Neutrinos in a gravitational background: a test for the universality of the gravitational interaction

In this work we propose an extended formulation for the interaction between neutrinos and gravitational fields. It is based on the parametrized post-Newtonian aproach, and includes a violation of the universality of the gravitational interaction which is non diagonal in the weak flavor space. We find new effects that are not considered in the standard scenario for violation of the equivalence principle. They are of the same order as the effects produced by the Newtonian potential, but they are highly directional dependent and could provide a very clean test of that violation. Phenomenological consequences are briefly discussed.Comment: 18 pages, revtex, no figure

Magnetoelectric ordering of BiFeO3 from the perspective of crystal chemistry

In this paper we examine the role of crystal chemistry factors in creating conditions for formation of magnetoelectric ordering in BiFeO3. It is generally accepted that the main reason of the ferroelectric distortion in BiFeO3 is concerned with a stereochemical activity of the Bi lone pair. However, the lone pair is stereochemically active in the paraelectric orthorhombic beta-phase as well. We demonstrate that a crucial role in emerging of phase transitions of the metal-insulator, paraelectric-ferroelectric and magnetic disorder-order types belongs to the change of the degree of the lone pair stereochemical activity - its consecutive increase with the temperature decrease. Using the structural data, we calculated the sign and strength of magnetic couplings in BiFeO3 in the range from 945 C down to 25 C and found the couplings, which undergo the antiferromagnetic-ferromagnetic transition with the temperature decrease and give rise to the antiferromagnetic ordering and its delay in regard to temperature, as compared to the ferroelectric ordering. We discuss the reasons of emerging of the spatially modulated spin structure and its suppression by doping with La3+.Comment: 18 pages, 5 figures, 3 table

Geometry of Pentaphenylantimony in Solution: Support for a Trigonal Bipyramidal Assignment from Xray Absorption Spectroscopy and Vibrational Spectroscopic Data

Pentaphenylantimony (SbPh5) has been previously crystallized in either a square pyramidal or trigonal bipyramidal geometry. Investigation of the solution-state structure of SbPh5 has been hampered by the extreme fluxionality of this compound, but previous vibrational spectroscopic studies concluded that it maintains a square pyramidal geometry in solution. This non-VSEPR-compliant geometry, which is also assumed by BiPh5 in the solid state, stands in contrast to the trigonal bipyramidal geometries of PPh5 and AsPh5. A range of phenomena have been invoked to explain this discrepancy, most notably, the increased importance of relativistic effects as group 15 is descended. We present crystallographic, spectroscopic, and computational data revealing that SbPh5 in fact assumes the VSEPR-compliant trigonal bipyramidal geometry in solution. In particular, Sb X-ray absorption spectroscopy (XAS) was used to obtain geometry-sensitive spectra that do not suffer from the slow spectroscopic time scale that has prevented NMR studies from elucidating the structure of this fluxional molecule. Sb K-edge and LIII-edge XAS spectra of crystalline solids featuring SbPh5 in either a square pyramidal (nonsolvate) or trigonal bipyramidal (cyclohexane hemisolvate or THF hemisolvate) form were compared to spectra of SbPh5 in solution. The solution-state spectra agree with those from solids containing trigonal bipyramidal SbPh5. The most diagnostic spectroscopic feature was the distribution of intensity in the Sb LIII pre-edge features. These distributions were rationalized using time-dependent density functional theory calculations that take into account spin-orbit coupling. Our use of Sb XAS not only resolves a long-standing physical inorganic question but also demonstrates more widely the utility of XAS in establishing the structures of fluxional main-group compounds. This conclusion was further supported by solid- and solution-state Raman data. Finally, we note that the present high-resolution diffraction data allow τ for nonsolvated SbPh5 to be revised to 0.216