Etude du Supraconducteur Magnetique (BaFe2-xNixAs2) en Radio et Hyperfréquences
The recent discovery of iron based arsenide superconductors has attracted much interest, especially those of the BaFe2As2 family. The compound parent BaFe2As2 exhibits simultaneous structural and magnetic phase transitions below 𝑇𝑆 = 𝑇𝑁 = 140 𝐾. Upon doping electrons via either Co or Ni substitution for Fe, these structural and magnetic phase transition temperatures decrease and became separated as 𝑇𝑆 > 𝑇𝑁 . A number of experimental works have been published recently concerning the surface impedance of iron based superconductors BaFe2- xCoxAs2 or BaFe2-xNixAs2, providing information on the superconducting pairing in these materials.
Our first contribution concerns ultrasonic measurements on superconducting crystals BaFe2-xNixAs2 with x = 0.07 and x = 0.15. The elastic constants 𝐶33 and 𝐶44 for the underdoped crystal BaFe1.93Ni0.07As2 shows 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. The elastic properties of the underdoped crystal are strongly affected by the structural instabilities competing with the purely electronic effects in the elastic properties near the superconducting phase transition. In the overdoped crystal BaFe1.85Ni0.15 As2, large anisotropic behaviour is observed in the temperature dependence of the sound velocity and attenuation of the 𝐶𝐿 longitudinal and 𝐶𝑇 shear modes around the superconducting transition 𝑇𝑐 = 13 𝐾 . Above the superconducting phase transition the softening of the 𝐶𝑇 shear mode is related to structural instabilities.
Unconventional behaviour of the ultrasonic attenuation is observed in the superconducting crystals BaFe2-xNixAs2.This result is in agreement with the softening of 𝐶66 observed in the BaFe2-xCoxAs2 crystals above the superconducting transition by the japanese Group of Yoshizawa and al.
Our second contribution reports measurements of the temperature dependence of the surface impedance and electrical conductivity in three different superconducting crystals: underdoped BaFe1.93Ni0.07As2, optimally doped BaFe1.9 Ni0.1 As2 and overdoped BaFe1.88Ni0.12 As2 crystals in the frequency range 10 MHz-1.5 GHz using a single coil technique. The temperature dependence of the London penetration depth follows a 𝑇2 law. The real conductivity 𝜍1 increases with decreasing temperature below 𝑇𝑐 in agreement with the results obtained for the optimally Co doped BaFe2-xCoxAs2 crystals. The increase of the real conductivity σ1 in the superconducting state is attributed to a rapidly decrease of the quasiparticle scattering time. In the underdoped crystal BaFe1.93Ni0.07As2 , the establishment of the antiferromagnetic order at 𝑇𝑁~50 𝐾 gives rise to anomalous increase of electron scattering time. Drude type conductivity yields 𝑋 and 𝑅 differ from each other. This result gives evidence of coexistence of superconductivity and antiferromagnetism.
A similar behavior was observed in Co doped BaFe2-xCoxAs2 by Cherpak and al. A strong temperature dependence of the quasiparticles scattering time is observed in many unconventional superconductors.
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