Cho et al. [Phys. Rev. B, 84, 174502 (2011)] have reported on the temperature dependence of the London penetration depth deduced from Tunnel Diode Oscillator (TDO) measurements in optimally doped Fe(Se,Te) single crystals. According to their analysis
, these measurements chould suggest a nodeless two-gap pairing symmetry with strong pair breaking effects. However, to reach this conclusion, the authors fit the temperature dependence of the superfluid density with a two band {it clean} limit model which is incompatible with the presence of strong pair breaking effects, deduced from the $T^n$ temperature dependence of the London penetration depth below $T_c/3$. Moreover they claim that their results are also ruling out the suggestion that surface conditions can significantly affect the TDO data but this conclusion is based on one very specific damaging process, and is completely ignoring the large dispersion in the previously published TDO data.
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 ($T_c sim 14$ K). The thermodynamic upper critical field $H_{c2}$ lines has been deduced from $C_p$ mea
surements 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 {it thermodynamic} $H_{c2}$ line presents a very strong downward curvature for $T rightarrow T_c$ which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirm that $H_{c2}$ is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to $T/T_c sim 0.99$ (for $H|ab$) which is the consequence of a very small value of the coherence length $xi_c(0) sim 4 AA$ (and $xi_{ab}(0) sim 15 AA$), confirming the strong renormalisation of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [Phys. Rev. Lett. 104, 097002 (2010)]. $H_{c1}$ measurements lead to $lambda_{ab}(0) = 430 pm 50$ nm and $lambda_c(0) = 1600 pm 200$ nm and the corresponding anisotropy is approximatively temperature independent ($sim 4$), being close to the anisotropy of $H_{c2}$ for $Trightarrow T_c$. The temperature dependence of both $lambda$ ($propto T^2$) and the electronic contribution to the specific heat confirm the non conventional coupling mechanism in this system.
The upper and lower critical fields have been deduced from specific heat and Hall probe magnetization measurements in non-optimally doped NdFeAs(O,F) single crystals ($T_c sim 32-35$K). The anisoptropy of the penetration depth ($Gamma_lambda$) is tem
perature independent and on the order of $4.0 pm 1.5$. Similarly specific heat data lead an anisotropy of the coherence lenght $Gamma_xi sim 5.5 pm 1.5$ close to $T_c$. Our results suggest the presence of rather large thermal fluctuations and to the existence of a vortex liquid phase over a broad temperature range ($sim 5$K large at 2T).
