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|Linear and dynamical optical properties of a silica-core gold-shell photonic crystal
|Thursday, 9 December 2004
|I will discuss the linear and ultrafast nonlinear properties of 3D fcc photonic crystal composed of SiO2/Au/SiO2 core-shell-outer-shell colloidal particles. Strong coupling between incident light and the surface plasmons of the spherical gold shell appears as sharp features in the observed reflectivity spectra in the visible. We observed a highly directional diffraction pattern with hexagonal symmetry. The transient changes in reflection spectra following ultrafast optical excitation are enhanced in the spectral range of the surface plasmon resonance and demonstrate subpicosecond dynamics associated with a rapid heating of the electron gas. The results are compared with theory.
|Giant phonon anomalies on TiOCl/TiOBr
|Wednesday, 1 December 2004
In the last years one of the most hot and interesting topics in the fields of solid state physics is the study of low dimensional systems. Among them Spin=1/2 based system on early transition metal with 3d1 electronic configuration are one of the most intensively studied system of this kind. TiOCl/Br provides indeed a powerful playground to study novel physics phenomena involving not only phonons and charge but also spin and orbital degrees of freedom. The different phases individuated in these compounds are still under debate and discussed in term of spin pierls transition and competing orbital ordering.
In this presentation we will report on an Raman study comparing TiOCl and TiOBr. The giant phonon anomalies reported in both the structure evidence indeed the interaction of phonons with spin and orbital degrees of freedom.
|Optical spectroscopy of Yb2Ti2O7 and Y2Ti2O7:Yb3+ and crystal-field parameters in rare-earth titanate pyrochlores
|Tuesday, 23 November 2004
|Optical absorption spectra of the Yb2Ti2O7 single crystals and luminescence spectra of the Y2Ti2O7:Yb (1%) polycrystalline samples were registered at temperatures 4.2 - 300 K. These spectra and earlier published data on magnetic properties of Yb3+ ions and on the temperature dependence of the electric field gradient at Yb nuclei in Yb2Ti2O7 were used to analyze the crystal-field parameters in rare-earth titanates with the pyrochlore structure. The self-consistent sets of crystal-field parameters for rare-earth ions in the 16d sites with the D3d symmetry that describe satisfactory all known single-ion magnetic properties and low-energy excitations in R2Ti2O7 crystals (R=Tb, Ho, Er, Tm, Yb) are presented.
|Nonlinear transport in ß-Na0.33V2O5
|Friday, 5 November 2004
|One-dimensional materials show a variety of unusual phenomena, one of the most spectacular ones being the occurrence of highly nonlinear transport properties. The recently rediscovered one-dimensional metal ß-Na0.33V2O5, is a material showing three phase transitions: a sodium ordering transition at 240 K, a metal-insulator transition at 136 K, and a magnetic ordering transition around 22 K. Around 8 GPa and 8 K this material becomes superconducting. The combination of the metal-insulator transition, the one-dimensional metallic conductivity, and the sodium ordering makes ß-Na0.33V2O5 an interesting material for transport studies. Here we report on the nonlinear transport properties of ß-Na0.33V2O5. The dc electrical conductivity has been measured in the temperature range from 65 K to 300 K. Below the metal-insulator transition temperature (TMI=136 K) a charge density wave behavior is observed. The charge density wave conductivity follows the empirical form scdw=sn(k/E)(E/ET-1)a, where sn is the conductivity of normal carriers . This form corresponds to the classical model of the charge density wave transport, where the wave is considered to be an object, which is moving in a periodic potential.
|Ringing in a gold nanoshell
|Friday, 29 October 2004
|We report on direct observation of coherent acoustic radial oscillations of a submicron gold shell loosely bound to a silica core. Our sample is composed of a periodic array of silica-core gold-shells forming a 3D photonic crystal. The vibrational mode is excited by an ultrafast optical pulse and appears as a pronounced modulation of the transient reflectivity on a subnanosecond time scale. To best of our knowledge, this is the first demonstration of acoustical vibrations of a free nanoscale shell detected in a pump-probe experiment. The observed oscillation properties of our gold-shell spheres are in excellent agreement with Lamb theory.