Electronic origin of the volume collapse in cerium

Abstract

The cerium α-γ phase transition is characterized by means of a many-body Jastrow-correlated wave function, which minimizes the variational energy of the first-principles scalar-relativistic Hamiltonian, and includes correlation effects in a non-perturbative way. Our variational ansatz accurately reproduces the structural properties of the two phases, and proves that even at temperature T = 0K the system undergoes a first order transition, with ab-initio parameters which are seamlessly connected to the ones measured by experiment at finite T . We show that the transition is related to a complex rearrangement of the electronic structure, with key role played by the p-f hybridization. The underlying mechanism unveiled by this work can hold in many Ce-bearing compounds, and more generally in other f-electron systems.PACS numbers: 71.20.Eh, 71.27.+a, 02.70.Ss Understanding the anomalous behavior of cerium, the prototypical f -electron system, is one of the main challenges in condensed matter physics. The 4f electrons are strongly localized and their on-site Coulomb repulsion is large compared to bandwidth. Among all lanthanides, cerium is particularly fascinating, due to the strong hybridization with the 6s6p5d bands, all present at the Fermi level. The origin of the cerium volume collapse along the isostructural α-γ transition has been a puzzle since its discovery in 1927 [1]. A microscopic comprehensive description of the transition is still lacking, because a direct comparison with the measured structural properties requires an accuracy below 10 meV. This challenges any ab-initio method, particularly in a regime of strong correlation. Model calculations have been performed in the Mott[2], Kondo [3,4] and dynamical mean field theory (DMFT) [5-9] frameworks, with input parameters either chosen ad-hoc or derived from first-principles density functional theory (DFT) and cRPA calculations [10]. Fully first-principle electronic structure schemes, such as DFT [11] or GW [12], grasp some features of the α and γ phases, but the quantitative agreement with experiment is generally quite poor.Experimentally, pure cerium undergoes the α-γ transition always at finite temperature T . Recently, very accurate X-ray diffraction measurements undoubtedly confirmed the first-order Fm3m isostructural character of the transition [13]. The first-order line extrapolates to zero-T at negative pressures. Nevertheless, the T =0K determination of its phase diagram is extremely important as it can shed light on the underlying electronic structure mechanism of the transition, and clarify some critical points still under debate. For instance, some experiments with cerium alloys seem to find a critical low-T end-point on the α-γ phase boundary [14], where the effect of alloying is expected to provide a negative chemical pressure on the cerium sites. However, it has also been proven that the end-point of the critical line can be tuned down to zero T by changing the bulk modulus through alloying, thus opening the way of new low-T scenarios, lik...

Polyamorphism of a Ce-based bulk metallic glass by high-pressure and high-temperature density measurements

“…In both cases, the density change is not accompanied by a rearrangement of the local geometry, nor by a modification of the coordination number, at variance with standard structural phase transitions. In a previous theoretical study of pure Ce [26] based on quantum Monte Carlo simulations, some of us pointed out that the Ce-Ce chemical bond variation, driven by strong local electron correlation and forbital hybridization, is key to understand the volume collapse. The corresponding phase transition is found already at zero temperature, even before the entropy effects dictated by the Kondo energy scale kick in, whose magnitude exponentially varies as a function of the f-electron hybridization [32].…”

“…The emergence of this new peak (labeled c) first demonstrates that a LDA-HDA transition occurs with local structure transformation probably related to the critical change of electronic properties of Ce atoms [25,26]. Secondly, the shift of the FSDP without intensity change shows that such transition does not significantly alter the mean distance order (taken, here, as the total coordination number).…”

Phase stability and properties of manganese oxide polymorphs: Assessment and insights from diffusion Monte Carlo

“…Scalar-relativistic energy-consistent HartreeFock pseudopotentials ([Ne] core for Mn) as implemented by Burkatzki, Filippi, and Dolg (BFD) 26 were used to remove the core electrons. These pseudopotentials are designed for use within QMC and there are now several indications in the literature that they are well-suited for DMC simulations of solids [27][28][29][30] . The rock salt structure of MnO has previously been studied within DMC 31 ; to this analysis we now provide a comparison between the ZB and RS polymorphs, physical insights into the electronic structure of the two phases, and statistical analysis of the many body wave functions to reveal the reasons for the failure of conventional and hybrid DFT to obtain the correct energy ordering.…”

Exact special twist method for quantum Monte Carlo simulations

“…QMC methods provide a truly first-principles approach to molecular as well as extended systems. These features, along with the negligible parallel overhead of the main QMC algorithms, allow an unprecedented level of accuracy on a wide range of systems ranging from small/medium size molecules [3][4][5] to strongly correlated materials as cerium 6 and iron 7 and, recently, several high-temperature superconductors [8][9][10][11] . Numerous studies have also been carried out on systems dominated by weak intermolecular forces such as Van der Waals interactions [12][13][14][15] and on adsorption phenomena 16,17 .…”