Ultrafast Melting of Spin Density Wave Order in BaFe$_{2}$As$_{2}$ Observed by Time- and Angle-Resolved Photoemission Spectroscopy with Extreme-Ultraviolet Higher Harmonic Generation

TL;DR

This study uses time- and angle-resolved photoemission spectroscopy with extreme-ultraviolet pulses to observe ultrafast melting of spin density wave order in BaFe₂As₂ within approximately 1 picosecond after optical excitation.

Contribution

It demonstrates the capability to probe entire Brillouin zone dynamics and directly observe the ultrafast melting of SDW order in an iron-based superconductor parent compound.

Findings

SDW order melts within ~1 ps after excitation

Optically excited electrons decay rapidly and thermalize

Recovery to SDW state occurs in ~0.60 ps

Abstract

Transient single-particle spectral function of BaFe$_{2}$As$_{2}$, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin-density-wave (SDW) ordered state within $\sim$ 1 ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above $E_{F}$ at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From the comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are $\sim$ 0.60 ps both around the zone center and the zone corner.