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Register a new userElectronic Structure of Heavy Fermion Uranium Compounds Studied by Core-Level Photoelectron Spectroscopy
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High-energy-resolution core-level and valence-band photoelectron spectroscopic studies were performed for the heavy Fermion uranium compounds UGe2, UCoGe, URhGe, URu2Si2, UNi2Al3, UPd2Al3, and UPt3 as well as typical localized and itinerant uranium compounds to understand the relationship between the uranium valence state and their core-level spectral line shapes. In addition to the main line and high-binding energy satellite structure recognized in the core-level spectra of uranium compounds, a shoulder structure on the lower binding energy side of the main lines of localized and nearly localized uranium compounds was also found. The spectral line shapes show a systematic variation depending on the U 5f electronic structure. The core-level spectra of UGe2, UCoGe, URhGe, URu2Si2, and UNi2Al3 are rather similar to those of itinerant compounds, suggesting that U 5f electrons in these compounds are well hybridized with ligand states. On the other hand, the core-level spectra of UPd2Al3 and UPt3 show considerably different spectral line shapes from those of the itinerant compounds, suggesting that U 5f electrons in UPd2Al3 and UPt3 are less hybridized with ligand states, leading to the correlated nature of U 5f electrons in these compounds. The dominant final state characters in their core-level spectra suggest that the numbers of 5f electrons in UGe2, UCoGe, URhGe, URu2Si2, UNi2Al3, and UPd2Al3 are close to but less than three, while that of UPt3 is close to two rather than to three.
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The electronic structure of the unconventional superconductor UTe$_2$ was studied by resonant photoelectron spectroscopy (RPES) and angle-resolved photoelectron spectroscopy (ARPES) with soft X-ray synchrotron radiation. The partial $mathrm{U}~5f$ density of states of UTe$_2$ were imaged by the $mathrm{U}~4d$--$5f$ RPES and it was found that the $mathrm{U}~5f$ state has an itinerant character, but there exists an incoherent peak due to the strong electron correlation effects. Furthermore, an anomalous admixture of the $mathrm{U}~5f$ states into the $mathrm{Te}~5p$ bands was observed at a higher binding energy, which cannot be explained by band structure calculations. On the other hand, the band structure of UTe$_2$ was obtained by ARPES and its overall band structure were mostly explained by band structure calculations. These results suggest that the $mathrm{U}~5f$ states of UTe$_2$ have itinerant but strongly-correlated nature with enhanced hybridization with the $mathrm{Te}~5p$ states.
We use angle-resolved photoemission spectroscopy to study heavy fermion superconductor Ce2RhIn8. The Fermi surface is rather complicated and consists of several hole and electron pock- ets. We do not observe kz dispersion of Fermi sheets, which is consistent with 2D character of the electronic structure. Comparison of the ARPES data and band structure calculations points to a localized picture of f electrons. Our findings pave the way for understanding the transport and thermodynamical properties of this material.
In an effort to explore the differences between rare-earth-based and uranium-based heavy Fermion (HF) compounds that reflect the underlying difference between local 4$f$ moments and itinerant 5$f$ moments we analyze scaling laws that relate the low temperature neutron spectra of the primary (Kondo-esque) spin fluctuation to the specific heat and susceptibility. While the scaling appears to work very well for the rare earth intermediate valence compounds, for a number of key uranium compounds the scaling laws fail badly. There are two main reasons for this failure. First, the presence of antiferromagnetic (AF) fluctuations, which contribute significantly to the specific heat, alters the scaling ratios. Second, the scaling laws require knowledge of the high temperature moment degeneracy, which is often undetermined for itinerant 5$f$ electrons. By making plausible corrections for both effects, better scaling ratios are obtained for some uranium compounds. We point out that while both the uranium HF compounds and the rare earth intermediate valence (IV) compounds have spin fluctuation characteristic energies of order 5 - 25 meV, they differ in that the AF fluctuations that are usually seen in the U compounds are never seen in the rare earth IV compounds. This suggests that the 5f itineracy increases the f-f exchange relative to the rare earth case.
We systemically investigate the nature of Ce 4f electrons in structurally layered heavy-fermion compounds CcmMnIn3m+2n (with M =Co, Rh, Ir, and Pt, m=l, 2, n=0 - 2), at low temperature using on-resonance angle-resolved photoemission spectroscopy. Three heavy quasiparticle bands f^0, f^1_7/2 and f^1_5/2 are observed in all compounds, but their intensities and energy locations vary greatly with materials. The strong f^0 states imply that the localized electron behavior dominates the Ce 4f states. The Ce 4f electrons are partially hybridized with the conduction electrons, making them have the dual nature of localization and itinerant. Our quantitative comparison reveals that the f^1_5/2 / f^0 intensity ratio is more suitable to reflect the 4f-state hybridization strength.
The valence state of UTe$_2$ was studied by core-level photoelectron spectroscopy. The main peak position of the U $4f$ core-level spectrum of UTe$_2$ coincides with that of UB$_2$, which is an itinerant compound with a nearly $5f^3$ configuration. However, the main peak of UTe$_2$ is broader than that of UB$_2$, and satellite structures are observed in the higher binding energy side of the main peak, which are characteristics of mixed-valence uranium compounds. These results suggest that the U 5$f$ state in UTe$_2$ is in a mixed valence state with a dominant contribution from the itinerant $5f^3$ configuration.
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