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Resonant inelastic x-ray scattering (RIXS) using an incident energy tuned to the uranium N 4,5 absorption edges is reported from epitaxial films of α-U3O8 and UN. Theory shows that for U3O8 the multiplets associated with a 5f 1 configuration with a ground state of 2 F 5/2 and the excited state of 2 F 7/2 are observed. However, the strong transition predicted at a transfer energy of 1.67 eV is not observed. We assume this is a consequence of the intermediate state lifetime broadening due to interaction with continuum states when the transferred energy exceeds the onset of the continuum in the presence of the core hole. This hypothesis is supported by the results obtained for the 5f-itinerant system UN, where no sharp transitions have been observed, although the broad scattering response centered at ∼ 1 eV is considered a signature of a predominantly 5f 3 configuration in this band-like semi-metallic system. These experiments and theory add important information on these materials, both of which have been investigated since the 1960s, as well as whether RIXS at the uranium N edge can become a valuable tool for actinide research.

Research on actinide materials, both basic and applied, has been greatly advanced by the general techniques available from high-intensity photon beams from x-ray synchrotron sources. The most important single reason is that such x-ray sources can work with minute (e.g., microgram) samples, and at this level, the radioactive hazards of actinides are much reduced. We start by discussing the form and encapsulation procedures used for different techniques, then discuss the basic theory for interpreting the results. By reviewing a selection of x-ray diffraction (XRD), resonant elastic x-ray scattering (REXS), x-ray magnetic circular dichroism (XMCD), resonant and non-resonant inelastic scattering (RIXS, NIXS), dispersive inelastic x-ray scattering (IXS), and conventional and resonant photoemission experiments, we demonstrate the potential of synchrotron radiation techniques in studying lattice and electronic structure, hybridization effects, multipolar order, and lattice dynamics in actinide materials., Comment: To be published in Reviews of Modern Physics; 57 pages, 36 figures, 475 references

Understanding the role of disorder and the correlations that exist within it, is one of the defining challenges in contemporary materials science. However, there are few material systems, devoid of other complex interactions, which can be used to systematically study the effects of crystallographic conflict on correlated disorder. Here, we report extensive diffuse x-ray scattering studies on the epitaxially stabilised alloy $\mbox{U}_{1-x}\mbox{Mo}_x$, showing that a new form of intrinsically tuneable correlated disorder arises from a mismatch between the preferred symmetry of a crystallographic basis and the lattice upon which it is arranged. Furthermore, combining grazing incidence inelastic x-ray scattering and state-of-the-art ab initio molecular dynamics simulations we discover strong disorder-phonon coupling. This breaks global symmetry and dramatically suppresses phonon-lifetimes compared to alloying alone, providing an additional design strategy for phonon engineering. These findings have implications wherever crystallographic conflict can be accommodated and may be exploited in the development of future functional materials., Comment: V3: Some supplementary content moved into main text and manuscript structure reformatted to accommodate. No substantive change to transferred material but linking statements added. Section III discussion expanded by one paragraph. No change to results, figures, or conclusions in either the main manuscript or supplementary material. Main text: 10 pg, 7 figs. SM: 14 pg, 5 figs, 1 table

Resonant inelastic x-ray spectroscopy at the uranium N

Single crystal [0 0 1], [1 1 0], and [1 1 1]-oriented UO2 thin films were utilised to investigate the orientational dependence of radiolytic dissolution. The films were exposed to water in the presence of a synchrotron X-ray source to induce dissolution, with X-ray reflectivity (XRR) used to observe changes in morphology as a function of exposure time. The [0 0 1] and [1 1 0]-oriented films were found to corrode at comparable rates, however, the [1 1 1] film was significantly more corrosion resistant, passivating after the initial 90 s dissolution period. This result shows the orientational dependence of UO2 dissolution, which may have important consequences for theoretical dissolution models.

