Multi-scale modelling of matter under extreme irradiation
- Jorge Kohanoff (Queen's University Belfast, United Kingdom)
- Antonio Rivera (Universidad Politecnica de Madrid, Spain)
- Tzveta Apostolova (Institute for Nuclear Research, Bulgaria)
- Stephen Fahy (University College Cork, Ireland)
To elaborate a roadmap for the development of computational approaches that address phenomena induced by intense electronic excitation. At the end of the workshop we expect to have identified:
(a) an array of strategies to connect computational methods that currently operate successfully on different time scales,
(b) areas where there is need for new methodologies,
(c) a set of key experiments to validate these computational approaches.
1. To identify the specific role of different radiation sources on electronic excitation-induced effects. This will serve to connect different communities that explore similar phenomena in parallel (e.g., intense laser and swift ion irradiation). It requires the participation of experimentalists and theoreticians from different fields.
2. To identify multi-scale strategies to connect computational methods on different timescales. This is particularly important when the different methods do not run consecutively but simultaneously, overlapping over a certain period of time. This will be a central point of the workshop, since several simulation methods operate reasonably well within their scope of applicability but their coupling to other methods is not straightforward.
3. To identify and propose experiments to validate simulations. Effects dependent on different consecutive or concomitant phenomena require dedicated time-resolved experiments to assess their role and the degree of coupling between them.
Specific ideas and topic to be discussed.
1. Timescale from sub-as to fs. Ionization processes, initial stages of electronic evolution. It requires the development of codes to treat non-adiabatic effects, e.g. NEGF and/or TDDFT. High-level descriptions, but limited to short timescales.
2. Timescale from fs to ps. Carrier scattering, carrier recombination. It requires the development of kinetic codes. In most situations a good description can be obtained by a Boltzmann-like equation with a Fokker-Planck term. Alternatively, less costly methods based on Monte Carlo simulations can be employed. The use of TDDFT or DF perturbation theory to describe ionization (when possible) within kinetic codes can provide a very accurate description of the electron evolution.
3. Timescale from ps to ns. Lattice evolution. Classical molecular dynamics (MD) coupled to the kinetic codes constitutes the best description of thermal effects. However, MD cannot account for certain processes such as charging effects or non-radiative exciton decay. In addition, a full coupling to kinetic codes can be very slow and thus, not justified. Alternative approaches to describe the thermal evolution based on rate equations (e.g., two-temperature models) are worth discussing.
4. Timescale exceeding ns and large spatial dimensions. The description of phenomena highly dependent on the electron evolution in long timescales must be described by hydrodynamic codes appropriately coupled to kinetic codes. This provides a good description of relevant cases, for instance, particle emission. Other alternatives based on kinetic Monte Carlo methods will be discussed.
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