arXiv subjects

G. H. Lander

Publications and source records attributed to G. H. Lander.

At least 37 records · Page 2Linked to original sources

arXiv2014

The malleability of uranium: manipulating the charge-density wave in epitaxial films

We report x-ray synchrotron experiments on epitaxial films of uranium, deposited on niobium and tungsten seed layers. Despite similar lattice parameters for these refractory metals, the uranium epitaxial arrangements are different and the strains propagated along the a-axis of the uranium layers are of opposite sign. At low temperatures these changes in epitaxy result in dramatic modifications to the behavior of the charge-density wave in uranium. The differences are explained with the current theory for the electron-phonon coupling in the uranium lattice. Our results emphasize the intriguing possibilities of producing epitaxial films of elements that have complex structures like the light actinides uranium to plutonium.

cond-mat.str-el↗

arXiv2013

Multipolar, magnetic and vibrational lattice dynamics in the low temperature phase of uranium dioxide

We report the results of inelastic neutron scattering experiments performed with triple-axis spectrometers to investigate the low-temperature collective dynamics in the ordered phase of uranium dioxide. The results are in excellent agreement with the predictions of mean-field RPA calculations emphasizing the importance of multipolar superexchange interactions. By comparing neutron scattering intensities in different polarization channels and at equivalent points in different Brillouin zones, we show the mixed magneto-vibrational-quadrupolar character of the observed excitations. The high energy resolution afforded by the cold triple-axis spectrometer allowed us to study in detail the magnon-phonon interaction giving rise to avoided crossings along the $[00ξ]$ reciprocal space direction.

cond-mat.str-el↗

arXiv2013

X-ray magnetic circular dichroism experiments and theory of transuranium Laves phase compounds

The actinide cubic Laves compounds NpAl2, NpOs2, NpFe2, and PuFe2 have been examined by X-ray magnetic circular dichroism (XMCD) at the actinide M4,5 absorption edges and Os L2,3 absorption edges. The XMCD experiments performed at the M4,5 absorption edges of Np and Pu allow us to determine the spectroscopic branching ratio, which gives information on the coupling scheme in these materials. In all materials the intermediate coupling scheme is found appropriate. Comparison with the SQUID data for NpOs2 and PuFe2 allows a determination of the individual orbital and spin magnetic moments and the magnetic dipole contribution mmd. The resulting orbital and spin magnetic moments are in good agreement with earlier values determined by neutron diffraction, and the values of mmd are non-negligible, and close to those predicted for intermediate coupling. There is a comparatively large induced moment on the Os atom in NpOs2 such that the Os contribution to the total moment per formula unit is ~30% of the total. The spin and orbital moments at the Os site are parallel, in contrast to the anti-parallel configuration of Os impurities in 3d ferromagnetic transition metals. Calculations using the LDA+U technique are reported. The ab initio computed XMCD spectra show good agreement with experimental spectra for small values (0-1eV) of the Hubbard U parameter, which underpins that 5f electrons in these compounds are relatively delocalized.

cond-mat.str-el↗

arXiv2013

Unexpected phase locking of magnetic fluctuations in the multi-k magnet USb

The spin waves in the multi-k antiferromagnet, USb, soften and become quasielastic well below the AFM ordering temperature, T_N. This occurs without a magnetic or structural transition. It has been suggested that this change is in fact due to de-phasing of the different multi-k components: a switch from 3-k to 1-k behaviour. In this work, we use inelastic neutron scattering with tri-directional polarisation analysis to probe the quasielastic magnetic excitations and reveal that the 3-k structure does not de-phase. More surprisingly, the paramagnetic correlations also maintain the same clear phase correlations well above T_N (up to at least 1.4T_N). This precursor regime has not been observed before in a multi-k system.

cond-mat.str-el↗

arXiv2011

Understanding the complex phase diagram of uranium: the role of electron-phonon coupling