This paper reports experiments investigating the reaction of H$_{2}$ with uranium metal-oxide bilayers. The bilayers consist of $\leq$ 100 nm of epitaxial $\alpha$-U (grown on a Nb buffer deposited on sapphire) with a UO$_{2}$ overlayer of thicknesses of between 20 and 80 nm. The oxides were made either by depositing via reactive magnetron sputtering, or allowing the uranium metal to oxidise in air at room temperature. The bilayers were exposed to hydrogen, with sample temperatures between 80 and 200 C, and monitored via in-situ x-ray diffraction and complimentary experiments conducted using Scanning Transmission Electron Microscopy - Electron Energy Loss Spectroscopy (STEM-EELS). Small partial pressures of H$_{2}$ caused rapid consumption of the U metal and lead to changes in the intensity and position of the diffraction peaks from both the UO$_{2}$ overlayers and the U metal. There is an orientational dependence in the rate of U consumption. From changes in the lattice parameter we deduce that hydrogen enters both the oxide and metal layers, contracting the oxide and expanding the metal. The air-grown oxide overlayers appear to hinder the H$_{2}$-reaction up to a threshold dose, but then on heating from 80 to 140 C the consumption is more rapid than for the as-deposited overlayers. STEM-EELS establishes that the U-hydride layer lies at the oxide-metal interface, and that the initial formation is at defects or grain boundaries, and involves the formation of amorphous and/or nanocrystalline UH$_{3}$. This explains why no diffraction peaks from UH$_{3}$ are observed. {\textcopyright British Crown Owned Copyright 2017/AWE}, Comment: Submitted for peer review

While x-ray absorption spectroscopy is mainly governed by electric-dipole transitions, the technique of nonresonant inelastic x-ray scattering (NIXS) offers the possibility to explore higher-order multipole transitions. These transitions obey different selection rules that can reach final states of higher angular momenta, opening complementary spectroscopic perspectives. Here, we investigate the suitability of NIXS to study transuranium compounds. We show that the $K$ edge of the Be encapsulation can be practically fully excluded by using the imaging capabilities of the technique arising from the position of the signals on the multidetector. Experimental results for the multipole transitions at the actinide ${O}_{4,5}$ edges (90--120 eV) in ${\mathrm{UO}}_{2}$, ${\mathrm{NpO}}_{2}$, ${\mathrm{PuO}}_{2}$, and ${\mathrm{Pu}}_{2}{\mathrm{O}}_{3}$ are compared with multielectronic calculations. The spectral features are shown to be very sensitive to the ratio of the triakontadipole and octupole transitions, which could potentially be used to assess the radial expansion of the $5f$ wave function, which is expected to occur in covalent mixing with the O $2p$ states.

We examine the magnetic ordering of UN and of a closely related nitride, ${\mathrm{U}}_{2}{\mathrm{N}}_{3}$, by preparing thin epitaxial films and using synchrotron x-ray techniques. The magnetic configuration and subsequent coupling to the lattice are key features of the electronic structure. The well-known antiferromagnetic (AF) ordering of UN is confirmed, but the expected accompanying distortion at ${\mathrm{T}}_{N}$ is not observed. Instead, we propose that the strong magnetoelastic interaction below ${\mathrm{T}}_{N}$ causes substantial changes in the strain in the sample being measured. These strains vary as a function of the sample form. As a consequence, the accepted AF configuration of UN may be incorrect. In the case of cubic $\ensuremath{\alpha}\ensuremath{-}{\mathrm{U}}_{2}{\mathrm{N}}_{3}$, no single crystals have been previously prepared, and we have determined the AF ordering wave vector. The AF ${\mathrm{T}}_{N}$ is close to that previously reported. In addition, resonant diffraction methods have identified an aspherical quadrupolar charge contribution in ${\mathrm{U}}_{2}{\mathrm{N}}_{3}$ involving the $5f$ electrons.

In 1966 Roger Cowley (together with Gerald Dolling) reported the first neutron inelastic scattering from the magnetic excitations from UO

Resonant inelastic x-ray spectroscopy at the uranium N4 absorption edge at 778 eV has been used to reveal the excitations in UO2 up to 1 eV. The earlier (1989) studies by neutron inelastic scattering of the crystal-field states within the 3H4 multiplet are confirmed. In addition, the first excited state of the 3F2 multiplet at ∼520 meV has been established, and there is a weak signal corresponding to the next excited state at ∼920 meV. This represents a successful application of soft x-ray spectroscopy to an actinide sample, and resolves an open question in UO2 that has been discussed for 50 years. The technique is described and important caveats are drawn about possible future applications.

