Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field ($H_{c2}$) of the iron-based superconductor FeSe ($T_c$ = 8.6 K) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of $H_{c2}$ for magnetic field applied both parallel (textbf{H} $|$ $ab$) and perpendicular (textbf{H} $|$ $c$) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for textbf{H} $|$ $c$, a Pauli paramagnetic contribution is evidenced for textbf{H} $|$ $ab$ with Maki parameter $alpha$ = 2.1, corresponding to Pauli field $H_{P}$ = 36.5 T
Temperature dependence of the upper critical magnetic field (Hc2) of single crystalline FeTe0.5Se0.5 (Tc = 14.5 K) have been determined by tunnel diode oscillator-based measurements in magnetic fields of up to 55 T and temperatures down to 1.6 K. The Werthamer-Helfand-Hohenberg model accounts for the data for magnetic field applied both parallel (H || ab) and perpendicular (H || c) to the iron conducting plane, in line with a single band superconductivity. Whereas Pauli pair breaking is negligible for H || c, Pauli contribution is evidenced for H || ab with Maki parameter alpha= 1.4, corresponding to Pauli field HP = 79 T. As a result, the Hc2 anisotropy (= Habc2 /Hcc2) which is already rather small at Tc (gamma = 1.6) further decreases as the temperature decreases and becomes smaller than 1 at liquid helium temperatures.
