting mergers collapse and
form stars.Comment: 22 pages, 11 figures. Accepted to ApJ. Link to animation update
Recombinant luciferase-expressing human cytomegalovirus (CMV) for evaluation of CMV inhibitors
Recombinant Towne CMV expressing luciferase under the control of CMV-DNA polymerase (POL) or the late pp28 (UL99) promoters were evaluated for potential application in high-throughput screening of anti-viral compounds. POL-and pp28-luciferase displayed maximal expression 48 and 72 hours post infection, respectively. The pp28-luciferase virus achieved a wider dynamic range of luciferase expression (6-7 logs) and was selected for testing of inhibition by five anti-viral compounds. Luciferase expression highly correlated with plaque reduction and real-time PCR. The pp28-luciferase reporter system is rapid, reproducible, and highly sensitive. It may be applied to screening of novel anti-CMV compounds
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