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The electric resistivity of single crystalline Fe-based superconductors FeSe and FeSeTe was measured in pulsed magnetic fields up to 55 T. Te-doped iron selenide, FeSeTe, shows rather isotropic behavior in both magnetoresistance (MR) and upper critical fields ((0)/(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 Hc2 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 Hc2 ((0)/(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 two-dimensional 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. [ABSTRACT FROM AUTHOR]

The article discusses research that resulted in the detection of a nontrivial pi geometric quantum phase, also known as a pi Berry's phase, in the bulk Rashba semiconductor BiTel via analysis of a Shubnikov-de Haas effect. Topics include phase shifts, Fermi surfaces, oscillations, and wave functions.