Anisotropic fluctuations and quasiparticle excitations inFeSe0.5Te0.5

Abstract

We present data for the temperature dependence of the magnetic penetration depth $\ensuremath{\lambda}(T)$, heat capacity $C(T)$, resistivity $\ensuremath{\rho}(T)$, and magnetic torque $\ensuremath{\tau}$ for highly homogeneous single-crystal samples of ${\text{Fe}}_{1.0}{\text{Se}}_{0.44(4)}{\text{Te}}_{0.56(4)}$. $\ensuremath{\lambda}(T)$ was measured down to 200 mK in zero field. We find $\ensuremath{\lambda}(T)$ follows a power law $\ensuremath{\Delta}\ensuremath{\lambda}\ensuremath{\sim}{T}^{n}$ with $n=2.2\ifmmode\pm\else\textpm\fi{}0.1$. This is similar to some 122 iron arsenides and likely results from a sign-changing pairing state combined with strong scattering. Magnetic fields of up to $B=55$ or 14 T were used for the $\ensuremath{\tau}(B)$ and $C(T)/\ensuremath{\rho}(T)$ measurements, respectively. The specific heat, resistivity, and torque measurements were used to map out the $(H,T)$ phase diagram in this material. All three measurements were conducted on exactly the same single-crystal sample so that the different information revealed by these probes is clearly distinguished. Heat-capacity data strongly resemble those found for the high-${T}_{c}$ cuprates, where strong fluctuation effects wipe out the phase transition at ${H}_{c2}$. Unusually, here we find the fluctuation effects appear to be strongly anisotropic.