Pulsed-field contactless mapping of the anisotropic upper critical field in LiFeAs superconducting crystals

Angle - resolved measurements of the upper critical field were performed using a tunnel diode resonator in the stoichiometric iron arsenide superconductor LiFeAs ($T_c$=18~K) in pulsed magnetic fields up to 50~T at temperatures down to 0.6~K. Complete $H^{\parallel c}_{c2}(T)$ and $H^{\bot c}_{c2}(T)$ curves with $T \to 0$ extrapolated values of $H^{\parallel c}_{c2}(0)=17 \pm 1 $ T and $H^{\bot c}_{c2}(T)=26 \pm 1$ T were obtained. The anisotropy, ${\gamma _{H_{c2}} } \equiv H_{c2}^{ \bot c}/H_{c2}^{\parallel c} \approx 2$, close to $T_c$ has revealed the essentially three-dimensional electronic structure of the material. Both temperature - dependent $H_{c2}(T)$ can be well fit within a single set of band structure, magnetism and scattering parameters. In a configuration with $H \parallel c$, $H^{\parallel c}_{c2} (T)$ is limited by orbital effects with modest scattering. In the perpendicular orientation, $H^{\bot c}_{c2}(T)$ shows a notable low-temperature saturation and a strong departure from the orbital Werthamer-Helfand-Hohenberg model. Instead, fitting results suggest paramagnetic Pauli limiting and the development of a spatially - modulated superconducting state.