Publications

2019

• Unique silica polymorph obtained under electron irradiation
  
  • Reghioua Imène
  • Lancry Matthieu
  • Cavani Olivier
  • Le Floch Sylvie
  • Neuville Daniel R.
  • Ollier Nadège
  Applied Physics Letters, American Institute of Physics, 2019, 115 (25), pp.251101. High purity synthetic silica glass (Suprasil F300) samples were densified by High Pressure/High Temperature (HP-HT) using three different pressure/temperature/duration values. Their relaxation process was studied by applying 2.5 MeV electron irradiation with doses varying from 1 MGy up to 11 GGy. At very high dose (11 GGy), all the densified silica samples exhibit the same density value (∼2.26 g/cm3) regardless of the densification conditions, referring to an equilibrium value known as an amorphous silica metamict phase. In detail, the HP-HT silica samples were progressively dedensified from 1 GGy irradiation dose until reaching this equilibrium. Whatever the initial topological disorder revealed through Raman spectra, all silica samples present identical Raman spectra at 11 GGy, with high intensities of D1 and D2 components, attesting they reach one unique silica polymorph. Regarding the irradiation at high temperature (600 MGy, 600 K), the results show that the thermal annealing during irradiation “accelerates” somehow the relaxation kinetics of both macroscopic density and vibrational signature. Furthermore, it seems that such high irradiation temperatures lead to increase the Non-Bridging Oxygen Hole Center point defects generation. (10.1063/1.5127836)
  DOI : 10.1063/1.5127836
• Quantification of irradiation-induced defects IN UO2 using Raman and positron annihilation spectroscopies
  
  • Mohun R.
  • Desgranges L.
  • Jegou C.
  • Boizot B.
  • Cavani O.
  • Canizares A.
  • Duval Florian
  • He C.
  • Desgardin Pierre
  • Barthe Marie-France
  • Simon Patrick
  Acta Materialia, Elsevier, 2019, 164, pp.512-519. In UO2, Raman spectroscopy has recently put into evidence the existence of a specific signature, referred to as the triplet defect bands, which is characteristic to irradiation damages. In this work, we perform a detailed experimental analysis to investigate how this Raman signature can be used to characterize irradiated nuclear fuels. For this purpose, an electron irradiation experiment of sintered UO2 disks coupled with ex situ Raman and positron annihilation spectroscopy measurements were carried out. The obtained findings showed that the Raman defect bands take their origin from the ballistic collisions of the incident electrons with the U and O atoms and are due to the formation of point defects. These defects induce the re-arrangement of UO2 lattice atoms giving rise to domains with symmetry lower than Fm-3m with the loss of one or more symmetry elements, such as translational symmetry, centering F, mirror or rotational symmetry operations”. (10.1016/j.actamat.2018.10.044)
  DOI : 10.1016/j.actamat.2018.10.044
• Long-range corrected exchange-correlation kernels to describe excitons in second-harmonic generation
  
  • Gauriot Nicolas
  • Véniard Valérie
  • Luppi Eleonora
  The Journal of Chemical Physics, American Institute of Physics, 2019, 151 (23), pp.234111. We investigate the role of excitons in second-harmonic generation (SHG) through the long-range corrected (LRC) exchange-correlation kernels: empirical LRC, Bootstrap, and jellium-with-a-gap model. We calculate the macroscopic second-order frequency-dependent susceptibility χ(2). We also present the frequency-dependent macroscopic dielectric function ϵM which is a fundamental quantity in the theoretical derivation of χ(2). We assess the role of the long-range kernels in describing excitons in materials with different symmetry types: cubic zincblende, hexagonal wurtzite, and tetragonal symmetry. Our studies indicate that excitons play an important role in χ(2) bringing a strong enhancement of the SHG signal. Moreover, we found that the SHG enhancement follows a simple trend determined by the magnitude of the long-range corrected α-parameter. This trend is material dependent. (10.1063/1.5126501)
  DOI : 10.1063/1.5126501
• Relaxation study of pre-densified silica glasses under 2.5 MeV electron irradiation
  
  • Ollier Nadège
  • Lancry Matthieu
  • Martinet Christine
  • Martinez Valérie
  • Le Floch Sylvie
  • Neuville Daniel R.
  Scientific Reports, Nature Publishing Group, 2019, 9, pp.1227. We examined the “relaxation properties” of pre-densified synthetic fused silica glass under 2.5 MeV electron irradiation. The densification of the glass was either obtained by hot compression (5 GPa-350 °C and 5 GPa-1000 °C) or via a thermal treatment increasing its fictive temperature (T_f = 1050, 1250 and 1400 °C). Under irradiation, the pre-densified silica glasses exhibit a relaxation of their macroscopic density with increasing integrated dose. Density was reduced for hot compressed silica and increased for T_f samples with different relaxation rates but it is remarkable that all sample densities follow a trend towards the same equilibrium value around 2.26 for a dose larger than 10 GGy despite a different final topology. After irradiation of hot compressed silica, the Raman spectra display a significant increment of 4 and almost 3-membered rings whereas they exhibit a glass density reduction; demonstrating that a D₂ band increase cannot be considered as an absolute marker of the glass compaction. The correlation between density and D₂ intensity remains valid until silica density remains lower than 2.26. In contrast, the FWHM of the main band peaking at 440 cm^-1 appears to remain correlated to the silica glass density for all investigated samples. (10.1038/s41598-018-37751-9)
  DOI : 10.1038/s41598-018-37751-9
• Theory of the Anomalous Tunnel Hall Effect at Ferromagnet-Semiconductor Junctions
  
  • Huong Dang T.
  • Quang To D.
  • Erina E.
  • Hoai Nguyen T.L.
  • Safarov V.I.
  • Jaffrès H.
  • Drouhin H.-J.
  Journal of Magnetism and Magnetic Materials, Elsevier, 2019, 459, pp.37-42. We report on theoretical investigations of carrier scattering asymmetry at ferromagnet-semiconductor junctions. By an analytical 2 × 2 spin model, we show that, when Dresselhaus interactions is included in the conduction band of III-V T d symmetry group semiconductors, the electrons may undergo a difference of transmission vs. the sign of their incident parallel wavevector normal to the in-plane magnetization. This asymmetry is universally scaled by a unique function independent of the spin-orbit strength. This particular feature is reproduced by a multiband k • p tunneling transport model. Astonishingly, the asymmetry of transmission persists in the valence band of semiconductors owing to the inner atomic spin-orbit strength and free of asymmetric potentials. We present multiband 14 × 14 and 30 × 30 k • p tunneling models together with tunneling transport perturbation calculations corroborating these results. Those demonstrate that a tunnel spin-current normal to the interface can generate a surface transverse charge current, the so-called Anomalous Tunnel Hall Effect. (10.1016/j.jmmm.2017.12.065)
  DOI : 10.1016/j.jmmm.2017.12.065