Vibrational anomalies in AFe${}_2$As${}_2$ (A$=$Ca, Sr, and Ba) single crystals

Abstract

The detailed behavior of the in-plane infrared-active vibrational modes has been determined in AFe${}_2$As${}_2$ (A$=$Ca, Sr, and Ba) above and below the structural and magnetic transition at $T_N=$172, 195 and 138 K, respectively. Above $T_N$, two infrared-active $E_u$ modes are observed. In all three compounds, below $T_N$ the low-frequency $E_u$ mode is observed to split into upper and lower branches; with the exception of the Ba material, the oscillator strength across the transition is conserved. In the Ca and Sr materials, the high-frequency $E_u$ mode splits into an upper and a lower branch; however, the oscillator strengths are quite different. Surprisingly, in both the Sr and Ba materials, below $T_N$ the upper branch appears be either very weak or totally absent, while the lower branch displays an anomalous increase in strength. The frequencies and atomic characters of the lattice modes at the center of the Brillouin zone have been calculated for the high-temperature phase for each of these materials. The high-frequency $E_u$ mode does not change in position or character across this series of compounds. Below $T_N$, the $E_u$ modes are predicted to split into features of roughly equal strength. We discuss the possibility that the anomalous increase in the strength of the lower branch of the high-frequency mode below $T_N$ in the Sr and Ba compounds, and the weak (silent) upper branch, may be related to the orbital ordering and a change in the bonding between the Fe and As atoms in the magnetically-ordered state.

13 pages, 3 figures, 2 tables

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Cited by in corpus (5)

• Optical and photoemission investigation of structural and magnetic transitions in the iron-based superconductor Sr${}_{0.67}$Na${}_{0.33}$Fe${}_2$As${}_2$
• Experimental investigation of electronic interactions in collapsed and uncollapsed LaFe2As2 phases
• Anisotropic optical properties of detwinned BaFe${}_2$As${}_2$
• Anomalous electronic energy relaxation and soft phonons in the Dirac semimetal Cd${}_3$As${}_2$
• Transient gap generation in BaFe${}_2$As${}_2$ driven by coherent lattice vibrations