Determination of the lower critical field in Hc1(T) FeSe single crystals by magnetization measurements

In a recent work, Abdel-Hafiez et al. [1] we have determined the temperature dependence of the lower critical field H-c1(T) of a FeSe single crystal under static magnetic fields H parallel to the crystallographic c axis. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding H-c1(T) was deduced by taking into account demagnetization factors. In general, the first vortex penetration field may not reflect the true H-c1(T) due to the presence of surface barriers. In this work we show that magnetic hysteresis loops are very symmetric close to the critical temperature T-c = 9 K evidencing the absence of surface barriers and thus validating the previously reported determination of H-c1(T) and the main observations that the superconducting energy gap in FeSe is nodeless. (C) 2014 Elsevier B.V. All rights reserved.