A manifestly MHV Lagrangian for N=4 Yang-Mills

We derive a manifestly MHV Lagrangian for the N=4 supersymmetric Yang-Mills theory in light-cone superspace. This is achieved by constructing a canonical redefinition which maps the N=4 superfield and its conjugate to a new pair of superfields. In terms of these new superfields the N=4 Lagrangian takes a (non-polynomial) manifestly MHV form, containing vertices involving two superfields of negative helicity and an arbitrary number of superfields of positive helicity. We also discuss constraints satisfied by the new superfields, which ensure that they describe the correct degrees of freedom in the N=4 supermultiplet. We test our derivation by showing that an expansion of our superspace Lagrangian in component fields reproduces the correct gluon MHV vertices.Comment: 37 pages, 1 figure. v2: minor changes, references adde

Geometric parameterization of the YBaCo4O7 structure type: implications for stability of the hexagonal form and oxygen uptake.

We explore the stability of the hexagonal form of MBaCo4O7 cobaltites in terms of geometric characteristics of the crystal structure and Global Instability Index (GII) based on the bond-valence considerations. Mismatch between an M3+/2+ and the three-dimensional network of CoO4 tetrahedra, whether expressed using an M ionic radii or GII, is shown to essentially determine both the temperature of structural transition to an orthorhombic modification and oxygen storage properties. A number of M cations not reported in the literature are identified to be suitable for the octahedral sites in an MBaCo4O7 structure. © 2010, Elsevier Ltd

Transitions between P2¹, P6³(√3A), and P6³22 modifications of SrAl²O4 by in situ high-temperature X-ray and neutron diffraction.

The results of in situ high-temperature X-ray and neutron powder diffraction experiments reconcile inconsistencies in previous reports on the symmetry of high-temperature phases of SrAl2O4. The material undergoes two reversible phase transitions P2(1) P6(3)(√3A) and P6(3)(√3A) P6(3)22 at similar to 680 and similar to 860°C, respectively, and the latter one is experimentally observed and characterized for the first time. The higher symmetry above the P6(3)(√3A) P6(3)22 transition is gained by disordering off-center split site of oxygen atoms around trigonal axis rather than by unbending Al-O-Al angle to the ideal value 180°C. The analysis of the literature suggests that it is a common feature of the P6(3)22 phases of stuffed tridymites. © 2007, Elsevier Ltd

Mossbauer spectroscopy analysis of Fe-57-doped YBaCo4O7+delta: effects of oxygen intercalation.

Mossbauer spectroscopy of layered YBaCo3.96Fe0.04O7+delta (delta=0.02 and 0.80), where 1% cobalt is substituted With 57 Fe isotope, revealed no evidence of charge ordering at 4-293 K. The predominant state of iron cations was found trivalent, irrespective of their coordination and oxygen stoichiometry variations determined by thermogravimetric analysis. The extremely slow kinetics of isothermal oxidation at 598 K in air, and the changes of Fe3+ fractions in the alternating triangular and Kagome layers in oxidized YBaCo3.96Fe0.04O7.80, may suggest that oxygen intercalation is accompanied with a substantial structural reconstruction stagnated due to sluggish cation diffusion. Decreasing temperature below 75-80 K leads to gradual freezing of the iron magnetic moments in inverse correlation with the content of extra oxygen. The formation of metal-oxygen octahedra and resultant structural distortions extend the temperature range where the paramagnetic and frozen states co-exist, down to 45-50 K. © 2008, Elsevier Ltd

Mixed conductivity and stability of CaFe2O4−δ.

The total conductivity of CaFe2O4-delta, studied in the oxygen partial pressure range from 10(-17) to 0.5 atm at 1023-1223 K, is predominantly p-type electronic under oxidizing conditions. The oxygen ion transference numbers determined by the steady-state oxygen permeation and faradaic efficiency measurements vary in the range of 0.2 to 7.2 x 10(-4) at 1123-1273 K, increasing with temperature. No evidence of any significant cationic contribution to the conductivity was found. The Mossbauer spectroscopy, thermogravimetry, and X-ray diffraction (XRD) showed that the orthorhombic lattice of calcium ferrite is essentially intolerant to the oxygen vacancy formation and to doping with lower-valence cations, such as Co and Ni. The oxygen nonstoichiometry (delta) is almost negligible, 0.0046-0.0059 at 973-1223 K and p(O-2) = 10(-5)-0.21 atm, providing a substantial dimensional stability of CaFe2O4-delta ceramics. The average linear thermal expansion coefficients, calculated from the controlled-atmosphere dilatometry and high-temperature XRD data, are (9.6-13.9) x 10(-6) K-1 in the oxygen pressure range from 10(-8) to 0.21 atm at 873-1373 K. Decreasing P(02) results in a modest lattice contraction and in the p-n transition indicated by the conductivity and Seebeck coefficient variations. The phase decomposition of CaFe2O4-delta occurs at oxygen chemical potentials between the low-p(O-2) stability limit of Ca2Fe2O5-delta brownmillerite and the hematite/magnetite boundary in binary Fe-O system. © 2008, Electrochemical Society Inc