Thermodynamic phase diagram of Fe(Se0.5Te0.5) 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 (Se0.5Te0.5) single crystals (T-c similar to 14 K). The thermodynamic upper critical field H-c2 lines has been deduced from C-p measurements up to 28 T for both H parallel to c and H parallel to 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 -> Tc 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 Hc2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T/T-c similar to 0.99 (for H parallel to ab) which is the consequence of a very small value of the coherence length xi(c) (0) similar to 4 angstrom [and xi(ab) (0) similar to 15 angstrom], confirming the strong renormalization of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [A. Tamai, A. Y. Ganin, E. Rozbicki, J. Bacsa, W. Meevasana, P. D. C. King, M. Caffio, R. Schaub, S. Margadonna, K. Prassides, M. J. Rosseinsky, and F. Baumberger, Phys. Rev. Lett. 104, 097002 (2010)]. H-c1 measurements lead to lambda(ab) (0) = 430 +/- 50 nm and lambda(c)(0) = 1600 + 200 nm and the corresponding anisotropy is approximatively temperature independent (similar to 4), being close to the anisotropy of H-c2 for T -> T-c. The temperature dependence of both lambda(proportional to T-2) and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system.