Acoustic and optical phonon dynamics from femtosecond time-resolved optical spectroscopy of superconducting iron pnictide Ca(Fe_0.944Co_0.056)_2As_2

TL;DR

This study investigates how acoustic and optical phonons in a superconducting iron pnictide change with temperature, revealing their coupling with quasiparticles and spin fluctuations across phase transitions.

Contribution

It provides detailed temperature-dependent measurements of phonon dynamics and introduces a strain pulse propagation model to extract optical constants and sound velocities in the material.

Findings

Phonon frequencies and dephasing times show anomalous behavior below T_SC.

Maximum acoustic mode amplitude occurs around 170 K, linked to spin fluctuations.

Coupling between phonons and quasiparticles is observed below T_SC.

Abstract

We report temperature evolution of coherently excited acoustic and optical phonon dynamics in superconducting iron pnictide single crystal Ca(Fe_0.944Co_0.056)_2As_2 across the spin density wave transition at T_SDW ~ 85 K and superconducting transition at T_SC ~20 K. Strain pulse propagation model applied to the generation of the acoustic phonons yields the temperature dependence of the optical constants, and longitudinal and transverse sound velocities in the temperature range of 3.1 K to 300 K. The frequency and dephasing times of the phonons show anomalous temperature dependence below T_SC indicating a coupling of these low energy excitations with the Cooper-pair quasiparticles. A maximum in the amplitude of the acoustic modes at T ~ 170 is seen, attributed to spin fluctuations and strong spin-lattice coupling before T_SDW.