Search

LuCuGaO₄ has magnetic Cu(2+) and diamagnetic Ga(3+) ions distributed on a triangular bilayer and is suggested to undergo a spin glass transition at Tg ∼ 0.4 K. Using μSR (muon spin rotation) and neutron scattering measurements, we show that at low temperature the spins form a short range correlated state with spin fluctuations detectable over a wide range of timescales: at 0.05 K magnetic fluctuations can be detected in both the μSR time window and also extending beyond 7 meV in the inelastic neutron scattering response, indicating magnetic fluctuations spanning timescales between ∼10(-5) and ∼10(-10) s. The dynamical susceptibility scales according to the form χ″(ω)T(α), with α = 1, throughout the measured temperature range (0.05-50 K). These effects are associated with quantum fluctuations and some degree of structural disorder in ostensibly quite different materials, including certain heavy fermion alloys, kagome spin liquids, quantum spin glasses, and valence bond glasses. We therefore suggest that LuCuGaO₄ is an interesting model compound for the further examination of disorder and quantum magnetism.

Neutron diffraction measurements on a powder sample of bundles of single-walled carbon nanotubes are used to gain insight into adsorption sites for methane molecules. In parallel, numerical methods, based on empirical force-fields, are employed to calculate the adsorption of methane in bundles of nanotubes and neutron diffraction patterns based on these structures. Adsorption sites in grooves on the outer surface of bundles and on the curved surfaces of the nanotubes are characterised by their binding energy and relative abundance. Comparison with published adsorption isotherms indicate that the more stable groove sites must also be complemented by interstitial sites. Such sites are found to be energetically unfavourable in homogeneous bundles composed of average diameter nanotubes, but can become favourable in bundles made of bigger diameter tubes and in more realistic, heterogeneous bundles. Nanotube deformation is an important factor, limiting the size of interstitial channels, but also allowing the stabilisation of adsorbed methane molecules. Diffraction patterns calculated for methane–nanotube structures with partially occupied interstitial sites are in best agreement with the experimental diffraction data. [Copyright &y& Elsevier]

Abstract. Elastic and inelastic neutron scattering are used to study the structure and dynamics of single-wall carbon nanorobes (SWNT) self-assembled into nanobundles (NBSWNT). Suspensions of NBSWNT arc characterized by small-angle neutron scattering. Neutron diffraction is used as a useful tool to study the structure of both the SWNT and NBSWNT. Calculations on finite-size bundles are compared to the data in order to estimate the distribution of tube diameters. Finally, we present time of flight inelastic scattering measurements of the phonon density of states and discuss the main features of the spectra in comparison with calculations. [ABSTRACT FROM AUTHOR]

Abstract: Neutron scattering experiments have been carried out on low-coverage films of the aromatic p-phenylene oligomer molecules (p-nP, C6n H10+4(n − 2), where n indicates the number of phenyl units per molecule) physisorbed onto the surface of graphite. The molecular arrangement within these films has been determined as a function of temperature, number of molecular layers, and molecular length. Analysis of the diffraction data reveals two-dimensional film structures consistent for molecules having their long-axis parallel to the underlying substrate. The experimental measurements provide evidence for a melting transition in a monolayer film and thermal expansion of a bilayer film of p-terphenyl (p-3P). The planes of the phenyl rings are approximately parallel to the substrate in the monolayer structure but rotate out of this plane by 60±20° in the bilayer film of p-3P. As the number of phenyl units is increased from n =3 to n =6, bilayer structures are observed having similar packing arrangements of the molecules but with lattice parameters that scale with molecular length. [Copyright &y& Elsevier]

Abstract: Heavy fermion CeCu4Al compound has been studied by inelastic neutron diffraction (INS), heat capacity and magnetic susceptibility measurements. A single Crystal Electric Field (CEF) peak has been detected in the INS spectra, which may be explained by a quasi-quartet state suggested by the analysis of the Schottky anomaly contributing to the magnetic part of the specific heat. The Kondo interactions have been included in the analysis of the magnetic part employing the simplified resonance level model. The resulting Kondo temperature of about 5–10 K is somewhat larger than in the previous studies. The magnetic susceptibility confirms the CEF level scheme of the type 0–93 K and provides the values of the CEF parameters. [Copyright &y& Elsevier]

It was recently demonstrated on high-quality single crystal that Ce3Pt23Si11 orders ferromagnetically at Tc = 440 mK. Extensive studies of the physical properties of this compound are reported. In the paramagnetic phase, heat capacity and electrical resistivity measurements on a single crystal confirm a good metal behavior with a residual resistivity of ρ0 = 12.17 µΩ cm, no Kondo-type anomaly, and a Debye temperature of 280 K, similar to the one of the the isostructural nonmagnetic compound La3Pt23Si11. In the ferromagnetic phase, the easy magnetization axis is the threefold one with a spontaneous magnetization of 0.91 μB/Ce at 100 mK. This value differs from the expectations within a cubic crystalline electric field (CEF) model. More surprisingly, the inelastic neutron scattering shows two CEF peaks, while neutron diffraction experiments reveal that the structure remains cubic down to 800 mK. These results are very reminiscent of those of CeAl2. Using the CEF formalism, the fits of the magnetic susceptibility lead to a quadruplet ground state, which is not consistent with the anisotropy found in the ferromagnetic state. This strongly suggests a splitting of the ground state by a dynamical Jahn-Teller effect, which retains the apparent cubic symmetry for static measurements. [ABSTRACT FROM AUTHOR]

Abstract: We have analyzed the mechanism of melting of layers adsorbed in cylindrical pores of porous materials. The goal was to understand the melting mechanism of simple fluids adsorbed in pores with heterogeneous wall surface. The studied system was a monolayer of methane molecules adsorbed in MCM-41 pore of diameter d =4nm. Both experimental (neutron scattering) and simulation (Monte Carlo) results proved extremely strong influence of the wall roughness on the melting mechanism. The most striking difference between melting on smooth and rough surfaces was in the temperatures of the transition. The transformation between solid-like and liquid-like monolayer phases adsorbed on a rough surface was observed in a very large temperature range and the solid like properties were observed even above the bulk methane melting temperature. [Copyright &y& Elsevier]