Jack Carpenter passed away on March 10, 2020 after falling ill 1 week earlier while vacationing in Florida. During his eventful life, he changed the field of neutron scattering.Jack, a native Penns...

Nonresonant inelastic x-ray scattering (NIXS) has been performed on single crystals of ${\mathrm{UO}}_{2}$ to study the directional dependence of higher-order-multipole scattering from the uranium ${O}_{4,5}$ edges (90--110 eV). By comparing the experimental results with theoretical calculations the symmetry of the ground state is confirmed directly as the crystal-field (CF) ${\mathrm{\ensuremath{\Gamma}}}_{5}$ triplet state within the $J=4$ manifold. The results demonstrate that the directional dichroism of the NIXS spectra is sensitive to the CF strength and establish NIXS as a tool for probing CF interactions quantitatively.

We report experiments to determine the effect of radiation damage on the phonon spectra of the most common nuclear fuel, UO$_2$. We have irradiated thin ($\sim$ 300 nm) epitaxial films of UO$_2$ with 2.1 MeV He$^{2+}$ ions to 0.15 dpa and a lattice swelling of $\Delta$a/a $\sim$ 0.6 %, and then used grazing-incidence inelastic X-ray scattering to measure the phonon spectrum. We succeeded to observe the acoustic modes, both transverse and longitudinal, across the Brillouin zone. The phonon energies, in both the pristine and irradiated samples, are unchanged from those observed in bulk material. On the other hand, the phonon linewidths (inversely proportional to the phonon lifetimes), show a significant broadening when comparing the pristine and irradiated samples. This effect is shown to increase with phonon energy across the Brillouin zone. The decreases in the phonon lifetimes of the acoustic modes are roughly consistent with a 50 % reduction in the thermal conductivity., Comment: 11 pages, 7 figures

Terry Willis left us on January 18, 2018 after a long and active life. Terry obtained an honors degree from Cambridge in 1948, and a PhD from Royal Holloway, University of London in 1951 with John ...

We measure x-ray magnetic circular dichroism (XMCD) spectra at the Pu M_{4,5} absorption edges from a newly prepared high-quality single crystal of the heavy-fermion superconductor ^{242}PuCoGa_{5}, exhibiting a critical temperature T_{c}=18.7 K. The experiment probes the vortex phase below T_{c} and shows that an external magnetic field induces a Pu 5f magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a superconducting quantum interference device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa_{5} have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence 5f electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the 5f magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local 5f moment.

We present resonant x-ray emission spectroscopic (RXES) data from the uranium intermetallics UPd$_3$, USb, USn$_3$ and URu$_2$Si$_2$, at the U $M_{4,5}$ edges and compare the data to those from the well-localized $5f^2$ semiconductor UO$_2$. The technique is especially sensitive to any oxidation of the surface, and this was found on the USb sample, thus preventing a good comparison with a material known to be $5f^3$. We have found a small energy shift between UO$_2$ and UPd$_3$, both known to have localized $5f^2$ configurations, which we ascribe to the effect of conduction electrons in UPd$_3$. The spectra from UPd$_3$ and URu$_2$Si$_2$,are similar, strongly suggesting a predominant $5f^2$ configuration for URu$_2$Si$_2$. The valence-band resonant inelastic x-ray scattering (RIXS) provides information on the U $P_3$ transitions (at about $18$~eV) between the U $5f$ and U $6p$ states, as well as transitions of between $3$ and $7$~eV from the valence band into the unoccupied $5f$ states. These transitions are primarily involving mixed ligand states (O $2p$ or Pd, Ru $4d$) and U $5f$ states. Calculations are able to reproduce both these low-energy transitions reasonably well., 11 pages, 8 figures

