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A thorough study of Mn K-edge x-ray-absorption near-edge spectroscopy ͑XANES͒ on several manganites has been performed. The spectra of La 1Ϫx Ca x MnO 3 have a similar shape for the whole series. The chemical shift for intermediate compositions lies between the two end members of the series showing an intermediate oxidation state of the Mn atom. The analysis of the spectra indicates that the Mn atom does not fluctuate between 3ϩ and 4ϩ states. We have also collected XANES spectra of magnetoresistive perovskites (x Ϸ0.33) at different temperatures and at different external magnetic fields. Very small differences in the XANES spectrum are induced by decreasing the temperature or by applying a magnetic field showing that the electronic state of the Mn atom across the metal-insulator transition remains essentially unaltered. X-ray circular magnetic dichroism at the Mn K edge in the ferromagnetic phase also shows a unique magnetic signal.

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We demonstrate experimentally all-optical switching on a silicon chip at telecom wavelengths. The switching device comprises a compact ring resonator formed by horizontal silicon slot waveguides filled with highly nonlinear silicon nanocrystals in silica. When pumping at power levels about 100 mW using 10 ps pulses, more than 50% modulation depth is observed at the switch output. The switch performs about 1 order of magnitude faster than previous approaches on silicon and is fully fabricated using complementary metal oxide semiconductor technologies.

The effect of the substitution for Mn with Al in the magnetoresistive perovskite La 2/3 Ca 1/3 MnO 3 has been studied by preparing the series La 2/3 Ca 1/3 Mn 1Ϫx Al x O 3 (xр0.2). A careful study of the magnetic, structural, and transport properties has been carried out by means of electrical resistance, magnetoresistance, ac magnetic susceptibility, x-ray-diffraction, and neutron-diffraction techniques. Up to xϭ0.05 the Curie temperature ͑and the associated metal-insulator transition͒ decreases drastically with Al doping and the magnetoresistive properties do not change very much. For xу0.1 the lattice spontaneously begins to lose oxygen atoms and for xϭ0.2, 3% of oxygen vacancies are present. This fact along with the random distribution of the Al atoms makes these compounds rather disordered from a structural and magnetic point of view. However, the magnetoresistance is enhanced, reaching colossal values of 10 7 % at Hϭ12 T at low temperatures for xϭ0.2.

The effects of magnetic field and pressure on the unusual spontaneous behavior of La 2/3 Ca 1/3 MnO 3 have been thoroughly investigated. Resistivity and volume thermal expansion, both under magnetic field and pressure, ac susceptibility under pressure, magnetostriction, magnetoresistance, and neutron diffraction measurements, have allowed us to determine the relevant underlying mechanisms in this system. Above T c the neutron measurements reveal short-range ferromagnetic correlations and the anomalous volume thermal expansion indicates that local distortions are present. Both experiments support the formation of magnetic polarons above T c . At T c the compound undergoes a paramagnetic-ferromagnetic transition accompanied by an insulatormetal-like transition with anomalies in the electrical and volume properties. Above T c the magnetic field and the pressure favor electrical conduction by enhancing the double-exchange interaction. Below T c the metallic state is favored by the magnetic field and the pressure in a different way.