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

Abstract

In a recent work, Abdel-Hafiez et al. [1] we have determined the temperature dependence of the lower critical field Hc1(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 Hc1(T) was deduced by taking into account demagnetization factors. In general, the first vortex penetration field may not reflect the true Hc1(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 Tc = 9K evidencing the absence of surface barriers and thus validating the previously reported determination of Hc1(T) and the main observations that the superconducting energy gap in FeSe is nodeless.