Ultrafast quasiparticle dynamics in superconducting iron pnictide CaFe1.89Co0.11As2

TL;DR

This study investigates the ultrafast quasiparticle relaxation dynamics in superconducting CaFe1.89Co0.11As2 using femtosecond pump-probe spectroscopy, revealing multiple decay channels, a reduced charge gap, and insights into electron-phonon coupling near phase transitions.

Contribution

It provides the first detailed analysis of quasiparticle dynamics in this specific iron pnictide superconductor, including gap measurement and electron-phonon coupling estimation.

Findings

Multiple electronic relaxation components with varying time scales.

Reduced charge gap of approximately 1.8 times the BCS value.

Electron-phonon coupling constant around 0.14.

Abstract

Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting CaFe1.89Co0.11As2 single crystal using femtosecond time-resolved pump-probe spectroscopy. The carrier dynamics reflects a three-channel decay of laser deposited energy with characteristic time scales varying from few hundreds of femtoseconds to order of few nanoseconds where the amplitudes and time-constants of the individual electronic relaxation components show significant changes in the vicinity of the spin density wave (T_SDW ~ 85 K) and superconducting (T_SC ~ 20 K) phase transition temperatures. The quasiparticles dynamics in the superconducting state reveals a charge gap with reduced gap value of 2$\Delta$_0/k_BT_SC ~ 1.8. We have determined the electron-phonon coupling constant $\lemda$ to be ~ 0.14 from the temperature dependent relaxation time in the normal state, a value close to those reported for other types of pnictides. From the peculiar temperature-dependence of the carrier dynamics in the intermediate temperature region between the superconducting and spin density wave phase transitions, we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures.