Intrinsic and extrinsic pinning in NdFeAs(O,F): vortex trapping and lock-in by the layered structure

Abstract

Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the REFeAs(O,F) (RE1111, RE being a rare-earth element) is the family with the highest critical temperature Tc but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 (Tc ∼ 55 K). Here we focus on the pinning properties of the lower-Tc Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density Jc at high temperatures is dominated by point-defect pinning centres, whereas at low temperatures surface pinning by planar defects parallel to the c-axis and vortex shearing prevail. When the field approaches the ab-planes, two different regimes are observed at low temperatures as a consequence of the transition between 3D Abrikosov and 2D Josephson vortices: one is determined by the formation of a vortex-staircase structure and one by lock-in of vortices parallel to the layers. This is the first study on FBS showing this behaviour in the full temperature, field, and angular range and demonstrating that, despite the lower Tc and anisotropy of Nd1111 with respect to Sm1111, this compound is substantially affected by intrinsic pinning generating a strong ab-peak in Jc.

Angular dependence of vortex instability in a layered superconductor: the case study of Fe(Se,Te) material

“…In the strong pinning regime for this multiband superconductor it has already been proven that the scaling procedure for the critical current works by using the temperature dependence of the anisotropy parameter γ J 8 . Our estimate gives at lower temperatures an anisotropy value ranging from 1 to 2 (see Supplemental Material), which confirms the lower anisotropic pinning character of this material grown on CaF 2 substrate 16 , more isotropic also with respect to the same superconductor grown on MgO 8 , as well as other layered 1111-IBS 4 , or 122-IBS 39,40 , too. Therefore in our case, it is possible to apply Blatter scaling rule to verify the anisotropy estimation for γ J parameter from the magnetic field dependence of the critical current (see Fig.…”

“…Although we are used to correlate flux pinning with critical current, and flux motion with dissipation, by gradually increasing the bias current above I c the vortex motion can reach a speed limit which is unavoidably affected by pinning mechanism itself 35 . Either due to material defects 4,36 , or to geometrical barrier 37,38 , pinning becomes crucial in flux flow motion at high velocities too. Can the intrinsic layered structure of this Fe(Se,Te) superconductor make feel its influence?…”

“…In HTS 1 as well as in Iron Based Superconductors (IBS) 3 , the goal of increasing J c and reducing their anisotropy passes through a deep understanding of a complex vortex matter and flux pinning landscapes. A critical issue for applications in high magnetic fields is the current carrying capability of these superconducting materials, which in the IBS case can even be surprising considering similarities and differences between the two classes of superconductors 4 . In particular, the elemental compound Fe(Se,Te) offers several advantages: few non-toxic elements, less complex layered crystal structure, and low anisotropy.…”

Angular flux creep contributions in YBa2Cu3O7−δ nanocomposites from electrical transport measurements

“…Therefore, we observe the appearance of the S(θ)-peak at this temperature, which spans an orientation width of 5° and widens as temperature decreases. However, at low temperatures (50 K in this case), a narrow S(θ)-dip arises at the center of the S(θ)-peak, indicative of a lock-in transition 27 , also reported in other electrical transport studies 23,25 . This latter effect shows that at low temperatures, the thermal energy k B T is not high enough to cause movement of vortices through DK formation when they lie almost parallel to the CuO 2 planes.…”

“…2(b)). Dissipation due to DK excitations in the regime of IP can be detected by performing J c (θ) electrical transport measurements around H||ab and analyzing the obtained I-V curves 19–25 . In these experiments, an inverse correlation between J c and N was obtained, with a J c (θ) dependence showing a peak for H||ab whereas N(θ) is showing a dip.…”