Martin Neumann

This study presents an analysis of the diffusional dynamics of H 2 impurities in liquid Ne using quasielastic neutron scattering (QENS) and quantum dynamical simulations. By applying a Lorentzian fitting procedure, we identify two key spectral features: peak width ( Γ ) and intensity ( A ), both of which are highly dependent on the momentum transfer Q . At low Q values, the experimental results for Γ align well with simulated self-diffusion coefficients. However, at higher Q , QENS data reveals a sublinear variation with respect to Q 2 that characterizes a jump diffusion process. Using the Hall and Ross model, we determine the mean residence time ( τ CM ) and the standard deviation of jump lengths ( l CM ). These values provide concrete evidence for the existence of Ne “pseudocages,” i.e., short-lived structures formed by neon atoms that trap H 2 molecules, confirming a hypothesis suggested in a previous research. The study highlights a strong coupling between the H 2 diffusion and its vibration within these pseudocages, driven by the high molecular density of the system. This finding underscores the necessity of moving beyond the well-known Gaussian approximation to accurately describe the microscopic dynamics of semiquantum fluids. In conclusion, this work improves the quality of available neutron spectra for H 2 in liquid Ne, correcting previous misinterpretations that were actually due to instrumental limitations rather than physical anomalies.

A recent simulation study of the transverse current autocorrelation of the Lennard-Jones fluid [Guarini et al., Phys. Rev. E 107, 014139 (2023)] revealed that this function can be perfectly described within the exponential expansion theory [Barocchi et al., Phys. Rev. E 85, 022102 (2012)]. However, above a certain wavevector Q, not only transverse collective excitations were found to propagate in the fluid, but a second oscillatory component of unclear origin (therefore called X) must be considered to fully account for the time dependence of the correlation function. Here, we present an extended investigation of the transverse current autocorrelation of liquid Au as obtained by ab initio molecular dynamics in the very wide range of wavevectors 5.7 ≤ Q ≤ 32.8 nm−1 in order to also follow the behavior of the X component, if present, at large Q values. A joint analysis of the transverse current spectrum and its self-portion indicates that the second oscillatory component arises from the longitudinal dynamics, as suggested by its close resemblance with the previously determined component accounting for the longitudinal part of the density of states. We conclude that such a mode, albeit featuring a merely transverse property, fingerprints the effect of longitudinal collective excitations on single-particle dynamics, rather than arising from a possible coupling between transverse and longitudinal acoustic waves.

XPS results on Mn0.95Dy0.05NiSb Heusler alloy are presented. The compound is single phase with a C1b type cubic structure. All the experimental results suggest that the rare earth atoms entered in the lattice and they are not forming second phases. An exchange splitting of Mn 3s core-level spectrum of about 4.6 eV was shown, giving a direct evidence of the local magnetic moments on Mn sites. The Ni 3d states are giving the major contribution to the XPS valence band spectra. The Ni 3d satellite peak situated around 6 eV was attributed to the fact that the Ni 3d states in the valence band are not fully occupied. However, the Ni 3d band is almost full in this alloy and the Stoner criterion for the existence of a magnetic moment on Ni sites is no longer fulfilled.