Doping-dependent critical current properties in K, Co, and P-doped BaFe2As2 single crystals

Abstract

* Both authors contributed equally to this work.In order to establish the doping dependence of the critical current properties in the iron-based superconductors, the in-plane critical current density (Jc) of BaFe2As2-based superconductors, Ba1-xKxFe2As2 (K-Ba122), Ba(Fe1-xCox)2As2 (Co-Ba122), and BaFe2(As1-xPx)2 (P-Ba122) in a wide range of doping concentration (x) was investigated by means of magnetization hysteresis loop (MHL) measurements on single crystal samples. Depending on the dopant elements and their concentration, Jc exhibits a variety of magneticfield (H)-and temperature (T)-dependences. (1) In the case of K-Ba122, the MHL of the under-doped samples (x ≤ 0.33) exhibits the second magnetization peak (SMP), which sustains high Jc at high H and high T, exceeding 10 5 A/cm 2 at T = 25 K and µ0H = 6 T for x = 0.30. On the other hand, the SMP is missing in the optimally-(x ~ 0.36-0.40) and over-doped (x ~ 0.50) samples, and consequently Jc rapidly decreases by more than one order of magnitude, although the change in Tc is within a few K. (2) For Co-Ba122, the SMP is always present over the entire superconducting (SC) dome from the under-(x ~ 0.05) to the over-doped (x ~ 0.12) region. However, the magnitude of Jc significantly changes with x, exhibiting a sharp maximum at x ~ 0.057, which is a slightly under-doped composition among Co-Ba122. (3) For P-Ba122, the highest Jc is attained at x = 0.30 corresponding to the highest Tc composition. For the over-doped samples, the MHL is characterized by a SMP located close to the irreversibility field Hirr. Common to the three doping variations, Jc becomes highest at the under-doping side of the SC dome near the phase boundary between the SC phase and the antiferromagnetic/orthorhombic (AFO) phase. Also, the peak appears in a narrow range of doping, distinct from the Tc dome with broad maximum. These similarities in the three cases indicate that the observed doping dependence of Jc is intrinsic to the BaFe2As2-based superconductors. The scaling analysis of the normalized pinning force density fp as a function of the reduced magnetic field h = H/Hirr (Hirr: irreversibility field) shows that the peak in the pinning force position (hmax) depends on x, indicating a change in pinning with x. On the other hand, high-Jc samples always attain similar hmax values of 0.40-0.45 for all the dopants, which may suggest that a common pinning source causes the highest Jc. A quantitative analysis of the Tdependent Jc indicates that the two pinning mechanisms, namely, the spatial variations in Tc (referred to as Tc pinning) and the fluctuations in the mean free path (l pinning), are enhanced for the under-doped samples, which results in the enhancement of Jc. Possible origins for the different pinning mechanism are discussed in connection with the x-dependence of Tc, the residual resistivity, AFO domain boundaries, and a possible quantum critical point.

Large and significantly anisotropic critical current density induced by planar defects in CaKFe4As4 single crystals

“…The magnetic field dependence of J c above 20 K becomes very weak, or J c even show a broad maximum at higher fields. Such a non-monotonic field dependence of J c is often referred to as the fish-tail effect and has been found in cuprate superconductors [22][23][24][25] and other IBSs [26][27][28]. In most cases, however, the evolution of the fish-tail peak is continuous and no crossings of M-H curves at different temperatures occur.…”

Ultrahigh critical current densities, the vortex phase diagram, and the effect of granularity of the stoichiometric high- Tc superconductor CaKFe4As4

“…In the 122 iron-based superconductors, the pinning mechanism depends on the doping level and dopant and h max tends to increase with increasing doping level 16 . The highest J c is achieved when h max ∼ 0.40 − 0.45 for the optimally-doped compounds 16 . Single crystal of Ba 0.6 K 0.4 Fe 2 As 2 shows only small-size normal pinning (h max = 0.33) 21 , whereas the values obtained for BaFe 1.8 Co 0.2 As 2 single crystal 16 (h max = 0.4, p = 1.67, q = 2) are correlated to a dense vortex pinning nano-structure, likely due to the inhomogeneous distribution of cobalt ions.…”

“…Different regimes of vortex dynamics 52,53 , marked elastic creep and plastic creep, separated by the H sp line, are shown in the phase diagram in Fig.6a. In general, the vortex phase diagram is determined by the competition between the elastic energy of the vortex lattice, the pinning energy and thermal energy 16 . The second magnetization peak effect is commonly associated with an order-disorder transition, which occurs at H min when the pinning energy exceeds the elastic energy 24 .…”

Doping evolution of the second magnetization peak and magnetic relaxation in (Ba1−xKx)FLiu1, Zhou2, Sun3 et al. 2018Phys. Rev. B12170View full textAdd to dashboardCite

“…After the field was ramped to the desired field, the superconducting coil was changed to persistent mode and the time dependence of magnetization, M vs t curve, was immediately recorded over a period of 1 10 seconds. 6 Temperature dependent magnetization data were recorded in both ZFC and FC procedures.…”