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Correlated 5f electronic states and phase stability in americium under high pressure: Insights from DFT+DMFT calculations

Haiyan Lu

TL;DR

This work applies a fully self-consistent DFT+DMFT approach with spin-orbit coupling to four experimentally known high-pressure phases of americium, revealing a progression from localized 5f states in Am-I and Am-II to increasing hybridization and partial delocalization in Am-III and Am-IV. Spectral functions show a robust -2.8 eV Am-I feature, with a shift and emergence of multi-peak structures as pressure grows, along with enhanced f–conduction hybridization and growing valence-state fluctuations, particularly in Am-IV. The study finds moderate electronic correlations (quasiparticle weights Z ~ 0.4–0.6, m* ~ 2–3 m_e), a trend toward jj angular-momentum coupling, and stabilization of the low-symmetry phases via a Peierls-like distortion that lowers energy through electronic reconstruction. These results offer a unified microscopic framework for 5f electron localization, hybridization, and structural stability in heavy actinides under extreme pressure, helping to resolve longstanding experimental-theoretical tensions.

Abstract

We investigate the electronic structure of americium (Am) across its four experimentally confirmed high-pressure phases Am-I (P63/mmc), Am-II (Fm-3m), Am-III (Fddd), and Am-IV (Pnma) up to 100 GPa, using density functional theory combined with embedded dynamical mean-field theory. Our results successfully reproduce the prominent localized 5f peak observed in ultraviolet photoelectron spectroscopy around -2.8 eV below the Fermi level in the Am-I phase. While 5f electrons in Am-I and Am-II remain strongly localized, those in Am-III and Am-IV manifest discernible signatures of increased hybridization: a noticeable shift of spectral weight toward the Fermi level, enhanced hybridization strength, and the emergence of distinct multi-peak structures. These changes indicate that 5f electrons begin to participate in bonding and undergo partial delocalization under pressure. Nevertheless, the spectral weight of 5f electrons near the Fermi level in Am-IV remains relatively low, indicating that, compared to U and Pu, Am retains stronger localized 5f electrons even under high pressure. Analysis of the electronic configurations reveals pressure-enhanced valence state fluctuation, characterized by the mixing of 5f5, 5f6, and 5f7 electronic configurations. The X-ray absorption branching ratio further shows that the angular-momentum coupling scheme approaches the jj limit. Additionally, we demonstrate that the stability of the low-symmetry high-pressure phases (Am-III and Am-IV) is governed by a Peierls-like distortion mechanism, which reduces the total energy through symmetry-lowering lattice distortions accompanied by electronic reconstruction. This study offers a new microscopic perspective on high-pressure phase transitions and emergent quantum phenomena in actinide materials.

Correlated 5f electronic states and phase stability in americium under high pressure: Insights from DFT+DMFT calculations

TL;DR

This work applies a fully self-consistent DFT+DMFT approach with spin-orbit coupling to four experimentally known high-pressure phases of americium, revealing a progression from localized 5f states in Am-I and Am-II to increasing hybridization and partial delocalization in Am-III and Am-IV. Spectral functions show a robust -2.8 eV Am-I feature, with a shift and emergence of multi-peak structures as pressure grows, along with enhanced f–conduction hybridization and growing valence-state fluctuations, particularly in Am-IV. The study finds moderate electronic correlations (quasiparticle weights Z ~ 0.4–0.6, m* ~ 2–3 m_e), a trend toward jj angular-momentum coupling, and stabilization of the low-symmetry phases via a Peierls-like distortion that lowers energy through electronic reconstruction. These results offer a unified microscopic framework for 5f electron localization, hybridization, and structural stability in heavy actinides under extreme pressure, helping to resolve longstanding experimental-theoretical tensions.

