Abstract
We investigate the ultrafast lattice dynamics in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ via time-resolved x-ray diffraction measurements using an x-ray free-electron laser at SPring-8 Angstrom Compact free-electron LAser (SACLA). The profile of the Bragg peaks substantially changes after strong photoexcitation, where the $c$ axis length of the lattice initially contracts, followed by expansion. These ultrafast lattice contractions and expansions significantly depend on the pump fluence, and it is suggested that a strong normal stress is applied along the $c$ axis. To obtain a phenomenological understanding of the observed lattice dynamics, we performed calculations based on a two-temperature model and an ultrafast thermoelasticity model. Our results showed that a steep gradient of the electron temperature induced by photoexcitation generates a blast force, which acts as a strong normal stress. We propose that a temporal stress can be applied by a strong optical pump, and this scheme can be another route for the application of uniaxial stress to superconductors.
Highlights
Ultrafast optical pulses have been intensively employed in condensed matter physics to induce numerous exotic states that can never appear under equilibrium [1]
We investigate the ultrafast lattice dynamics in BaFe2As2 via time-resolved x-ray diffraction measurements using an x-ray free-electron laser at SPring-8 Angstrom Compact free-electron LAser (SACLA)
Our results showed that a steep gradient of the electron temperature induced by photoexcitation generates a blast force, which acts as a strong normal stress
Summary
Ultrafast optical pulses have been intensively employed in condensed matter physics to induce numerous exotic states that can never appear under equilibrium [1]. For BaFe2As2, controlling the lattice profile by isovalent substitution [24] or physical pressure [25] substantially alters the electronic properties, resulting in the emergence of superconductivity In this respect, photoexcitation has a strong advantage over other techniques for ion-based superconductors because it can instantaneously change the physical properties without contact or fabrication. One of the most spectacular phenomena is photo-induced superconductivity, which has been suggested to be realized in iron-based superconductors measured by time- and angle-resolved photoemission spectroscopy (TARPES) [26,27] In these studies, the lattice modulation was found to play an important role for realizing superconducting-like states, which strongly motivated us to directly observe the lattice dynamics. Our results suggest that photoexcitation can be used as an alternative way to apply normal stress on an ultrafast time scale without using static conventional methods
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