Publications

2007

• Origin of the Optical Contrast in Phase-Change Materials
  
  • Welnic W.
  • Botti Silvana
  • Reining L.
  • Wuttig M.
  Physical Review Letters, American Physical Society, 2007, 98, pp.236403. Several chalcogenide alloys exhibit a pronounced contrast between the optical absorption in the metastable rocksalt and in the amorphous phase. This phenomenon is the basis for their application in optical data storage. Here we present ab initio calculations of the optical properties of GeTe and Ge1Sb2Te4 in the two phases. The analysis of our computations and experimental data reveal the correlation between local structural changes and optical properties as well as the origin of the optical contrast in these materials. We find that the change in optical properties cannot be attributed to a smearing of transition energies as commonly assumed for amorphous semiconductors: the optical contrast between the two phases can only be explained by significant changes in the transition matrix elements. (10.1103/PhysRevLett.98.236403)
  DOI : 10.1103/PhysRevLett.98.236403
• Positron annihilation characteristics in UO$_2$: For lattice and vacancy defects induced by electron irradiation
  
  • Barthe M.-F.
  • Labrim H.
  • Gentils A.
  • Desgardin P.
  • Corbel C.
  • Esnouf S.
  • Piron J.P.
  Physica Status Solidi C: Current Topics in Solid State Physics, Wiley, 2007, 4 (10), pp.3627-3632. Annealing of sintered polished UO$_2$ disks at high temperature (1700DC) has been performed in order to eliminate the defects created both during the sintering and the polishing of the disks. Several sets of such UO$_2$ disks coming from different origins have been studied in the bulk using positron lifetime spectroscopy, and near the surface using a slow positron beam coupled with Doppler broadening spectrometry. They appear homogeneous and similar using positron annihilation spectroscopy. Results show the presence of negative ions, that may be negatively charged oxygen atom located in interstitial site. The positrons annihilation characteristics of the UO$_2$ lattice have been determined and are equal to SL(UO$_2$) = 0.371, WL(UO$_2$)=0.078, $\tau_L$(UO$_2$)=169ps. (10.1002/pssc.200675752)
  DOI : 10.1002/pssc.200675752