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  • Title
    Physicist
  • Email
    yang1@llnl.gov
  • Phone
    (925) 424-4153

Research Interests

My primary research interest is to develop and apply efficient quantum simulation methods for equation of state and materials strength modellings under extreme conditions. The current research projects include quantum molecular dynamics (QMD) simulations for low-Z materials, transition-metal oxides and actinides.

Quantum molecular dynamics simulations for equations of state modeling

The quantum-based theoretical framework for obtaining high-pressure and high-temperature phase diagrams and multiphase equations of state (EOS) for metals treats cold, ion-thermal, and electron-thermal contributions to phase stability and the EOS separately. For d- and f-electron metals, however, there can be a high density of electronic states at the Fermi level, leading to a strong coupling between the ion- and electron-thermal components for temperatures as low as melt. This effectively leads to temperature-dependent forces on the ions. Consequently, the high-temperature phase diagram and EOS, the melt curve, and the liquid EOS can all be significantly affected. To treat the electrons and ions on an equal footing I have been developing rigorous ab-initio QMD simulations for d- and f-electron metals, so the additional ion-electron coupling and temperature-dependent forces in question are rigorously treated [1]. The areas of interest for this work are: (i) the development of robust QMD algorithms and pseudopotentials to treat electrons at high temperature and density regimes; (ii) the study of important physical phenomena, including high-temperature phase stability [2, 3] , melting, and liquid structure for equation of state modelling [4]; (iii) the development of QMD-based electron-thermal and ion-thermal models.

Multiscale modeling of materials strength

The predictive modeling across length scales all the way from the atomic level to the continuum level to achieve a physics-based multiscale description of mechanical properties such as plasticity, strength and other mechanical properties requires an accurate atomistic description of defect properties as input into higher length-scale simulations such as 3D dislocation dynamics (DD) of single-crystal plasticity at the microscale. Especially important is the accurate atomistic modeling of the structure, motion, and interaction of individual dislocations, as well as the accurate modeling of the relevant aspects of elasticity, including elastic moduli and the limits of elastic stability. To accomplish this task fully, one not only needs to understand the underlying qualitative mechanisms that control plastic deformation, but also needs to be able to calculate the quantitative parameters that will allow a predictive description of plasticity and strength properties in real materials under various conditions. The latter is particularly important in regimes where experimental data are scarce or nonexistent such as under the extreme conditions of pressure, temperature, strain, and strain rate of current interest to many modern applications. Especially interesting in this regard is the regime of high pressure, a regime in which dislocation-driven plasticity has been heretofore largely unexplored from a fundamental perspective. My contribution of this work is to help fill that void. Specifically, we elaborate here a predictive multiscale description of dislocation behavior and single-crystal plasticity in bcc transition metals (e.g., Pb) over a wide range of pressures, ranging from ambient all the way up to many hundreds of gigapascals (GPa) [5].

References

  1. Anharmonicity-induced first-order isostructural phase transition of zirconium under pressure, E. Stavrou, L.H. Yang, P. Söderlind, D. Aberg, H.B. Radousky, M.R. Armstrong, Physical Review B 98, 220101 (2018).
  2. Quantum-mechanical interatomic potentials with electron temperature for strong-coupling transition metals. J.A. Moriarty, R.Q. Hood, L.H. Yang, Physical Review Letters 108 (3), 036401 (2012).
  3. High-temperature phonon stabilization of g-uranium from relativistic first-principles theory, P. Söderlind, B. Grabowski, L. Yang, A. Landa, T. Björkman, P. Souvatzis, O. Eriksson, Physical Review B 85, 060301 (2012).
  4. Equation of state of boron nitride combining computation, modeling, and experiment, S. Zhang et al. Physical Review B 99, 165103 (2019).
  5. Modeling laser-driven high-rate plasticity in BCC lead, R.E. Rudd, L.H. Yang, P.D. Powell, P. Graham, A. Arsenlis, R.M. Cavallo, ..., AIP Conference Proceedings 1979, 070027 (2018).

Ph.D., Physics, University of California at Davis