Elastic anomalies in BaFe2-xNixAs 2 crystals

We present ultrasonic measurements on superconducting BaFe 2-xNixAs2 crystals with x = 0.07 and x = 0.15. The elastic constants C33 and C44 for the underdoped crystal (x = 0.07) show a large softening related to the structural phase transition at high temperatures. Anomalies in the sound velocity and the ultrasonic attenuation have been found at the superconducting phase transition Tc = 17 K. Ultrasonic attenuation exhibits a peak at the superconducting transition in contrast with the attenuation in conventional superconductors. In the overdoped crystal (x = 0.15) a minimum of C66 is found at a temperature just above the superconducting temperature T c = 13 K. Superconducting energy gap values have been tentatively extracted from the longitudinal ultrasonic attenuation. Unconventional behaviour of the ultrasonic attenuation is observed in the superconducting BaFe 2-xNixAs2 crystals. © 2012 Elsevier B.V. All rights reserved.