First-principles calculations of heat capacities of ultrafast laser-excited electrons in metals

TL;DR

This paper uses first-principles density functional calculations to determine how electronic heat capacities vary with temperature in different metals, highlighting the limitations of free electron models especially for transition metals with localized d electrons.

Contribution

It provides a detailed analysis of electronic heat capacities across various metals using finite-temperature DFT, evaluating the effects of exchange-correlation functionals and semicore electrons, and assessing the validity of free electron models.

Findings

Free electron models work well for simple metals.

Localized d electrons in transition metals cause deviations at high energies.

Electronic screening effects influence heat capacity at high temperatures.

Abstract

Ultrafast laser excitation can induce fast increases of the electronic subsystem temperature. The subsequent electronic evolutions in terms of band structure and energy distribution can determine the change of several thermodynamic properties, including one essential for energy deposition; the electronic heat capacity. Using density functional calculations performed at finite electronic temperatures, the electronic heat capacities dependent on electronic temperatures are obtained for a series of metals, including free electron like, transition and noble metals. The effect of exchange and correlation functionals and the presence of semicore electrons on electronic heat capacities are first evaluated and found to be negligible in most cases. Then, we tested the validity of the free electron approaches, varying the number of free electrons per atom. This shows that only simple metals can be correctly fitted with these approaches. For transition metals, the presence of localized d electrons produces a strong deviation toward high energies of the electronic heat capacities, implying that more energy is needed to thermally excite them, compared to free sp electrons. This is attributed to collective excitation effects strengthened by a change of the electronic screening at high temperature.