We measure x-ray magnetic circular dichroism (XMCD) spectra at the Pu M_{4,5} absorption edges from a newly prepared high-quality single crystal of the heavy-fermion superconductor ^{242}PuCoGa_{5}, exhibiting a critical temperature T_{c}=18.7 K. The experiment probes the vortex phase below T_{c} and shows that an external magnetic field induces a Pu 5f magnetic moment at 2 K equal to the temperature-independent moment measured in the normal phase up to 300 K by a superconducting quantum interference device. This observation is in agreement with theoretical models claiming that the Pu atoms in PuCoGa_{5} have a nonmagnetic singlet ground state resulting from the hybridization of the conduction electrons with the intermediate-valence 5f electronic shell. Unexpectedly, XMCD spectra show that the orbital component of the 5f magnetic moment increases significantly between 30 and 2 K; the antiparallel spin component increases as well, leaving the total moment practically constant. We suggest that this indicates a low-temperature breakdown of the complete Kondo-like screening of the local 5f moment.

Per Anker Lindgaard (born Mogensen) passed away on December 15, 2019. Lindgaard was a prominent theoretical physicist and worked for more than 40 years as the “in-house” theoretical physicist in th...

Some 6 years ago we established a website for the history of neutron scattering: http://neutronsources.org/about/history.html. Some of the articles were described in my earlier columns NN 25(2), 39...

The recent PNAS paper by Migliori et al. (1) attempts to explain the unusually strong temperature dependence of the bulk modulus of fcc plutonium (δ-Pu) by use of the disordered local moment (DLM) model. It is our opinion that this approach does not correctly incorporate the dynamic magnetism of δ-Pu. First, we note that, contrary to the comments in Migliori et al. (1), the results of our recent inelastic neutron experiments (2) do not contradict the results of earlier experiments (3⇓–5), which show that the ground state is nonmagnetic. All of these experiments are consistent with the interpretation that δ-Pu has a nonmagnetic fluctuating valence ground state. … [↵][1]1To whom correspondence should be addressed. Email: mjanoschek{at}lanl.gov. [1]: #xref-corresp-1-1

We have performed high-resolution powder x-ray diffraction measurements on a sample of $^{242}$PuCoGa$_{5}$, the heavy-fermion superconductor with the highest critical temperature $T_{c}$ = 18.7 K. The results show that the tetragonal symmetry of its crystallographic lattice is preserved down to 2 K. Marginal evidence is obtained for an anomalous behaviour below $T_{c}$ of the $a$ and $c$ lattice parameters. The observed thermal expansion is isotropic down to 150 K, and becomes anisotropic for lower temperatures. This gives a $c/a$ ratio that decreases with increasing temperature to become almost constant above $\sim$150 K. The volume thermal expansion coefficient $\alpha_{V}$ has a jump at $T_{c}$, a factor $\sim$20 larger than the change predicted by the Ehrenfest relation for a second order phase transition. The volume expansion deviates from the curve expected for the conventional anharmonic behaviour described by a simple Gr\"{u}neisen-Einstein model. The observed differences are about ten times larger than the statistical error bars but are too small to be taken as an indication for the proximity of the system to a valence instability that is avoided by the superconducting state., Comment: 6 pages, 4 figures

SQUID magnetometry and polarised neutron reflectivity measurements have been employed to characterise the magnetic properties of U/Fe, U/Co and U/Gd multilayers. The field dependence of the magnetisation was measured at 10K in magnetic fields from -70kOe to 70kOe. A temperature dependent study of the magnetisation evolution was undertaken for a selection of U/Gd samples. PNR was carried out in a field of 4.4kOe for U/Fe and U/Co samples (at room temperature) and for U/Gd samples (at 10K). Magnetic 'dead' layers of about 15 Angstrom were observed for U/Fe and U/Co samples, consistent with a picture of interdiffused interfaces. A large reduction in the magnetic moment, constant over a wide range of Gd layer thicknesses, was found for the U/Gd system (about 4 Bohr magnetons compared with 7.63 for the bulk metal). This could be understood on the basis of a pinning of Gd moments arising from a column-like growth mechanism of the Gd layers. A study of the effective anisotropy suggests that perpendicular magnetic anisotropy could occur in multilayers consisting of thick U and thin Gd layers. A reduction in the Curie temperature was observed as a function of Gd layer thickness, consistent with a finite-size scaling behaviour., 18 pages, 8 figures, submitted to J. Phys.: Condens. Matter