Abstract

We investigate the electronic structure of americium (Am) across its four experimentally confirmed high-pressure phases Am-I (P63/mmc), Am-II (Fm-3m), Am-III (Fddd), and Am-IV (Pnma) up to 100 GPa, using density functional theory combined with embedded dynamical mean-field theory. Our results successfully reproduce the prominent localized 5f peak observed in ultraviolet photoelectron spectroscopy around -2.8 eV below the Fermi level in the Am-I phase. While 5f electrons in Am-I and Am-II remain strongly localized, those in Am-III and Am-IV manifest discernible signatures of increased hybridization: a noticeable shift of spectral weight toward the Fermi level, enhanced hybridization strength, and the emergence of distinct multi-peak structures. These changes indicate that 5f electrons begin to participate in bonding and undergo partial delocalization under pressure. Nevertheless, the spectral weight of 5f electrons near the Fermi level in Am-IV remains relatively low, indicating that, compared to U and Pu, Am retains stronger localized 5f electrons even under high pressure. Analysis of the electronic configurations reveals pressure-enhanced valence state fluctuation, characterized by the mixing of 5f5, 5f6, and 5f7 electronic configurations. The X-ray absorption branching ratio further shows that the angular-momentum coupling scheme approaches the jj limit. Additionally, we demonstrate that the stability of the low-symmetry high-pressure phases (Am-III and Am-IV) is governed by a Peierls-like distortion mechanism, which reduces the total energy through symmetry-lowering lattice distortions accompanied by electronic reconstruction. This study offers a new microscopic perspective on high-pressure phase transitions and emergent quantum phenomena in actinide materials.
Paper Structure (11 sections, 4 equations, 7 figures, 2 tables)

This paper contains 11 sections, 4 equations, 7 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Methods
  3. Results
  4. Comparison with experimental photoemission spectroscopy
  5. Quasiparticle resonance peaks
  6. Band structure and hybridization function
  7. Atomic eigenstate probabilities
  8. Self-energy functions
  9. Angular momentum coupling scheme
  10. Discussion
  11. Conclusions

Figures (7)

  • Figure 1: (Color online) Comparisons of theoretical and experimental density of states for Am. The total density of states (green-solid line) after considering the Fermi-Dirac distribution and 5$f$ partial density of states (color-filled regions) of Am-I phase at $T = 290$ K obtained by the DFT + DMFT method. The experimental data (filled blue circles) are taken from PhysRevLett.52.1834. The Fermi level $E_F$ is represented by vertical dashed line.
  • Figure 2: (Color online) Total and 5$f$ partial density of states of Am at $T = 290$ K by DFT + DMFT calculations. (a) Total density of states A($\omega$) (in solid lines) and 5$f$ partial density of states A$_{5f}$($\omega$) (in colored shadow regions). (b) and (c) Orbital-resolved (or $j$-resolved) 5$f$ partial density of states $j$=5/2 and $j$=7/2. The Fermi levels $E_{F}$ are represented by vertical dashed lines.
  • Figure 3: (Color online). Momentum-resolved spectral functions A(${\bf k}$, $\omega$) of Am at $T = 290$ K obtained by the DFT + DMFT method. (a) Am-I. (b) Am-II. (c) Am-III. (d) Am-IV. The horizontal dashed lines denote the Fermi levels.
  • Figure 4: (Color online) Hybridization functions of Am at $T = 290$ K obtained by the DFT + DMFT method. (a) For the Am-5$f$$j$=5/2 components. (b) For the Am-5$f$$j$=7/2 components.
  • Figure 5: (Color online) Valence state histograms of Am at $T = 290$ K by DFT + DMFT calculations. (a)–(d) Atomic eigenstate probabilities of Am-I, Am-II, Am-III, and Am-IV. The atomic eigenstates are denoted by using good quantum numbers $N$ (total occupancy), $J$ (total angular momentum) and $\gamma$ (rest of good quantum numbers), i.e., $|\psi_\Gamma\rangle=|N, J, \gamma \rangle$. The distributions of atomic eigenstate probabilities with respect to different $N$ are displayed in the legends.
  • ...and 2 more figures