Anisotropic fluctuations and quasiparticle excitations in FeSe _0.5 Te_0.5

We present data for the temperature dependence of the magnetic penetration depth λ (T), heat capacity C (T), resistivity ρ (T), and magnetic torque τ for highly homogeneous single-crystal samples of Fe_1.0 Se_0.44 (4) Te_0.56 (4). λ (T) was measured down to 200 mK in zero field. We find λ (T) follows a power law Δλ∼ Tn with n=2.2±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 τ (B) and C (T) /ρ (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- Tc cuprates, where strong fluctuation effects wipe out the phase transition at Hc2. Unusually, here we find the fluctuation effects appear to be strongly anisotropic.