Well-defined U/Fe multilayers of varying layer thicknesses and bilayer repeat numbers were prepared by a dc magnetron sputtering method. Polarized neutron reflectometry, off-specular neutron diffraction and magnetic moment measurements were used to determine the physical properties of the multilayers leading to an evaluation of the magnetic moments associated with the U and Fe atoms. The multilayers exhibit ferromagnetic behaviour with the easy axis in the plane of the multilayer. The saturation magnetization was found to increase with increasing Fe-layer thickness and the magnetic moment averaged over the structured iron layers was below the bulk value of 2.2 μB/Fe atom. No anomalies were observed in the magnetization from 4.2 to 375 K in temperature-dependent scans at 0.005 and 0.1 T or in magnetic field scans from 0 to 7 T at 4.2 and 295 K. The hysteresis curves exhibited a small degree of perpendicular magnetic anisotropy. The Curie temperatures for the multilayers were.determined from ac susceptibility measurements and were found to be less than the bulk Fe value of 1043 K.

Epitaxial layers of uranium have been grown on a variety of buffer/seed layers on sapphire substrates by UHV magnetron sputtering and their structure determined using x-ray diffraction. The buffer layers were epitaxial layers of niobium, tungsten and niobium covered by a seed layer of hcp gadolinium, on which uranium layers were grown to a thickness of 600 A. The x-ray diffraction results establish that the α-orthorhombic phase of uranium grows epitaxially in the (110) orientation on the niobium (110) buffer, while on the tungsten (110) buffer the growth planes of the α-uranium were (002) and for the growth on the gadolinium buffer the α-uranium was predominantly (021) oriented. These results show that epitaxial uranium films in selected orientations can be grown by using an appropriate buffer. To our knowledge this is the first report of epitaxial α-uranium films, and it is significant because of the difficulty of growing single crystals of α-uranium due to the occurrence of high temperature structural transformations.

X-ray magnetic circular dichroism (XMCD) measurements were performed at the U M-4,M-5 edges and Fe K edge on well-defined uranium/iron multilayers with different compositions. The multilayers have layer thicknesses in the range 9-40 A for uranium and 9-34 A for iron. At both 10 K and room temperature, the U layers are magnetically polarized in all of the multilayers studied. To deduce the magnetic moment on the uranium from the XMCD results requires assumptions about the magnetic dipole operator in the analysis, and this is discussed in detail. Given the most likely scenario of strong hybridization between the U 5f and Fe 3d states, the largest value of the induced U moment is similar to 0.12 mu(B), which is located primarily at the interface and oscillates within the uranium layer.

We report an experimental and theoretical investigation of the lattice dynamics and thermal properties of the actinide dioxide NpO$_2$. The energy-wavevector dispersion relation for normal modes of vibration propagating along the $[001]$, $[110]$, and $[111]$ high-symmetry lines in NpO$_2$ at room temperature has been determined by measuring the coherent one-phonon scattering of X-rays from a $\sim$1.2 mg single-crystal specimen, the largest available single crystal for this compound. The results are compared against ab initio phonon dispersions computed within the first-principles density functional theory in the generalized gradient approximation plus Hubbard $U$ correlation (GGA+$U$) approach, taking into account third-order anharmonicity effects in the quasiharmonic approximation. Good agreement with the experiment is obtained for calculations with an on-site Coulomb parameter $U = 4$ eV and Hund's exchange $J= 0.6$ eV in line with previous electronic structure calculations. We further compute the thermal expansion, heat capacity, thermal conductivity, phonon linewidth, and thermal phonon softening, and compare with available experiments. The theoretical and measured heat capacities are in close agreement with another. About 27% of the calculated thermal conductivity is due to phonons with energy higher than 25 meV ($\sim$ 6 THz ), suggesting an important role of high-energy optical phonons in the heat transport. The simulated thermal expansion reproduces well the experimental data up to about 1000 K, indicating a failure of the quasiharmonic approximation above this limit., 12 pages, 10 figures

