Modeling Thermal and Quantum Magnetic Fluctuations in Correlated Materials

Babak Sadigh | 17-ERD-041


Quantum as well as classical molecular-dynamics simulations are indispensable tools for exploring fundamental atomistic processes in chemistry and materials science. However, a similar methodology for quantitative understanding of magnetic fluctuations and their role in determining thermal, mechanical, and electromagnetic properties of materials is still not available.

Our project aimed to develop an important capability in response to two pressing needs at the Lawrence Livermore National Laboratory: 1) to model the mechanical and thermal properties of plutonium and its alloys as they age, and 2) to understand the kinetics of volume-collapse transitions in magnetic, e.g., iron, and correlated systems, e.g., actinides, under dynamic loading. The common denominator in the physics of these two problems is the importance of microscopic quantum/thermal spin/valence fluctuations whose contributions cannot be simply taken into account in a mean-field fashion. As a result, we developed a suite of codes that have enabled an unprecedented capability to the study of spin dynamics and coupled spin-lattice fluctuations from ab initio electronic structure calculations. Coarse-grained Hamiltonians for spin and displacive disorder, and their coupling, can now be parameterized from spin-constrained density-functional theory (DFT) and beyond-DFT calculations. This parameterization can be performed with mathematical rigor via the combined use of compressive sampling, thus avoiding the limitations of heuristic intuition-based approaches of the past. These basic components have been integrated with software for force-biased Monte Carlo and molecular dynamics simulations. We used this novel technology to study finite-temperature phonon magnon coupling in paramagnetic iron as well as non-collinear magnetism in different phases of plutonium metal at ambient condition, correctly taking account of relativistic effects as well as orbital polarization.

Impact on Mission

Our work on plutonium supports the Laboratory's nuclear weapons stockpile stewardship mission focus area. We will be able to better address plutonium aging issues as well as temperature-dependent properties heretofore unattainable from first-principles modeling. The work on shock-induced phase transformation of iron supports the Laboratory's high-energy-density science core competency. Our work developing computational code, as well as overcoming the computational challenges presented by this project, also supports the Laboratory's high-performance computing, simulation, and data science core competencies.

Publications, Presentations, Etc.

Soderlind, P. 2016. "Phase Stability, Elasticity, and Phonons for Plutonium from Electronic-Structure Theory." Plutonium Futures: The Science 2016, Baden-Baden, Germany, September 2016. LLNL-ABS-690920.

––– . 2017. "Lattice Dynamics and Elasticity for Epsilon-Plutonium." Scientific Reports 7: 1116, April 2017. LLNL-JRNL-695423.

––– . 2018. "Phase Stability, Elasticity, and Phonons for Plutonium from Electronic-Structure Theory." LLNL-PRES-736878.

Soderlind, P. and B. Sadigh. 2018. "Free-Energy Calculations for Plutonium." Pu Futures, San Diego, CA, September 2018. LLNL-ABS-749944.

––– . 2019. "Free-Energy Calculations for Plutonium." Actinide Research Quarterly , Third Quarter 2019. LLNL-ABS-749944.

Soderlind, P., et al. 2016. "Density-Functional Theory for Plutonium: Phase Stability, Elasticity, Phonons, and Magnetic Structure." 2016 MRS Spring Meeting & Exhibit, Phoenix, AZ, United States, March 28 through April 1, 2016. LLNL-ABS-678576.

––– . 2019. "Density-Functional Theory for Plutonium." Advances in Physics 68(1) 1-47. LLNL-JRNL-764803.

Zhou, F. and V. Ozolins. 2018. "A Unified Treatment of Derivative Discontinuity, Delocalization and Static Correlation Effects in DFT: The LDA Plus Density Matrix Minimization (DMM) Method." LLNL-PRES-740899.

Zhou, F., et al. 2018. "Compressive Sensing Lattice Dynamics. II. Efficient Phonon Calculations." arXiv . Feb. 16, 2019. LLNL-JRNL-726598.