Dynamic crossover across the peak effect in Ba 1−x K x Fe2As2 superconductor for H ||c -axis and H ||ab -plane

Abstract

Abstract The vortex matter properties of a Ba 1 − x K x Fe2As2 single crystal ( T c = 38.2 K) were studied, by employing both isofield and isothermal ac-susceptibility measurements, χ n ( f , H 0 ) = χ n ′ ( f , H 0 ) − i χ n ′ ′ ( f , H 0 ) , in a wide range of frequencies and amplitudes of the applied ac magnetic field. The irreversibility line ( H irr ( T , θ ) ), formally defined by the onset of the third harmonic, is recorded for both H ∥ c -axis and H ∥ a b -plane. It can be reproduced from the empirical equation, H irr ( T ) = H 0 ( 1 − T / T c ) n , with n c = n a b = 1.24 and μ 0 H 0 c = 210 Tesla, μ 0 H 0 a b = 540 Tesla. The isofield measurements of the first harmonic revealed a narrow diamagnetic peak, related to a local peak of the critical current below the irreversibility line for both H ∥ c -axis and H ∥ a b -plane. The local peak for μ 0 H < 0.2 Tesla is transformed to a sudden drop before it completely disappears. Detailed ac-susceptibility measurements were conducted for frequencies ranging within f = 0.1–10 kHz. From these data, the pinning potential, U, is deduced both as function of temperature and dc magnetic field. These results revealed that the ac response of vortex matter exhibits three distinct dynamic behaviors. By employing a model proposed by Mikitik and Brandt (2001 Phys. Rev. B 64 184514), that is based on a Lindemann type criterion and the collective pinning theory, we reproduced the experimentally recorded vortex matter phase diagram by taking into account both thermal fluctuations and random point disorder. To this effect, we adopted a δ T c pinning mechanism, c L = 0.25 , D 0 / c L = 1.1–1.5 and a Ginzburg number G i = 10 − 3 .