Electrical resistivity measurements of Americium metal under pressure in the normal and in the superconducting state are reported up to 27 GPa and down to temperatures of 0.4 K under magnetic field (

Recent experiments have indicated that the high-temperature properties of α-uranium may be strongly influenced by the formation of randomly distributed intrinsically localized vibrational modes, just a few atoms across in size. One observation was a loss of mechanical ductility that coincided with the formation of the intrinsically localized mode (ILM). Here, we consider this observation in more detail. In particular, we use the anisotropic thermal expansion behaviour to estimate the strains associated with each ILM and consider the implications for the forces between ILMs and the defects responsible for mechanical deformation. In the process we also suggest that an unusual transition from positive to negative thermal expansion along the [0 1 0] direction may be caused by the formation of ILMs.

Although the birth of quantum mechanics and relativity are rightly hailed as the greatest achievements in physics of the 20th century, the 10 years between 1932 and 1942 witnessed a new understanding in the structure of the nucleus that had, and still has, momentous consequences for mankind. This article gives a short account of those years as seen through the lives of James Chadwick (1891–1974) and Lise Meitner (1878–1968).

As interest in low-temperature physics increases, whether for the study of frustrated magnets or for quantum effects, neutron spin-echo will become increasingly important, because of the high-energy resolution achievable. The behaviour of quasielastic scattering for low temperatures relative to the energy scale of interest is investigated. In addition, we note that momentum-transfer selective magnetic scattering may be susceptible to parasitic echoes in certain experimental configurations.

The anisotropy of magnetic fluctuations propagating along the high-symmetry directions in cubic systems with 3- k magnetic order is analyzed within the random-phase approximation assuming anisotropic exchange interactions. Both transverse and longitudinal structures are considered, with reference to the UO 2 and USb compounds, respectively. In the case of UO 2 , the spin-waves polarizations calculated for acoustic and optical branches are favorably compared with three-dimensional polarization analysis experiments carried out on a triple axis spectrometer. The overall spin-waves polarization behavior emerges as a consequence of the 3- k nature of the magnetic order, whatever the strength of the exchange coupling assumed.

Trivalent americium has a nonmagnetic (J=0) ground state arising from the cancellation of the orbital and spin moments. However, magnetism can be induced by a large molecular field if Am^{3+} is embedded in a ferromagnetic matrix. Using the technique of x-ray magnetic circular dichroism, we show that this is the case in AmFe_{2}. Since ⟨J_{z}⟩=0, the spin component is exactly twice as large as the orbital one, the total Am moment is opposite to that of Fe, and the magnetic dipole operator ⟨T_{z}⟩ can be determined directly; we discuss the progression of the latter across the actinide series.

This topical review presents results from neutron inelastic scattering experiments on single crystals of UPd2Al3. The focus is on the experimental situation, while the sequel paper advances theoretical perspectives. We present a detailed and complete characterization of the wavevector- and energy-dependent magnetization dynamics in UPd2Al3 as measured by neutron inelastic scattering primarily in the form of extensive surveys in energy–momentum space under a wide range of experimental conditions, and put our observations in context with data that has been published previously by two independent groups. In this way we emphasize the robustness of the results, which indicate the intricate nature of the dynamic magnetic susceptibility of this material. This study yields unique insight into the low-temperature ground state, which exhibits a microscopic coexistence of antiferromagnetism and superconductivity, making UPd2Al3 one of the most accessible heavy-fermion superconductors that can be fully characterized by neutron spectroscopy.

International audience; We report the observation by neutron diffraction of phase coherent Bragg reflections in a multi-k magnetic configuration with a spatial periodicity outside the conventional scattering cross-section. The peaks, which exist in the 3-k state of UAs0.8Se0.2, display long-range order with a wavevector dependence characteristic of a magnetic interaction. The results confirm the long-range order and temperature dependence reported in an earlier study of similar peaks in this material using x-ray resonant scattering by (a) the non-trivial extension to the technique of neutron diffraction, and (b) the observation of similar 3-k phase-coherent reflections in other samples by x-ray resonant scattering. The importance of the neutron diffraction results lies primarily in the fact that magnetic neutron diffraction is well established as a weak probe operating on thermodynamic time scales. This alleviates concern that the rapid (10-15 - 10-14 s), strong interaction, characteristic of the resonant x-ray technique, is imaging a transient or non-equilibrium configuration. Likewise, the extension of the x-ray resonant scattering results to other samples establishes the generality of this effect. The enigma of how to understand the observed diffraction, which appears to lie strictly outside both the conventional neutron and x-ray scattering cross sections, remains.

