Thermodynamic phase diagram of Fe(Se${}_{0.5}$Te${}_{0.5}$) single crystals up to 28 Tesla

Abstract

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 $C_p$ measurements up to 28 T for both $H\Vert c$ and $H\Vert ab$, 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 $T\to T_c$ 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 $H\Vert ab$) which is the consequence of a very small value of the coherence length ${\xi }_c(0)\sim 4\mathring{A}$ (and ${\xi }_{ab}(0)\sim 15\mathring{A}$), 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 $T\to T_c$. 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.

10 pages, 5 figures

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