Low temperature specific heat of 12442-type KCa_2Fe_4As_4F_2 single crystals

Abstract

Low-temperature specific heat (SH) is measured for the 12442-type KCa${}_2$Fe${}_4$As${}_4$F${}_2$ single crystal under different magnetic fields. A clear SH jump with the height of $\Delta C/T{|}_{T_c}$ = 130 mJ/mol K${}^2$ is observed at the superconducting transition temperature $T_c$. It is found that the electronic SH coefficient $\Delta \gamma (H)$ quickly increases when the field is in the low-field region below 3 T 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 indicates the presence of the $T^2$ 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 placed this system in the middle between the BCS limit and the Bose-Einstein condensation.

6 pages, 4 figures, accepted by SCIENCE CHINA Physics, Mechanics & Astronomy