An asymmetric shift in the position of the magnetic Bragg peak with respect to the fiducial lattice has been observed by resonant x-ray scattering in a diverse series of antiferromagnetic compounds. A possible explanation is given in terms of a generalized Berry phase correction.

The most remarkable advance that one can report as regards transuranium systems is certainly the discovery of superconductivity above 18 K in PuCoGa5. Motivated by this discovery, we have investigated bulk samples of both PuCoGa5 and its Rh analogue. Characterizations of polycrystalline Co and Rh compounds are reported. The new PuRhGa5 compound is also found to become superconducting above 8 K.

The magnetic structure and excitation in a localized 5f system U3Pd20Si6 were studied by means of neutron scattering. Our neutron diffraction experiments on a single-domain sample uniquely determined a remarkable collinear structure due to higher-order exchange interaction. We discovered a new type of spin-flop transition with collinear coupling. The localized nature of uranium 5f electrons with a 5f2 Γ5 ground state was clearly demonstrated by the observed crystalline electric field excitation. We observed beautiful ferromagnetic and antiferromagnetic spin-wave excitation over a whole Brillouin zone. Surprisingly, however, a clear low-energy quasi-elastic component was also observed around the antiferromagnetic zone centre. This low-energy quasi-elastic response is concluded to be the excitation of the quasi-particles due to hybridization between 5f and conduction electrons. The observation of this localized and itinerant dual nature in magnetic excitation indicates the possibility of partial localization based on an itinerant and two localized 5f electrons proposed theoretically.

The magnetic moment orientation of UGa3 has been revealed by means of both polarized and unpolarized neutron beam experiments under uniaxial pressure. The uniaxial pressure induces a anisotropic domain structure whose populations are quantitatively obtained by polarized beam experiments. We have concluded that the magnetic moment of UGa3 is along the [011] direction.

The structural behaviour of the actinide metals with pressure is being studied using diamond-anvil cells and the high-brilliance and angle-dispersive techniques now available at the ID30 beamline of the European Synchrotron Radiation Facility. A review is given of recent work at room temperature and up to 100 GPa. The results illustrate clearly the difference between the earlier metals (Pa, U) and those further across the actinide series (Am, Cm, and Cf). The complex structures at atmospheric pressure of the lighter actinides are a consequence of itinerant 5f electrons, and these metals show less compressibility (i.e. exhibit larger bulk moduli) and few (if any) phase transitions under pressure. In contrast, the transplutonium metals do not have 5f electrons contributing to their cohesive energies at atmospheric pressure, are therefore 'soft' (have lower bulk moduli), and show multiple phase transitions before their 5f electrons become itinerant. After delocalization of their 5f electrons due to pressure, they acquire structures displayed by the light actinides.

The phase transition at T0 = 25 K in NpO2 and the single-ion nature of the Np 5f electrons is examined in the light of the results of resonant x-ray scattering experiments at the M4 Np edge. These experiments exclude usual magnetic dipole ordering at T0, and provide direct evidence of long-range order of the electric quadrupole moment with ?5 symmetry. The phase transition is purely electronic and does not involve either internal or external crystallographic distortions, so the symmetry of the system remains cubic. The primary order parameter (OP) is associated with ?4t magnetic octupoles, ordering in a triple-q longitudinal structure defined by the three wavevectors of the 001 star. Magnetic octupolar order breaks invariance under time reversal and induces the order of electric quadrupoles as the secondary OP. The resulting ground state is a singlet with zero dipole magnetic moment.

