Gap Structure of 12442-Type KCa2(Fe1-xCox)4As4F2(x=0,0.1)Revealed by Temperature Dependence of Lower Critical Field

We report an in-depth investigation on the out-of-plane lower critical field Hc1 of the KCa2(Fe1-xCox)4As4F2(12442-type,x=0,0.1).The multi-gap feature is revealed by the kink in the temperature-dependent Hc1(T)curve for the two samples with different doping levels.Based on a simplified two-gap model,the magnitudes of the two gaps are determined to beΔ1=1.2 meV andΔ2=5.0 meV for the sample with x=0,Δ1=0.86 meV andΔ2=2.8 meV for that with x=0.1.With the cobalt doping,the ratio of energy gap to critical transition temperature(Δ/kBTc)remains almost unchanged for the smaller gap and is suppressed by 20%for the larger gap.For the undoped KCa2Fe4As4F2,the obtained gap sizes are generally consistent with the results of angle-resolved photoemission spectroscopy experiments.

Low temperature specific heat of 12442-type KCa2Fe4As4F2 single crystals

Low-temperature specific heat(SH)is measured for the 12442-type KCa2Fe4As4F2 single crystal under different magnetic fields.A clear SH jump with the height of?C/T|Tc=130 mJ mol-1 K-2 is observed at the superconducting transition temperature Tc.It is found that the electronic SH coefficient?γ(H)quickly increases when the field is in the low-field region below 3T and then considerably slows down the increase with a further increase in the field,which indicates a rather strong anisotropy or multi-gap feature with a small minimum in the superconducting gap(s).The temperature-dependent SH data indicate the presence of the T2 term,which supplies further information and supports the picture with a line-nodal gap structure.Moreover,the onset point of the SH transition remains almost unchanged under the field as high as 9 T,which is similar to that observed in cuprates,and places this system in the middle between the BCS limit and the Bose-Einstein condensation.

Strong Pauli paramagnetic effect in the upper critical field of KCa2Fe4As4F2

Recently, 12442 system of Fe-based superconductors has attracted considerable attention owing to its unique double-Fe As-layer structure. A steep increase in the in-plane upper critical field with cooling has been observed near the superconducting transition temperature, Tc, in KCa2Fe4As4F2 single crystals. Herein, we report a high-field investigation on upper critical field of this material over a wide temperature range, and both out-of-plane(H∥c, Hc2c) and in-plane(H∥ab, Hc2ab ) directions have been measured.A sublinear temperature-dependent behavior is observed for the out-of-plane Hc2c , whereas strong convex curvature with cooling is observed for the in-plane Hc2ab . Such behaviors could not be described by the conventional Werthamer-Helfand-Hohenberg(WHH) model. The data analysis based on the WHH model by considering the spin aspects reveals a large Maki parameter α=9,indicating that the in-plane upper critical field is affected by a very strong Pauli paramagnetic effect.

In situ annealing effects on magnetic properties and variable-range hopping of iron-based ladder material BaFe2S3

All iron-based superconductors,regardless of their detailed crystal structure,share the same conductive Fe_2X_2(X=As,P,Se,and S)layers as the crucial structural unit[1-8].In the Fe_2X_2 layers,edge-shared FeX_4 tetrahedra form a square lattice in quasi-two-dimensional(quasi-2D)planes.One may wonder what kind of properties will be manifested in the quasi-one-dimensional(quasi-1D)counterparts exhibiting an identical coordination as the above-mentioned