Vibrational anomalies in AFe2As2 (A=Ca, Sr, and Ba) single crystals

2018/02/28 by C. C. Homes, Yaomin Dai, Y. M. Dai +4
Chemistry · Materials Science · Physics and Astronomy · #Atomic physics #Brillouin zone #Chemistry #Condensed matter physics #Crystallography #Infrared #Inorganic Chemistry and Materials #Iron-based superconductors research #Lattice (music) #Optics #Oscillator strength #Physics #Quantum mechanics #Rare-earth and actinide compounds #Spectral line #cond-mat.str-el #cond-mat.supr-con

paper · pdf · doi:10.1103/physrevb.98.035103

published as Phys. Rev. B 98, 035103 (2018) · 13 pages, 3 figures, 2 tables

arxiv created 2018/02/28 · openalex created_date 2018/03/06 · openalex publication_date 2018/07/02 · arxiv updated 2018/07/11 · openalex updated_date 2026/08/06

Abstract

The detailed behavior of the in-plane infrared-active vibrational modes has been determined in AFe₂As₂ (A\phantom\rule0.16em0ex=\phantom\rule0.16em0exCa, Sr, and Ba) above and below the structural and magnetic transitions 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 to 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.

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