We report an experimental determination of the dispersion of the soft phonon mode along [1,0,0] in uranium as a function of pressure. The energies of these phonons increase rapidly, with conventional behavior found by 20 GPa, as predicted by recent theory. New calculations demonstrate the strong pressure (and momentum) dependence of the electron-phonon coupling, whereas the Fermi-surface nesting is surprisingly independent of pressure. This allows a full understanding of the complex phase diagram of uranium, and the interplay between the charge-density wave and superconductivity.

cond-mat.str-el↗

arXiv2010

5d-5f Electric-multipole Transitions in Uranium Dioxide Probed by Non-resonant Inelastic X-ray Scattering

Non-resonant inelastic x ray scattering (NIXS) experiments have been performed to probe the 5d-5f electronic transitions at the uranium O(4,5) absorption edges in uranium dioxide. For small values of the scattering vector q, the spectra are dominated by dipole-allowed transitions encapsulated within the giant resonance, whereas for higher values of q the multipolar transitions of rank 3 and 5 give rise to strong and well-defined multiplet structure in the pre-edge region. The origin of the observed non-dipole multiplet structures is explained on the basis of many-electron atomic spectral calculations. The results obtained demonstrate the high potential of NIXS as a bulk-sensitive technique for the characterization of the electronic properties of actinide materials.

cond-mat.str-el↗

arXiv2007

Profile of the U 5f magnetization in U/Fe multilayers

Recent calculations, concerning the magnetism of uranium in the U/Fe multilayer system have described the spatial dependence of the 5f polarization that might be expected. We have used the x-ray resonant magnetic reflectivity technique to obtain the profile of the induced uranium magnetic moment for selected U/Fe multilayer samples. This study extends the use of x-ray magnetic scattering for induced moment systems to the 5f actinide metals. The spatial dependence of the U magnetization shows that the predominant fraction of the polarization is present at the interfacial boundaries, decaying rapidly towards the center of the uranium layer, in good agreement with predictions.

cond-mat.other↗

arXiv2007

Resonant x-ray scattering study of NpRhGa_5 and NpCoGa_5

We report a resonant x-ray scattering (RXS) study of antiferromagnetic neptunium compounds NpCoGa_5 and NpRhGa_5 at the Np M_4 and Ga K-edges. Large resonant signals of magnetic dipole character are observed below the Neel temperatures at both edges. The signals at the Np edges confirm the behaviour determined previously from neutron diffraction, i.e. the moments along [001] in NpCoGa_{5} and in NpRhGa_5 a reorientation of the moments from the c-axis direction to the ab plane. In the latter material, on application of magnetic field of 9 Tesla along the [010] direction we observe a change in the population of different [110]-type domains. We observe also a magnetic dipole signal at the Ga K-edge, similarly to the reported UGa_3 case, that can be interpreted within a semi-localized model as an orbital polarization of the Ga 4p states induced via strong hybridization with the Np 5f valence band. Quantitative analysis of the signal shows that the Ga dipole on the two different Ga sites follows closely the Np magnetic moment reorientation in NpRhGa_5. The ratios of the signals on the two inequivalent Ga sites are not the same for the different compounds.

cond-mat.str-el↗

arXiv2007

Magnetic Excitations in NpCoGa5

We report the results of inelastic neutron scattering experiments on NpCoGa$_{5}$, an isostructural analogue of the PuCoGa$_{5}$ superconductor. Two energy scales characterize the magnetic response in the antiferromagnetic phase. One is related to a non-dispersive excitation between two crystal field levels. The other at lower energies corresponds to dispersive fluctuations emanating from the magnetic zone center. The fluctuations persist in the paramagnetic phase also, although weaker in intensity. This supports the possibility that magnetic fluctuations are present in PuCoGa$_{5}$, where unconventional d-wave superconductivity is achieved in the absence of magnetic order.

cond-mat.str-el↗

arXiv2006

Multi-k magnetic structures in USb_{0.9}Te_{0.1} and UAs_{0.8}Se_{0.2} observed via resonant x-ray scattering at the U M4 edge

