Gap-dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe2As2 across the spin density wave phase transition

TL;DR

This study investigates ultrafast quasiparticle dynamics and coherent acoustic phonons in CaFe2As2 across its spin density wave transition, revealing a BCS-like gap and elastic properties through femtosecond spectroscopy.

Contribution

It provides the first detailed analysis of quasiparticle relaxation and phonon behavior in CaFe2As2 across the SDW transition using ultrafast spectroscopy.

Findings

Identified a BCS-like charge gap of ~1.6k_B T_SDW.

Estimated an electron-phonon coupling constant of ~0.13.

Determined optical constants, sound velocity, and elastic modulus across 3 K to 300 K.

Abstract

We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165 K using femtosecond time-resolved pump-probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T_SDW. From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW, whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of ~100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.