A study of the porosity of nuclear graphite using small-angle neutron scattering

Small angle neutron scattering (SANS) measures porosity in nuclear graphites, including both open pores, caused by escaping decomposition gases, and internal cracks (in coke particles) generated by anisotropic thermal contraction along the c-direction (Mrozowski Cracks). Porosity changes on the length scale observable by SANS must control the development of internal stresses and hence of cracking in AGR graphite due to irradiation (both fast neutron displacements of carbon atoms and radiolytic corrosion by CO2). Such cracking may cause premature reactor shutdown. SANS measurements show that porosity is fractal on a length scale between ~0.2-300 nm, presumably due to Mrozowski cracks – because the fractal index of the SANS signal depends only on the porosity of the graphitic filler. We report here two novel uses of the SANS technique as applied to reactor graphite – contrast matching with D-toluene (to measure the fraction of the porosity open to the surface) and the temperature dependence of the scattering (to measure pore width changes up to 2000 °C). These results provide important new information on AGR graphite porosity and its evolution during irradiation

Heat capacity of the quantum magnet TiOCl

Measurements of the heat capacity C(T,H) of the one-dimensional quantum magnet TiOCl are presented for temperatures 2K < T < 300K and magnetic fields up to 5T. Distinct anomalies at 91K and 67K signal two subsequent phase transitions. The lower of these transitions clearly is of first order and seems to be related to the spin degrees of freedom. The transition at 92K probably involves the lattice and/or orbital moments. A detailed analysis of the data reveals that the entropy change through both transitions is surprisingly small (~ 0.1R), pointing to the existence strong fluctuations well into the non-ordered high-temperature phase. No significant magnetic field dependence was detected.Comment: 4 pages, 2 figure

Spin-Peierls transition in TiOCl

Temperature-dependent x-ray diffraction of the low-dimensional spin 1/2 quantum magnet TiOCl shows that the phase transition at T_{c2} = 90 K corresponds to a lowering of the lattice symmetry. Below T_{c1} = 66 K a twofold superstructure develops, that indicates the formation of spin-singlet pairs via direct exchange between neighboring Ti atoms, while the role of superexchange is found to be negligible. TiOCl thus is identified as a spin-Peierls system of pure 1D chains of atoms. The first-order character of the transition at T_{c1} is explained by the competition between the structurally deformed state below T_{c2} and the spin-Peierls state below T_{c1}.Comment: Phys. Rev. B (Rapid Communications) in pres

Gas adsorption/desorption in silica aerogels: a theoretical study of scattering properties

We present a numerical study of the structural correlations associated to gas adsorption/desorption in silica aerogels in order to provide a theoretical interpretation of scattering experiments. Following our earlier work, we use a coarse-grained lattice-gas description and determine the nonequilibrium behavior of the adsorbed gas within a local mean-field analysis. We focus on the differences between the adsorption and desorption mechanisms and their signature in the fluid-fluid and gel-fluid structure factors as a function of temperature. At low temperature, but still in the regime where the isotherms are continuous, we find that the adsorbed fluid density, during both filling and draining, is correlated over distances that may be much larger than the gel correlation length. In particular, extended fractal correlations may occur during desorption, indicating the existence of a ramified cluster of vapor filled cavities. This also induces an important increase of the scattering intensity at small wave vectors. The similarity and differences with the scattering of fluids in other porous solids such as Vycor are discussed.Comment: 16 pages, 15 figure

Pressure-induced insulator-to-metal transition in low-dimensional TiOCl

We studied the transmittance and reflectance of the low-dimensional Mott-Hubbard insulator TiOCl in the infrared and visible frequency range as a function of pressure. The strong suppression of the transmittance and the abrupt increase of the near-infrared reflectance above 12 GPa suggest a pressure-induced insulator-to-metal transition. The pressure-dependent frequency shifts of the orbital excitations, as well as the pressure dependences of the charge gap and the spectral weight of the optical conductivity above the phase transition are presented.Comment: 4 pages, 6 figure

Are the renormalized band widths in TTF-TCNQ of structural or electronic origin? - An angular dependent NEXAFS study

We have performed angle-dependent near-edge x-ray absorption fine structure measurements in the Auger electron yield mode on the correlated quasi-one-dimensional organic conductor TTF-TCNQ in order to determine the orientation of the molecules in the topmost surface layer. We find that the tilt angles of the molecules with respect to the one-dimensional axis are essentially the same as in the bulk. Thus we can rule out surface relaxation as the origin of the renormalized band widths which were inferred from the analysis of photoemission data within the one-dimensional Hubbard model. Thereby recent theoretical results are corroborated which invoke long-range Coulomb repulsion as alternative explanation to understand the spectral dispersions of TTF-TCNQ quantitatively within an extended Hubbard model.Comment: 6 pages, 5 figure

Spin dynamics in the low-dimensional magnet TiOCl

We present detailed ESR investigations on single crystals of the low-dimensional quantum magnet TiOCl. The anisotropy of the g-factor indicates a stable orbital configuration below room temperature, and allows to estimate the energy of the first excited state as 0.3(1) eV ruling out a possible degeneracy of the orbital ground state. Moreover, we discuss the possible spin relaxation mechanisms in TiOCl and analyze the angular and temperature dependence of the linewidth up to 250 K in terms of anisotropic exchange interactions. Towards higher temperatures an exponential increase of the linewidth is observed, indicating an additional relaxation mechanism.Comment: 10 pages, 8 figures; accepted for publication in Phys. Rev.

Nanostructure and paramagnetic centres in diamond-like carbon: Effect of Ar dilution in PECVD process

Diamond-like carbon (DLC) films were deposited utilising plasma enhanced chemical vapour deposition (PECVD) with acetylene precursor, diluted with 0 – 45% argon. Electron paramagnetic resonance (EPR) measurements show the presence of one paramagnetic centre with no change in spin population over the range of film deposition conditions. However, the EPR linewidth decreases with increasing argon content of the precursor mix, suggesting an enhancement of motional narrowing due to an increase in electron delocalization, related to an increase in the sp2 cluster size. Atomic force microscopy (AFM) measurements indicate the surface of the DLC is formed of nanoscale asperities of material. With radii of tens of nanometres for films deposited with zero argon, the size of the features increases with the argon dilution of the acetylene. Energy dispersive x-ray analysis and electrical measurements further elucidate the changes in film structure