Multitemperature atomic ensemble: Nonequilibrium evolution after ultrafast electronic excitation.

Area of Science:

• Condensed matter physics
• Materials science
• Physical chemistry

Background:

• Ultrafast laser radiation and particle beams excite electronic systems in solids, leading to transient thermal non-equilibrium.
• Both electronic and atomic subsystems can exist far from equilibrium during this process.

Purpose of the Study:

• To define atomic temperatures in electronically excited solids from first principles.
• To investigate the behavior of atomic temperatures during non-thermal phase transitions induced by electronic excitation.
• To propose a formulation for multitemperature heat transport equations.

Main Methods:

• Derivation of atomic temperature definitions for excited ensembles.
• Analysis of kinetic temperature in the momentum subspace.
• Proposal and application of configurational atomic temperature for electronic-temperature-dependent interatomic potentials, including ab initio molecular dynamics.
• Study of temperature evolution during non-thermal phase transitions.

Main Results:

• The kinetic temperature of atoms in the momentum subspace remains unaffected by electronic excitation.
• A configurational atomic temperature is defined for situations where electronic and atomic temperatures differ.
• Solids under ultrafast irradiation exhibit a temporary multitemperature state with distinct momentum and configurational equilibria.
• Complete equilibration of atomic temperatures occurs over longer timescales, leading to energy equipartition.

Conclusions:

• The study provides a theoretical framework for understanding atomic temperatures in non-equilibrium excited solids.
• The proposed configurational temperature is crucial for simulations like ab initio molecular dynamics under electronic excitation.
• The identification of a temporary multitemperature state necessitates the development of advanced heat transport models.