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Register a new userExperimental electronic heat capacities of $alpha-$ and $delta-$Plutonium; heavy-fermion physics in an element
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We have measured the heat capacities of $delta-$Pu$_{0.95}$Al$_{0.05}$ and $alpha-$Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduction of the electronic contribution to the heat capacity of $delta-$Pu$_{0.95}$Al$_{0.05}$; we find $gamma = 64 pm 3$ mJK$^{-2}$mol$^{-1}$ as $T to 0$. This is a factor $sim 4$ larger than that of any element, and large enough for $delta-$Pu$_{0.95}$Al$_{0.05}$ to be classed as a heavy-fermion system. By contrast, $gamma = 17 pm 1$ mJK$^{-2}$mol$^{-1}$ in $alpha-$Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of $delta-$Pu$_{0.95}$Al$_{0.05}$, one or both of which may be associated with the formation of the $alpha-$ martensitic phase. We suggest that the large $gamma$-value of $delta-$Pu$_{0.95}$Al$_{0.05}$ may be caused by proximity to a quantum-critical point.
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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.
An understanding of the phase diagram of elemental plutonium (Pu) must include both the effects of the strong directional bonding and the high density of states of the Pu 5f electrons, as well as how that bonding weakens under the influence of strong electronic correlations. We present for the first time electronic-structure calculations of the full 16-atom per unit cell alpha-phase structure within the framework of density functional theory (DFT) together with dynamical mean-field theory (DMFT). Our calculations demonstrate that Pu atoms sitting on different sites within the alpha-Pu crystal structure have a strongly varying site dependence of the localization-delocalization correlation effects of their 5f electrons and a corresponding effect on the bonding and electronic properties of this complicated metal. In short, alpha-Pu has the capacity to simultaneously have multiple degrees of electron localization/delocalization of Pu 5f electrons within a pure single-element material.
Plutonium (Pu), in which the 5$f$ valence electrons always wander the boundary between localized and itinerant states, exhibits quite complex crystal structures and unprecedentedly anomalous properties with respect to temperature and alloying. Understanding its chemical and physical properties, especially its 5$f$ electronic structure is one of the central and unsolved topics in condensed matter theory. In the present work, the electronic structures of the six allotropes of Pu (including its $alpha$, $beta$, $gamma$, $delta$, $delta$, and $epsilon$ phases) at ambient pressure are studied comprehensively by means of the density functional theory in combination with the single-site dynamical mean-field theory. The band structures, total and partial density of states, valence state histograms, 5$f$ orbital occupancies, X-ray branching ratios, and self-energy functions are carefully studied. It is suggested that the $alpha$, $beta$, and $gamma$ phases of Pu are typical Racah metals in which the atomic multiple effect dominates near the Fermi level. The calculated results reveal that not only the $delta$ phase, but also all the six allotropes are archetypal mixed-valence metals with remarkable atomic eigenstate fluctuation. In consequence of that, the 5$f$ occupancy $n_{5f}$ is around 5.1 $sim$ 5.4, which varies with respect to the atomic volume and electronic correlation strength of Pu. The 5$f$ electronic correlation in Pu is moderately orbital-dependent. Moreover, the 5$f$ electrons in the $delta$ phase are the most correlated and localized.
Plutonium is a critically important material as the behavior of its 5f-electrons stands midway between the metallic-like itinerant character of the light actinides and localized atomic-core-like character of the heavy actinides. The delta-phase of plutonium (delta-Pu), while still itinerant, has a large coherent Kondo peak and strong electronic correlations coming from its near-localized character. Using sophisticated Gutwiller wavefunction and dynamical mean-field theory correlated theories, we study for the first time the Fermi surface and associated mass renormalizations of delta-Pu together with calculations of the de Haas-van Alphen (dHvA) frequencies. We find a large (200%) correlation-induced volume expansion in both the hole and electron pockets of the Fermi surface in addition to an intermediate mass enhancement. All of the correlated electron theories predict, approximately, the same hole pocket placement in the Brillouin zone, which is different from that obtained in conventional density-functional band-structure theory, whereas the electron pockets from all theories are in, roughly, the same place.
Calorimetry experiments under high pressure were used to clarify the interplay between different states such as superconductivity and antiferromagnetism in CeRhIn5, spin density wave and large moment antiferromagnetism in URu2Si2. Evidences are given on the re-entrance of antiferromagnetism under magnetic field in the superconducting phase of CeRhIn5 up to pc = 2.5 GPa where the Neel temperature will collapse in the absence of superconductivity. For URu2Si2 measurements up to 10 GPa support strongly the coexistence of spin density wave and large moment antiferromagnetism at high pressures.
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