Abstract
Pulsed ultrasonic experiments performed on the parent compounds of FeAs based superconductors Fe (, Ca) revealed elastic anomalies that agree with a two-step process for the structural/magnetic transitions. Upon cooling, a pronounced velocity softening of longitudinal phonons propagating along the axis is observed at the structural transition due to a distortion of the lattice. Below a slightly lower temperature , a biquadratic coupling between the distortion and an antiferromagnetic order parameter produces a steep velocity stiffening upon further cooling, the stiffening being five times larger in than in . Microwave resistivity measurements at 16.5 GHz suggest that, upon cooling, the lattice distortion at is driven by magnetic fluctuations in , while it is rather due to lattice fluctuations in . This suggestion appears consistent with a second-order character of the transitions in and a first-order one in .
- Received 25 February 2014
- Revised 7 April 2014
DOI:https://doi.org/10.1103/PhysRevB.89.155129
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