Quasiparticle relaxation dynamics in spin-density-wave and superconducting SmFeAsO_{1-x}F_{x} single crystals

TL;DR

This study uses femtosecond spectroscopy to analyze quasiparticle relaxation in SmFeAsO_{1-x}F_{x} crystals, revealing distinct dynamics associated with spin-density wave order and superconductivity, including a pseudogap feature.

Contribution

It provides new insights into the quasiparticle relaxation processes and electronic structure in SDW and superconducting SmFeAsO_{1-x}F_{x} crystals, highlighting the presence of a pseudogap and multiple electronic subsystems.

Findings

Critical slowing down at SDW transition temperature.

Presence of a pseudogap above T_c with 2Δ_PG ≈ 120 meV.

Independent relaxation channels for superconducting and pseudogap states.

Abstract

We investigate the quasiparticle relaxation and low-energy electronic structure in undoped SmFeAsO and near-optimally doped SmFeAsO_{0.8}F_{0.2} single crystals - exhibiting spin-density wave (SDW) ordering and superconductivity respectively - using pump-probe femtosecond spectroscopy. In the undoped single crystals a single relaxation process is observed, showing a remarkable critical slowing down of the QP relaxation dynamics at the SDW transition temperature T_{SDW}\simeq125{K}. In the superconducting (SC) crystals multiple relaxation processes are present, with distinct SC state quasiparticle recombination dynamics exhibiting a BCS-like T-dependent superconducting gap, and a pseudogap (PG)-like feature with an onset above 180K indicating the existence of a pseudogap of magnitude 2\Delta_{\mathrm{PG}}\simeq120 meV above T_{\mathrm{c}}. From the pump-photon energy dependence we conclude that the SC state and PG relaxation channels are independent, implying the presence of two separate electronic subsystems. We discuss the data in terms of spatial inhomogeneity and multi-band scenarios, finding that the latter is more consistent with the present data.