Measurements of x-ray magnetic circular dichroism (XMCD) and x-ray absorption near-edge structure (XANES) spectroscopy at the Pu ${M}_{4,5}$ edges of the ferromagnet PuSb are reported. Using bulk magnetization measurements and a sum rule analysis of the XMCD spectra, we determine the individual orbital [${\ensuremath{\mu}}_{\mathrm{L}}=2.8(1){\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Pu}$] and spin moments [${\ensuremath{\mu}}_{\mathrm{S}}=\ensuremath{-}2.0(1){\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Pu}$] of the Pu $5f$ electrons. Atomic multiplet calculations of the XMCD and XANES spectra reproduce well the experimental data and are consistent with the experimental value of the spin moment. These measurements of $\ensuremath{\langle}{L}_{z}\ensuremath{\rangle}$ and $\ensuremath{\langle}{S}_{z}\ensuremath{\rangle}$ are in excellent agreement with the values that have been extracted from neutron magnetic form factor measurements, and confirm the local character of the $5f$ electrons in PuSb. Finally, we demonstrate that a split ${M}_{5}$ as well as a narrow ${M}_{4}$ XMCD signal may serve as a signature of $5f$ electron localization in actinide compounds.

Structural, magnetic and electrical transport properties of a new compound, NpIrSn, crystallizing in the ZrNiAl-type hexagonal structure are reported. NpIrSn orders antiferromagnetically below T N =40 K and undergoes a second magnetic phase transition around 22 K. The antiferromagnetic order is not affected by magnetic fields up to 9 T. Powder neutron diffraction revealed an incommensurate magnetic structure in both magnetic phases.

Neutron spectroscopy investigations into the magnetic dynamical response of the heavy Fermion superconductor UPd2Al3 support the notion of an interplay between magnetism and superconductivity in strongly correlated f-electron systems. After introducing the scientific framework we review the neutron experiments performed over the last few years. These studies highlight the experimental challenges and allow us to identify the instrumental requirements for such investigations. We conclude our paper on the recent technical achievements as well as future developments for three-axis spectroscopy (TAS) at the ILL.

Trivalent americium has a nonmagnetic (J=0) ground state arising from the cancellation of the orbital and spin moments. However, magnetism can be induced by a large molecular field if Am^{3+} is embedded in a ferromagnetic matrix. Using the technique of x-ray magnetic circular dichroism, we show that this is the case in AmFe_{2}. Since ⟨J_{z}⟩=0, the spin component is exactly twice as large as the orbital one, the total Am moment is opposite to that of Fe, and the magnetic dipole operator ⟨T_{z}⟩ can be determined directly; we discuss the progression of the latter across the actinide series.

In the majority of magnetic systems the surface is required to order at the same temperature as the bulk. In the present study, we report a distinct and unexpected surface magnetic phase transition, uniquely at a lower temperature than the N\'eel temperature. Employing grazing incidence X-ray resonant magnetic scattering we have observed the near surface behavior of uranium dioxide. UO$_2$ is a non-collinear, triple-q, antiferromagnet with the U ions on an face-centered-cubic lattice. Theoretical investigations establish that at the surface the energy increase, due to the lost bonds, reduces when the spins near the surface rotate, gradually losing their normal to the surface component. At the surface the lowest-energy spin configuration has a double-q (planar) structure. With increasing temperature, thermal fluctuations saturate the in-plane crystal field anisotropy at the surface, leading to soft excitations that have ferromagnetic $XY$ character and are decoupled from the bulk. The structure factor of a finite two-dimensional $XY$ model, fits the experimental data well for several orders of magnitude of the scattered intensity. Our results support a distinct magnetic transition at the surface in the Kosterlitz-Thouless universality class., Comment: 7 pages including Supplemental material, 4 figures

Studies of the actinide elements and compounds were (and are) motivated by the need to characterize their structural and thermodynamic properties for the development of nuclear fuels and the treatment of waste, whether it be for long-term storage or ideas involving transmutation in high-powered accelerators. For the most part, tables giving these data exist, although the data for transuranium compounds are rather sparse. A much more difficult task is to understand the data and develop theories that have predictive power in this part of the periodic table. In doing this, however, we are confronted with the extremely complicated electronic structure of the 5f shell and the great paucity of high-quality data on materials containing transuranium isotopes.