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A new approach to observe the radiative decay of the $^{229}$Th nuclear isomer, and to determine its energy and radiative lifetime, is presented. Situated at a uniquely low excitation energy, this nuclear state might be a key ingredient for the development of a nuclear clock, a nuclear laser and the search for time variations of the fundamental constants. The isomer's $\gamma$ decay towards the ground state will be studied with a high-resolution VUV spectrometer after its production by the $\beta$ decay of $^{229}$Ac. The novel production method presents a number of advantages asserting its competitive nature with respect to the commonly used $^{233}$U $\alpha$-decay recoil source. In this paper, a feasibility analysis of this new concept, and an experimental investigation of its key ingredients, using a pure $^{229}$Ac ion beam produced at the ISOLDE radioactive beam facility, is reported., Comment: 11 pages, 5 figures, 2 tables

The crystal structure of Lanthanum Cerium Oxide (La$_2$Ce$_2$O$_7$) is investigated using \textit{ab initio} density functional theory (DFT) calculations. The relative stability of fluorite- and pyrochlore-like structures is studied through comparison of their formation energies. These formation energies show the pyrochlore structure to be favored over the fluorite structure, apparently contradicting the conclusions based on experimental neutron and X-ray diffraction (XRD). By calculating and comparing XRD spectra for a set of differently ordered and random structures, we show that the pyrochlore structure is consistent with diffraction experiments. For these reasons, we suggest the pyrochlore structure as the ground state crystal structure for La$_2$Ce$_2$O$_7$. %we show that among the structures considered in this work, the pyrochlore geometry is clearly favorable over the disordered fluorite geometry., Comment: 10 pages, 5 tables, 5 figures

Fluorite CeO$_2$ doped with group IV elements is studied within the DFT and DFT+U framework. Concentration dependent formation energies are calculated for Ce$_{1-x}$Z$_x$O$_2$ (Z= C, Si, Ge, Sn, Pb, Ti, Zr, Hf) with $0\leq x \leq 0.25$ and a roughly decreasing trend with ionic radius is observed. The influence of the valence and near valence electronic configuration is discussed, indicating the importance of filled $d$ and $f$ shells near the Fermi level for all properties investigated. A clearly different behavior of group IVa and IVb dopants is observed: the former are more suitable for surface modifications, the latter are more suitable for bulk modifications.\\ \indent For the entire set of group IV dopants, there exists an inverse relation between the change, due to doping, of the bulk modulus and the thermal expansion coefficients. Hirshfeld-I atomic charges show that charge transfer effects due to doping are limited to the nearest neighbor oxygen atoms., Comment: 11 pages, 6 tables, 5 figures

A new value for the hyperfine magnetic field of copper impurities in iron is obtained by combining resonance frequencies from experiments involving {\beta}-NMR on oriented nuclei on 59-Cu, 69-Cu, and 71-Cu with magnetic moment values from collinear laser spectroscopy measurements on these isotopes. The resulting value, i.e., Bhf(CuFe) = -21.794(10) T, is in agreement with the value adopted until now but is an order of magnitude more precise. It is consistent with predictions from ab initio calculations. Comparing the hyperfine field values obtained for the individual isotopes, the hyperfine anomalies in Fe were determined to be 59{\Delta}69=0.15(9)% and 71{\Delta}69=0.07(11)%., Comment: 6 pages, 2 figures, 3 tables

Applying time differential perturbed angular correlation (TDPAC) spectroscopy and \emph{ab initio} calculations, we have investigated possible lattice instabilities in Sr$_{2}$RuO$_{4}$ by studying the electric quadrupole interaction of a $^{111}$Cd probe at the Ru site. We find evidence for a dynamic lattice distortion, revealed from the observations of: (i) a rapidly fluctuating electric-field gradient (EFG) tensor of which the main component decreases with decreasing temperature, and (ii) a monotonic increase of the EFG asymmetry ($\eta$) below 300 K. We argue that the observed dynamic lattice distortion is caused by strong spin fluctuations associated with the inherent magnetic instability of Sr$_{2}$RuO$_{4}$., Comment: 6 pages, 2 figures, 3 tables

We present a comparison between accurate \textit{ab initio} calculations and a high-quality experimental data set (1986-2002) of electric-field gradients and magnetic hyperfine fields of Cd at different sites on Ni, Cu, Pd and Ag surfaces. Experiments found a systematic rule to assign surface sites on (100) and (111) surfaces based on the main component of the electric-field gradient, a rule that does not work for (110) surfaces. Our calculations show that this particular rule is a manifestation of a more general underlying systematic behavior. When looked upon from this point of view, (100), (111) \emph{and} (110) surfaces behave in precisely the same way. The experimentally observed parabolic coordination number dependence of the Cd magnetic hyperfine field at Ni surfaces is verified as a general trend, but we demonstrate that individual cases can significantly deviate from it. It is shown that the hyperfine fields of other 5sp impurities at Ni surfaces have their own, typical coordination number dependence. A microscopic explanation for the different dependencies is given in terms of the details of the s-DOS near the Fermi level., Comment: 13 pages, 8 figures, submitted to Phys. Rev. B

In RIn3 and RSn3 the rare earth (R) is trivalent, except for Eu and Yb, which are divalent. This was experimentally determined in 1977 by perturbed angular correlation measurements of the electric-field gradient on a 111Cd impurity. At that time, the data were interpreted using a point charge model, which is now known to be unphysical and unreliable. This makes the valency determination potentially questionable. We revisit these data, and analyze them using ab initio calculations of the electric-field gradient. From these calculations, the physical mechanism that is responsible for the influence of the valency on the electric-field gradient is derived. A generally applicable scheme to interpret electric-field gradients is used, which in a transparent way correlates the size of the field gradient with chemical properties of the system., Comment: 10 pages

Using TDPAD experiment and local spin density calculations, we have observed large 4d moments on isolated Mo atoms at substitutional and octahedral interstitial lattice sites in Yb metal, showing Curie-Weiss local susceptibility and Korringa like spin relaxation rate. As a surprising feature, despite strong hybridization with the Yb neighbours, interstitial Mo atoms show high moment stability with small Kondo temperature. While, magnetism of Mo, at substitutional site is consistent with Kondo type antiferromagnetic d-sp exchange interaction, we suggest that moment stability at the interstitial site is strongly influenced by ferromagnetic polarization of Yb-4f5d band electrons., Comment: 4 pages, 4 figures