Luis A. Morales

Plutonium-based superconductivity with a transition temperature above 18 K

Plutonium is a metal of both technological relevance and fundamental scientific interest. Nevertheless, the electronic structure of plutonium, which directly influences its metallurgical properties, is poorly understood. For example, plutoniums 5f electrons are poised on the border between localized and itinerant, and their theoretical treatment pushes the limits of current electronic structure calculations. Here we extend the range of complexity exhibited by plutonium with the discovery of superconductivity in PuCoGa5. We argue that the observed superconductivity results directly from plutoniums anomalous electronic properties and as such serves as a bridge between two classes of spin-fluctuation-mediated superconductors: the known heavy-fermion superconductors and the high-Tc copper oxides. We suggest that the mechanism of superconductivity is unconventional; seen in that context, the fact that the transition temperature, Tc ≈ 18.5 K, is an order of magnitude greater than the maximum seen in the U- and Ce-based heavy-fermion systems may be natural. The large critical current displayed by PuCoGa5, which comes from radiation-induced self damage that creates pinning centres, would be of technological importance for applied superconductivity if the hazardous material plutonium were not a constituent.

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.

Reactions of plutonium dioxide with water and hydrogen–oxygen mixtures: Mechanisms for corrosion of uranium and plutonium

Abstract Investigation shows that plutonium dioxide interacts chemically with water and catalytically with oxygen–hydrogen mixtures to form water. Water adsorbs strongly on the oxide below 120°C and desorbs as the temperature is increased to 200°C. Hydroxide formed by dissociative adsorption of water promotes formation of the higher oxide (PuO 2+ x ) plus H 2 , and in the presence of O 2 , drives a water-catalyzed cycle that accelerates formation of PuO 2+ x by the PuO 2 +O 2 reaction. Results are consistent with kinetic control of plutonium corrosion by the adherent oxide layer on the metal and imply that moisture-enhanced oxidation is driven by the water-catalyzed cycle. Evaluation of experimental results and literature data for U and Pu lead to a comprehensive corrosion mechanism applicable to both metals in dry air, water vapor, and moist air. In all cases, corrosion proceeds by diffusion of oxide ions through the oxide product, not by transport of hydrogen or hydroxide. Rates vary as changes in the concentration of adsorbed water determine the rate of O 2− formation and the gradient in oxygen concentration across the oxide diffusion barrier. Results account for a sharp decrease in the corrosion rate of Pu by water and moist air as the temperature approaches 200°C.

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.

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.

μSRμSR studies of the superconducting order parameter in PuCoGa5PuCoGa5

We present transverse-field (TF) measurements of the in-plane magnetic-field penetration depth λ(T)λ(T) in single-crystalline PuCoGa5PuCoGa5 (Tc=18.5K) for 0.06T applied field (≈2–5×Hc1)(≈2–5×Hc1). We find that the temperature dependence of the superfluid density, and therefore Δλ(T)=λ(T)-λ(0)Δλ(T)=λ(T)-λ(0), is ∝T∝T for T/Tc⩽0.5T/Tc⩽0.5. We estimate that the measured λ(0)=241(3)nm. ZF measurements find no evidence for time-reversal symmetry violation. The ZF and TF measurements are consistent with an even-parity (pseudo-spin singlet), d-wave pairing state.

Electronic structure of correlated electron materials from photoemission in high-quality single crystals

Much of our understanding of heavy fermion systems over the past two decades has been based on the single impurity model and its approximate solutions. We show with numerous examples of photoelectron spectra, especially with YbInCu , 4 that this model is not applicable to stoichiometric heavy fermion compounds. There is overwhelming evidence that the correct description of heavy fermions must include very narrow, hybridized bands which exist already at temperatures far above the thermodynamically determined Kondo temperature, and that these bands are relatively temperature independent. Some form of the periodic Anderson model (PAM) is needed, one which results in very narrow renormalized LDA bands. We compare our data to one form of the PAM.

ARPES in strongly correlated 4f and 5f systems: Comparison to the periodic Anderson model (PAM)

Abstract The electronic structure of both Ce and U heavy fermions appears to consist of extremely narrow, nearly temperature-independent bands (i.e., no spectral weight loss or transfer with temperature). A small dispersion of the f-bands above the Kondo temperature is easily measurable so that a Kondo resonance, as defined by NCA, is not evident. Preliminary results, however, indicate that the Periodic Anderson Model (PAM) captures some of the essential physics. Angle-integrated resonant photoemission results on δ-Pu indicate a narrow 5f feature at E F , similar in width to f-states in Ce and U compounds, but differing in that PES cross-section as a function of h ν suggests substantial 6d admixture.

The electronic structure of CeRhIn5 and LaRhIn5 from ARPES

Abstract In the heavy-fermion CeRhIn5 and the isostructural compound LaRhIn5 the extra 4f-electron in Ce may alter the band structure near EF, suggesting that the 4fs participate in band formation. ARPES data indicates that correlation effects are mostly evident along the Γ–Z direction in the Brillouin zone. Very good agreement to GGA band calculations is found.

Strongly correlated f-electron systems: A PES study

The term heavy fermions refers to materials (thus far only compounds with elements having an unfilled 4f or 5f shells) whose large specific heat {gamma}-values suggest that the conduction electrons at low temperatures have a very heavy effective mass. Magnetic susceptibility measurements, {chi}, generally yield a Curie-Weiss behavior at high temperatures with a well developed moment, which would be consistent with localized behavior of the f-electrons. Thus, the f-electrons appear to behave as non-interacting single impurities at elevated temperature. Below a characteristic Kondo temperature, T{sub K}, the susceptibility levels off or even decreases. This is interpreted as a compensation of the f-moment by the ligand conduction electrons that are believed to align anti-parallel to form a singlet state and has led to the widespread use of the Anderson Impurity Hamiltonian and the Single Impurity Model (SIM). Weak hybridization with these conduction electrons yields a narrow, highly temperature dependent, DOS at the Fermi energy, often referred to as the Kondo resonance (KR). At still lower temperatures it is generally agreed that in stoichiometric compounds a lattice of these singlet states finally results in extremely narrow bands at the Fermi energy, whose bandwidth is of the order k{sub B}T{sub K}. Clearly coherent bands cannot form above T{sub K} owing to the narrow width. A model for periodic Kondo systems will inevitably have to include the lattice. Preliminary PAM calculations indicate that this inclusion yields results differing qualitatively, rather than just quantitatively, from the SIM predictions. The photoemission data on single crystal heavy fermions are consistent with the following PAM predictions: (1) the temperature dependence of the KR is much slower than expected from the SIM; indeed, it is primarily7 due to broadening and Fermi function truncation; (2) the spectral weight of the KR relative to the localized 4f feature (not discussed here) is much larger than the SIM expectations (equivalently, n{sub f} values are far too small); (3) the KR and its sidebands does not lose spectral weight with T, but rather only broadens; (4) f-electrons in both Ce and U systems form narrow bands already far above T{sub K} (the jury is still out for Yb systems); (5) the width of these bands is much larger than k{sub B}T{sub K}; (6) f-character is obtained in only some regions of the Brillouin zone; i.e., momentum dependence of the KR above T{sub K}. While the PAM seems to predict the correct trends, they have no reason yet to rule out other models, such as those of Liu and Sheng and Cooper. Such discrimination may occur when the models develop sufficiently to allow real system calculations.