Experiments with resonant photons at the U M4 edge have been performed on a sample of USb_{0.9}Te_{0.1}, which has an incommensurate magnetic structure with k = 0.596(2) reciprocal lattice units. The reflections of the form , as observed previously in a commensurate k = 1/2 system [N. Bernhoeft et al., Phys. Rev. B 69 174415 (2004)] are observed, removing any doubt that these occur because of multiple scattering or high-order contamination of the incident photon beam. They are clearly connected with the presence of a 3k configuration. Measurements of the reflections from the sample UAs_{0.8}Se_{0.2} in a magnetic field show that the transition at T* ~ 50 K is between a low-temperature 2k and high-temperature 3k state and that this transition is sensitive to an applied magnetic field. These experiments stress the need for quantitative theory to explain the intensities of these reflections.

cond-mat.str-el↗

arXiv2005

Nature of non-magnetic strongly-correlated state in delta-plutonium

Ab-initio relativistic dynamical mean-field theory is applied to resolve the long-standing controversy between theory and experiment in the "simple" face-centered cubic phase of plutonium called delta-Pu. In agreement with experiment, neither static nor dynamical magnetic moments are predicted. In addition, the quasiparticle density of states reproduces not only the peak close to the Fermi level, which explains the large coefficient of electronic specific heat, but also main 5f features observed in photoelectron spectroscopy.

cond-mat.str-el↗

arXiv2005

Superconductivity in Actinide Materials

Actinide materials play a special role in condensed matter physics, spanning behaviours of itinerant d-electron and localized 4f-electron materials. An intermediate state, found notably in Pu-based materials whose 5f electrons are neither fully localized nor itinerant, is particularly challenging to understand. Superconductivity appearing in some actinide materials provides clues to the nature of the 5f electrons. PuCoGa5, the first Pu-based superconductor, is superconducting at Tc=18.5 K. This relatively high Tc is unprecedented in any other actinide system but is typical of itinerant electron compounds in which superconductivity is mediated by phonons. Recent studies of PuCoGa5 show that its superconductivity is not phonon-mediated; rather, these experiments are consistent with superconductivity produced by antiferromagnetic fluctuations of nearly localized 5f electrons. Similarities of PuCoGa5 with the superconducting and normal states of isostructural 4f analogues CeMIn5 (M=Co, Rh, Ir) and high-Tc cuprates enable new perspectives on the 5f electrons of Pu.

cond-mat.str-el↗

arXiv2005

Direct observation of electric-quadrupolar order in UO$_{2}$

We report direct experimental evidence for long-range antiferro ordering of the electric-quadrupole moments on the U ions. Resonant x-ray scattering experiments at the uranium $M_{4}$ absorption edge show a characteristic dependence in the integrated intensity upon rotation of the crystal around the scattering vector. Although quadrupolar order in uranium dioxide was advocated already in the 1960s no experimental evidence for this phenomenon was provided until now. We conclude with a possible model to explain the phase diagram of the solid solutions of UO$_{2}$ and NpO$_{2}$. We suggest that in the region $0.30 < x < 0.75$ neither the transverse nor the longitudinal quadrupole ordering can dominate, leading to frustration and only short-range ordering.

cond-mat.str-el↗

arXiv2004

Magnetisation dynamics in the normal and condensate phases of UPd2Al3 I: Surveys in reciprocal space using neutron inelastic scattering

This paper, I, presents new results from neutron inelastic scattering experiments on single crystals of UPd2Al3. The focus is on the experimental position whilst the sequel, II, advances theoretical perspectives. We present a detailed and complete characterisation of the wavevector- and energy-dependent magnetisation 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 previously published by two independent groups. In this way we emphasize the commonality and robust nature of the data which indicate the intricate nature of the dynamic magnetic susceptibility of this material. Our results yield 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 characterised by neutron spectroscopy.

