471 research outputs found
Test of hybrid metric-Palatini f(R)-gravity in binary pulsars
We developed the parameterized post-Keplerian formalism for hybrid
metric-Palatini -gravity. We obtained analytical expressions in the
generel eccentric case for four PPK parameters: , ,
and . Using observational data of PSR J0737-3039 and PSR J1903+0327 we
imposed restrictions on the parameters of hybrid f(R)-gravity and showed that
this theory is not ruled out by the observations in strong field regime. In
addition we obtained predictions for masses of systems components and found
that considered astrophysical objects will be heavier than in GR.Comment: 12 pages, 4 figure
Superconductivity and Cobalt Oxidation State in Metastable Na(x)CoO(2-delta)*yH2O (x ~ 1/3; y ~ 4x)
We report the synthesis and superconducting properties of a metastable form
of the known superconductor NaxCoO2*yH2O (x ~ 1/3, y ~ 4x). Instead of using
the conventional bromine-acetonitrile mixture for sodium deintercalation, we
use an aqueous bromine solution. Using this method, we oxidize the sample to a
point that the sodium cobaltate becomes unstable, leading to formation of other
products if not controlled. This compound has the same structure as the
reported superconductor, yet it exhibits a systematic variation of the
superconducting transition temperature (Tc) as a function of time. Immediately
after synthesis, this compound is not a superconductor, even though it contains
appropriate amounts of sodium and water. The samples become superconducting
with low Tc values after ~ 90 h. Tc continually increases until it reaches a
maximum value (4.5 K) after about 260 h. Then Tc drops drastically, becoming
non-superconducting approximately 100 h later. Corresponding time-dependent
neutron powder diffraction data shows that the changes in superconductivity
exhibited by the metastable cobaltate correspond to slow formation of oxygen
vacancies in the CoO2 layers. In effect, the formation of these defects
continually reduces the cobalt oxidation state causing the sample to evolve
through its superconducting life cycle. Thus, the dome-shaped superconducting
phase diagram is mapped as a function of cobalt oxidation state using a single
sample. The width of this dome based on the formal oxidation state of cobalt is
very narrow - approximately 0.1 valence units wide. Interestingly, the maximum
Tc in NaxCoO2*yH2O occurs when the cobalt oxidation state is near 3.5. Thus, we
speculate that the maximum Tc occurs near the charge ordered insulating state
that correlates with the average cobalt oxidation state of 3.5.Comment: 22 pages, 9 figures, 1 tabl
Harmonic analysis on spherical homogeneous spaces with solvable stabilizer
For all spherical homogeneous spaces G/H, where G is a simply connected
semisimple algebraic group and H a connected solvable subgroup of G, we compute
the spectra of the representations of G on spaces of regular sections of
homogeneous line bundles over G/H.Comment: v2: 14 pages, minor correction
Crystal Chemistry of Carbon-Substituted MgB2
Neutron powder diffraction has been used to characterize a sample of
C-substituted MgB2 synthesized from Mg and B4C (with isotopically enriched
11B). The sample is multiphase, with the major phase [73.4(1) wt.%] being
Mg(B1-xCx)2 with x=0.10(2). Minor phases include MgB2C2, Mg, and MgO. The major
Mg(B1-xCx)2 phase displays diffraction peak widths as sharp as for pure MgB2,
indicating good C homogeneity. There is no evidence for ordering of the
substituted C atoms or distortion of the host structure other than contraction
of the a axis and slight expansion of the c axis. The observed changes in
lattice parameters vs. C concentration provide a means for estimating the C
concentration in other Mg(B1-xCx)2 samples. The reduction in Tc resulting from
10% C substitution is much larger than previously reported, suggesting that
previous reports of the C concentration in Mg(B1-xCx)2 are overestimated. The
Mg site occupancy is determined to be 0.990(4) which is consistent with full Mg
occupancy. Given these results, the stoichiometry Mg(B0.9C0.1)2 should be used
by future attempts (band structural or otherwise) to explain (i) the dramatic
suppression of Tc (Tc ~ = 22 K) and (ii) the persistence of the
two-superconducting-gap feature in the specific heat data.Comment: 14 pages, 3 figure
Infra-Red Stable Supersymmetry in Chern-Simons Theories with Matter and Quenched Disorder
We study Abelian Chern-Simons field theories with matter fields and global
SU(N) symmetry in the presence of random weak quenched disorder. In the absence
of disorder these theories possess N=2 supersymmetric fixed points and N=1
supersymmetric fixed lines in the infra-red limit. We show that although the
presence of disorder forbids any supersymmetry of the bare action, infra-red
stable supersymmetric fixed points and fixed lines are realized in the
disorder-averaged effective theories.Comment: 8 pp., LaTeX. Explanatory remarks and references added. Version to
appear in Phys. Rev. Let
Decoupling of the -scalar mass in softly broken supersymmetry
It has been shown recently that the introduction of an unphysical
-scalar mass is necessary for the proper renormalization
of softly broken supersymmetric theories by dimensional reduction (\drbar).
In these theories, both the two-loop -functions of the scalar masses and
their one-loop finite corrections depend on . We find, however,
that the dependence on can be completely removed by slightly
modifying the \drbar renormalization scheme. We also show that previous \drbar
calculations of one-loop corrections in supersymmetry which ignored the
contribution correspond to using this modified scheme.Comment: 7 pages, LTH-336, NUB-3094-94TH, KEK-TH-40
The Four-Loop Konishi in N=4 SYM
We present the result of a full direct component calculation for the planar
four-loop anomalous dimension of the Konishi operator in N =4 Supersymmetric
Yang-Mills theory. Our result confirms the results obtained from superfield
(arXiv:0712.3522, arXiv:0806.2095) and superstring (arXiv:0807.0399)
computations, which take into account finite size corrections to the all-loop
asymptotic Bethe ansatz for the integrable models describing the spectrum of
the anomalous dimensions of the gauge-invariant operators and the spectrum of
the string states in the framework of the gauge/string duality.Comment: 7 pages, some detailes of calculations adde
Gauge and parametrization dependencies of the one-loop counterterms in the Einstein gravity.
The parametrization and gauge dependencies of the one-loop counterterms on
the mass-shell in the Einstein gravity are investigated. The physical meaning
of the loop calculation results on the mass shell and the parametrization
dependence of the renormgroup functions in the nonrenormalizable theories are
discussed.Comment: 14 pages in LATEX (Some references added
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