Quantum charge density fluctuations and the phase transition in Ce

Abstract

We have calculated the quantum quadrupolar interaction due to charge density fluctuations of localized 4f -electrons in γ−Ce by taking into account the angular dependence, the degeneracy of the localized 4f −orbitals and the spin-orbit coupling. The calculated crystal field of 4f electronic states is in good agreement with neutron diffraction measurements. We show that orientational ordering of quantum quadrupoles drives a Fm3m → Pa3 phase transition at ∼ 86 K which we assign with the γ−α transformation. In the Pa3 phase the centers of mass of the Ce atoms still form a face centered cubic lattice. The theory accounts for the first order character of the transition and for the cubic lattice contraction which accompanies the transition. The transition temperature increases linearly with pressure. Our approach does not involve Kondo spin fluctuations as the significant process for the phase transition. PACS. 71.10.-w Theories and models of many electron systems -71.27.+a Strongly correlated electron systems; heavy fermions -71.45.-d Collective effects -64.70.Kb Solid-solid transitions

Molecular terms, magnetic moments, and optical transitions of molecular ions C60m±

“…(The spin-orbit coupling is a single particle operator and in principle can be included in the calculation [15].) We start with a pair of t 1u electrons and label the twoelectron basis ket-vectors by a single index I which incorporates a pair of one-electron indices (i 1 , i 2 ),…”

Electronic structure calculations with dynamical mean-field theory

“…However, unlike the ␣ → ␥ transition which is believed to be isostructural, or perhaps having a small symmetry change ͑Eliashberg and Capellmann, 1998; Nikolaev andMichel, 1999, 2002͒, the volume changing transitions in actinides are accompanied by changes in the structure.…”

The puzzle of the $ \gamma\to\alpha$ and other phase transitions in cerium

“…However, there is another approach [73][74][75] which treats the phase change as a hidden structural transformation (see Sec. 6).…”

“…[73][74][75]37] a distinctive approach is presented, which considers the  phase transition as a hidden structural transformation. Note that its intrinsic feature is softening of bulk modulus and elastic anomalies [69,78,80] (Sec.…”

“…The idea on the nonisomorphic nature of the  transition was independently formulated in Refs. [74,75,37], where a quadrupolar ordering of the electron density in the -phase was considered as the driving force for the phase transformation. Although the quadrupolar model can be viewed as a continuation of the approach of Eliashberg and Capellmann, already in the first work [74] it was claimed that there is no distortions of the fcc lattice of cerium nuclei, and all structural changes are associated with electronic density of the valence electrons.…”