cond-mat.str-el↗

arXiv2004

Magnetisation dynamics in the normal and condensate phases of UPd2Al3 II: Inferences on the nodal gap symmetry

This paper provides an analysis of neutron inelastic scattering experiments on single crystals of UPd2Al3. The emphasis is on establishing robust, general, inferences on the joint antiferromagnetic-superconducting state which characterises UPd2Al3 at low temperatures. A distinction is drawn between these conclusions and various theoretical perspectives of a more model sensitive nature that have been raised in the literature.

cond-mat.str-el↗

arXiv2004

Absence of magnetic moments in plutonium

Many theories published in the last decade propose that either ordered or disordered local moments are present in elemental plutonium at low temperatures. We present new experimental data and review previous experimental results. None of the experiments provide any evidence for ordered or disordered magnetic moments (either static or dynamic) in plutonium at low temperatures, in either the alpha- or delta-phases. The experiments presented and discussed are magnetic susceptibility,electrical resistivity, NMR, specific heat, and both elastic and inelastic neutronscattering. Many recent calculations correctly predict experimentally observed atomic volumes, including that of delta-Pu. These calculations achieve observed densities by the localization of electrons, which then give rise to magnetic moments. However, localized magnetic moments have never been observed experimentally in Pu. A theory is needed that is in agreement with all the experimental observations. Two theories are discussed that might provide understanding of the ensemble of unusual properties of Pu, including the absence of experimental evidence for localized magnetic moments; an issue that has persisted for over 50 years.

cond-mat.mtrl-sci↗

arXiv2004

Resonant X-ray Scattering Study of Magnetic and Electric-quadrupole Order in U$_{0.75}$Np$_{0.25}$O$_2$

We have used element specific X-ray resonant scattering to investigate the M edge resonances in a single crystal of \unpo. Earlier neutron diffraction and Mössbauer studies had shown the presence of long-range AF order below T$_{N}$ = 19 K, with sizeable magnetic moment both on the U and the Np ions. RXS results confirm the presence of an ordered dipole magnetic moment on the Np ions, but also reveal the presence of an anti-ferro arrangement of the electric quadrupole moments on both U and Np ions, with the same propagation vector that defines the magnetic structure. From the azimuthal dependence of the intensities we are able to determine the exact configuration of the quadrupolar ordering. The intensities of Bragg peaks associated with magnetic dipole and electric quadrupole order have different temperature dependences. On cooling below T$_{N}$, the magnetic dipole order develops faster on the uranium ions, with magnetic order on the Np ions increasing at a lower rate. At the same temperature, quadrupolar order on both the U and Np ions occurs along with an internal Jahn-Teller lattice distortion.

cond-mat.str-el↗

arXiv2003

Multi-$\vec{k}$ Configurations

Using resonant x-ray scattering to perform diffraction experiments at the U M$_{4}$ edge novel reflections of the generic form $ $ have been observed in UAs$_{0.8}$Se$_{0.2}$ where $\vec{k} = < k00>$, with $k = {1/2}$ reciprocal lattice units, is the wave vector of the primary (magnetic) order parameter. The $ $ reflections, with $10^{-4}$ of the $ $ magnetic intensities, cannot be explained on the basis of the primary order parameter within standard scattering theory. A full experimental characterisation of these reflections is presented including their energy, azimuthal and temperature dependencies. On this basis we establish that the reflections most likely arise from the electric dipole operator involving transitions between the core 3d and partially filled $5f$ states. The temperature dependence couples the $ $ peak to the triple-$\vec{k}$ region of the phase diagram: Below $\sim 50$ K, where previous studies have suggested a transition to a double-$\vec{k}$ state, the intensity of the $ $ is dramatically reduced. Whilst we are unable to give a definite explanation of how these novel reflections appear, this paper concludes with a discussion of possible ideas for these reflections in terms of the coherent superposition of the 3 primary (magnetic) order parameters.

cond-mat↗