John J. Joyce

Photoemission and the electronic structure of PuCoGa5.

The electronic structure of the first Pu-based superconductor PuCoGa5 is explored using photoelectron spectroscopy and a novel theoretical scheme. Exceptional agreement between calculation and experiment defines a path forward for understanding the electronic structure aspects of Pu-based materials. The photoemission results show two separate regions of 5f electron spectral intensity, one at the Fermi energy and another centered 1.2 eV below the Fermi level. The results for PuCoGa5 clearly indicate 5f electron behavior on the threshold between localized and itinerant. Comparisons to delta phase Pu metal show a broader framework for understanding the fundamental electronic properties of the Pu 5f levels in general within two configurations, one localized and one itinerant.

Effect of spin-orbit coupling on the actinide dioxides AnO2 (An=Th, Pa, U, Np, Pu, and Am): A screened hybrid density functional study

We present a systematic comparison of the lattice structures, electronic density of states, and band gaps of actinide dioxides, AnO(2) (An=Th, Pa, U, Np, Pu, and Am) predicted by the Heyd-Scuseria-Ernzerhof screened hybrid density functional (HSE) with the self-consistent inclusion of spin-orbit coupling (SOC). The computed HSE lattice constants and band gaps of AnO(2) are in consistently good agreement with the available experimental data across the series, and differ little from earlier HSE results without SOC. ThO(2) is a simple band insulator (f(0)), while PaO(2), UO(2), and NpO(2) are predicted to be Mott insulators. The remainders (PuO(2) and AmO(2)) show considerable O2p/An5f mixing and are classified as charge-transfer insulators. We also compare our results for UO(2), NpO(2), and PuO(2) with the PBE+U, self interaction correction (SIC), and dynamic mean-field theory (DMFT) many-body approximations.

Dispersion in the Mott insulator UO2: A comparison of photoemission spectroscopy and screened hybrid density functional theory

We present a comparison between the screened hybrid density functional theory of Heyd, Scuseria, and Enzerhof (HSE06) and high‐resolution photoemission (PES) measurement on a single crystal of UO2. Angle‐resolved photoemission data show a slight dispersion in the f‐orbital derived bands in good agreement with the HSE band structure. The effect of spin‐orbit coupling on the HSE band gap has also been calculated and found to be negligible.

Thermal work function shifts for polycrystalline metal surfaces

Abstract A new approach is presented for the calculation of the thermal shifts of the work functions of polycrystalline metal surfaces by including the thermal expansion of the metal into the electron density parameter as well as Fermi energy. Calculations of d ϕ /d T as a function of temperature are performed for 74 elements, encompassing a total of 103 cases of various temperature ranges, phases and (in a few cases) orientations. It is shown that for a majority of metals d ϕ /d T is not a constant over a broad temperature range. Comparison of theoretical and experimental values indicates that the thermal lattice vibrations do not influence d ϕ /d T significantly. Our results would suggest that the use of a Kelvin probe to measure the thermal variation of ϕ may be problematic.

Imaging the three-dimensional Fermi-surface pairing near the hidden-order transition in URu2Si2 using angle-resolved photoemission spectroscopy.

Condensed Matter and Magnet Science Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Department of Physics and Astronomy, Uppsala University, Box 516, S-75120 Uppsala, Sweden Kamerlingh Onnes Laboratory, Leiden University, NL-2300 RA Leiden, The Netherlands Department of Physics, Temple University Philadelphia, PA 19122, USA Wolfram Research Inc., Champaign, IL 61820, USA (Dated: Wednesday 20 February, 2013)

Angle-resolved photoemission study of Usb2: The 5f band structure

Single crystal antiferromagnetic USb 2 was studied at 15 K by angle-resolved photoemission with an overall energy resolution of 24 meV. The measurements unambiguously show the dispersion of extremely narrow bands situatednear the Fermi level. The peak at the Fermi level represents the narrowest feature observed in 5f-electron photoemission to date. The natural linewidth of the feature just below the Fermi level is not greater than 10 meV. Normal emission data indicate a three dimensional aspect to the electronic structure of this layered material.

Electronic structure of heavy fermions: Narrow temperature-independent bands☆

The electronic structure of both Ce and U heavy fermions appears to consist of extremely narrow temperature independent bands. There is no evidence from photoemission for a collective phenomenon normally referred to as the Kondo resonance. In uranium compounds a small dispersion of the bands is easily measurable.

Observation of a kink in the dispersion of f-electrons

Strong interactions in correlated electron systems may result in the formation of heavy quasiparticles that exhibit kinks in their dispersion relation. Spectral weight is incoherently shifted away from the Fermi energy, but Luttingers theorem requires the Fermi volume to remain constant. Our angle-resolved photoemission study of USb2 reveals a kink in a noncrossing 5f band, representing the first experimental observation of a kink structure in f-electron systems. The kink energy scale of 21 meV and the ultra-small peak width of 3 meV are observed. We propose the novel mechanism of renormalization of a point-like Fermi surface, and that Luttingers theorem remains applicable.

Resonance photoelectron spectroscopy using a tunable laser plasma light source

We report the first resonance photoemission experiments on plutonium metal, with surface preparation accomplished by means of laser ablation. The photoemission experiments were conducted using the newly commissioned tunable light source for transuranic research at Los Alamos. The light source is a tunable, laser plasma system capable of producing photons in the energy range 30-150 eV, with a photon flux of order 10 9 photons s -1 at 0.1% bandwidth. First results on δ-phase plutonium reveal the 5d-5f resonance enhancement to be maximum at a photon energy of 113 eV. Lower energy photoemission results show a narrow peak very near the Fermi level, with a linewidth of order 100 meV. This peak is verified by resonance photoemission to be of 5f character but also having strong 6d-7s contributions. The combination of high-energy resolution, tunable energies for orbital character determination and surface sensitivity of the UPS energy range make the laser plasma light source ideal for surface and interface studies.

A tunable bench top light source for photoelectron spectroscopy : first results for alpha and delta Pu

Abstract We report the first ever utilization of a laser plasma light source for photoelectron spectroscopy. Using a continuous mercury stream as the target, the light source is essentially debris free with intensities rivaling some second generation syncrotrons. Its primary use is likely to be in connection with hazardous materials which are undesirable at synchrotrons, such as Pu. Toward this end we report the first photoemission data on α- and δ-Pu. Both phases display a 5f-related feature at E F , which is much sharper in δ-Pu than in α-Pu, suggestive of heavy fermion behavior. In both phases, however, the hν-dependence of the intensity of this feature can only be understood if there exists substantial 6d-admixture.