We present data for the temperature dependence of the magnetic penetration depth $\lambda \left(T\right)$, heat capacity $C\left(T\right)$, resistivity $\rho \left(T\right)$, and magnetic torque $\tau$ for highly homogeneous single-crystal samples of ${\text{Fe}}_{1.0}{\text{Se}}_{0.44\left(4\right)}{\text{Te}}_{0.56\left(4\right)}$. $\lambda \left(T\right)$ was measured down to 200 mK in zero field. We find $\lambda \left(T\right)$ follows a power law $\Delta \lambda \sim T^n$ with $n=2.2\pm 0.1$. 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 $\tau \left(B\right)$ and $C\left(T\right)/\rho \left(T\right)$ measurements, respectively. The specific heat, resistivity, and torque measurements were used to map out the $\left(H,T\right)$ 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.

• Received 22 July 2010

DOI:https://doi.org/10.1103/PhysRevB.82.104514