Ultrafast Structural Dynamics of the Fe-Pnictide Parent CompoundBaFe2As2

Abstract

Using femtosecond time-resolved x-ray diffraction we investigate the structural dynamics of the coherently excited A1g phonon mode in the Fe-pnictide parent compound BaFe2As2. The fluence dependent intensity oscillations of two specific Bragg reflections with distinctly different sensitivity to the pnictogen height in the compound allow us to quantify the coherent modifications of the Fe-As tetrahedra, indicating a transient increase of the Fe magnetic moments. By a comparison with timeresolved photoemission data we derive the electron-phonon deformation potential for this particular mode. The value of ∆µ/∆z = −(1.0 − 1.5) eV/Å is comparable with theoretical predictions and demonstrates the importance of this degree of freedom for the electron-phonon coupling in the Fe pnictides. In the Fe pnictides, the complex interplay of the electronic, spin, orbital/ising-nematic and lattice degrees of freedoms leads to the emergence of a complex phase diagram, including structural transitions, spin-density wave (SDW) phases and high-temperature superconductivity. These phases emerge for electron and hole doping, but also for isovalent doping and external pressure [1,2]. The electronic structure and the magnetic properties depend very sensitively on the exact shape and size of the Fe-As tetrahedra, where an important degree of freedom is the pnictogen height h above the Fe layers, which changes the Fe-As tetrahedra angle α (see Fig. 1(a)). A high sensitivity of the Fe magnetic moments on the pnictogen height with a rate of 6.8 µ B /Å has been predicted [3], signifying a strong magneto-structural coupling [2,4,5]. Similarly, an increase of the electron-phonon (e-ph) coupling strength was been found in calculations including magnetic ordering [6,7]. Indeed, a universal relation between the Fe-As tetrahedra angle and the superconducting critical temperature T c has been proposed for the Fe pnictides [2,8], underlining the importance of structural degrees of freedom in these compounds.The role of e-ph coupling for the mechanism of superconductivity in the Fe pnictides is still controversial. The average e-ph coupling constant has been found to be relatively weak both experimentally [9][10][11] and theoretically [7,12] with λ 0.35, insufficient to explain the high critical temperatures found in the pnictides in a conventional pairing scheme. However, due to the strong interplay of structural and magnetic degrees of freedom, a few phonon modes with enhanced magneto-structural coupling could still play an important role in the superconducting pairing mechanism [5,13].One such mode is the Raman active A 1g mode at the zone center corresponding to a displacement of the As ions perpendicular to the Fe layers with ω ≈ 22 meV ( Fig. 1(a)). This mode directly modulates the pnictogen height h and thereby the Fe-As tetrahedra angle α. Coherent excitation of this mode has recently been observed using femtosecond (fs) time-resolved optical spectroscopy [14], and time-resolved THz spectroscopy demonstrated a transient resurrection of the mag...

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“…Conceptual sketch of one mechanism of DECP, in rough correspondence to recent observations in BaFe 2 As 2 33,34 . Panels (a)–(d) show the density of electronic states (DOS) at various points in time relative to a pump laser that suddenly raises the electronic temperature at t  = 0.…”

“…This is well illustrated by a pair of recent experiments on single crystals of BaFe 2 As 2 , a material known as a parent compound for Fe-pnictide superconductors 33,34 . In both experiments, a femosecond laser pulse at 800 nm center wavelength excites electronic states in this poorly conducting metallic material.…”

“…In addition to the time-dependent shift of μ , the corresponding atomic motion can be seen by x-ray diffraction 33 . Figure 7 shows a sample of the data, where measurements of the intensity of both the (1 0 5) and (2 0 6) reflections show time-dependent changes in the structure factor due to the A 1 g mode.…”

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