Anisotropy in the upper critical field of FeSe and FeSe _0.33 Te _0.67 single crystals

The electric resistivity of single crystalline Fe-based superconductors FeSe and FeSe0.33Te0.67 was measured in pulsed magnetic fields up to 55 T. Te-doped iron selenide, FeSe0.33Te0.67, shows rather isotropic behavior in both magnetoresistance (MR) and upper critical fields (H _c2 ^∥ (0)/ H _c2 ^⊥ (0) = 48 T/ 42 T ∼ 1.14), when an external magnetic field is applied parallel and perpendicular to the crystallographic c-axis. These isotropic behaviors are frequently observed in an iron-based superconductor, and the isotropy in H _c2 can likely be described by the Pauli effect. In contrast, our measurements elucidate that the undoped iron selenide, FeSe, exhibits a clear anisotropy in both MR and H _c2 (H _c2 ^⊥ (0)/ H _c2 ^∥ (0) = 27 T/ 15 T ∼ 1.8). This behavior occurs because the clean in-plane conduction (RRR = 35) allows to form a closed electron orbit within the twodimensional Fermi sheets only for the magnetic field parallel to the c-axis, which results in an appearance of an intrinsic (orbital-effect limited) upper critical field for the undoped FeSe. Doping of Te breaks the clean in-plane conduction and leads to a crossover from an orbital limit dominating behavior to a Pauli limit dominating one.