The Effect of Ordinary Portland Cement on Nuclear Waste Glass Dissolution

AbstractTo evaluate the durability of the glass matrix and to identify the key mechanisms responsible for glass alteration in the cementitious environment imposed by the Supercontainer design, glass leach tests were conducted at 30°C under Ar atmosphere in suspensions of Ordinary Portland Cement and synthetic young cement water with the high pH of 13.5. The cement appears to trigger the glass dissolution by consumption of glass matrix components leading to a fast glass dissolution at a constant rate with the formation of a porous gel layer on the glass. The key mechanism driving the long-term glass dissolution is secondary phase formation with Si and Al from the glass matrix. The two main reactions are the reaction of Si released by the glass with portlandite, leading to the formation of Calcium Silicate Hydrate (C-S-H) phases, and the further conversion of C-S-H phases with Al from the glass to form Calcium Aluminum Silicate Hydroxide (C-A-S-H) phases. After consumption of the portlandite, the glass alteration rate is expected to decrease

Relation between Kitaev magnetism and structure in α\alpha-RuCl3_3

Raman scattering has been employed to investigate lattice and magnetic excitations of the honeycomb Kitaev material $\alpha$-RuCl$_3$ and its Heisenberg counterpart CrCl$_3$. Our phonon Raman spectra give evidence for a first-order structural transition from a monoclinic to a rhombohedral structure for both compounds. Significantly, only $\alpha$-RuCl$_3$ features a large thermal hysteresis, consistent with the formation of a wide phase of coexistence. In the related temperature interval of $70-170$ K, we observe a hysteretic behavior of magnetic excitations as well. The stronger magnetic response in the rhombohedral compared to the monoclinic phase evidences a coupling between the crystallographic structure and low-energy magnetic response. Our results demonstrate that the Kitaev magnetism concomitant with fractionalized excitations is susceptible to small variations of bonding geometry.Comment: 9 pages, 8 figures, To appear in PR

Investigation of the oxohalogenide Cu4Te5O12Cl4 with weakly coupled Cu(II) tetrahedra

The crystal structure of the copper(II) tellurium(IV) oxochloride Cu$_{4}$Te$_{5}$O$_{12}$Cl$_{4}$ (Cu-45124) is composed of weakly coupled tetrahedral Cu clusters and shows crystallographic similarities with the intensively investigated compound Cu$_{2}$Te$_{2}$O$_{5}$X$_{2}$, with X~=~Cl, Br (Cu-2252). It differs from the latter by a larger separation of the tetrahedra within the crystallographic ab plane, that allows a more direct assignment of important inter-tetrahedra exchange paths and the existence of an inversion center. Magnetic susceptibility and specific heat evidence antiferromagnetic, frustrated correlations of the Cu spin moments and long range ordering with $T_{c}$=13.6 K. The entropy related to the transition is reduced due to quantum fluctuations. In Raman scattering a well structured low energy magnetic excitation is observed at energies of $\approx$50K (35cm$^{-1})$. This energy scale is reduced as compared to Cu-2252.Comment: 11 pages, 9 figures, further information see http://www.peter-lemmens.d

Longitudinal magnon in the tetrahedral spin system Cu2Te2O5Br2 near quantum criticality

We present a comprehensive study of the coupled tetrahedra-compound Cu2Te2O5Br2 by theory and experiments in external magnetic fields. We report the observation of a longitudinal magnon in Raman scattering in the ordered state close to quantum criticality. We show that the excited tetrahedral-singlet sets the energy scale for the magnetic ordering temperature T_N. This energy is determined experimentally. The ordering temperature T_N has an inverse-log dependence on the coupling parameters near quantum criticality

Electronic Raman scattering of Tl-2223 and the symmetry of the supercon- ducting gap

Single crystalline Tl2Ba2Ca2Cu3O10 was studied using electronic Raman scattering. The renormalization of the scattering continuum was investigated as a function of the scattering geometry to determine the superconducting energy gap 2Delta(k). The A1g- and B2g-symmetry component show a linear frequency behaviour of the scattering intensity with a peak related to the energy gap, while the B1g-symmetry component shows a characteristic behaviour at higher frequencies. The observed frequency dependencies are consistent with a dx^2-y^2-wave symmetry of the gap and yield a ratio of 2Delta/k_BT_c=7.4. With the polarization of the scattered and incident light either parallel or perpendicular to the CuO2-planes a strong anisotropy due to the layered structure was detected, which indicates an almost 2 dimensional behaviour of this system.Comment: 2 pages, Postscript-file including 2 figures. Accepted for publication in the Proceedings of the M^2SHTSC IV Conference, Grenoble (France), 5-9 July 1994. Proceedings to be published in Physica C. Contact address: [email protected]

Giant phonon anomalies in the pseudo-gap phase of TiOCl

We report infrared and Raman spectroscopy results of the spin-1/2 quantum magnet TiOCl. Giant anomalies are found in the temperature dependence of the phonon spectrum, which hint to unusual coupling of the electronic degrees of freedom to the lattice. These anomalies develop over a broad temperature interval, suggesting the presence of an extended fluctuation regime. This defines a pseudo-gap phase, characterized by a local spin-gap. Below 100 K a dimensionality cross-over leads to a dimerized ground state with a global spin-gap of about 2$\Delta_{spin}\approx$~430 K.Comment: 4 pages, 3 figures, for further information see http://www.peter-lemmens.d

Revealing the correlation between real-space structure and chiral magnetic order at the atomic scale

We image simultaneously the geometric, electronic and magnetic structure of a buckled iron bilayer film that exhibits chiral magnetic order. We achieve this by combining spin-polarized scanning tunneling microscopy and magnetic exchange force microscopy (SPEX), to independently characterize the geometric as well as the electronic and magnetic structure of non-flat surfaces. This new SPEX imaging technique reveals the geometric height corrugation of the reconstruction lines resulting from strong strain relaxation in the bilayer, enabling the decomposition of the real-space from the eletronic structure at the atomic level, and the correlation with the resultant spin spiral ground state. By additionally utilizing adatom manipulation, we reveal the chiral magnetic ground state of portions of the unit cell that were not previously imaged with SP-STM alone. Using density functional theory (DFT), we investigate the structural and electronic properties of the reconstructed bilayer and identify the favorable stoichiometry regime in agreement with our experimental result