Abstract
The anisotropic upper critical field, , curves for FeSe are determined over a wide range of temperatures down to 1.5 K and magnetic fields up to 60 T. Anisotropic initial slopes of T/K and T/K for magnetic field applied along axis and plane, respectively, were observed. Whereas the axis () increases quasilinearly with decreasing temperature, the plane () shows a flattening, starting near 25 K above 30 T. This leads to a nonmonotonic temperature dependence of the anisotropy parameter . The anisotropy parameter is 2 near K and rises to a maximum 3.6 around 27 K. For lower temperatures, decreases with in a linear fashion, dropping to by K. Despite the apparent differences between the FeSe and (BaK)FeAs or Ba(FeCo)As, in terms of the magnetic state and proximity to an insulating state, the curves are remarkably similar.
- Received 2 March 2011
DOI:https://doi.org/10.1103/PhysRevB.83.100514
©2011 American Physical Society