Thermodynamic phase diagram of Fe ( Se_0.5 Te_0.5 ) single crystals in fields up to 28 tesla

We report on specific heat (C_p), transport, Hall probe, and penetration depth measurements performed on Fe (Se_0.5 Te _0.5) single crystals (T_c ∼14K). The thermodynamic upper critical field H_c2 lines has been deduced from C_p measurements up to 28 T for both H||c and H||ab, and compared to the lines deduced from transport measurements (up to 55 T in pulsed magnetic fields). We show that this thermodynamic H_c2 line presents a very strong downward curvature for T→ T_c which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that H_c2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T/ T_c ∼0.99 (for H||ab) which is the consequence of a very small value of the coherence length ξ_c (0) ∼4Å [and ξ_ab (0) ∼15Å], confirming the strong renormalization of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements. H_c1 measurements lead to λ_ab (0) =430±50nm and λ_c (0) =1600±200nm and the corresponding anisotropy is approximatively temperature independent (∼4), being close to the anisotropy of H _c2 for T→ T_c. The temperature dependence of both λ (∝ T2) and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system.