Ultrasonic and microwave investigation of the structural and magnetic transitions in CaFe2As2 and BaFe2As2 single crystals

作者：Poirier Mario^*; Bilodeau Maxime; Lefebvre Simon; Karki Amar B; Jin Rongying

来源：Physical Review B, 2014, 89(15): 155129.

DOI：10.1103/PhysRevB.89.155129

Pulsed ultrasonic experiments performed on the parent compounds of FeAs based superconductors AFe(2)As(2) (A = Ba, 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 c axis is observed at the structural transition T-s due to a distortion of the lattice. Below a slightly lower temperature T-N, 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 CaFe2As2 than in BaFe2As2. Microwave resistivity measurements at 16.5 GHz suggest that, upon cooling, the lattice distortion at Ts is driven by magnetic fluctuations in BaFe2As2, while it is rather due to lattice fluctuations in CaFe2As2. This suggestion appears consistent with a second-order character of the transitions in BaFe2As2 and a first-order one in CaFe2As2 .

• 出版日期2014-4-22