Limitations to testing the equivalence principle with satellite laser ranging

We consider the possibility of testing the equivalence principle (EP) in the gravitational field of the Earth from the orbits of LAGEOS and LAGEOS II satellites, which are very accurately tracked from ground by laser ranging. The orbital elements that are affected by an EP violation and can be used to measure the corresponding dimensionless parameter η are semimajor axis and argument of pericenter. We show that the best result is obtained from the semimajor axis, and it is limited—with all available ranging data to LAGEOS and LAGEOS II—to η ≃ 2 × 10⁻⁹, more than 3 orders of magnitude worse than experimental results provided by torsion balances. The experiment is limited because of the non uniformity of the gravitational field of the Earth and the error in the measurement of semimajor axis, precisely in the same way as they limit the measurement of the product GM of the Earth. A better use of the pericenter of LAGEOS II can be made if the data are analyzed searching for a new Yukawa-like interaction with a distance scale of one Earth radius. It is found that the pericenter of LAGEOS II is 3 orders of magnitude more sensitive to a composition dependent new interaction with this particular scale than it is to a composition dependent effect expressed by the η parameter only. Nevertheless, the result is still a factor 500 worse than EP tests with torsion balances in the gravitational field of the Earth (i.e. at comparable distance), though a detailed data analysis has yet to be performed. While EP tests with satellite laser ranging are not competitive, laser ranging to the Moon has been able to provide a test of the EP almost 1 order of magnitude better than torsion balances. We show that this is due to the much greater distance of the test masses (the Earth and the Moon) from the primary body (the Sun) and the correspondingly smaller gradients of its gravity field. We therefore consider a similar new experiment involving the orbit of LAGEOS: testing LAGEOS and the Earth for an EP violation in the gravitational field of the Sun. We show that this test may be of interest, though it is a factor 300 less sensitive than in the case of the Moon due to the fact that LAGEOS is closer to the Earth than the Moon and consequently its orbit is less affected by the Sun. The limitations we have pointed out for laser ranging can be overcome by flying in low Earth orbit a spacecraft carrying concentric test masses of different composition with the capability, already demonstrated in ground laboratories, to accurately sense in situ any differential effects between them. An erratum to this article can be found at     

Self-organization of a tetrasubstituted tetrathiafulvalene (TTF) in a silica based hybrid organic-inorganic material

A hybrid organic inorganic nanostructured material containing a TTF core substituted by four arms exhibited a high level of both condensation at silicon (96%) and self-organization as evidenced by X-ray diffraction and an unprecedented birefringent behaviour.     

Azimuthal anchoring energy at the interface between a nematic liquid crystal and a PTFE substrate

The azimuthal anchoring energy of the nematic liquid crystal 4-pentyl-4'-cyanobiphenil (5CB) on a poly(tetrafluoroethylene) (teflon, PTFE) film is measured for the first time. The PTFE film is deposed using the Wittmann and Smith technique which consists on rubbing a bar of this polymer against a glass substrate at a controlled temperature and pressure. Measurements of the azimuthal anchoring energy are made with a reflectometric technique which provides high accuracy and sensitivity. The dependence of the azimuthal anchoring energy on temperature and on the rubbing pressure is investigated. The extrapolation length remains virtually constant in the whole temperature range of the nematic phase except for an increase of 25% close to the clearing temperature. The azimuthal anchoring energy is somewhat strong and increases with increasing the deposition pressure of PTFE. The observation of a relevant pre-transitional anisotropy of the reflection coefficients in the isotropic phase proves that the surface interactions favor an excess of orientational order. Ageing of the anchoring energy and gliding of the easy axis are experimentally observed. Both these phenomena suggest the presence of an anisotropic adsorbed layer of nematic molecules on the PTFE film.     

Fe3O4/Graphene Composite Anode Material for Fast-Charging Li-Ion Batteries

Composite anode material based on Fe₃O₄ and reduced graphene oxide is prepared by base-catalysed co-precipitation and sonochemical dispersion. Structural and morphological characterizations demonstrate an effective and homogeneous embedding of Fe₃O₄ nanoparticles in the carbonaceous matrix. Electrochemical characterization highlights specific capacities higher than 1000 mAh g⁻¹ at 1C, while a capacity of 980 mAhg⁻¹ is retained at 4C, with outstanding cycling stability. These results demonstrate a synergistic effect by nanosize morphology of Fe₃O₄ and inter-particle conductivity of graphene nanosheets, which also contribute to enhancing the mechanical and cycling stability of the electrode. The outstanding capacity delivered at high rates suggests a possible application of the anode material for high-power systems.     

Correlation of AC-impedance and in situ X-ray spectra of LiCoO2

In-situ X-ray and AC-impedance spectra have been obtained simultaneously during the deintercalation of lithium from LiCoO2 using a specially designed electrochemical cell. The AC-dispersions have been correlated with the cell parameters obtained from the X-ray spectra. The correlation confirms previous hypothesis on the interpretation of the AC-dispersions in terms of an equivalent circuit comprising an element that relates the change of the intrinsic electronic conductivity, occurring at the early stages of deintercalation, to the semiconductor to metal transition caused by the change of the cell parameters.     

Synchrotron X-ray study of the q-fold quasicrystalline symmetry of the smectic C twist grain boundary phase (TGBC)

We report the first X-ray scattering investigation of spatial variations of the q-fold quasicrystalline symmetry (so-called commensurability) of well-aligned samples. A spatial resolution of was achieved using the ESRF microfocus beamline. The liquid crystal samples, contained between glass plates which were either parallel or in a wedge geometry, were scanned in order (i) to probe the mosaicity and (ii) to continuously change the balance between surface and volume effects. In the case of parallel plate cells, commensurability was observed everywhere throughout the sample, hence ruling out possible effects of mosaicity to explain the q-fold symmetry of the diffraction patterns previously reported when probed with a spatial resolution of mm² in rotating anode experiments. In the case of wedge cells, the evolution of X-ray patterns with thickness suggested that commensurate lockin occurs for sufficiently thick samples with a width that is statistical. Received 18 January 1999