Anisotropic fluctuations and quasiparticle excitations in FeSe0.5Te0.5

We present data for the temperature dependence of the magnetic penetration depth lambda(T), heat capacity C(T), resistivity rho(T), and magnetic torque tau for highly homogeneous single-crystal samples of Fe1.0Se0.44(4)Te0.56(4). lambda(T) was measured down to 200 mK in zero field. We find lambda(T) follows a power law Delta lambda similar to T-n 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 tau(B) and C(T)/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.