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Title: Vibrational anomalies in AFe 2As 2 ( A=Ca, Sr, and Ba) single crystals

In this study, the detailed behavior of the in-plane infrared-active vibrational modes has been determined in AFe 2As 2 ( A=Ca, Sr, and Ba) above and below the structural and magnetic transitions at TN = 172, 195 and 138 K, respectively. Above TN, two infrared-active Eu modes are observed. In all three compounds, below TN the low-frequency Eu 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 Eu 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 TN 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 Eu mode does not change in position or character across this series of compounds. Below TN, the Eu modes are predicted to split into features of roughly equal strength. Finally,more » we discuss the possibility that the anomalous increase in the strength of the lower branch of the high-frequency mode below TN 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.« less
ORCiD logo [1] ;  [2] ;  [3] ;  [4] ;  [4]
  1. Brookhaven National Lab. (BNL), Upton, NY (United States). Condensed Matter Physics and Materials Science Division
  2. Nanjing Univ. (China). Center for Superconducting Physics and Materials, Department of Physics
  3. Univ. of Geneva (Switzerland). DQMP
  4. Ames Lab. and Iowa State Univ., Ames, IA (United States). Department of Physics and Astronomy
Publication Date:
Report Number(s):
BNL-205789-2018-JAAM; IS-J-9712
Journal ID: ISSN 2469-9950
Grant/Contract Number:
SC0012704; AC02-07CH11358; PHY1066293
Accepted Manuscript
Journal Name:
Physical Review B
Additional Journal Information:
Journal Volume: 98; Journal Issue: 3; Journal ID: ISSN 2469-9950
American Physical Society (APS)
Research Org:
Brookhaven National Laboratory (BNL), Upton, NY (United States); Ames Laboratory (AMES), Ames, IA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division; USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1458595; OSTI ID: 1461657